DEPAR TMENT OF AGRICULT URE

United States 

Department of 

Agriculture

Forest Service

Pacific Northwest 

Research Station

General Technical 

Report

PNW-GTR-870

December 2012

Effects of Climatic Variability and 

Change on Forest Ecosystems:

A Comprehensive Science 

Synthesis for the U.S. Forest 

Sector
Editors

James M. Vose is a research ecologist, U.S. Department of Agriculture, Forest Service, Southern Research Station, 

Center for Integrated Forest Science and Synthesis at North Carolina State University, Department of Forestry and 

Environmental Resources, Campus Box 8008, Raleigh, NC 27695; David L. Peterson is a research biological scientist, U.S. 

Department of Agriculture, Forest Service, Pacific Northwest Research Station, Pacific Wildland Fire Sciences Laboratory, 

400 N 34th Street, Suite 201, Seattle, WA 98103; Toral Patel-Weynand is a national coordinator for bioclimatology, U.S. 

Department of Agriculture, Forest Service, Research and Development, 1601 N Kent Street, RPC 4, Arlington, VA 22209. 

The Forest Service of the U.S. Department of Agriculture is dedicated to the principle of multiple use management of the 

Nation’s forest resources for sustained yields of wood, water, forage, wildlife, and recreation. Through forestry research, cooperation with the States and private forest owners, and management of the national forests and national grasslands, it strives—

as directed by Congress—to provide increasingly greater service to a growing Nation.

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, 

national origin, sex, religion, age, disability, sexual orientation, marital status, family status, status as a parent (in education 

and training programs and activities), because all or part of an individual’s income is derived from any public assistance program, or retaliation. (Not all prohibited bases apply to all programs or activities).

If you require this information in alternative format (Braille, large print, audiotape, etc.), contact the USDA’s TARGET Center at 

(202) 720-2600 (Voice or TDD).

If you require information about this program, activity, or facility in a language other than English, contact the agency office responsible for the program or activity, or any USDA office.

To file a complaint alleging discrimination, write USDA, Director, Office of Civil Rights, 1400 Independence Avenue, S.W., 

Washington, D.C. 20250-9410, or call toll free, (866) 632-9992 (Voice). TDD users can contact USDA through local relay or 

the Federal relay at (800) 877-8339 (TDD) or (866) 377-8642 (relay voice users). You may use USDA Program Discrimination 

Complaint Forms AD-3027 or AD-3027s (Spanish) which can be found at: http://www.ascr.usda.gov/complaint_filing_cust.html 

or upon request from a local Forest Service office. USDA is an equal opportunity provider and employer.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Effects of Climatic Variability and Change on 

Forest Ecosystems:

A Comprehensive Science Synthesis for the 

U.S. Forest Sector

James M. Vose, David L. Peterson, and Toral Patel-Weynand, Editors

Published by:

U.S. Department of Agriculture 

Pacific Northwest Research Station

Forest Service

Portland, Oregon

General Technical Report PNW-GTR-870

December 2012
ii

GENERAL TECHNICAL REPORT PNW-GTR-870

Abstract

Vose, James M.; Peterson, David L.; Patel-Weynand, Toral, eds. 2012. Effects of climatic variability and change on forest 

ecosystems: a comprehensive science synthesis for the U.S. forest sector. Gen. Tech. Rep. PNW-GTR-870. Portland, OR: 

U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 265 p. 

This report is a scientific assessment of the current condition and likely future condition of forest resources in the United 

States relative to climatic variability and change. It serves as the U.S. Forest Service forest sector technical report for the 

National Climate Assessment and includes descriptions of key regional issues and examples of a risk-based framework for assessing climate-change effects. 

By the end of the 21st century, forest ecosystems in the United States will differ from those of today as a result of changing climate. Although increases in temperature, changes in precipitation, higher atmospheric concentrations of carbon dioxide 

(CO2), and higher nitrogen (N) deposition may change ecosystem structure and function, the most rapidly visible and most 

significant short-term effects on forest ecosystems will be caused by altered disturbance regimes. For example, wildfires, 

insect infestations, pulses of erosion and flooding, and drought-induced tree mortality are all expected to increase during the 

21st century. These direct and indirect climate-change effects are likely to cause losses of ecosystem services in some areas, 

but may also improve and expand ecosystem services in others. Some areas may be particularly vulnerable because current 

infrastructure and resource production are based on past climate and steady-state conditions. The ability of communities 

with resource-based economies to adapt to climate change is linked to their direct exposure to these changes, as well as to 

the social and institutional structures present in each environment. Human communities that have diverse economies and are 

resilient to change today will also be prepared for future climatic stresses. 

Significant progress has been made in developing scientific principles and tools for adapting to climate change through 

science-management partnerships focused on education, assessment of vulnerability of natural resources, and development of 

adaptation strategies and tactics. In addition, climate change has motivated increased use of bioenergy and carbon (C) sequestration policy options as mitigation strategies, emphasizing the effects of climate change-human interactions on forests, as 

well as the role of forests in mitigating climate change. Forest growth and afforestation in the United States currently account 

for a net gain in C storage and offset approximately 13 percent of the Nation’s fossil fuel CO2 production. Climate change 

mitigation through forest C management focuses on (1) land use change to increase forest area (afforestation) and avoid 

deforestation, (2) C management in existing forests, and (3) use of wood as biomass energy, in place of fossil fuel or in wood 

products for C storage and in place of other building materials. Although climate change is an important issue for management and policy, the interaction of changes in biophysical environments (e.g., climate, disturbance, and invasive species) and 

human responses to those changes (management and policy) will ultimately determine outcomes for ecosystem services and 

people. 

Although uncertainty exists about the magnitude and timing of climate-change effects on forest ecosystems, sufficient 

scientific information is available to begin taking action now. Building on practices compatible with adapting to climate 

change provides a good starting point for land managers who may want to begin the adaptation process. Establishing a foundation for managing forest ecosystems in the context of climate change as soon as possible will ensure that a broad range of 

options will be available for managing forest resources sustainably.

Keywords: Adaptation, carbon, climate change, climate-change effects, climate-smart management, ecological disturbance, forest ecosystems, mitigation, National Climate Assessment.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

English Equivalents

When you know: Multiply by: To find:

Millimeters (mm) 0.0394 Inches

Centimeters (cm) .394 Inches

Meters (m) 3.28 Feet

Kilometers (km) .621 Miles

Hectares (ha) 2.47 Acres

Square meters (m2

) 10.76 Square feet

Square kilometers (km2

) .386 Square miles

Megagrams (Mg) 1 Tons

Teragrams (Tg) 1,102,311.31 Tons

Degrees Celsius (C) 1.8 °C + 32 Degrees Fahrenheit

Acknowledgments

The editors express their profound thanks to the many 

authors who contributed excellent material to this report 

under a tight deadline. We also thank the following technical 

reviewers who provided insightful comments and suggestions on earlier drafts, thus improving the quality of the 

final publication: Craig Allen, Dan Binkley, Barry Bollenbacher, Paul Bolstad, Marilyn Buford, Catherine Dowd, 

Daniel Fagre, Bruce Goines, Margaret Griep, Sue Hagle, 

Edie Sonne Hall, Jeffrey Hicke, Randy Johnson, Evan 

Mercer, David Meriwether, James Morrison, Charles Sams, 

Amy Snover, and David Theobald. Ellen Eberhardt assisted 

with formatting the report and tracking many details of the 

editorial process. Patricia Loesche carefully edited the entire 

report and provided many helpful suggestions on writing 

quality. Robert Norheim created and revised several maps. 

Participants in the Forest Sector stakeholders workshop in 

Atlanta, Georgia, in July 2011 provided helpful input on 

the overall content of the report, owing in large part to the 

skillful facilitation and workshop design provided by Nancy 

Walters. Financial support for the editors and for publication 

was provided by the U.S. Forest Service Southern Research 

Station (James M. Vose), Pacific Northwest Research Station 

(David L. Peterson), and national research and development 

staff (Toral Patel-Weynand).
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GENERAL TECHNICAL REPORT PNW-GTR-870

Executive Summary

The forest sector technical report is a sector-wide scientific assessment of the current condition and likely future 

condition of forest resources in the United States relative to 

climatic variability and change. The assessment provides 

technical input to the National Climate Assessment (NCA) 

and serves as a framework for managing forest resources in 

the United States. The report provides technical input to the 

2013 NCA developed by the U.S. Global Change Research 

Program (USGCRP).

The Global Change Research Act of 1990 requires the 

USGCRP to produce the NCA for the President and the Congress every four years, analyzing the effects of global change 

on multiple sectors and regions in the United States. The 

USGCRP is responsible for preparing the report based on 

technical information provided by public agencies and nongovernmental organizations. The NCA evaluates the effects 

of global change on agriculture, forests, energy production 

and use, land and water resources, transportation, human 

health and welfare, human social systems, and biological diversity, projecting major trends forward for up to 100 years.

In addition, the USGCRP is tasked with providing a coordinated strategy and implementation plan for assessing the 

effects of a changing climate on the Nation. This strategy is 

being developed to provide support to the NCA and establish 

a mechanism for an ongoing assessment capability beyond 

the 2013 report. 

The forest sector technical report is the key technical 

input to the NCA forest sector chapter. To provide national 

stakeholder input to the forest sector technical report, a 

workshop was held in Atlanta, Georgia, on July 12–14, 

2011, to solicit input from public, private, and tribal forest 

stakeholders, nongovernmental organizations, academics, 

professional organizations, private corporations, and federal 

agencies. These stakeholder suggestions helped to frame 

the subject matter content and management options in the 

report, ensuring relevance for decisionmakers and resource 

managers. 

The forest sector technical report builds on the portion 

of the 2009 NCA that discussed forest ecosystems and 

incorporates new findings from scientific and management 

perspectives. The introduction provides an overview and

discusses interrelated aspects of biophysical and socioeconomic phenomena in forested ecosystems that may be 

affected by climatic variability and change, followed by 

these chapters:

• Effects of Climatic Variability and Change

• Climate Change, Human Communities, and Forests

in Rural, Urban, and Wildland-Urban Interface 

Environments 

• Adaptation and Mitigation

• Improving Scientific Knowledge

• Future Assessment Activities

• Conclusions

It is difficult to conclude whether recently observed 

trends or changes in ecological phenomena are the result of 

human-caused climate change, climatic variability, or other 

factors. Regardless of the cause, forest ecosystems in the 

United States at the end of the 21st century will differ from 

those of today as a result of changing climate. Below we 

discuss the most important issues that have emerged from 

the report, including a brief summary of regional issues.

Effects of Climate Change on 

Ecosystem Structure, Function,

and Services

A gradual increase in temperature will alter the growing 

environment of many tree species throughout the United 

States, reducing the growth of some species (especially in 

dry forests) and increasing the growth of others (especially 

in high-elevation forests). Mortality may increase in older 

forests stressed by low soil moisture, and regeneration 

may decrease for species affected by low soil moisture and 

competition with other species during the seedling stage. 

Most models preject that species habitat will move upward 

in elevation and northward in latitude and will be reduced in 

current habitats at lower elevations and lower latitudes. New 

climatic conditions may “move” faster in some locations 

than tree species can disperse, creating uncertainty about the 

future vegetation composition of these new habitats.

The high genetic diversity of most tree species confers 

tolerance of a broad range of environmental conditions, 

including temperature variation. Therefore, in many species, tree growth and regeneration may be affected more 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

by extreme weather events and climatic conditions than by 

gradual changes in temperature or precipitation. Longer dry 

seasons and multiyear droughts will often become triggers 

for multiple stressors and disturbances (e.g., fire, insects, 

invasive species, and combinations thereof). These pulses 

of biophysical disturbance will change the structure and 

function of ecosystems across millions of hectares over a 

short period of time, focusing pressure on the regeneration 

stage of forest ecosystems. Increased atmospheric carbon 

dioxide (CO2) and nitrogen deposition will potentially alter 

physiological function and productivity of forest ecosystems, 

with considerable variation in response among species and 

regions.

The effects of climate change on water resources will 

differ by forest ecosystem and local climatic conditions, as 

mediated by local management actions. Higher temperature 

during the past few decades has already decreased snow 

cover depth, duration, and extent, a trend that will probably 

continue with further warming. Decreased snow cover will 

exacerbate soil moisture deficit in some forests, which may 

decrease tree vigor and increase susceptibility of forests to 

insects and pathogens. As climatic extremes increase and 

forest ecosystems change, water produced from forest lands 

may become more variable and of lower quality. 

Forest growth and afforestation in the United States currently account for a net gain in carbon (C) storage, offsetting 

approximately 13 percent of the Nation’s fossil fuel CO2

production. During the next few decades, Eastern forest 

ecosystems are expected to continue to sequester C through 

favorable response to elevated CO2

 and higher temperature, although retention of C will depend on maintaining 

or increasing total forest area. Western forest ecosystems 

may begin to emit C if wildfire area and insect disturbance 

increase as expected.

Future changes in forest ecosystems will occur on both 

public and private lands and will challenge our ability to 

provide ecosystem services desired by society, especially 

as human populations continue to grow and demands for 

ecosystem services increase. Climate change effects in 

forests are likely to cause losses of ecosystem services in 

some areas (e.g., timber production, water supply, recreational skiing), but they may improve and expand ecosystem 

services in others (e.g., increased growth of high-elevation 

trees, longer duration of trail access in high-snow regions). 

Some areas may be particularly vulnerable because current 

infrastructure and resource production are based on past 

climate and the assumption of steady-state natural resource 

conditions. Any change in forest ecosystems that affects 

water resources will typically result in a significant loss of 

ecosystem services.

Effects of Disturbance Regimes

The most rapidly visible and significant short-term effects 

on forest ecosystems will be caused by altered disturbance 

regimes, often occurring with increased frequency and severity. Interacting disturbances will have the biggest effects 

on ecosystem responses, simultaneously altering species 

composition, structure, and function. The type and magnitude of disturbances will differ regionally and will pose 

significant challenges for resource managers to mitigate and 

reduce damage to resource values:

• Wildfire will increase throughout the United States, 

causing at least a doubling of area burned by the mid-21st

century.

• Insect infestations, such as the current advance of bark 

beetles in forests throughout the Western United States 

and Canada, will expand, often affecting more land area 

per year than wildfire.

• Invasive species will likely become more widespread, 

especially in areas subject to increased disturbance and 

in dry forest ecosystems.

• Increased flooding, erosion, and movement of sediment 

into streams will be caused by (1) higher precipitation 

intensity in some regions (e.g., Southern United States), 

(2) higher rain:snow ratios in mountainous regions 

(western mountains), and (3) higher area burned (western 

dry forests). These increases will be highly variable in 

space and time, affecting decisions about management of 

roads and other infrastructure, as well as access for users 

of forest land. 

• Increased drought will exacerbate stress complexes that 

include insects, fire, and invasive species, leading to 

higher tree mortality, slow regeneration in some species, 

and altered species assemblages.
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GENERAL TECHNICAL REPORT PNW-GTR-870

Managing Risk and Adapting to 

Climate Change

A risk-management framework for natural resources identifies risks and quantifies the magnitude and likelihood of 

environmental and other effects. Although risk management 

frameworks have been used (often informally) in natural 

resource management for many years, it is a new approach 

for projecting climate change effects, and some time may be 

needed for scientists and resource managers to feel comfortable with this approach. Risk assessment for climate change 

must be specific to a particular region and time period, and it 

needs to be modified by an estimate of the confidence in the 

projections being made.

Ecosystem services derived from forests are produced 

in (1) rural areas, where human population densities are 

low and forest cover dominates; (2) urban settings, where 

trees may exist in low densities but provide high value for 

direct ecosystem services; and (3) transition zones between 

rural and urban settings (wildland-urban interface [WUI]). 

Climate change will alter ecosystem services, perceptions 

of value, and decisions regarding land uses. Outcomes for 

people will be determined by the interaction between changes in biophysical environments (e.g., climate, disturbance, 

and invasive species) and human responses to those changes 

(management and policy). In recent years, C sequestration 

policy options and increased use of bioenergy emphasize 

both climate change-human interactions on forests and the 

role of forests in mitigating climate change.

Land use shifts in rural areas under climate change 

could involve conversion of forests to agricultural uses, 

depending on market conditions. Climate change is expected 

to alter productivity (local scale) and prices (market scale). 

The extent of WUI areas and urban areas are projected to 

increase, often at the expense of rural forests. Higher temperature coupled with population growth will increase the 

extent and value of urban trees for mitigating climate change 

effects, but these two factors may also increase the difficulty 

of keeping trees healthy in urban environments. 

The ability of communities with resource-based 

economies to adapt to climate change is linked to their 

direct exposure to these changes, as well as to the social and 

institutional structures present in each environment. Human 

communities that have diverse economies and are resilient to 

change today will also be better prepared for future climatic 

stresses, especially if they implement adaptation strategies 

soon. Federal agencies have made significant progress in developing scientifically based principles and tools for adapting to climate change. These tools and techniques are readily 

available in recent materials that can be supplied to public, 

private, and tribal land owners and managers for their use in 

forest management.

Regional Effects of Climatic 

Variability and Change

The report incorporates a regional perspective and highlights 

key issues for the forest sector in the NCA regions.

Alaska

• Alaskan forests are regionally and globally significant, 

and changes in disturbance regimes will directly affect 

the global climate system through greenhouse gas 

emissions and altered surface energy budgets. 

• Climate-related changes in Alaskan forests have societal 

consequences, because some forests are in proximity to 

(urban and rural) communities and provide a diversity of 

ecosystem services. 

• In interior Alaska, the most important effects of climate 

change are permafrost thawing and changes in fire 

regimes. 

• South-central Alaska will be sensitive to climate change 

because of its confluence of human population growth 

and changing disturbance regimes (insects, wildfire, 

invasive species). 

• In southeast Alaska, climatic warming will affect forest 

ecosystems primarily through effects on precipitation 

(i.e., snow versus rain).

Hawaii and the Pacific Territories

• Pacific islands are vulnerable to climate change because 

of (1) the diversity of climate-related stressors; (2) low 

financial, technological, and human resource capacities 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

to adapt to or mitigate projected effects; and (3) diverse 

and often more pressing concerns affecting island 

communities.

• Island societies and cultures based on traditional 

knowledge and institutions have provided resilience 

to these communities during past stressful periods. 

Contributing to resilience are locally based ownerships 

and management, subsistence economies, tight linkages 

between landowners and government, and opportunities 

for migration. 

• The direct effects of changing climate on forests will 

be significant and strongly dependent on interactions 

with disturbances, especially novel fire regimes that are 

expanding into new areas because of flammable invasive 

species.

• For low-lying islands, enhanced storm activity and 

severity and sea level rise will cause the relocation 

of entire communities, with the first climate refugees 

already having to relocate from homelands in the region. 

For high islands, higher temperature, expanded cover 

of invasive species, and higher fire frequency and 

severity will affect ground-water recharge, downstream 

agriculture, urban development, and tourism. 

Northwest

• Based on projections of distribution of tree species and 

forest biomes, widespread changes in the distribution 

and abundance of dominant forest species are expected, 

although the results of modeling studies differ. Forest 

cover will change faster via disturbance and subsequent 

regeneration responses, rather than through slow 

adjustment to gradual warming.

• Climate is projected to become unfavorable for Douglasfir (Pseudotsuga menziesii (Mirb.) Franco) over 32 

percent of its current range in Washington, and up to 85 

percent of the range of some pine species may be outside 

the current climatically suitable range.

• Area burned and biomass consumed by wildfire will 

greatly increase, leading to changes in ecosystem 

structure and function, resource values in the WUI, and 

expenditures for fire suppression and fuels management. 

• A combination of higher temperature and dense, lowvigor stands have increased vulnerability to bark beetles 

and other insects, and mortality is currently high in some 

dry forests.

Southwest

• Disturbance processes facilitated by climatic extremes, 

primarily multiyear droughts, will dominate the effects 

of climatic variability and change on both short- and 

long-term forest dynamics. 

• Although diebacks in species other than pinyon pine 

(Pinus edulis Engelm.) are not widespread, large fires 

and insect outbreaks appear to be increasing in frequency 

and spatial extent throughout the Southwest.

• Increased disturbance from fire and insects, combined 

with lower forest productivity at most lower elevation 

locations, will result in lower C storage in most forest 

ecosystems. The fire-insect stress complex may keep 

many low-elevation forests in younger age classes in 

perpetuity. 

• Increased fire followed by high precipitation (in winter 

in California, in early summer in much of the rest of 

the Southwest) may result in increased erosion and 

downstream sediment delivery. 

Great Plains

• Trees occur along streams, on planted woodlots, as 

isolated forests such as the Black Hills of South Dakota, 

and near the biogeographic contact with the Rocky 

Mountains and Eastern deciduous forests, providing 

significant value in riparian areas, at higher elevations, 

and within agroforestry systems.

• Tree species in mountainous regions are expected to 

gradually become redistributed to higher elevations, with 

disturbances mediating rapid change in some locations.

• Climate-driven changes in hydrology are expected to

reduce the abundance of dominant, native, earlysuccessional tree species and increase herbaceous, 

drought-tolerant, late-successional woody species 

(including nonnative species), leading to reduced habitat 

quality for riparian fauna.
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GENERAL TECHNICAL REPORT PNW-GTR-870

• The potential for increased wildfire hazard, longer 

droughts, insect outbreaks, and fungal pathogens, 

individually and in combination, could significantly 

reduce forest cover and vigor. Reduced tree distribution 

will likely have a negative effect on agricultural systems, 

given the important role of shelterbelts and windbreaks 

in reducing soil erosion.

Midwest

• Northern and boreal tree species at the southern edge 

of their current range will decrease in abundance and 

extent as their current habitat becomes less suitable (and 

moves northward) and reestablishment in a warmer 

climate becomes more difficult. Some forested wetlands 

may also disappear as the climate warms. Some oak 

and hickory species tolerant of low soil moisture may 

become more abundant.

• Increased drought and fire occurrence are expected 

to have rapid and extensive effects on the structure 

and function of forest ecosystems. Oak decline and 

invasive species are expected to become more common, 

contributing to stress complexes that include nearly two 

centuries of land use activities.

• Increased disturbance will tend to fragment forest 

landscapes that are already highly fragmented in terms 

of species, structure, and ownerships. This will reduce 

habitat connectivity and corridors for species movement.

• The large amount of private land and fine-scale 

fragmentation of forest landscapes will make it 

challenging to implement climate change adaptation. 

Outreach to private land owners will be necessary to 

ensure that climate preparedness is effective.

Northeast

• Stress complexes are especially important in northeastern 

forests, where climate interacts with nitrogen (N) deposition, tropospheric ozone, land use, habitat fragmentation, 

invasive species, insects, pathogens, and fire.

• A warmer climate will cause a major reduction of 

spruce-fir forest, moderate reduction of maple-birchbeech forest, and expansion of oak-dominated forest. 

Projections of change in suitable habitat indicate that, of 

the 84 most common species, 23 to 33 will lose suitable 

habitat under low- and high-emission scenarios, 48 to 50 

will gain habitat, and 1 to 10 will experience no change. 

• Warmer temperature will increase rates of microbial 

decomposition, N mineralization, nitrification, and 

denitrification, resulting in higher short-term availability 

of calcium, magnesium, and N for forest growth, as well 

as elevated losses of these nutrients to surface waters.

• Migratory bird species that require forest habitat are 

arriving earlier and breeding later in response to recent 

warming, with consequences for the annual production 

of young and their survival. Many bird species have 

already expanded their ranges northward.

Southeast

• Red spruce (Picea rubens Sarg.) and eastern hemlock 

(Tsuga canadensis [L.] Carrière), already declining 

in some areas, are projected to be extirpated from the 

southeast by 2100 as a result of the combined stresses of 

warming, air pollution, and insects.

• The majority of the Nation’s pulp and timber supply is 

produced in the southeast, but if temperature continues 

to increase and precipitation becomes more variable, 

conditions for pine growth may begin to deteriorate. 

Even if regional forest productivity remains high, the 

center of forest productivity could shift northward 

into North Carolina and Virginia, causing significant 

economic and social impacts.

• Increasing demand for water from a rapidly growing 

urban population, combined with increased drought 

frequency could result in water shortages in some areas 

of the Southeast. 

• Warmer temperature may increase decomposition of 

soil organic matter and emissions of CO2

, reducing the 

potential for C sequestration.

• Increased fire hazard and insect outbreaks will provide 

significant challenges for sustainable management of 

forests for timber and other uses, but may also motivate 

restoration of fire-tolerant longleaf pine (Pinus palustris

Mill.) forests.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

An Imperative for Action

Climate change will generally reduce ecosystem services 

because most human enterprises are based on past climatic 

environments and the assumption of static natural resource 

conditions. Increased forest disturbance will, at least temporarily, reduce productivity, timber value, and C storage, 

and, in some cases, will increase surface runoff and erosion. Changes in forest ecosystems that affect hydrology 

and water supply will typically result in a significant loss of 

resource value. Scientific principles and tools for adapting 

to these climate change effects focus on education, vulnerability assessment of natural resources, and development of 

adaptation strategies and tactics. The hallmark of successful 

adaptation efforts in the United States has been sciencemanagement partnerships that work collaboratively within 

public agencies and externally with various stakeholders. 

Several recent case studies of adaptation for national forests 

and national parks are now available and can be emulated by 

other land management organizations.

Although uncertainty exists about the magnitude and 

timing of climate change effects on forest ecosystems, sufficient scientific information is available to begin taking 

action now. Managing simultaneously for C and for on-theground implementation of adaptation plans is challenging 

in both public and private sectors; however, implementation 

can be increased through effective exchange of information 

and success stories. Land managers are already using “climate-smart” practices, such as thinning and fuel treatments 

that reduce fire hazard, reduce intertree competition, and increase resilience in a warmer climate. Building on practices 

compatible with adapting to climate change provides a good 

starting point for land managers who may want to begin the 

adaptation process. Establishing a framework for managing 

forest ecosystems in the context of climate change as soon as 

possible will ensure that a broad range of options is available 

for managing forest resources sustainably.

We are optimistic that a proactive forest sector will 

make the necessary investments to work across institutional 

and ownership boundaries by developing, sharing, and 

implementing effective adaptation approaches. This will be 

accomplished by (1) embracing education on climate science 

for resource professionals and the general public; (2) ensuring accountability and infusing climate change into organizational efforts (e.g., management plans and projects); (3) 

implementing an all-lands approach through collaboration 

across administrative, political, and ownership boundaries; 

and (4) streamlining planning processes and establishing 

projects on the ground. The twofold challenge of adapting 

to climate change and managing C in the broader context 

of sustainable forest management will require creativity by 

future generations of forest resource managers. In the short 

term, management strategies that are relatively inexpensive, 

have few institutional barriers, and produce timely results 

can be rapidly implemented. For adaptation, examples 

include reducing nonclimatic stressors in forests (e.g., nonnative pathogens), implementing fuel reduction, and reducing stand densities. For C management, examples include 

reducing deforestation, increasing afforestation, reducing 

wildfire severity, increasing tree growth, and increasing use 

of wood-based bioenergy. Specific strategies and actions 

will differ by location, inherent forest productivity, and local 

management objectives.

Coordinating adaptation and C management will help 

optimize implementation across specific landscapes. For 

example, fuel reduction treatments can reduce wildfire 

severity in dry forests (adaptation) and provide material for 

local bioenergy use (C management). In the near term, we 

anticipate that federal agencies will continue to lead the development of science-management partnerships and collaborative approaches to climate-smart management, although 

(static) legal, regulatory, and institutional constraints will 

continue to deter timely responses to (dynamic) climatecaused changes in forest ecosystems. Successful adaptation 

strategies and C management will likely accelerate across 

large landscapes as community-based partnerships integrate 

climate change-related concerns into sustainable stewardship 

of natural resources.
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GENERAL TECHNICAL REPORT PNW-GTR-870

Contents

1 Chapter 1: Introduction 

David L. Peterson and James M. Vose

4 Literature Cited

7 Chapter 2: Effects of Climatic Variability and Change 

Michael G. Ryan and James M. Vose

7 Introduction

8 Projected Changes in Future Climate

8 Scenarios for Projecting Future Climate

9 Temperature Projections

15 Sea Level Rise

16 Key Findings

16 Key Information Needs

16 Effects of Climate Change on Disturbance Regimes

16 Fire

18 Key Findings

18 Key Information Needs

18 Insects and Pathogens

18 General Concepts

21 Climate and Biotic Disturbances

27 Impacts and Interactions With Other Disturbances

28 Future Vulnerabilities and Opportunities

28 Key Findings

28 Key Information Needs

28 Invasive Plants

32 Interactions Between Climate Change and Plant Invasion

32 Temperature, Precipitation, and Carbon Dioxide

33 Disturbance and Resource Availability

34 Key Findings

34 Key Information Needs

34 Erosion, Landslides, and Precipitation Variability

35 Erosion and Landslides

36 Drought and Water Supply

36 Key Findings

36 Key Information Needs

36 Disturbance Interactions

36 Thresholds

38 Stress Complexes: From Conceptual to Quantitative Models

40 Uncertainties

42 Key Findings
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

42 Key Information Needs

42 Effects of Climate Change on Forest Processes

42 Carbon and Nutrient Cycling

43 Response of Forest Carbon Cycling Processes to Increased Temperature, Changes in Precipitation,

Increased Carbon Dioxide, Nitrogen Deposition, and Tropospheric Ozone

46 Effects on Nutrient Cycling

46 Key Findings

47 Key Information Needs

47 Forest Hydrological Processes

47 Forest Evapotranspiration and Streamflow

47 Elevated Atmospheric Carbon Dioxide

48 Warmer Temperatures and Drought

48 Changing Species Composition

49 Snowmelt

50 Soil Infiltration, Ground Water Recharge, and Lateral Redistribution

50 Carbon and Water Tradeoffs

50 Key Findings

50 Key Information Needs

50 Forest Tree Species Distributions

51 Species Distribution Models

52 Process Models

52 Demography Studies

57 Dispersal and Migration Models

57 Assisted Migration

58 Key Findings

58 Key Information Needs

58 Effects of Altered Forest Processes and Functions on Ecosystem Services

61 Conclusions

63 Literature Cited

97 Chapter 3: Climate Change, Human Communities, and Forests in Rural, Urban, and Wildland-Urban 

Interface Environments

David N. Wear and Linda A. Joyce

97 Introduction

97 Socioeconomic Context: Ownership, Values, and Institutions

98 Forest Ownership Patterns in the United States

101 Economic Contributions of Forests

103 Policy Context of Forest Management in Response to Climate Change

105 Interactions Among Forests, Land Use Change, and Climate Change

105 Interactions Among Forests, Agricultural Land Use, and Climate Change

in Rural Environments
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GENERAL TECHNICAL REPORT PNW-GTR-870

107 Interactions Between Trees and Climate in Urban Environments

108 Effects of Climate Change on Urban Trees

108 Effects of Urban Trees on Climate Change

109 Interactions Between Climate Change and the Wildland-Urban Interface

110 Disturbances in the Wildland-Urban Interface

111 Multiple Stressors on Wildlands in the Interface

112 Social Interactions With Forests Under Climate Change

112 Natural Resource-Based Communities

114 Tribal Forests

116 Social Vulnerability and Climate Change

117 Conclusions

119 Literature Cited

125 Chapter 4: Adaptation and Mitigation

Constance I. Millar, Kenneth E. Skog, Duncan C. McKinley, Richard A. Birdsey, Christopher W. 

Swanston, Sarah J. Hines, Christopher W. Woodall, Elizabeth D. Reinhardt, David L. Peterson, 

and James M. Vose

125 Strategies for Adapting to Climate Change

125 Principles for Forest Climate Adaptation

125 Successful Climate Adaptation Planning and Implementation

125 Education and Training

126 Science-Management Partnerships

126 Risk and Uncertainty

127 Toolkit Approach

127 No-Regrets Decisionmaking

127 Flexibility and Adaptive Learning

127 Mixed-Models Approach

128 Integration With Other Priorities and Demands of Forest Management

128 Placing Adaptation in Context

128 Biogeography and Bioclimate

128 Scale

130 Institutional and Regulatory Contexts

130 Adaptation Strategies and Implementation

130 Overview of Forest Adaptation Strategies

134 Tools and Resources for Adaptation and Implementation

134 Web Sites

135 Tools

136 Institutional Responses

136 President’s Directive

136 U.S. Forest Service

137 U.S. Department of the Interior (DOI)
xiii

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

140 Regional Integrated Sciences and Assessment (RISA)

142 State and Local Institutions

142 Western Governors’ Association (WGA)

142 Minnesota State Climate Response

143 North Carolina State Climate Response

143 State University and Academic Response

144 Industrial Forestry

144 Native American Tribes and Nations

145 Nongovernmental Organizations

146 Ski Industry

146 Examples of Regional and National Responses

146 Western United States

147 Southern United States

148 Northern United States

151 National Example

151 Challenges and Opportunities

151 Assessing Adaptation Response

152 Existing Constraints

154 Vision for the Future

156 Path to the Vision

156 Carbon Management

157 Status and Trends in Forest-Related Carbon

160 Monitoring and Evaluating Effects of Carbon Management

164 Carbon Mitigation Strategies

167 In Situ Forest Carbon Management

169 Ex Situ Forest Carbon Management

172 Mitigation Strategies: Markets, Regulations, Taxes, and Incentives

175 The Role of Public Lands in Mitigation

176 Managing Forests in Response to Climate Change

179 Literature Cited

193 Chapter 5: Improving Scientific Knowledge

James M. Vose and David L. Peterson

195 Chapter 6: Future Assessment Activities

Toral Patel-Weynand

195 Introduction

196 Regional Issues

196 Developing a Risk-Based Framework

196 Social Issues

197 Management Options
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GENERAL TECHNICAL REPORT PNW-GTR-870

197 Ecological Disturbances and Extreme Events

197 Coordination With Other Assessment Activities

198 Effects on Tribal Lands

198 Carbon Estimation

199 Conclusions

199 Literature Cited

201 Chapter 7: Conclusions

David L. Peterson and James M. Vose

201 Introduction

201 Forest Disturbance

202 Forest Processes

203 Species Distributions

203 Risk and Social Context

204 Preparing for Climate Change

205 Appendix 1: Regional Summaries

205 Alaska

210 Hawaii and U.S.-Affiliated Pacific Islands

215 Northwest

219 Southwest

223 Great Plains

227 Midwest

231 Northeast

238 Southeast

243 Appendix 2: Risk-Based Framework and Risk Case Studies

243 Risk-based Framework for Evaluating Changes in Response Thresholds and Vulnerabilities 

Dennis S. Ojima, Louis R. Iverson, and Brent L. Sohngen

246 Risk Case Study: A Framework for Assessing Climate Change Risks to Forest Carbon Stocks 

Christopher W. Woodall and Grant M. Domke

249 Risk Assessment for Wildfire in the Western United States

David L. Peterson and Jeremy S. Littell

253 Risk Assessment for Forested Habitats in Northern Wisconsin 

Louis R. Iverson, Stephen N. Matthews, Anantha Prasad, Matthew P. Peters, and Gary W. Yohe

256 Risk Assessment for Two Bird Species in Northern Wisconsin 

Megan M. Friggens and Stephen N. Matthews

259 Appendix 3: Western Mountain Initiative Synthesis

259 Response of Western Mountain Ecosystems to Climatic Variability and Change: A Synthesis From the Western 

Mountain Initiative

Crystal L. Raymond
1

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Chapter 1

Introduction

David L. Peterson and James M. Vose1

Projected changes in climate (temperature and precipitation means and extreme events), increased atmospheric 

carbon dioxide (CO2), and increased nitrogen deposition are 

likely to affect U.S. forests throughout this century. Effects 

will be both direct (e.g., effects of elevated CO2 on forest 

growth and water use) and indirect (e.g., altered disturbance 

regimes), and will differ temporally and spatially across the 

United States. Some of these effects may already be occurring. For example, large insect outbreaks and large wildfires 

during the past decade (Bentz et al. 2009, Turetsky et al. 

2010) are a wake-up call about the potential effects of a rapidly changing climate on forest ecosystems. Individually and 

in combination, these two major disturbance phenomena are 

reshaping some forest landscapes and may be causing longterm, possibly permanent changes in forest structure, function, and species composition (Hicke et al. 2012, McKenzie 

et al. 2004). Combined with other stressors, such as invasive 

species and air pollution (McKenzie et al. 2009), and a 

legacy of fire exclusion and other land management activities, maintaining resilience and restoring forest ecosystems 

in the face of climate change will be a major challenge for 

the 21st century and beyond (Peterson et al. 2011).

In this document, we provide a scientific assessment of 

the current condition and likely future condition of forest 

resources in the United States relative to climatic variability 

and change. This assessment, which is conducted periodically by the U.S. Global Change Research Program as a 

component of a broader assessment of the effects of climate 

change on natural resources (U.S. Global Change Research 

Program 2012), is scheduled to be completed in 2013. The 

most recent assessment of the forest sector (Ryan and

1 David L. Peterson is a research biological scientist, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research 

Station, Pacific Wildland Fire Sciences Laboratory, 400 N 34th

Street, Suite 201, Seattle, WA 98103; James M. Vose is a research 

ecologist, U.S. Department of Agriculture, Forest Service, Southern 

Research Station, Center for Integrated Forest Science and Synthesis 

at North Carolina State University, Department of Forestry and 

Environmental Resources, Campus Box 8008, Raleigh, NC 27695.

Archer 2008) provides a foundation and point of departure 

for this document. We focus on the latest observations of effects of climatic variability and change in forest ecosystems, 

supported by scientific literature, with emphasis on issues 

and solutions relevant for sustainable management of forest 

resources. 

It is difficult to conclude whether recently observed 

trends or changes in ecological phenomena are the result 

of human-caused climate change or climatic variability. 

Regardless of the cause, we emphasize the response of forest 

resources to climatic patterns observed over the past few 

decades because they are similar to climatic phenomena 

expected for the rest of the 21st century. Compared to most 

of the 20th century, these more recent patterns are associated 

with periods of warmer temperature throughout the United 

States, and to multiyear droughts (low soil moisture) in 

arid and semiarid regions of the Western United States and 

many areas of the Eastern United States (Karl et al. 2009). 

For example, Breshears et al. (2005) concluded that dieback 

of pinyon pine (Pinus edulis Engelm.) in the Southwestern 
2

GENERAL TECHNICAL REPORT PNW-GTR-870

United States was caused by “global-change type drought.” 

If extended drought will indeed be more common in the 

future, then it is reasonable to infer that this type of dieback 

will also be more common. “Global-change type” climatic 

phenomena provide a reasonable context for projecting the 

effects of climate change on forest ecosystems. 

In this document, we develop inferences from smallscale experiments (e.g., soil warming or CO2

 enrichment 

studies) and time series of natural resource data when available, while recognizing the challenges and uncertainties 

of scaling small-scale and site-specific studies in time and 

space (Peterson and Parker 1998). We also use the results of 

simulation modeling to project the effects of climate change 

on species distribution and abundance, ecosystem processes, 

ecological disturbance, and carbon dynamics. The results 

of both empirical (statistical) and process-based (mechanistic) models are presented, and we emphasize that these 

results are projections (proposed or calculated), rather than 

predictions (forecast or foretold about the future). Trends 

established by empirical data, combined with results from 

robust modeling, are a good combination on which to base 

inferences about climate change effects (e.g., Araújo et al. 

2005). In this document, climate change effects are rarely 

projected beyond 2100, the limit for most current global climate models and emission scenarios (Solomon et al. 2007). 

We have high confidence in projections through the mid-21st

century, beyond which agreement among global climate 

models diverges. 

Forest ecosystems are inherently resilient to variability 

in climate at time scales ranging from daily to millennial. 

For example, forest species distribution and abundance have 

shifted over long time scales by responding individually to 

variability in temperature, precipitation (Brubaker 1986), 

and climatic influences on wildfire and other disturbance 

regimes (Prichard et al. 2009, Whitlock et al. 2008). Gradual 

changes in mean climate or atmospheric environment 

produce gradual changes in ecosystems. However, a rapid 

increase in temperature will increase the number of extreme 

climatic periods (e.g., extended droughts), leading to more 

frequent and intense ecological disturbances, which in turn 

lead to rapid change in the composition and dynamics of 

forests (McKenzie et al. 2009). Therefore, this assessment 

often focuses on extreme events and ecological disturbance, 

because these phenomena usually produce faster, larger, 

and more persistent changes than does a gradual increase in 

temperature.

Although the short-term effects of the El Niño-Southern 

Oscillation on natural resources have been well documented, 

the effects of dominant modes of climatic variability (Atlantic Monthly Oscillation, Pacific Decadal Oscillation, Pacific 

North American pattern) provide a better understanding 

about the potential effects of climate change, because periods of warmer (and cooler) and drier (and wetter) conditions 

are experienced over two to three decades at a time. For example, in some areas of the Western United States, the warm 

phase of the Pacific Decadal Oscillation is associated with 

more area burned by wildfire than in the cool phase (Hessl 

et al. 2004, Schoennagel et al. 2007). Studies of longer term 

modes of climatic variability thus provide a window into the 

nature of a permanently warmer climate, including quantitative relationships among temperature, precipitation, and 

area burned, on which projections of the effects of different 

climatic conditions can be based.

Forests that experience frequent disturbance often have 

characteristics that enhance their capacity to survive disturbance events (resistance) or facilitate recovery after disturbance (resilience) (Millar et al. 2007). Despite this inherent 

capacity, current thinking suggests that the rapid pace and 

magnitude of climate change will exceed the resistance and 

resilience capacity of many forests, and novel ecosystems 

without historical analogs will develop (Hobbs et al. 2009, 

Williams and Jackson 2007). A significant challenge for 

resource managers is to identify areas where forests are most 

vulnerable to change (i.e., have low resistance and resilience) and where the effects of change on critical ecosystem 

services will be greatest. Among the most obvious locations 

for vulnerable forest ecosystems (and species) are those 

near the limits of their biophysical requirements (Parmesan 

and Yohe 2003). However, the complexities of fragmented 

landscapes and multiple co-occurring stressors are likely 

to change response thresholds in many forest ecosystems 

(Fagre et al. 2009), with outcomes that may be unpredictable and unprecedented (Anderson et al. 2009, Scheffer et 

al. 2009). Under these conditions, traditional approaches 
3

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

to forest management that focus on historical conditions or 

protection of rare species or communities are likely to fail. 

Management approaches that instead anticipate and respond 

to change by guiding development and adaptation of forest 

ecosystem structures and functions will be needed to sustain 

desired ecosystem services and values across large landscapes and multiple decades (Millar et al. 2007, Seastedt et 

al. 2008). In this document, we discuss new management 

approaches along with specific tools and case studies.

Uncertainty and risk are frequently discussed in this document, as mandated by general guidance for the National 

Climate Assessment. Important sources of uncertainty 

include short time series of climatological and forest effects 

data, limited spatial extent of many types of measurements, 

lack of understanding of complex ecological processes, and 

simulation models that cannot accurately represent a wide 

range of ecosystem dynamics. Risk is generally associated 

with the likelihood of exposure or effects at specific points 

in time, combined with the magnitude of the consequence 

of a particular biophysical change (Mastrandrea et al. 2010, 

Yohe and Oppenheimer 2011). Risk is inherently associated 

with human judgments and ranking (e.g., high, medium, 

low) and human values related to ecosystem services and 

perceptions (good vs. bad). When clearly articulated, in 

either qualitative or quantitative terms, uncertainty and risk 

are useful concepts for natural resource managers, decisionmakers, and policymakers (Spittlehouse and Stewart 2003). 

Incorporating risk into discussions of climate change effects 

is relatively new for the forest resources community, but we 

are optimistic that doing so will improve our ability to apply 

climate change science to the management of forest ecosystems, including the development of adaptation options.
4

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Brubaker, L.B. 1986. Responses of tree populations to 

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Hessl, A.E.; McKenzie, D.; Schellhaas, R. 2004. Drought 

and Pacific Decadal Oscillation linked to fire occurrence 

in the inland Pacific Northwest. Ecological Applications. 

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Hicke, J.A.; Allen, C.D.; Desai, A.R. [et al.]. 2012. Effects 

of biotic disturbances on forest carbon cycling in the 

United States and Canada. Global Change Biology. 18: 

7–34.

Hobbs, R.J.; Higgs, E.; Harris, J.A. 2009. Novel ecosystems: implications for conservation and restoration. 

Trends in Ecology and Evolution. 24: 599–605.

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knowledge report from the U.S. Global Change Research 

Program. New York: Cambridge University Press. 192 p.

Mastrandrea, M.D.; Field, C.B.; Stocker, T.F. [et al.]. 

2010. Guidance note for lead authors of the Intergovernmental Panel on Climate Change 5th assessment report on 

consistent treatment of uncertainties. http://www.ipcc.ch/

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warming and stress complexes in forests of western North 

America. In: Bytnerowicz, A.; Arbaugh, M.J.; Riebau, 

A.R.; Andersen, C., eds. Wildland fires and air pollution. 

The Hague, Netherlands: Elsevier Publishers: 317–337.

Millar, C.I.; Stephenson, N.L.; Stephens, S.L. 2007. 

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D.L. 2009. Holocene fire and vegetation dynamics in a 

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Ryan, M.G.; Archer, S.R. 2008. Land resources: forests 

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7

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Chapter 2

Effects of Climatic Variability and Change

Michael G. Ryan and James M. Vose1

Contributors

Matthew P. Ayres, Lawrence E. Band, Chelcy R. Ford, Paul J. Hanson, Jeffrey A. Hicke, Louis R. Iverson, Becky 

K. Kerns, Steven L. Klein, Jeremy S. Littell, Charles H. Luce, Don McKenzie, David N. Wear, and Aaron S. Weed2

1 Michael G. Ryan is a research forest ecologist, U.S. Department 

of Agriculture, Forest Service, Rocky Mountain Research Station, 

240 W Prospect Road, Fort Collins, CO 80526; James M. Vose is a 

research ecologist, U.S. Department of Agriculture, Forest Service, 

Southern Research Station, Center for Integrated Forest Science 

and Synthesis at North Carolina State University, Department of 

Forestry and Environmental Resources, Campus Box 8008, Raleigh, 

NC 27695.

2 Matthew P. Ayres is a professor of biological sciences, Dartmouth 

College, Hanover, NH 03755; Lawrence E. Band is a professor of 

geography, University of North Carolina at Chapel Hill, Campus 

Box 3220, Chapel Hill, NC 27559; Chelcy R. Ford is a research 

ecologist, U.S. Department of Agriculture, Forest Service, Southern 

Research Station, Coweeta Hydrologic Laboratory, 3160 Coweeta 

Lab Road, Otto, NC 28763; Paul J. Hanson is a group leader, 

Oak Ridge National Laboratory, Environmental Sciences Division, 

Ecosystem and Plant Services Group, Bethel Valley Road, Building 

1062, Oak Ridge, TN 37831; Jeffrey A. Hicke is an assistant professor, University of Idaho, Moscow, ID 83844; Louis R. Iverson

is a landscape ecologist, U.S. Department of Agriculture, Forest 

Service, Northern Research Station, 359 Main Road, Delaware, OH 

43015; Becky K. Kerns is a research ecologist, U.S. Department 

of Agriculture, Forest Service, Pacific Northwest Research Station, 

3200 SW Jefferson Way, Corvallis, OR 97331; Steven L. Klein is 

a research forester, U.S. Environmental Protection Agency, Western 

Ecology Division, 200 SW 35th Street, Corvallis, OR 97333; Jeremy 

S. Littell is a research ecologist, U.S. Department of the Interior, 

Geological Survey, Alaska Climate Science Center, 4210 University Drive, Anchorage, AK 99508; Charles H. Luce is a research 

hydrologist, U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station, Boise Aquatic Sciences Lab, 322 East 

Front Street, Suite 401, Boise, ID 83702; Don McKenzie is a 

biological scientist, U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Research Station, Pacific Wildland Fire Sciences Laboratory, 400 N 34th Street, Suite 201, Seattle, WA 98103; 

David N. Wear is a research forest economist, U.S. Department 

of Agriculture, Forest Service, Southern Research Station, Center 

for Integrated Forest Science and Synthesis at North Carolina State 

University, Department of Forestry and Environmental Resources, 

Campus Box 8008, Raleigh, NC 27695; Aaron S. Weed is a postdoctoral fellow, Dartmouth College, Department of Biological 

Sciences, Hanover, NH 03755.

Introduction 

Climate profoundly shapes forests. Forest species composition, productivity, availability of goods and services, 

disturbance regimes, and location on the landscape are all 

regulated by climate. Much research attention has focused on 

the problem of projecting the response of forests to changing 

climate, elevated atmospheric carbon dioxide (CO2

) concentrations, and nitrogen deposition, deepening our understanding since the publication of the last forest sector assessment 

(Ryan et al. 2008). We have many new examples of how 

changes in climate over the period 1971–2000 have affected 

forest ecosystems, including long-term monitoring data on 

forest change, multifactor experiments that document the 

potential interactions between temperature and elevated 

CO2

, and new modeling approaches that project the effect 

of projected changes in climate on forest ecosystems, their 

goods and services, and their disturbance regimes. Climate 

projections are being done on a finer spatial scale, and global 

climate models include more detail and feedbacks with terrestrial processes. Downscaled estimates from these models 

are more readily available and have been used for more 

regional and local assessments. Despite the large amount 

of new research, this new information has not substantially 

altered the primary projections made in the last assessment (Ryan et al. 2008). In this assessment, we have added 

more detail about the effects covered in the last assessment 

(especially altered disturbance regimes and potential effects 

on hydrologic processes), provided more information about 

regional effects, and covered additional topics.

Climate change, higher CO2 concentrations, and increased nitrogen (N) deposition have already significantly 

affected the Nation’s forests. These effects are projected to 

get even larger in the future as the climate warms throughout 

this century and moves further from the historical climate. 

Although projecting the response of forest ecosystems to 

global change is difficult and complex, we have a high 

degree of confidence in many of the projections made for 

larger scales and for the next few decades. Our confidence 

comes from the observed changes that have occurred in 

response to the relatively small changes in climate over the 

past 30 years. Predicted future climate will likely bring even 
8

GENERAL TECHNICAL REPORT PNW-GTR-870

more dramatic effects, because temperatures are expected 

to be 2.5 to 5.3 °C warmer than in 1971 to 2000, and large 

effects have been seen with less than 1 °C warming over 

the past 30 years. For example, snowpack is melting earlier 

in the spring, forest fires are becoming larger, bark beetles 

are moving higher in elevation and attacking species that 

were climatically protected in the past, bark beetle and other 

insect outbreaks have become larger and more frequent without very cold winters to stop them, and drought has killed 

trees in the drier regions of tree species’ ranges. For many 

factors, the aggregate ecosystem response over large areas 

is well understood, perhaps even better understood than the 

projections of future climate, which differ from model to 

model, are less certain about precipitation than temperature, 

and have less certain regional and local projections.

Sometimes, we do not know enough about the science 

to make good projections. For example, how do increased 

temperature and drought interact to affect tree mortality? Will mature trees respond to elevated CO2

? For these 

problems, further research will improve our projections. In 

addition, many outcomes rely on complex interactions and 

contingencies, making projections difficult, highly uncertain, 

or sometimes, impossible. Some of the projected climates 

will be novel, with no historical analog and hence, we 

have limited experience or data on how ecosystems might 

respond. Trees are long-lived organisms and individuals of 

some species may remain in place long after an altered climate would favor the establishment of different species. This 

is because seeds for replacement species may not move into 

an altered environment, so the best-adapted species to the 

new climate may not be available. The interaction of climate 

and disturbance will substantially alter forest ecosystems. 

As a result, species and forest ecosystem processes may not 

have time to adapt to a rapidly changing climate, and multiple disturbance and stressor interactions will make it even 

more difficult to understand and project responses to climate 

change. 

Predicting outcomes for a particular location is very uncertain, because in general, projections of future climate and 

ecosystem response for a given area are very uncertain. Over 

a very large area, patterns that are obscured by interannual 

variability at an individual location begin to emerge. For 

example, in the Western United States, the annual area 

burned by fire has increased and snowmelt has occurred earlier as temperatures have warmed. However, projecting how 

fire or snowmelt will be affected at the local or forest-stand 

scale is much more subject to contingencies and local factors 

that were not assessed in developing regional relationships, 

making the projections very uncertain at smaller spatial 

scales. 

Predictions for the long term are also uncertain. Projections of future climate differ among both the global climate 

models used and the different emission scenarios. For ecosystems, longer time periods allow more time for contingencies and unanticipated factors to shape the future, adding 

additional uncertainty.

In this chapter, we review studies that were either 

published after the last forest assessment (Ryan et al. 2008) 

or not previously covered. We summarize the state-ofknowledge on projected changes in future climate. Next, we 

discuss the potential effects of climate change on disturbance 

regimes and forest processes and their interactions. Finally, 

connections between biophysical responses and socioeconomic responses are discussed in the context of ecosystem 

services.

Projected Changes in Future Climate 

Scenarios for Projecting Future Climate

Projected changes in future climate are based on output 

from 15 global climate models (GCMs) (box 2.1).All model 

runs used future scenarios of economic growth, population 

growth, and greenhouse gas emissions scenarios that were 

intended to represent the high (A2) and low (B1) ends of 

future emissions. A2 describes a world with continuous high 

population growth, slow economic development, slow technological change, and independently operating, self-reliant 

nations. B1 describes an environmentally friendly world 

with an emphasis on global solutions to economic, social, 

and environmental stability; a global population that peaks 

in mid-century and then declines, and with rapid changes 

in the economy toward a service and information economy, 

reductions in material intensity, and the introduction of clean 

and resource-efficient technologies. For models of effects, 
9

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Box 2.1—Global climate models and emission scenarios

Most of the climate projections used to describe future climatic conditions in this report are based on model ensembles, 

that is, syntheses of the output of various global climate models (GCMs).3 The report includes output from four specific 

GCMs, as summarized below:

CCSM2 (Community Climate System Model, version 2)—U.S. National Center for Atmospheric Research 

(http://www.CESM.NCAR.edu).

CSIRO Mk3—Australian Commonwealth Scientific Industrial Research Organisation (Gordon et al. 2002).

Hadley (versions 1 to 3)—United Kingdom Hadley Center (Burke et al. 2006).

PCM (Parallel Climate Model)—U.S. National Center for Atmospheric Research (Washington et al. 2000).

This report also uses terminology that refers to standard greenhouse gas (GHG) emission scenarios as described by the 

Intergovernmental Panel on Climate Change (IPCC). Emission scenarios cited in the report are described below, in which 

A scenarios have higher GHG emissions and higher projected temperature increases than B scenarios.

A2—A2 scenarios represent a more divided world, characterized by independently operating, self-reliant nations; continuously increasing population, and regionally oriented economic development.

A1F1—A1 scenarios represent a more integrated world, characterized by rapid economic growth, a global 

population that reaches 9 billion in 2050 and then gradually declines, quick spread of new and efficient technologies, a world in which income and way of life converge between regions, and extensive social and cultural interactions worldwide. A1F1 emphasizes the use of fossil fuels.

A1B—Same as A1F1, except it emphasizes a balance of energy sources.

B1—B1 scenarios represent a more integrated, ecologically friendly world, characterized by rapid economic 

growth as in A1, but with rapid changes toward a service and information economy, population rising to 9 billion 

in 2050 and then declining as in A1, reductions in material intensity and the introduction of clean and resource efficient technologies, and an emphasis on global solutions to economic, social, and environmental instability.

B2—B2 scenarios represent a more divided but more ecologically friendly world, characterized by continuously increasing population but at a slower rate than in A2; emphasis on local rather than global solutions to economic, 

social, and environmental instability; intermediate levels of economic development; and less rapid and more fragmented technological change than in A1 and B1.

The forthcoming Fifth IPCC Assessment, scheduled for publication in 2014, will use representative concentration pathways (RCP) rather than the emission scenarios that were used in the Fourth Assessment (Solomon et al. 2007). The RCPs 

are four GHG concentrations (not emissions), named after a possible range of radiative forcing (increased irradiance 

caused by GHGs) values at the Earth’s surface in the year 2100: RCP2.6, RCP4.5, RCP6, and RCP8.5, which represent 

2.6, 4.5, 6.0, and 8.5 W·m-2, respectively (Moss et al. 2008). Current radiative forcing is approximately 1.6 W·m-2, which 

is equivalent to a global-scale warming effect of 800 terawatts.

3

 Kunkel, K.E.; Stevens, L.E.; Sun, L. [et al.]. [N.d.]. Climate of the contiguous United States. Tech. Memo. National

Oceanographic and Atmospheric Administration. On file with: North Carolina State University, 151 Patton Avenue,

Asheville, NC 28801.

some additional scenarios and GCMs were used in this 

report and are noted where appropriate. 

Trends in temperature and precipitation from weather 

stations show that the United States has warmed over the 

past 100 years, but the trends differ by region (Backlund et 

al. 2008). The southeastern United States has cooled slightly 

(<0.7 °C), and Alaska has warmed the most (~4.5 °C); other 

Northern and Western U.S. regions also show a warming 

trend (~1.5 °C). Much of the Eastern and Southern United 

States now receives more precipitation than 100 years ago, 

whereas other areas, especially in the Southwest, now receive less (Backlund et al. 2008).

Temperature Projections

Average annual air temperatures across the continental 

United States are likely to steadily increase over the next 

century under the two emission scenarios (fig. 2.1). Compared to 1971 through 2000, average annual air temperature 

will likely increase from 0.8 to 1.9 °C by 2050, from 1.4 to 

3.1 °C by 2070, and from 2.5 to 5.3 °C by 2099. The range 
10

GENERAL TECHNICAL REPORT PNW-GTR-870

Figure 2.1—Multimodel mean annual differences in temperature between the three future periods compared to 1971 to 2000, from 

15 global climate models using two greenhouse gas emission scenarios (A2 and B1). The A2 scenario is for higher greenhouse gas 

emissions than for B1 (see text). For most interior states, models project a 1.4 to 1.9 °C temperature increase, rising to 2.5 to 3.6 °C 

for 2051 to 2071, and to > 4.2 °C for 2071 to 2099, depending on the emission scenario. (Kunkel, K.E.; Stevens, L.E.; Sun, L. [et al.]. 

[N.d.]. Climate of the contiguous United States. Manuscript in preparation. On file with: NOAA’s National Climate Data Center, 151 

Patton Avenue, Asheville, NC 28801.)
11

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

of these estimated temperatures is bounded by the B1 and 

A2 emission scenarios. Within each scenario, the magnitude 

of increase depends on both latitude and proximity to coastal 

areas. Greater warming is projected in more northern and 

interior locations. For example, the largest temperature increases are projected for the upper Midwest, and the smallest 

temperature increases are projected for peninsular Florida. 

Seasonally, these two constraints on the magnitude of warming are also apparent. For the higher emission scenario, the 

least amount of warming is expected for autumn (1.9 to 3.1 

°C) and spring seasons (1.4 to 2.5 °C). Winter season shows 

the most pronounced warming across the United States, with 

little change across the South and increases up to 3.6 °C in 

the North. During the summer, greater warming is projected 

for more interior locations (up to 3.6 °C warming across the 

central United States from Kentucky to Nevada).

In addition to overall warming over the next century, 

both the number of days when maximum temperatures 

exceed 35 °C and when heat waves occur (defined as the 

number of consecutive days with maximum temperatures 

Figure 2.2—Spatial distribution of the mean change in the annual number of days with a maximum

temperature above 35 °C (A), and in the annual number of consecutive days with a maximum temperature greater than 35 °C (B) between 1971 to 2000 and 2041 to 2070. Models project that much of 

the Southeastern and Southwestern United States will experience more days with maximum temperature exceeding 35 °C, and longer runs of those days. Results are for the high (A2) emission scenario 

only, from the North American Regional Climate Change Assessment Program multimodel means (n 

= 9 GCMs). (Kunkel, K.E; Stevens, L.E.; Sun, L. [et al.]. [N.d.]. Climate of the contiguous United 

States. Manuscript in preparation. On file with: NOAA’s National Climate Data Center, 151 Patton 

Avenue, Asheville, NC 28801.)
12

GENERAL TECHNICAL REPORT PNW-GTR-870

exceeding 35 °C) are likely to increase over the next century 

(fig. 2.2). Under the higher greenhouse gas (GHG) emission 

scenario, the southeast will likely experience an additional 

month of days with maximum temperatures exceeding 35 

°C, and the Pacific Northwest and Northeast regions will 

likely experience 10 more of these days per year. Under 

future GHG emission scenarios, the United States will likely 

experience longer heat waves. In the Southwest, the average 

length of the annual longest heat wave will likely increase 

by 20 days or more. Little or no change is predicted for heat 

waves in the northwest, northeast and northern parts of the 

Great Plains and Midwest regions. Most other areas will 

likely see longer heat waves of 2 to 20 additional days. 

Precipitation projections—

Precipitation differs even more than temperature across the 

United States and through seasons and years. Any longterm trends in precipitation are less apparent within the high 

variation across years and decades. Observed data from the 

past century across the United States show that mean annual 

precipitation has significant interannual variability, with 

two particularly dry decades (1930s and 1950s) followed by 

a few relatively wet decades (1970–99); the overall result 

is a century-long increase in precipitation (Groisman et al. 

2004). 

Over the next century, multimodel mean projections

of precipitation across the entire United States generally 

predict little or no net change in precipitation, although the 

variance among models is high (fig. 2.3). Some models predict a significantly drier future (at least in some regions), and 

others a significantly wetter future. The agreement among 

models in the future forecasts for precipitation is high for 

some models (Solomon et al. 2007). For example, there is 

general consensus among GCMs that annual precipitation 

in the Southwest will decrease by 6 to 12 percent (fig. 2.4), 

whereas precipitation in the northern states will increase by 

6 to 10 percent (Easterling et al. 2000a, 2000b; Groisman 

et al. 2004; Huntington 2006; Pachuri and Reisinger 2007; 

Solomon et al. 2007). 

Many regions of the United States have experienced

increases in precipitation extremes, droughts, and floods 

over the last 50 years (Easterling et al. 2000a, 2000b;

Groisman et al. 2004; Huntington 2006; Pachuri and

Figure 2.3—Mean annual percentage of precipitation change for 

three future time periods, relative to a 1971 to 2000 reference period. Little change in annual precipitation is projected for the continental United States as a whole, but individual model projections 

differ widely. Model projections for the high (A2) and low (B1) 

emission scenarios for all three time periods used 15 GCMs. Also 

shown are results for the North American Regional Climate Change 

Assessment Program simulations for 2041–2070 and the four 

GCMs used in the NARCCAP experiment (A2 only). Plus signs 

are values for each individual model; circles show overall means. 

(Kunkel, K.E.; Stevens, L.E.; Sun, L. [et al.]. [N.d.]. Climate of 

the contiguous United States. Manuscript in preparation. On file 

with: NOAA’s National Climate Data Center, 151 Patton Avenue, 

Asheville, NC 28801.)

Reisinger 2007; Solomon et al. 2007). In most GCMs, as 

the climate warms, the frequency of extreme precipitation 

events increases across the globe, resulting in an intensification of the hydrologic cycle (Huntington 2006). For example, the upper 99th percentile of the precipitation distribution 

is projected to increase by 25 percent with a doubling of CO2

concentration (Allen and Ingram 2002). The timing and spatial distribution of extreme precipitation events are among 

the most uncertain aspects of future climate scenarios (Allen 

and Ingram 2002, Karl et al. 1995).

Drought projections—

As the climate warms from increasing GHGs, both the proportion of land experiencing drought and the duration

of drought events will likely increase (Burke et al. 2006). 

The spatial distribution of changes in drought over the 

21st century using the A2 scenario predicts significant
13

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 2.4—Mean percentage of annual differences in U.S. precipitation between three future periods relative to a 1971 to 

2000 reference period. The Northeast, northern Midwest and Northwest are projected to have slightly more precipitation, 

and the Southwest is projected to have 2 to 12 percent less precipitation, depending on the emission scenario, location, 

and time period. Means are for all 15 GCMs. (Kunkel, K.E.; Stevens, L.E.; Sun, L. [et al.]. [N.d.]. Climate of the contiguous United States. Manuscript in preparation. On file with: NOAA’s National Climate Data Center, 151 Patton Avenue, 

Asheville, NC 28801.)

drying over the United States (fig. 2.5). Globally, the Palmer 

Drought Severity Index is predicted to decrease by 0.3 per 

decade (indicating increased drought) for the first half of the 

21st century. Relative to historical figures, the percentage of 

the land surface in drought annually is predicted to increase 

in 2010–2020 from 1 to 3 percent for the extreme droughts, 

from 5 to 10 percent for the severe droughts, and from 20 to 

28 percent for the moderate droughts (fig. 2.6). This drying 

trend continues throughout the 21st century. By the 2090s, 

the percentage of the land area in drought is predicted to 

increase for extreme, severe, and moderate droughts to 

30 percent, 40 percent, and 50 percent, respectively. For 

extreme and severe droughts, the number of drought events 

is projected to double; for moderate drought the number of 

events remains stable. The duration of all forms of drought 

events also increases.
14

GENERAL TECHNICAL REPORT PNW-GTR-870

Figure 2.5—The trend in the Palmer Drought Severity 

Index (PDSI) per decade for (a) observed data and the 

mean of (b) the first half and (c) the second half of the 

21st century. The PDSI is projected to decrease by 0.5 to 1 

unit per decade for the period 2050–2096. For the PDSI, 

-1.9 to 1.9 is near normal, -2 to -2.9 is moderate drought, 

-3 to -3.9 is severe drought, and less than -4 is extreme 

drought. Projections are made by the third version of the 

Hadley Centre coupled ocean–atmosphere global climate 

model (HadCM3) with the A2 emission scenario. Figure 

from Burke et al. (2006). © British Crown Copyright 

2006, Met Office.

Figure 2.6—The projected average annual proportion of the global land surface in drought each month shows drought increasing over the current century. Drought is defined as extreme, severe, or moderate, which represents 1 percent, 5 percent, and 20 

percent, respectively, of the land surface in drought under present-day conditions. Results from the three simulations are from 

the third version of the Hadley Centre coupled ocean–atmosphere GCM (HadCM3) with the A2 emission scenario. Figure from 

Burke et al. (2006). © British Crown Copyright 2006, Met Office.
15

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 2.7—Four scenarios of projections of sea level rise from Parris et al. (2011) show sea level increasing from 0.2 to 2.0 m by 2100. The low scenario (dark blue line) is a quadratic extrapolation to the period 

1990 to 2100 of historical trends in sea level rise over the entire period of observations (1880 to 2009). The 

two intermediate scenarios (high and low) are based on averages of the A2 simulation (orange line) and B1 

simulation (green line), respectively, of four semiempirical studies. The high scenario (red line) is based on 

analysis of plausible ice melting required for a large sea level rise (2 m) to occur by 2100.

Sea Level Rise 

Global sea level rise results from changing the ocean’s 

water volume because of changes in temperature, salinity, ice melting, and land surface runoff. Global sea level 

responds to climate cycles of alternating glacial and interglacial conditions over millions of years (Kawamura et al. 

2007). Mean sea level rose by 120 m since the most recent 

ice age, at a rate of about 1 m per century. For the last 6,000 

years, sea level has remained relatively stable, with observed 

data indicating a global mean level increase of 0.17 m per 

century (Grinsted et al. 2010). As the temperature rises in 

GHG emission scenarios, a combination of factors (e.g., 

polar ice sheet melting) contributes to sea level rise. Four 

scenarios of projections of sea level rise are shown in fig. 2.7 

(Parris et al. 2011). The low scenario is a linear extrapolation of historical trends (1.7 mm·yr -1) in sea level rise over 

the entire period of tidal observations (1880 through 2009); 

the two intermediate scenarios (A2 and B1 simulations) are 

quadratic extrapolations of four semiempirical studies based 

on average sea level rise for 2100; and the high scenario is a 

quadratic extrapolation based on analysis of plausible glaciological conditions required for large sea level rise (2 m) to 

occur by 2100. Depending on the scenario, global sea level 

is projected to rise 0.2 to 2.0 m by 2100.

Satellite altimetry records show that the mean sea level 

rise since the middle of the 19th century is not uniform (fig. 

2.8). The Pacific Coast of the United States showed little sea 

level rise, consistent with tide gage records (see discussion 

in Parris et al. 2011). In contrast, sea level rise in the Gulf of 

Mexico has averaged 3.2 mm·yr -1 since 1992. Whether the 

observed spatially explicit trends will continue in the future 

is a topic of active research. For example, the spatial trend 

in the Pacific is thought to be a combination of wind stress 

patterns associated with the short-term climatic factors of 

the Pacific Decadal Oscillation (PDO) and El Niño-Southern 

Oscillation (ENSO). Because PDO and ENSO regularly shift 

phases, the likelihood is low that the observed sea level rise 

trends will continue with the same magnitude and direction.
16

GENERAL TECHNICAL REPORT PNW-GTR-870

Key Findings

• Using the A1 and B2 emission scenarios, average 

annual temperatures will likely increase from 2.5 to 5.3 

°C by 2100 relative to 1971 to 2000, and the highest 

temperature increases will likely be in the northern and 

interior United States; days with temperature higher than 

35 °C will also likely increase.

• Average annual precipitation in the Southwest will 

likely decrease 6 to 12 percent by 2100 and increase for 

northern states by 6 to 10 percent.

• Drought will likely increase and the increase will likely 

intensify as temperature increases.

• Global sea level will likely rise between 0.2 and 2.0 m by 

2100. 

Key Information Needs

• Improved projections of the timing, spatial distribution, 

and severity of extreme precipitation events.

• Expanded and more coordinated monitoring networks 

and data accessibility to enable detection and evaluation 

of changes in meso- and small-scale microclimatic 

conditions. 

Effects of Climate Change on 

Disturbance Regimes

Disturbances such as fire, insect outbreaks, disease, drought, 

invasive species, and storms are part of the ecological history of most forest ecosystems, influencing vegetation age and 

structure, plant species composition, productivity, carbon (C) 

storage, water yield, nutrient retention, and wildlife habitat. 

Climate influences the timing, frequency, and magnitude 

of disturbances (Dale et al. 2001). As the climate continues 

to change, we should expect increased disturbance through 

more frequent extreme weather events, including severe 

storms, drought, tornadoes, hurricanes, and ice storms. Indirect effects may amplify these changes, with conditions that 

favor fire, insect and pathogen outbreaks, and invasive species. In this section, we focus primarily on indirect effects of 

climate change on important forest disturbances across the 

United States.

Fire

Climate and fuels are the two most important factors

controlling patterns of fire within forest ecosystems. Climate 

controls the frequency of weather conditions that promote 

fire, whereas the amount and arrangement of fuels influences 

fire intensity and spread. Climate influences fuels on longer 

time scales by shaping species composition and productivity (Marlon et al. 2008, Power et al. 2008) and large-scale 

Figure 2.8—Geographic variability in the rate of global sea level change (1992–2010) based on three satellite 

records (TOPEX, Jason 1 and Jason 2) shows that little sea level rise occurred for the coastal United States 

during that period. Figure from NOAA Laboratory for Satellite Altimetry – Accessed November 2, 2011.
17

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

climatic patterns, such as the ENSO, PDO, Atlantic Multidecadal Oscillation, and Arctic Oscillation (Kitzberger et al. 

2007) (interior West: Collins et al. 2006; Alaska: Duffy et 

al. 2005, Fauria and Johnson 2006) are important drivers of 

forest productivity and susceptibility to disturbance. 

Current and past land use, including timber harvest, 

forest clearing, fire suppression, and fire exclusion through 

grazing (Allen et al. 2002, Swetnam and Betancourt 1998) 

have affected the amount and structure of fuels in the United 

States. For example, in the montane forests in the Southwest 

(Allen et al. 2002) and other drier forest types in the interior 

West, removal of fine fuels by grazing and fire suppression 

has increased the number of trees and fuels; these changed 

forest conditions have increased fire size and intensified fire 

behavior. In colder or wetter forests in the Western United 

States, such as subalpine forests in Yellowstone National 

Park and forests in the maritime Northwest, grazing and fire 

suppression have not altered fire regimes as extensively. 

Forests in the Northeasten United States (Foster et al. 2002) 

and the upper Midwest developed after widespread timber 

harvest, land clearing, and forest regrowth after land abandonment. These forests burn less often and with smaller fires 

than forests in other regions of the United States. Forests in 

the Southeastern United States are often managed for timber, 

and prescribed fire is generally more prevalent than uncontrolled ignitions (National Interagency Coordination Center 

2011). Prescribed fire occurs every 2 to 4 years in some firedependent ecosystems in the southeast (Mitchell et al. 2006). 

Fire suppression and deer herbivory in the central hardwoods section of the Eastern United States has pushed the 

composition towards more mesic and fire-intolerant species 

(e.g., oak-dominated to maple-dominated) (Nowacki and 

Abrams 2008).

Weather remains the best predictor of how much area 

will burn, despite the changes in land use and the resulting 

effects on fuels. Correlations between weather and either the 

area burned by fire or the number of large fires are similar 

for both presettlement fires and fires of the last few decades. 

These syntheses of fire-weather relationships for both presettlement and modern records exist in several subregions 

of the West (Northwest: Hessl et al. 2004; Heyerdahl et al. 

2002, 2008a; Southwest: Grissino-Mayer and Swetnam 

2000, Swetnam and Betancourt 1998; Northern Rockies: Heyerdahl et al. 2008b; Westwide: Littell et al. 2009; 

Westerling et al. 2003, 2006) and East (Hutchinson et al. 

2008). Presettlement fire-weather relationships are derived 

from trees scarred by fires or age classes of trees established 

after fire and independently reconstructed climate, and 

modern fire-weather comparisons are derived from observed 

fire events and observed weather occurring in the seasons 

leading up to and during the fire. These studies agree that 

drought and increased temperature are the basic mechanisms 

that promote large fires, but the effects differ by forest and 

region (Littell et al. 2009, Westerling et al. 2003). Weather 

can also influence fire through higher precipitation, increasing understory vegetation growth, which later becomes fuel 

(Littell et al. 2009, Swetnam and Betancourt 1998). Fire in 

some forests responds to drought and to precipitation enhancement of fine fuels (Littell et al. 2009). Increased temperature and altered precipitation also affect fuel moisture 

during the fire season and the length of time during which 

wildfires can burn during a given year. 

The potential effects of climate change on forest fire 

area have been assessed using statistical models that project 

area burned from climatic variables, and by using global 

climate models to predict future climatic variables (Westwide: McKenzie et al. 2004, Spracklen et al. 2009, Littell et 

al. 2010; Northwest: Littell et al. 2010; Yellowstone region: 

Westerling et al. 2011). Estimated future increases in annual 

area burned range from less than 100 percent to greater than 

500 percent, depending on the region, timeframe, methods, 

and future emissions and climatic scenario. Dynamic vegetation models have also been used to project future fire activity. Based on climate projections derived from global climate 

models over the West, these projections suggest a wide range 

of changes in biomass area burned (from declines of 80 

percent to increases of 500 percent, depending on region, climate model, and emissions scenario) (Bachelet et al. 2001). 

Future fire potential is expected to increase in summer and 

autumn from low to moderate in eastern regions of the 

South, and from moderate to high levels in western regions 

of the South (Liu et al 2010). Models have not yet estimated 

the effects of future climate on fire severity (i.e., the proportion of overstory mortality). These effects are 
18

GENERAL TECHNICAL REPORT PNW-GTR-870

less certain because severity may be more sensitive than area 

burned to arrangement and availability of fuels 

The risk posed by future fire activity in a changing 

climate can be assessed by its likely effects on human and 

ecological systems. At the wildland-urban interface, higher 

population and forest density have created forest conditions 

that are likely to experience more area burned and possibly 

higher fire severity than in the historical record. Fire risk is 

likely to increase in a warmer climate because of the longer 

duration of the fire season, and the greater availability of 

fuels if temperature increases and precipitation does not 

sufficiently increase to offset summer water balance deficit. 

Where fuels management is common, forest fuel reduction 

and restoration to presettlement tree density and ground fire 

regimes help to mitigate fire hazard under current and future 

climatic conditions. However, with current resources, only a 

small portion of the landscape can be treated. Finally, future 

fire risk may depend on whether extreme fire weather conditions will change in step with monthly to seasonal climate 

changes. Even if fire weather and ignitions do not change, 

it is likely that risk driven only by seasonal climate changes 

will increase—particularly in the wildland-urban interface 

and managed forests, where fire has been historically rare or 

fully suppressed and climate has not been as strong an influence as in wildland fires. The current increase in annual area 

burned may be partially related to increased fuels in frequent-fire forest types, in addition to more frequent weather 

conditions conducive to fire. The effects of climate change 

intersecting with these increased fuel loads in frequent-fire 

forests will be an exceptional management challenge. 

Key Findings

• Annual area burned and length of the fire season will 

likely increase throughout the United States, altering 

the structure, function, and potentially the species 

composition of forest ecosystems.

• Increased fire in the wildland-urban interface will likely 

create social and economic challenges, including higher 

fire-suppression costs.

• Hazardous fuel treatments and forest restoration will 

likely reduce fire severity at the local scale, but it is 

unlikely that treatments can be applied widely enough to 

modify fuels across large landscapes.

• Concentrating precipitation into more intense storms 

may increase fire risk through development of fine fuels 

and longer drought periods. 

Key Information Needs

• Quantifiable effects of increased fire occurrence on 

natural resource conditions and ecosystem services, 

including wildlife, water, fisheries, and C dynamics.

• Improved accuracy and spatial and temporal resolution 

of models that project extreme fire events.

• Additional empirical data on and models for interactions 

among seasonal hydrology, fuels, and fire occurrence in 

mountain environments.

Insects and Pathogens

Biotic disturbances are natural features of forests that play 

key roles in ecosystem processes (Adams et al. 2010, Boon 

2012, Hicke et al. 2012a). Epidemics by forest insects and 

pathogens affect more area and result in greater economic 

costs than other forest disturbances in the United States 

(Dale et al. 2001). By causing local to widespread tree mortality or reductions in forest productivity, insect and pathogen outbreaks have broad ecological and socioeconomic 

effects (Pfeifer et al. 2011, Tkacz et al. 2010). 

The first National Climate Assessment (Melillo et al. 

2000) projected increased disturbance in forests, especially 

from insects, and especially from bark beetles, because of 

their high physiological sensitivity to climate, short generation times, high mobility, and explosive reproductive 

potential. These projections have been upheld, and current 

observations suggest that disturbances are occurring more 

rapidly and dramatically than imagined a decade ago (boxes 

2.2 and 2.3). Understanding how these disturbances are influenced by climate change is therefore critical for quantifying and projecting effects.

General Concepts

The powerful general effect of temperature on insects

and pathogens is among the best known facts of biology 

(Gillooly et al. 2002), and recognition of climate change 
19

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Box 2.2—Mountain pine beetle and five-needle pines 

Five-needle pines, including whitebark (Pinus albicaulis Engelm.), limber (P. flexilis James), and bristlecone (P. aristata 

Engelm.) pines, play key roles in forest ecosystems of the West. They provide food resources for wildlife, affect snow 

distribution and melt, stabilize the soil, provide cover for other vegetation (Jewett et al. 2011, Logan et al. 2010), and 

are valued by the public for these services (Meldrum et al. 2011). However, these conifers are currently subjected to a 

climatically induced increase in biotic disturbance that is expected to continue in the coming decades. Mountain pine 

beetles (Dendrotonus ponderosae Hopkins) are attacking five-needle pines across the West; aerial surveys indicate that 

1 million ha were affected by five-needle pine mortality during 1997 through 2010. Research has identified higher temperatures and drier conditions as important climate drivers (Jewett et al. 2011, Logan et al. 2010, Perkins and Swetnam 

1996). These factors influence winter survival and development rate and population synchronization of beetles (Logan et 

al. 2010) as well as susceptibility of host trees (Perkins and Swetnam 1996). 

Similar epidemics occurred in the 1930s (Perkins and Swetnam 1996), also associated with a period of warmer 

years, but several differences exist between the mortality then and today. Most importantly, a cooler period followed the 

1930s that was less suitable for the beetle (Logan and Powell 2001). In contrast, the current warming trend which has 

persisted for several decades, with resultant increases in climate suitability (Logan et al. 2010) for mountain pine beetle, 

is expected to continue for decades to come (Littell et al. 2010, Logan et al. 2010). The recent beetle epidemics in fiveneedle pine stands are already more extensive than in the 1930s and are killing very old trees that survived previous outbreaks (Logan et al. 2010). Finally, white pine blister rust is predisposing whitebark pines to lethal attacks by mountain 

pine beetle (Six and Adams 2007). 

What is the future of these five-needle pine ecosystems? Given the trajectory of future warming, strong ties between temperature and beetle epidemics, and extensive mortality that has already occurred in some areas such as the 

Greater Yellowstone Ecosystem, significant consequences are expected for these forests and the ecosystem services that 

they provide (Logan et al. 2010). The recent decision of the U.S. Court of Appeals to re-list grizzly bears (Ursus arctos) 

as an endangered species in the Greater Yellowstone area cited the expectation of reduced food for bears because of 

climatic release of mountain pine beetle into whitebark pine forests.1

1 Greater Yellowstone Coalition v. State of Wyoming. No. 09-361000, 10-35043, 10-35052, 10-35053, 10-35054.

16 U.S.C. 1533(a)(1)(D). (2011).

Area affected by mortality in stands of 

whitebark, limber, and bristlecone pine in 

1997–2010 as detected by aerial surveys 

conducted by the USDA Forest Service. 

Affected area includes live and dead trees. 

Gray shading indicates locations of forest. 

Inset shows whitebark pine mortality in 2004 

in Yellowstone National Park. Credits: Polly 

Buotte, University of Idaho (map), Jeffrey 

Hicke (photo).
20

GENERAL TECHNICAL REPORT PNW-GTR-870

Box 2.3—The southern pine beetle reaches New Jersey Pinelands 

The southern pine beetle (Dendroctonus frontalis Zimmermann) is the most destructive herbivore in the most productive forests of the United States (Pye et al. 2011). Like the closely related mountain pine beetle (D. ponderosae

Hopkins), it uses aggregation pheromones to coordinate mass attacks that overwhelm the resin defenses of otherwise 

healthy trees; virtually every attacked tree dies within weeks. It has multiple generations per year (at least four to five 

in the warm Gulf Coast region), so the aggregations that typically form in spring can expand throughout the year as 

growing “spots” of tree mortality within forest landscapes. Effective suppression of these epidemics involves locating 

the spots and cutting the infested trees (Billings 2011). Effective prevention involves silvicultural thinning to reduce the 

occurrence of stands with high basal 

area (overstocked) that are especially 

suitable for beetle population growth. 

Monitoring, suppression, and prevention of southern pine beetle are 

integral to the management of pine 

ecosystems in the southeastern United 

States.

The northern distribution of 

southern pine beetle is constrained 

by the occurrence of lethal winter 

temperatures (Ungerer et al. 1999). 

As part of the first National Climate 

Assessment (Ayres and Lombardero 2000), it was estimated that 

an increase of 3 °C in minimum 

annual temperature would permit a 

northern expansion of about 180 km 

for this beetle. In fact, there was a 

regional increase of just over 3 °C 

from 1960 through 2005, and beetle 

populations are now epidemic in 

the New Jersey Pinelands, about 

200 km north of forests with a long 

history of such epidemics (Trần et 

al. 2007). Warming winters did not 

cause the current epidemic but may 

have permitted it. Given the natural 

population dynamics of southern 

pine beetle and the continued absence of lethal winter temperatures 

(which should be expected), the 

New Jersey Pinelands has entered 

a new phase where southern pine 

beetle will be influencing all aspects 

of forest ecology and management, 

as they have throughout the Southeastern United States. 

A view in October 2011 of one of many infestations of southern pine beetle in the 

New Jersey Pinelands. Aerial photo by Bob Williams, Land Dimensions. Close-up of 

southern pine beetle by Erich Vallery, USDA Forest Service. (Bottom)—Southern pine 

beetles die when winter air temperatures drop below about -17.7 °C. A subcontinental 

pattern of warmer winters has eliminated a climatic barrier to occupancy of the New 

Jersey Pinelands by the beetle and permitted an epidemic that is presently growing and 

expanding northward. 
21

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

has motivated scientific inquiry into climatic effects on the 

extent and severity of forest disturbances by insects and 

diseases. Clear examples exist of climatic effects on insects 

(boxes 2.2 through 2.4), yet the most important insects and 

pathogens of American forests remain poorly studied with 

respect to the interaction with climate and resulting effects 

on forests (tables 2.1 and 2.2).

Climate and atmospheric changes associated with 

increasing GHGs can influence biotic disturbances of forests through effects on (1) the physiology of insects and 

pathogens that cause changes in their abundance and 

distribution, (2) tree defenses and tolerance, and (3) interactions between disturbance agents and their own enemies, 

competitors, and mutualists (fig. 2.9). Current and projected 

increases in temperature can enhance forest disturbance by 

reducing winter mortality of insects and increasing their 

range northward (Paradis et al. 2008, Safranyik et al. 2010, 

Trần et al. 2007), and by increasing the development rate of 

insects and pathogens during the growing season (Bentz et 

al. 2010, Gillooly et al. 2002). Temperature increases can 

also alter phenology, such as bringing leaf maturation into 

synchrony with insect feeding (Jepsen et al. 2011) or changing the life cycle synchrony of bark beetles, which depend 

on mass attack to overwhelm tree defenses (Bentz et al. 

2010, Friedenberg et al. 2007). 

A broader set of atmospheric drivers affect tree defenses 

against, and tolerance to, herbivores and pathogens (BidartBouzat and Kliebenstein 2008, Lindroth 2010, Sturrock et 

al. 2011). Deficiencies of water or mineral nutrients can both 

increase and decrease tree defenses, depending on the severity of the deficiency, biochemical pathways, and the type of 

defense (Breshears et al. 2005, Herms and Mattson 1992, 

Lombardero et al. 2000, Worrall et al. 2008a). In addition, 

tree mortality from severe drought may permit an increase 

in bark beetles, which then become abundant enough to 

successfully attack healthy trees (Greenwood and Weisberg 

2008, Raffa et al. 2008). Limited understanding exists on 

the effects of climate on tree-pathogen interactions, despite 

a theoretical expectation for strong effects from temperature 

and moisture (Sturrock et al. 2011). Climatic sensitivity related to the joint phenology of plants, their pathogens, and 

the environment is not well studied (Grulke 2011, RohrsRichey et al. 2011). Outbreak dynamics of forest insects 

respond to interactions between herbivores and their enemies 

(Dwyer et al. 2004), and these interactions should be sensitive to temperature (Berggren et al. 2009, Klapwijk et al. 

2012), but empirical studies are rare (Siegert et al. 2009). 

Similarly, for the many forest insects that involve mutualisms with fungi, it is logical that outbreak dynamics will be 

sensitive to climatic effects on the mutualism, but studies are 

limited (Evans et al. 2011, Hofstetter et al. 2007, Lombardero et al. 2000, Six and Bentz 2007). Such interactions may 

not be predictable because of complexity and contingency.

Recent climatic patterns are likely affecting forest disturbance by insects and pathogens in North America (Raffa 

et al. 2008, Trần et al. 2007). Given the range of mechanisms 

(most still poorly studied) by which climate change can affect forest disturbance, existing scientific knowledge almost 

certainly captures only some of the current effects. 

Climate and Biotic Disturbances

Bark beetles—

Multiple species of indigenous bark beetles affect millions 

of hectares of coniferous forests in North America (table 

2.1). Major species include mountain pine beetle (Dendroctonus ponderosae Hopkins), the most important disturbance 

agent of pines in the Western United States (box 2.2), 

southern pine beetle (D. frontalis Zimmermann), the analog 

in the productive pine forests of the southeastern United 

States (box 2.3), and spruce beetle (D. rufipennis Kirby). In 

the early 2000s, severe drought, coupled with several species 

of bark beetles, killed trees of several conifer species in the 

Southwest (Ganey and Vojta 2011), most notably pinyon 

pines (Pinus edulis Engelm.) attacked by pinyon ips (Ips 

confusus LeConte) across 1.2 million ha (Breshears et al. 

2005). 

All of these bark beetles are native to North America, 

have population dynamics that are innately explosive, and 

have been exerting powerful effects on American forests for 

millennia. However, their outbreak tendencies are sensitive 

to climatic variation, and the massive extent and expanding distribution of recent outbreaks have been permitted 

or exacerbated by increasing temperatures during recent 
22

GENERAL TECHNICAL REPORT PNW-GTR-870

Box 2.4—Hemlock woolly adelgid 

Invasive insects and pathogens are an important class of biotic disturbance to American forests. A subset of invasives 

causes extensive tree mortality owing to lack of genetic resistance in host trees and the absence of natural enemies. 

Thus, nonindigenous insects and pathogens may be especially likely to cause the loss of native tree species and produce 

other substantial effects on forests, wildlife, biodiversity, and the many services provided by forest ecosystems. Climate 

change can exacerbate the effects of established invasives by permitting their expansion into previously unsuitable 

climatic regions (as with the expansion of the hemlock woolly adelgid [Adelges tsugae Amand] into New England) and 

by producing mismatches between mature trees and their new climate. Perhaps most importantly, warming is increasing 

the ports of entry where new potential invasives can become established in American forests. 

The hemlock woolly adelgid, accidentally introduced from Japan sometime before 1951, is a major biotic disturbance within American forests that has been killing eastern hemlock (Tsuga canadensis [L.] Carrière) and Carolina 

hemlock (T. caroliniana Engelm.) in advancing waves from its point of establishment in Virginia (Orwig et al. 2002). 

Hemlock woolly adelgid is an aphid-like insect that kills its American host trees slowly but inevitably. Since establishment, this insect has largely eliminated hemlocks from a large swath of eastern forests, including national icons such 

as the Shenandoah and Great Smoky Mountains National Parks. Consequences include lost value to property owners 

(Holmes et al. 2010) and persistent alterations to hydrological regimes, soil biogeochemistry, carbon stores, biodiversity, and forest composition, including promoting the establishment of undesirable invasive plants (Knoepp et al. 2011, 

Orwig et al. 2008, Peltzer et al. 2010, Stadler et al. 2006). 

Hemlocks north of the infested regions have thus far been spared by winter temperatures that are lethal to hemlock 

woolly adelgid (Parker et al. 1998). However, these conditions are changing with the amelioration of extreme winter 

temperatures in the Eastern United States (see also box 2.3), and projections under even conservative climate scenarios 

predict the loss of hemlock forests through most of the current range of hemlock (Dukes et al. 2009, Fitzpatrick et al. 

2012, Paradis et al. 2008). 

Dead mature eastern hemlocks 

killed by hemlock woolly 

adelgid in western North 

Carolina (photo: Forest Health 

Management International. 

Bugwood.org. http://www.

invasive.org/browse/detail.

cfm?imgnum=2167012. (4 

December 2012). (photo: 

http://upload.wikimedia.org/

wikipedia/commons/a/a0/

Adelges_tsugae_3225077.

jpg).
23

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Syndrome Herbivore Hosts

 References

General references Studies related to climatea

Defoliation by autumnal 

moth

Epirrita autumnatab Many broadleaved trees and 

conifers

Selas et al. 2001, 

Tenow et al. 2007

Jepsen et al. 2008, Peterson and

Nilssen 1996 (T), Virtanen et al. 

1998 (T)

Defoliation by gypsy moths 

and tussock moths

Lymantria dispar,b

 Orgyia 

spp.

Quercus spp., many other 

broadleaved trees and 

conifers

Mason 1996 Lindroth et al. 1993 (CO2

), Williams 

and Liebhold 1995 (P,T)

Defoliation by budworms Choristoneura fumiferana, C. occidentalis, 

C. pinus

Abies spp., Pseudotsuga spp., 

Picea spp., Pinus spp.

Royama 1984 Fleming 1996 (T), Rauchfuss et al. 

2009 (P,T), Ryerson et al. 2003 (P), 

Volney and Fleming 2000 (T)

Defoliation by gracillariid 

leaf miners

Archips pinus,

 Micrurapteryx salicifoliella, Phyllocnistis 

populiella

Populus tremuloides, Salix 

spp.

Furniss et al. 2001, 

Wagner et al. 

2008

Defoliation by loopers Enypia griseata, Nepytia 

spp.

Abies spp., Pseudotsuga spp., 

Picea spp., Pinus spp., 

Thuja spp.

Munroe 1963, 

Rindge 1967, Stevens et al. 1983

Defoliation by tent caterpillars

Malacosoma spp. Prunus spp., Populus spp., 

Betula spp., Nyssa spp., 

other broadleaved trees

Rejmánek et al. 

1987

Frid and Myers 2002 (T), Lindroth et 

al. 1993 (CO2), Volney and Fleming 

2000 (T)

Infestations by Asian longhorned beetle

Anoplophora glabripennisb

Acer spp., Ulmus spp., Populus spp.

Cavey et al. 1998, 

Dodds and Orwig 

2011

Keena 2006 (T), Keena and Moore 

2010 (T), Peterson et al. 2004 (T)

Infestations by bronze birch 

borer

Agrilus anxius Betula spp. Nielsen et al. 2011 Jones et al. 1993 (P,T)

Infestations by emerald ash 

borer

Agrilus planipennisb Fraxinus spp. Cappaert et al. 2005 Crosthwaite et al. 2011 (T)

Infestations by goldspotted 

oak borer

Agrilus auroguttatus Quercus spp. Coleman et al. 2011

Infestations by mountain 

pine beetle

Dendroctonus ponderosae Pinus spp. Safranyik and Carroll 2006 

Bentz et al. 2010 (P,T), Powell et al. 

2000 (T), Raffa et al. 2008 (P,T), 

Regnière and Bentz 2007 (T)

Infestations by pine engraver beetles

Ips spp. Pinus spp. Schenk and Benjamin 1969 

Breshears et al. 2005 (P,T), Lombardero et al. 2000 (T), Raffa et al. 

2008 (T)

Infestations by southern 

pine beetle

Dendroctonus frontalis Pinus spp., chiefly southern 

pine

Reeve et al. 1995 Friedenberg et al. 2007 (T), Lombardero et al. 2000 (T), Tran et al. 2007 

(T), Ungerer et al. 1999 (T), Waring 

et al. 2009 (T)

Infestations by spruce 

aphid

Elatobium abietinumb Picea spp. Lynch 2004 Powell 1974 (T), Powell and Parry 

1976 (T)

Infestations by spruce 

beetle

Dendroctonus rufipennis Picea spp. Allen et al. 2006 Bentz et al. 2010 (T), Berg et al. 2006 

(T), Hebertson and Jenkins 2008 

(P,T) 

Infestations by western 

pine beetle

Dendroctonus brevicomis Pinus spp., chiefly P. ponderosa

Liebhold et al. 1986 Evangelista et al. 2011 (T)

Infestations by white pine 

weevil

Pissodes strobi Pinus spp., Picea spp. Lavallée et al. 1996 Sullivan 1961 (T)

Infestations by woolly 

adelgids

Adelges piceae,b A. 

tsugaeb

Abies fraseri, A. balsamea, 

Tsuga spp.

McClure 1991 Butin et al. 2005 (T), Evans and

Gregoire 2007 (T), McClure 1989 

(T), Paradis et al. 2008 (T), Trotter 

and Shields 2009 (T)

Browsing by deer, elk, 

hares, and moose

Odocoileus spp., Cervus 

canadensis, Alces alces

Many broadleaved trees and 

some conifers

Gill 1992, Pease

et al. 1979, Ross 

et al. 1970

Simard et al. 2010 (T)

a Letters following references denote studies considering the effects of precipitation (P), temperature (T), or carbon dioxide (CO2

). b

 Nonindigenous to North America.

Source: Updated from Ayres and Lombardero (2000).

Table 2.1—Insects that are notable agents of biological disturbance in North American forests and therefore 

candidates for consequential changes to disturbance regimes as a result of climate change
24

GENERAL TECHNICAL REPORT PNW-GTR-870

 References

Syndrome Pathogen/parasite/decline Hosts General references Studies related to climatea

Alder canker Valsa melanodiscus Alnus spp. Worrall et al. 2009 Worrall et al. 2010 (T)

Annosum root rot Heterobasidion annosum Most conifers, some 

broadleaved trees

Stanosz et al. 1995 Boland et al. 2004, Witzell 

et al. 2011 (T)

Anthracnose leaf disease Discula destructiva, Glomerella cingulata, Colletotrichum gloeosporioides,

others

Quercus spp., Fraxinus

spp., Platanus spp., 

Cornus spp.

Stanosz 1993 Chakraborty et al. 2000 

(CO2

), Holzmueller et al. 

2006 (P)

Armillaria root rot Armillaria spp. Broadleaved trees and 

conifers, e.g., Acer

spp., Picea spp.

Entry et al. 1991, 

Smith et al. 1994

Dukes et al. 2009, Sturrock 

et al. 2011

Beech bark disease Nectria spp. (and associated scale insects 

Cryptococcus fagisugab

 and Xylococculus betulae)

Fagus grandifolia Busby and Canham 

2011, Garnas et al. 

2011a

Dukes et al. 2009, Garnas 

et al. 2011b (P,T)

Butternut canker Ophiognomonia clavigignenti-juglandacearumab

 (= Sirococcus clavigignenti-juglandacearumab)

Juglans cinerea Broders et al. 2011, 

Harrison et al. 1998 

Chestnut blight Cryphonectria parasiticab Castanea dentata McKeen 1995

Dothistroma needle blight Dothistroma septosporum and D. pini Many conifers, Pinus

spp.

Welsh et al. 2009 Sturrock et al. 2011, Watt

et al. 2009 (P,T), Woods

et al. 2005 (P)

Dutch elm disease Ophiostoma novoulmib (and associated 

bark beetles Hylurgopinus rufipes and 

Scolytus multistriatusb

)

Ulmus spp. Holmes 1980 Boland et al. 2004

Dwarf mistletoe Arceuthobium spp. Pinus spp. Synder et al. 1996 Brandt et al. 2004 (T), 

Stanton 2007 (P,T)

Fusiform rust Cronartium quercuum Pinus spp., chiefly 

southern pine

Doudrick et al. 1996, 

Nelson et al. 1996

Runion et al. 2010 (CO2

)

Laurel wilt Raffaelea lauricola (and associated bark 

beetle Xyleborus glabratus)b

Lauraceae Fraedrich et al. 2008, 

Harrington et al. 

2011

Koch and Smith 2008 (T)

Oak wilt disease Ceratocystis fagacearum Quercus spp. Juzwik et al. 2008 Boland et al. 2004, Tainter 

1986 (T) 

Phytophthora root disease Phytophthora cinnamomib Quercus spp., Castanea spp., Abies spp.

Griffin et al. 2009, 

Hardham 2005 

Zentmyer et al. 1979 (T), 

Bergot et al. 2004 (T)

Pitch canker Fusarium circinatumb Pinus spp. Gordon et al. 1996 Ganley et al. 2009 (P,T),

Inman et al. 2008 (T),

Runion et al. 2010 (CO2

), 

Watt et al. 2011 (P,T)

Procera, black stain, and 

other Leptographium root 

diseases

Leptographium spp. Many conifers, e.g., 

Pinus spp.

Harrington and Cobb 

1983, Jacobi et al. 

2008

Scleroderris canker Gremmeniella abietina (= Sclerroderris 

lagerbergii and Ascocalyx abietina)b

Conifers Hamelin et al. 1993, 

Laflamme 2005

Boland et al. 2004,

Donaubauer 1972, Venier

et al. 1998 (P,T) 

Sudden aspen decline — Populus tremuloides Hogg and Schwarz 

1999

Hogg et al. 2002, 2008 

(P,T), Rehfeldt et al. 

2009 (P,T), Worrall et al. 

2008ab, 2010 (P,T)

Sudden oak death Phytophthora ramorumb Quercus spp., Lithocarpus spp.

Spaulding and Rieske 

2011, Vaclavik et al. 

2010

Venette and Cohen 2006 

(P,T) 

Swiss needle cast Phaeocryptopus gaeumannii Pseudotsuga menziesii Hansen et al. 2000 Manter et al. 2005 (P,T), 

Stone et al. 2008 (P,T)

Thousand cankers disease Geosmithia morbida (and associated 

bark beetle Pityophthorous juglandis)

Juglans spp. Grant et al. 2011, 

Kolarik et al. 2011

White pine blister rust Cronartium ribicolab Five-needle pines, e.g., 

Pinus strobus, 

P. albicaulis

Keane et al. 1990, 

Kinloch 2003

Sturrock et al. 2011

Alaska cedar decline — Callitropsis 

nootkatensis

Wooton and

Klinkenberg 2011

Hennon et al. 2006 

(P,T),Sturrock et al. 2011

Table 2.2—Pathogens, parasites, and declines that are notable agents of biological disturbance in North 

American forests and therefore candidates for consequential changes to disturbance regimes as a result of 

climate change

— = none. a

 Letters following references denote studies considering the effects of carbon dioxide (CO2), precipitation (P), or temperature (T). b

 Nonindigenous to North America.

Source: Updated from Ayres and Lombardero (2000).
25

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

decades (Breshears et al. 2005, Raffa et al. 2008, Sherriff 

et al. 2011). Recent range expansions of bark beetles have 

been particularly notable (boxes 2.2, 2.3). Greater effects 

on forest ecosystems should be anticipated from these range 

expansions into areas with novel and naïve hosts (Cudmore 

et al. 2010). Mexican pine beetle (D. mexicanus Hopkins), 

previously known only in Mexico, has been recorded in the 

southwestern United States (Moser et al. 2005) and represents one of several species of Mexican bark beetles that 

may expand into U.S. forests with continued warming trends 

(Bentz et al. 2010, Salinas-Moreno et al. 2010). In general, 

climate change is anticipated to continue to reshape the patterns of bark beetle outbreaks in U.S. forests, with outbreak 

tendencies increasing for some species in some regions and 

decreasing in others (Bentz et al. 2010, Evangelista et al. 

2011, Littell et al. 2010). For example, the unprecedented 

absence of southern pine beetle activity since the late 1990s 

in Louisiana and east Texas may be related to climatic 

warming (Friedenberg et al. 2008).

Defoliating insects—

Defoliating insects are another broad class of continentally 

important biotic disturbances in American forests (table 2.1). 

For example, western spruce budworm (Choristoneura occidentalis Freeman) is currently important in the West (USDA 

FS 2010), and eastern boreal forests have been affected by 

many cycles of spruce budworm (C. fumiferana Clemens) 

outbreaks (Candau and Fleming 2005). Other important 

defoliators include tussock moths, tent caterpillars, gypsy 

moths, and jack pine budworm (Archips pinus Freeman) 

(table 2.1). Like bark beetles, most of the important defoliating insects are indigenous to American forests (but not gypsy 

moths). Many have cyclical outbreak dynamics involving predators, parasitoids, and pathogens of the herbivore 

(Dwyer et al. 2004). Climatic effects on these predator-prey 

interactions remain largely unstudied (Klapwijk et al. 2012). 

In general, it is less clear than with the bark beetles how climatic patterns influence the frequency, extent, and geographic distribution of defoliators in American forests. There have 

been signals from some systems of climatic effects on winter 

populations (Kemp et al. 1985, Thomson and Benton 2007, 

Thomson et al. 1984, Williams and Liebhold 1995a; but see 

Reynolds et al. 2007), drought stress of host trees (Campbell 

et al. 2006, Williams and Liebhold 1995b), and phenological 

synchronization of larval emergence and bud break (Thomson et al. 1984). Considerable uncertainty remains about 

future responses of defoliators to climate change (Dukes et 

al. 2009, Rodenhouse et al. 2009).

Plant pathogens—

We identified 21 plant pathogens that are notable agents 

of disturbance in U.S. forests and therefore top candidates 

for consequential responses to climate change (table 2.2). 

Climatic effects on these agents are far less well studied than 

for forest insects, but it can be expected from first principles 

that the severity of at least some of these pathogens will be 

affected directly by climatic influences on sporulation and 

infection, indirectly by predisposing trees to infection, or 

both (Sturrock et al. 2011). For pathogens that involve associations with insects, climatic effects on the animal associates may also be important. 

Figure 2.9—General pathways by which atmospheric 

changes associated with increasing greenhouse gases 

can influence forest disturbance from insects and 

pathogens. CO2

 = carbon dioxide, CH4 = methane.
26

GENERAL TECHNICAL REPORT PNW-GTR-870

Examples of pathogens where there is some understanding of climatic effects include Swiss needle cast, caused by 

a foliar pathogen (Phaeocryptopus gaeumannii [T. Rohde] 

Petr.) in the Pacific Northwest and which is influenced by 

winter warming and spring precipitation. Climate projections suggest an increase in Swiss needle cast distribution 

and severity (Stone et al. 2008). The susceptibility of alder 

to a cankering pathogen is related to the phenology of the 

plant, the pathogen, and water availability (Grulke 2011, 

Rohrs-Richey et al. 2011). Quaking aspen (Populus tremuloides Michx.) and Alaska cedar (Callitropsis nootkatensis

[D. Don] D.P. Little) are declining and experiencing elevated 

mortality in large areas in the United States. Sudden aspen 

decline appears to be related to drought stress (Worrall et 

al. 2010b), suggesting substantial future mortality with 

continued climate change in forests near the aridity limit for 

this species (Rehfeldt et al. 2009). Alaska cedar decline has 

been attributed to earlier snowmelt, which exposes roots to 

damage from lower temperatures (Hennon et al. 2010), and 

projected future warming is expected to cause additional 

mortality from freezing-induced root damage (Sturrock et 

al. 2011). Outbreaks of some virulent invasive pathogens 

can also be enhanced by climate (e.g., sudden oak death; 

Sturrock et al. 2011), whereas others are not very sensitive to 

climate (Garnas et al. 2011b).

The potential effects of climate change on root pathogens are difficult to project (Ayres and Lomardero 2000), but 

it will be important to understand this relationship because 

endemic root diseases are widespread in the United States 

and often have a major influence on forest dynamics and 

management. One would expect root diseases to be affected 

by both the distribution of host species and the effects of a 

changing climate on susceptibility of host species and prevalence of fungal pathogens. If a warmer climate increases 

physiological stress in a particular tree species, then it may 

be less resistant to some root diseases, potentially causing 

lower tree vigor, higher mortality in mature trees and seedlings, and lower C storage. Although some initial modeling 

of future changes in root pathogens has been attempted 

(Armillaria spp.; Klopfenstein et al. 2009), geographic specificity for host-pathogen relationships are highly uncertain 

based on current knowledge. Planting of nonhost species is

a standard silvicultural approach to avoid root disease.

Nonnative and emerging insects and pathogens—

On a global scale, biological invasions by nonindigenous 

species are at least as important as climate change for the 

sustainability of forest ecosystems and the goods and services that they provide (Seppälä et al. 2009). This pattern 

is evident in the United States, where invasive insects and 

pathogens are becoming an increasingly important component of forest disturbance (box 2.4) (Lovett et al. 2006). 

Warming, shifts in precipitation, and other alterations associated with climate change can affect forest vulnerability 

to these disturbance agents (Paradis et al. 2008, Sturrock et 

al. 2011). For example, the geographic range and incidence 

of dothistroma needle blight (Dothistroma septosporum 

[Dorog.] M. Morelet and D. pini Hulbary), which reduces 

growth of many conifers by causing premature needle 

defoliation, may shift with changing precipitation patterns 

(Woods et al. 2010). 

The primary cause of biological invasions is from global 

commerce, not climate change. However, climate change is 

strongly connected to risks from continuing invasions. Increasing temperatures are generally expanding the geographic zones where potential invasive species could survive and 

reproduce if they arrive, for example, at ports of entry on 

the Eastern Seaboard and in the Great Lakes Waterway. The 

specter of global, climate-driven increases in invasion risks 

has prompted international organizations to discuss changes 

in trade restrictions to manage associated phytosanitation 

risks (Standards and Trade Development Facility 2009). 

Outbreaks of lesser known forest insects have recently 

occurred in U.S. forests. Aspen leaf miner, (Phyllocnistis 

populiella Chambers) which reduces longevity of aspen 

leaves, has damaged 2.5 million ha of quaking aspen in

Alaska since 1996 (Wagner et al. 2008). Large areas of 

willows were damaged during two eruptive outbreaks of 

the willow leafblotch miner (Micurapteryx salicifoliella

Chambers)in the 1990s in two major river drainages in 

Alaska (Furniss et al. 2001); outbreaks of this leaf miner 

had not been previously reported. Substantial defoliation 
27

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

by Janet’s looper (Nepytia janetae Rindge) of stressed trees 

in southwestern spruce-fir forests was preceded by uncharacteristically warm winters.4

 Defoliation by Janet’s looper 

encouraged attack by opportunistic bark beetles. These 

examples are of previously rare native insects that displayed 

new eruptive behavior and caused notable forest disturbances. Our inability to anticipate disturbances by formerly 

innocuous native forest insects or pathogens is a major 

concern to forest health and monitoring.

Impacts and Interactions With Other 

Disturbances

Through their effects on tree growth and mortality, insects 

and pathogens have broad effects on ecosystem processes. 

Discussion of disturbance effects on biogeochemical cycling 

processes is presented in the “Effects of Climate Change on 

Forest Processes” section. In addition, insects and pathogens, by virtue of their host preferences, almost inevitably 

alter tree species composition within stands, can remove 

most host trees from many U.S. landscapes (tables 2.1, 2.2) 

(Lovett et al. 2006), and can modify forest types (e.g., from 

conifers to hardwoods) (Collins 2011, Orwig et al. 2002, 

Veblen et al. 1991). Because insects and pathogens often 

have size and age preferences for hosts, stands shift toward 

younger, smaller trees after biotic disturbances (Garnas 

et al. 2011a, Tchakerian and Couslon 2011, Ylioja et al. 

2005). Wildlife habitat and biodiversity are altered by forest 

insects and pathogens, especially those that kill trees (ChanMcLeod 2006). Modified food supply, such as increases in 

insects and reductions in foliage, can affect multiple trophic 

levels (Chan-McLeod 2006, Drever et al. 2009). Both positive and negative effects occur depending on species, time 

since disturbance, surviving vegetation, ecosystem type, and 

spatial extent of outbreak (Chan-McLeod 2006).

Trees damaged by insects and pathogens can have substantial socioeconomic effects. However, valuation of those 

effects remains a challenge because of nonmarket costs 

and accounting for long-term losses (Aukema et al. 2011; 

Holmes et al. 2010; Kovacs et al. 2011a, 2011b). 

4

 Ann Lynch. 2011. Personal communication. Research entomologist, Rocky Mountain Research Station, Southwest Forest Science 

Complex, 2500 S. Pine Knoll Road, Flagstaff, AZ 86001.

The economic effect of forest disturbances is difficult to 

quantify because insect and pathogen outbreaks have immediate effects on timber and pulp supply to the market and, 

if the outbreak is extensive, influence the future economic 

potential of forests. 

Valuation of forest resources is further complicated by 

difficulty in quantifying nonmarket values such as ecosystem 

services (Holmes et al. 2010). Regions with dead and dying 

trees have reduced aesthetic value (Sheppard and Picard 

2006) and housing prices (Holmes et al. 2010, Price et al. 

2010). Direct economic effects occur owing to tree removal 

and replacement, such as the $10 billion spent after emerald ash borer (Agrilus planipennis Fairmaire) infestations 

(Kovacs et al. 2010, 2011b). Indirect effects include reduced 

quality of life, enhanced perceived risk of wildfire and other 

infrastructure damage, and increased conflict regarding community responses (Flint 2006). 

Fire and biotic disturbances interact in several ways. 

Fires lead to younger stands that may be less susceptible 

to attack, and killed trees provide a food resource for some 

insects and pathogens (Parker et al. 2006). Insect-killed trees 

influence fuels and therefore fire behavior, although the effect depends on a number of factors, including the number 

of attacked trees within a stand and time since outbreak (e.g., 

Ayres and Lombardero 2000, Jenkins et al. 2008, Simard 

et al. 2011), and fire-induced increases in tree defenses can 

mitigate bark beetle risks (Lombardero and Ayres 2011). 

Extreme soil water deficits (drought) arise because of 

reduced precipitation and increased temperatures, and these 

strongly affect tree defenses against and tolerance of herbivores and pathogens (Lorio 1993). Although water limitations that reduce tree growth might also reduce tree defenses 

(Bentz et al. 2009, Sturrock et al. 2011), theory and data suggest that there may be either no effect (Gaylord et al. 2007, 

McNulty et al. 1997) or the opposite effect (Herms and 

Mattson 1992, Lombardero et al. 2000). Drought decreases 

inducible plant defenses even as it increases constitutive 

plant defenses (Lombardero et al. 2000). Thus, drought may 

increase tree susceptibility to pathogens, which generally 

evoke inducible defenses (Sturrock et al. 2011; Worrall et 

al. 2010a, 2010b). Drought facilitates population increases 

of western bark beetles. Some aggressive species such as 
28

GENERAL TECHNICAL REPORT PNW-GTR-870

mountain pine beetle are able to maintain epidemics after return to normal conditions, whereas others such as pinyon ips 

decline with alleviation of drought stress (Raffa et al. 2008). 

Future Vulnerabilities and Opportunities

Geographic changes in climate and disturbance place forests 

at risk, because mortality converts a large proportion of live 

biomass to dead, decomposing biomass, and because the 

new forest may have to establish under less climatically favorable conditions. Observations show, and theory predicts, 

that changing climate is altering biotic disturbance and will 

likely continue to do so. A changing climate may lead to 

more stressed trees that are susceptible to attack by insects 

and pathogens (Bentz et al. 2009, Sturrock et al. 2011). 

Climatic warming and elevated CO2

, through their positive effects on tree growth, may increase forest maturation 

rates in some regions of the United States (McMahon et al. 

2010, Salzer et al. 2009, Wang et al. 2011), leading to a more 

rapid transition to stands susceptible to some disturbance 

agents. Decreased disturbance by individual species may 

occur in some regions when year-round temperatures lead to 

maladaptive conditions for some bark beetles (Bentz et al. 

2010), such as the recent decreases in southern pine beetle 

damage (Friedenberg et al. 2008).

Changing climates also introduce practical problems 

for mitigation of disturbance, because geographic mismatches occur between risks and management expertise. For 

example, suppression of the pine beetle epidemic in New 

Jersey is hindered both by limited local experience with bark 

beetles and because administrative boundaries (physical and 

perceived) exist between different organizations. 

Key Findings

• Tree mortality caused by forest insects and pathogens 

likely exceeds other causes of disturbance for U.S. 

forests.

• Climate change will likely increase epidemics of forest 

insects and pathogens and related tree mortality, with 

broad consequences for forest ecosystems and their 

services. 

Key Information Needs

• Improved monitoring of biotic disturbance agents; more 

accurate quantification of the extent, severity, and types 

of effects to forests from biotic disturbance; evaluation 

of the efficacy of management responses to current 

epidemics.

• Increased understanding of how climate alters the 

abundance and effects of forest insects and pathogens, 

including interactions with other insects, pathogens, and 

disturbances, to project future biotic disturbance. 

• Increased capacity to manage risks from potential new 

invasive species, including identifying the most likely 

pathways of entry.

• Better understanding of the socioeconomic costs 

associated with biotic disturbance to forests. 

Invasive Plants

Invasive plants are recent introductions of nonnative, exotic, 

or nonindigenous species that are (or have the potential to 

become) successfully established or naturalized, and that 

spread into new localized natural habitats or ecoregions 

with the potential to cause economic or environmental harm 

(Lodge et al. 2006). This definition of ‘‘invasive’’ (1) does 

not consider native species that have recently expanded their 

range, such as juniper (Juniperus spp.) in the Western United 

States (Miller and Wigand 1994, Miller et al. 2005), (2) involves defined temporal and spatial scales, and (3) considers 

social values related to economic and environmental effects. 

An estimated 5,000 nonnative plant species exist in 

U.S. natural ecosystems (Pimentel et al. 2005) (table 2.3). 

In general, the effects of invasive plants include a reduction 

in native biodiversity, changes in species composition, loss 

of habitat for dependent species (e.g., wildlife), changes in 

biogeochemical cycling, changes in ecosystem water use, 

and alteration of disturbance regimes. Billions of dollars 

are spent every year to mitigate invasive plants or control 

their effects (Pimentel et al. 2005). Negative environmental 

effects are scale-dependent (Powell et al. 2011), with some 

subtle beneficial properties (Sage et al. 2009), on ecosystem 

function (Myers et al. 2000, Zavaleta et al. 2001). For
29

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Species Common name Growth form Environmental impacts

Acer platanoides L. Norway maple Tree Reduces abundance and diversity of native species; alters of community 

structure (e.g., shading of understory)

Ailanthus altissima Desf. Tree of heaven Tree Alters ecosystem processes (e.g., increases soil nitrogen, alters successional trajectories); displaces native vegetation; allelopathic; roots can 

damage buildings and sewer lines; risk to human health (pollen allergies, sap-caused dermatitis)

Alliaria petiolata (m. Bieb.) 

Cavara and Grande

Garlic mustard Biennial forb Reduces abundance and diversity of native species; potentially

allelopathic

Berberis thunbergii DC. Japanese barberry Shrub Displaces native shrubs; changes soil properties (alters soil microbial 

composition, increases nitrate concentration); alters successional patterns; potentially increases fire risk (owing to increased biomass)

Bromus tectorum L. Cheatgrass Annual grass May cause community type conversion; alters community structure, 

process, and function (e.g., decreases diversity, changes fire disturbance 

regime frequency, alters successional patterns and nutrient cycling)

Celastrus orbiculatus Thunb. Oriental bittersweet Vine Alters soil chemistry (e.g., increased pH, increased calcium levels), plant 

succession and stand structure (e.g., shades out understory, increases 

continuity of overstory vegetation); decreases native plant diversity; 

reduces productivity in managed systems

Centaurea solstitialis L. Yellow star-thistle Annual forb Displaces native plants, reduces native wildlife and forage; decreases 

native diversity; depletes soil moisture, altering water cycle; reduces 

productivity in agricultural systems (lowers yield and forage quality of 

rangelands)

Centaurea stoebe L. Spotted knapweed Biennial/perennial Reduces plant richness, diversity, cryptogam cover, soil fertility; reduces 

forage production; poisonous to horses; increases bare ground and 

surface water runoff, and can lead to stream sedimentation; allelopathic

Cirsium arvense (L.) Scop. Canada thistle Perennial forb Possible allelopathy; displaces native vegetation; alters community structure and composition; reduces diversity; reduces forage and livestock 

production

Cytisus scoparius (L.) Link Scotch broom Shrub Interferes with conifer establishment; reduces growth and biomass of 

trees; alters community composition and structure (increases stand 

density, often creating monospecific stands); alters soil chemistry 

(increases nitrogen); toxic to livestock

Hedera helix L. English ivy Vine Alters community structure; displaces native ground flora; weakens or 

kills host trees; potential to reduce water quality and increase soil erosion and soil nitrogen

Imperata cylindrical (L.) P. 

Beauv.

Cogongrass Grass Alters ecosystem structure (e.g., decreases growth and increases mortality of young trees) and function and decreases diversity; shortens fire 

return intervals and increases fire intensity, interferes with pine and oak 

regeneration

Ligustrum sinense Lour. Chinese privet Shrub Interferes with native hardwood regeneration; alters species composition 

and community structure (forms dense monospecific stands)

Lonicera japonica Thunb. Japanese honeysuckle Vine Alters forest structure and species composition; inhibits pine regeneration 

potentially weakens or kills host trees; suppresses native vegetation; 

provides food for wildlife; early- and late-season host for agricultural 

pests

Lygodium japonicum 

(Thunb. Ex Murr.) Sw.

Japanese climbing fern Climbing fern Reduces native understory vegetation; potentially weakens or kills host 

trees; interferes with overstory tree regeneration

Microstegium vimineum 

(Trin.) A. Camus

Japanese stiltgrass, 

Nepalese browntop

Annual grass Reduces ecosystem function (alters soil characteristics and microfaunal 

composition, decreases diversity, alters stand structure); reduces timber 

production; possibly allelopathic

Pueraria montana var. 

lobata

Kudzu Vine Potentially eliminates forest cover; overtops, weakens, and kills host 

trees; reduces timber production; increases winter fire risk

Triadica sebifera (Willd.) 

Maesen and S.M. Alemeida ex Sanjappa and 

Predeep

Chinese tallow,

tallowtree

Tree Displaces native species and reduces diversity; increases soil nutrient 

availability; reduces fire frequency and intensity

Table 2.3—Summary of common invasive plant species and environmental impacts for forests and woodlands 

in the United States
30

GENERAL TECHNICAL REPORT PNW-GTR-870

example, some consider species in the genus Tamarix to be 

among the most aggressively invasive and detrimental exotic 

plants in the United States (Stein and Flack 1996), but others 

point out benefits, including sediment stabilization and the 

creation of vertebrate habitat in riparian areas that can no 

longer support native vegetation (Cohn 2005). 

The spatial extent of many invasive plants at any point 

in time has been difficult to determine, limiting assessment 

of overall consequences of invasive plants. One assessment (Duncan et al. 2004) for the Western United States 

indicates that 16 invasive plants account for most current 

invasive plant problems. Centaurea species are particularly 

widespread and persistent in the West (table 2.3) (box 2.5). 

Cogongrass (Imperata cylindrica [L.] Raeusch.) (table 2.3), 

which has invaded extensive forested areas of the Southeast, 

is considered to be one of the most problematic invasive 

plants in the world (box 2.6). Mountain ecosystems tend 

to have fewer invasive plant species than other regions because of a short growing season, limited settlement history, 

relatively low frequency of seed sources, and prevalence of 

closed-canopy conifer forests that limit light in the understory and acidify the soil (Parks et al. 2005). 

Box 2.5—Centaurea invasion in the Western United States 

Eurasian forbs in the genus Centaurea are the most abundant invasive plants in the Western United States, covering 

over 7 million ha (3 million ha in California). Collectively known as knapweeds and star-thistles, 12 Centaurea species are listed as noxious in at least one U.S. state (5 species account for most of the damage). Although these species 

are usually associated with grasslands, they also affect forest ecosystems, particularly in open areas and after fire or 

other disturbances. In the northwestern United States, many forest ecosystems are susceptible to invasion by Centaurea, 

although some form of disturbance is typically required, particularly at higher elevations (Parks et al. 2005). 

Yellow-star thistle has a strong growth and competitive response to elevated carbon dioxide (CO2) (Dukes 2002). 

In one study, its aboveground biomass increased more than sixfold in response to elevated CO2, which allowed it to 

compete aggressively with native species, although supplemental precipitation reduced its establishment in the field 

(Dukes et al. 2011). Predictive models project various changes in the range of Centaurea species in a warmer climate. 

Broennimann and Guisan (2008) projected a northern shift and reduced invasion extent for spotted knapweed by 2080 

using the hot, dry HadCM-A1FI scenario, but Bradley et al. (2009) suggested that the distribution of yellow-star thistle 

was likely to increase in a warming West. Cumming (2007) found that small increases in temperature and precipitation 

would expand the suitable habitat for spotted knapweed in the short term, but large increases (4.5 °C, 10 cm) would 

decrease suitable habitat in Montana in the long term (several decades). Model output contains considerable uncertainty 

regarding potential changes in the geographic range of Centaurea species and thus represents potential vulnerabilities 

rather than predictions. 

Tyrol knapweed (Centaurea nigrescens Willd.), shown here in a dry, mixed-conifer forest in eastern Washington, is listed 

as a noxious weed in four Western States. It is also found in the Midwestern and Northeastern United States, invading 

forests along roadsides and in disturbed or open areas. (Photos by Gabrielle Snider.)
31

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Box 2.6—Invasive grasses, fire, and forests 

Species such as cogongrass (Imperata cylindrica [L.] P. Beauv.) in the Southeastern United States and cheatgrass (Bromus tectorum L.) in the West are invaders that alter fire regimes and are some of the most important ecosystem-altering 

species on the planet. Cogongrass threatens native ecosystems and forest plantations in the southeast, generally invading 

areas after a disturbance (e.g., mining, timber harvest, highway construction, natural fire, or flood). It is a major problem for forest industry, invading and persisting in newly established pine plantations (Jose et al. 2002). In sandhill plant 

communities, cogongrass provides horizontal and vertical fuel continuity, shifting surface fire regimes to crown fire 

regimes and increasing fire-caused mortality in longleaf pine (Pinus palustris Mill.) (Lippincott 2000), potentially shifting a species-diverse pine savanna to a grassland dominated by cogongrass. Cogongrass does not tolerate low temperatures, but increased warming could increase the threat of cogongrass invasion into new areas. Empirical modeling has 

shown that climatic habitat for cogongrass could greatly increase, although it would still be restricted to the Gulf Coast 

(Bradley et al. 2010). 

Cheatgrass is widely distributed in western North America and dominates many steppe communities (Mack 1981). 

After disturbance, this species can invade low-elevation forests (Keeley and McGInnis 2007, Keeley et al. 2003, Kerns 

et al. 2006), creating surface fuel continuity from arid lowlands into forested uplands. After establishment, cheatgrass 

contributes heavily to fine, continuous, and highly combustible fuel components that dry out early in the year, thus 

increasing the length of the fire season in some forests. Empirical modeling indicates that future changes in the climatic 

habitat of cheatgrass will depend on precipitation as well as temperature (Bradley et al. 2009). Climate models based 

on decreased precipitation, especially in summer, project expansion of cheatgrass area, and a reduction in the area of 

suitable climatic habitat, by up to 45 percent in Colorado, Montana, Utah, and Wyoming. Models based on increased 

precipitation, however, project reduction of cheatgrass area by as much as 70 percent. Elevated carbon dioxide increases 

cheatgrass productivity, a phenomenon that may already be contributing to the vigor and spread of this species (Ziska et 

al. 2005). Increased productivity causes higher fuel loads, potentially resulting in higher intensity fires. These consequences, in combination with more area burned by wildfire as caused by higher temperatures (Littell et al. 2009), can 

alter fire regimes in dry Western forests. 

A severe infestation of cogongrass in a longleaf pine upland in central Florida. (Photo by James R. Meeker,

U.S. Forest Service, available from Forestry Images, http://www.forestryimages.org/browse/detail.

cfm?imgnum=3970058).
32

GENERAL TECHNICAL REPORT PNW-GTR-870

Interactions Between Climate Change and 

Plant Invasion

Plant invasions can be influenced by warmer temperatures, 

earlier springs and earlier snowmelt, reduced snowpack, 

changes in fire regimes, elevated N deposition, and elevated 

CO2 concentrations. The responses of invasive plants to 

climate change should be considered separately from those 

of native species, because invasive plants (1) have characteristics that may differ from native species, (2) can be 

highly adaptive (Sexton et al. 2002), (3) have life-history 

characteristics that facilitate rapid population expansion, and 

(4) often require different management approaches than for 

native species (Hellmann et al. 2008). Successful invasion of 

a natural community depends on environment, disturbance, 

resource availability, biotic resistance, and propagule pressure (D’Antonio et al. 2001, Davis et al. 2000, Eschtruth and 

Battles 2009, Levine et al. 2004, Pauchard et al. 2009). Climate change may influence all of these drivers of invasion, 

with high variability across space and time. 

Temperature, Precipitation, and Carbon 

Dioxide

Climate change will alter the abiotic conditions under which 

plant species can establish, survive, reproduce, and spread. 

Key environmental consequences of climate change are 

increased temperature, longer growing seasons, less snow, 

and more frequent drought. These effects are expected to increase plant stress and decrease survival in the drier, warmer, 

and lower elevation portions of species’ ranges (Allen and 

Breshears 1998). Abiotic factors probably constrain the 

range of many invasive plants and limit their successful establishment (Alpert et al. 2000, Pauchard et al. 2009). With 

climate change, however, new habitat, once too cold or wet, 

may become available, enabling plants to survive outside 

their historical ranges and expand beyond their

current ranges. 

Many native plants are projected to move northward 

or upward in elevation with climate change. Examples of 

invasive plants projected to follow this pattern are rare, 

but information on species tolerances can provide insight 

on potential responses. For example, the northern limit of 

Japanese barberry (Berberis thunbergii DC.) (table 2.3), an 

invasive shrub in the Eastern United States, is determined 

by low temperature tolerance, the southern limit by cold 

stratification requirements for germination, and the western 

limit by drought tolerance (Silander and Klepeis 1999). The 

widespread invasive tree of heaven (Ailanthus altissima [P. 

Mill.] Swingle) is limited by cold and prolonged snow cover 

to lower mountain slopes, but it may be able to colonize during several successive years of mild climate, conditions that 

may become more common under climate change (Miller 

1990). Temperature change is not the only driver for plant 

range expansion or contraction. Soil water availability and 

regional changes in climatic water balance may be important 

for plant invasions, particularly at lower elevations (Chambers et al. 2007, Crimmins et al. 2011). Besides changes in 

range, species growth, productivity, and reproduction may 

also change as climatic conditions change. For example, 

invasive plants may be better able to adjust to rapid changes 

in abiotic conditions by tracking seasonal temperature trends 

and shifting their phenologies (e.g., earlier spring warming) 

(Willis et al. 2010). 

Increased productivity in response to elevated CO2

 has 

been documented under controlled conditions for several 

invasive plant species, including cheatgrass (Bromus tectorum L.), Canada thistle (Cirsium arvense [L.] Scop.), spotted 

knapweed (Centaurea melitensis L.), yellow star-thistle (C. 

solstitialis L.), and kudzu (Pueraria montana [Lour.] Merr.) 

(Dukes et al. 2011, Ziska and Dukes 2011, Ziska and George 

2004) (table 2.3) (boxes 2.5, 2.6). Response to CO2 enrichment is less predictable when plants are grown in the field 

(Dukes and Mooney 1999, Ziska and Dukes 2011), where 

response may be limited by nutrients and water availability. 

Carbon dioxide enrichment can also increase efficiency of 
33

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

water use, which can partially ameliorate conditions associated with decreased water availability, particularly for C3 

plants (Eamus 1991).5 This phenomenon may be partially 

responsible for global patterns of C3 encroachment in grasslands dominated by C4

 plants or mixed species (Bond and 

Migdley 2000).6

Disturbance and Resource Availability

Disturbances such as fire, landslides, volcanic activity, logging, and road building open forest canopies, reduce competition, and expose mineral soil, increasing light and nutrient 

availability (D’Antonio et al. 1999, Elton 1958, Hobbs and 

Huenneke 1992). Invasive plants are generally well adapted 

to use increased resources. Fluctuating resource availability, 

coinciding with available propagules, facilitates regeneration 

and establishment of both native and exotic invasive species 

associated with forest development after disturbance (Davis 

et al. 2000, Halpern 1989, Parks et al. 2005). Opportunities 

for invasions may also be created by forest thinning, fuel 

treatments, and biofuel harvesting done to adapt or mitigate climate change (Bailey et al. 1998, Nelson et al. 2008, 

Silveri et al. 2001). However, the spatial extent of invasions 

may be limited (Nelson et al. 2008), especially for shadeintolerant species in closed-canopy western forests. 

The reintroduction of fire is a high priority for restoration and management of fire-adapted forests such as ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & C. 

Lawson), longleaf pine (Pinus palustris Mill.), and loblolly 

pine (Pinus taeda L.). Invasive plants, especially annual 

grasses (box 2.6), can spread rapidly after fire, particularly in 

high-severity burns (D’Antonio 2000, Keeley and McGinnis 

2007, Kerns et al. 2006). Forest sites treated with prescribed 

fire, which are often near the wildland-urban interface and 

roads, are also well positioned for invasive plant introduction and spread (Keeley et al. 2003).

5

 C3 plants are those in which photosynthetic C fixation occurs in a 

metabolic process that converts CO2 and ribulose biphospate into 

3-phosphoglycerate. This phenomenon may be partially responsible for global patterns of C3

 encroachment in C4 plants or mixed 

grassland species. 

6

 Photosynthetic C fixation occurs in a metabolic process that uses 

the enzyeme PEP carboxylase to add CO2

 to phosphoeonlpyruvate, 

producing a 4-C compound prior to subsequent transport and use in 

the Calvin cycle.

Biotic factors—

The success of plant invasions is regulated by competition 

from resident plants, often measured as species richness and 

abundance (Levine 2000, Seabloom et al. 2003), and land 

managers can alter postdisturbance (logging, fire) invasive 

establishment by seeding to increase native plant competition. Although native plant competition can be overwhelmed 

by invasive plant seed abundance (D’Antonio et al. 2001, 

Lonsdale 1999), resistance related to soil properties is more 

likely to withstand seed abundance. Native plant competition 

with invasive plants can also be affected by the effects of 

predation, herbivory, and pathogens associated with native 

species. Native plant competition may change as temperature and ambient CO2

 increase; numerous studies have documented that weedy plants are more productive in an elevated 

CO2 environment (Ziska and George 2004). 

Propagule pressure—

Propagule pressure, which includes seed size, numbers, and 

temporal and spatial patterns, is perhaps the most important 

driver of successful invasions in forest ecosystems (Colautti 

et al. 2006, Eschtruth and Battles 2009, Simberloff 2009, 

Tilman 1997). For invasive plants, propagule pressure is 

largely controlled by factors other than climate. For example, the most critical factors projecting plant invasion in 

eastern hemlock (Tsuga canadensis [L.] Carrière) forests in 

the Eastern United States are overstory canopy disturbance 

and propagule pressure (Eschtruth and Battles 2009). However, little is known about how biotic and abiotic resistance 

factors interact with propagule supply to influence exotic 

plant invasion (D’Antonio et al. 2001, Lonsdale 1999).

Atmospheric CO2

 may influence seed production, 

through enhanced flowering under elevated CO2, increasing the probability that a smaller seed can establish a viable 

population (Simberloff 2009). Of greater concern is how 

climate change may alter human activities that transfer 

seeds. For example, climate change could alter tourism and 

commerce, enhance survival of seeds during transport (Hellmann et al. 2008), and shift recreation to higher elevations. 

Changes in atmospheric circulation patterns could also alter 

wind-dispersed species, allowing new species to arrive in 

areas that previously had few seeds. 
34

GENERAL TECHNICAL REPORT PNW-GTR-870

Vulnerabilities—

Future climate change may increase the likelihood of invasion of forest lands for several reasons: the potential for 

increased ecological disturbance, the effect of warming on 

species distributions, enhanced competitiveness of invasive 

plants owing to elevated CO2, and increased stress to native 

species and ecosystems (Breshears et al. 2005, Dukes and 

Mooney 1999, Pauchard et al. 2009, Ziska and Dukes 2011). 

The potential for warming itself to increase the risk of invasion in temperate mountainous regions is greater than in 

other regions because cold temperature has tended to limit 

the establishment of invasive plants in forests. Future actions 

to control invasive plants may also become less effective. 

Some herbicides are less effective on plants grown in elevated CO2

 (Ziska and Teasdale 2000), and some biocontrol 

methods may no longer be effective in a warmer climate 

(Hellmann et al. 2008). 

Empirical models used to assess the potential change in 

suitable climatic conditions for invasive plants suggest that 

a warmer climate could result in both range expansion and 

contraction for common invasive plants (Bradley et al. 2009, 

Kerns et al. 2009, Pattison and Mack 2008, Sasek and Strain 

1990). However, a weakness of empirically driven species distribution models is that they can be created without 

prior knowledge about species ecophysiology, autecology, 

synecology, and biotic interactions. Process-based models 

may ultimately prove more robust for prediction, although 

model parameters are quantified from experimental data or 

the research literature, which themselves have uncertainties. 

Regardless of whether the models are empirical or processbased, all model results have considerable uncertainty regarding their ability to project potential changes in the geographic 

range of invasive plants (Littell et al. 2011). 

The idea that climate change may increase the success 

of biological invaders has been a key concept for more than 

a decade (Dukes and Mooney 1999), although empirical 

documentation of this phenomenon is largely absent (but see 

Willis et al. 2010). It is critical to understand the response of 

the most detrimental invasive plants to individual climatic 

factors, interactions between those factors, and interactions 

among diverse biological factors. For management responses to plant invasions to be cost-effective and successful, 

assertive action is needed in the early phase of invasion. A 

potentially useful approach is a climate change-based modification of the Early Detection and Rapid Response System 

(National Invasive Species Council 2001). For example, risk 

assessment could be done over broader geographic areas 

than has been performed in the past (Hellman et al. 2008). 

The successful control of invasive plants over broad areas 

of forest lands ultimately depends on knowledge about resistance of native species to invasion and our ability to limit 

propagule pressure.

Key Findings

• Climate change will likely increase the establishment of 

invasive plants in U.S. forests. 

• Risk of exotic invasive plants entering forests is likely 

highest in mountainous ecosystems, where cooler 

temperatures and closed-canopy forests may have limited 

invasives under historical climate.

Key Information Needs

• Increased understanding of the responses of the most 

detrimental invasive plants to climate and biological 

factors. 

• Better models for projecting potential distributions of 

invasive plants.

Erosion, Landslides, and 

Precipitation Variability

Based on analysis of recent climate records and the projections of climate change simulations, hydroclimate extremes 

will become more prominent with a warming climate 

(O’Gorman and Schneider 2009, Trenberth et al. 2009),

with potential increases in flood frequency, droughts and

low flow conditions, saturation events, landslide occurrence, and erosion. Ecosystems are expected to differ in their 

response to changes in precipitation intensity and interstorm 

length because of differences in geomorphic conditions, climate, species assemblages, and susceptibility to drought. For 

erosion, these differences may be predictable with a general 
35

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

mass balance framework, but other processes are poorly 

understood, such as the effect of drought on short- and longterm tree mortality, the resistance to insects and pathogens of 

vegetation, and subsequent feedbacks to erosion processes. 

The indirect effects of disturbances (e.g., fire, insect infestations, pathogens) to shifts in water balance will complicate 

the response of erosion and need to be incorporated into assessments of effects. Changing species composition will also 

have potential effects on forest ecosystem water balance, as 

discussed in the “Forest Hydrological Processes” section.

Erosion and Landslides

Changes in precipitation intensity, and in the magnitude and 

frequency of precipitation events that saturate the soil and 

cause runoff, will interact with mass wasting and erosion in 

both direct and indirect ways. Expected changes in annual 

precipitation differ across the United States and are uncertain over much of it, particularly at the local scale. Potential 

annual increases and decreases will directly contribute to 

the amount of water available to drive mass wasting at both 

seasonal and event scales. Increases in extremes of precipitation intensity (Easterling et al. 2000a, Karl and Knight 1998), 

rain-on-snow during mid-winter melt (Hamlet and Lettenmaier 2007, Lettenmaier and Gan 1990, Wenger et al. 2011), and 

transport of moisture in atmospheric rivers (Dettinger 2011, 

Ralph et al. 2006) are all likely mechanisms for increasing 

sufficient pore water pressure or hillslope, thus increasing the 

risk of landslides, erosion, and gully formation for individual 

storms. Seasonal to annual changes in precipitation will 

contribute to soil moisture and groundwater levels, which can 

amplify or mitigate individual events. Although we have a 

significant understanding of erosion and landslide processes, 

the ability to predict or manage high-risk areas is limited by 

uncertainty in predicting changes in precipitation amount, 

frequency, and location of extreme rainfall events.

Direct effects of some climatic changes on sediment 

yield and mass wasting may be overshadowed by longer 

term, indirect effects through vegetation response (Bull

1991, Collins and Bras 2008, Goode et al. 2011,

Istanbulluoglu and Bras 2006, Kirkby and Cox 1995,

Langbein and Schumm 1958, Tucker and Bras 1998).

Although decreasing precipitation in some places might suggest reduced risks of erosion or landslides, this change may 

have indirect effects on mortality and thinning of vegetation 

and fire risk; these effects could have much greater consequences for erosion and landslides, through reductions in 

root reinforcement of soil and greater exposure of soil to 

precipitation effect and runoff. For example, paleoclimatic 

and paleoecological evidence links periods of drought and 

severe fire to severe erosion events (Briffa 2000, Marlon et 

al. 2006, Meyer and Pierce 2003, Meyer et al. 1992, Pierce 

et al. 2004, Swetnam and Betancourt 1998, Whitlock et al. 

2003). At shorter time scales, years of widespread fire are 

linked to severely dry and warm years (Heyerdahl et al. 2008, 

Holden et al. 2011b, Littell et al. 2009, McKenzie et al. 2004, 

Morgan et al. 2008). As we shift toward a drier and warmer 

climate in the Western United States, more areas are likely to 

burn annually (e.g., Holden et al. 2011b, Littell et al. 2009, 

Running, 2006, Spracklen et al. 2009), with resulting postfire 

debris flows (Cannon et al. 2010, Luce 2005, Meyer and 

Pierce 2003, Moody and Martin 2009, Shakesby and Doerr 

2006). Breshears et al. (2005) documented drought-induced 

canopy mortality of ponderosa pine, followed by erosional 

loss of topsoil and nutrients, with subsequent species replacement by pinyon pine and juniper. These types of state transitions may indicate the type of complex feedbacks that will 

lead to permanent canopy shifts, rather than to disturbance 

and recovery.

Adjustment of canopy density and root distributions to 

longer interstorm periods may increase the efficiency of use 

of rain or snowmelt (Brooks et al 2011, Hwang et al 2009). 

The response of both annual runoff and runoff from extreme 

events may be amplified or mitigated by forest canopy 

adjustment to temperature, moisture, N, and atmospheric 

CO2

. Increased precipitation intensity and amount, combined 

with lower root biomass from a drier climate, can yield more 

unstable slopes. An important interaction needing additional 

research is the effect of drought on adjustments of forest 

canopy leaf area and belowground allocation of C to hydrologic flowpaths and root reinforcement of soil. Shifts in 

species dominance can also result in major changes in root 

depth and cohesion (Hales et al. 2009). The spatial pattern 
36

GENERAL TECHNICAL REPORT PNW-GTR-870

of unstable slope conditions that can lead to landslides is 

influenced by interactions among the lateral redistribution of 

soil water in large events, the resulting pattern of high pore 

pressures with topographic slope, and root cohesive strength 

(Band et al. 2011).

Drought and Water Supply

Decreased precipitation and runoff is projected for substantial portions of the globe (Milly et al. 2005). Projections 

of the drought extent over the next 75 years show that the 

proportion of global land mass experiencing drought will 

double from 15 percent to 30 percent (Burke et al. 2006), 

and on most land masses, dry season precipitation is expected to decline by 15 percent (Solomon et al. 2009). 

Many projected declines in precipitation are in semiarid regions at mid-latitudes, where forests are at the limits 

of their ranges. Projections for the strongest declines in the 

United States are in the Southwest, strongly affecting water 

supply (Barnett and Pierce 2008, Rajagopalan et al. 2009). 

Further decreases in precipitation will probably increase 

both forest mortality (Allen et al. 2010, Holden et al. 2011a) 

and fire risk (Westerling et al. 2011); however, forest 

mortality may not substantially mitigate runoff reductions 

associated with decreased precipitation (Adams et al. 2011). 

Historical observations of interannual variability in precipitation in the Western United States have shown substantial 

increases in variability in the last half-century (Luce and 

Holden 2009, Pagano and Garen 2005), even in portions of 

the Western United States not projected to show precipitation declines. Short-term severe droughts have consequences 

for vegetation (Holden et al. 2011b, van Mantgem et al. 

2009) and water supply. Although there has been interest in 

using forest harvest to augment water supplies, three factors 

limit the utility of the approach: (1) most increases in water 

yield after harvest occur in wet years (Brown et al. 2005, 

Ford et al. 2011b, Troendle and King 1987); (2) water yield 

increases in snow environments occur earlier in the year, 

exacerbating flow timing issues by climate change (Troendle 

and Leaf 1981, Troendle et al. 2010); and (3) in warmer and 

moister locations, increases in water yields can be replaced 

by decreases as young vegetation reestablishes within a few 

years (Brown et al. 2005, Ford et al. 2011b).

Key Findings

• Concentrating precipitation in more intense storms will 

likely increase erosion and landslide risk, but the ability 

to project effects at meaningful spatial and temporal 

scales is limited by uncertainties in projecting future 

precipitation regimes. 

• Increases in drought frequency and severity will likely 

increase tree mortality and reduce streamflow.

Key Information Needs

• Improved understanding of the effects of tree mortality 

and changing species composition on soil stability. 

• Improved projections of changes in precipitation amount, 

and spatial and temporal distribution of extreme events. 

Disturbance Interactions

A particular challenge is to understand interactions among 

disturbance regimes (Bigler et al. 2005, Busby et al. 

2008). How will massive outbreaks of bark beetles, which 

kill drought-stressed trees by feeding on cambial tissues, 

increase the potential for large severe wildfires in a warming climate (fig. 2.10)? Interactions between processes can 

amplify or mute the overall effects of changes in complex 

forest ecosystems. The predominance of negative and positive feedbacks within and between processes will determine 

the stability or instability of the system. 

Thresholds

Disturbance interactions may rapidly bring ecosystems to 

thresholds (Groffman et al. 2006). For example, Allen and 

Breshears (1998) and Breshears et al. (2005) documented 

rapid dieback of pinyon pine across the arid Southwest. 

Mature trees were pushed over a threshold by a combination of “global-change type drought” (Breshears et al. 2005) 

and an opportunistic bark beetle invasion. Regeneration of 

pinyon pine will determine whether this mortality represents 

a threshold for the ecosystem. Characteristic patterns of 

patchiness or continuity may indicate thresholds that have 

been approached or crossed (Scheffer et al. 2009) (table 2.4).

For example, the invasion of sagebrush steppe by cheatgrass 
37

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 2.10—Mountain pine beetle outbreak in the years 

before the Tripod Complex Fire (2006) in north-central 

Washington created a “perfect storm” in higher elevation 

lodgepole pine stands, which burned with exceptionally 

high intensity. This figure shows how the timing of other 

disturbances can exacerbate or mitigate wildfire severity. 

Cause/response Predictable Unexpected

Continuous A tipping point is known from previous experience or modeling, and trends in the controlling 

factor(s) are measured. Example: gradual loss of 

habitat toward a point at which metapopulation 

models predict extirpation.

Controlling factor is changing gradually, but ecosystem 

effects or interactions of response variables are too 

complex to predict. Example: increases in an invasive 

nonnative species with unknown effects on biotic 

interactions of natives or grazing pressure.

Abrupt Pulses in a controlling factor precipitate an inevi- table response. Example: large disturbance or 

invasion (perhaps unprecedented) changes struc- ture and composition of a landscape with a loss of 

90 percent of potential nesting trees for northern 

spotted owls.

Pulses in a controlling factor (or very likely multiple 

controls) are expected to produce surprises. Example: 

changing fire frequency and mountain pine beetle outbreaks may have sudden consequences for vegetation, 

animals, or landscape pattern.

Table 2.4—Characteristics of continuous versus abrupt thresholdsSusan Prichard
38

GENERAL TECHNICAL REPORT PNW-GTR-870

(Fischer et al. 1996) and of the Sonoran Desert by buffelgrass (Esque et al. 2007) provide fuel continuity and the 

potential for much more extensive wildfires than noninvaded 

areas with patchy fuels. 

A notable threshold response to multiple stressors is the 

reproductive cycle of mountain pine beetle (Logan and Powell 2001), whose outbreaks have killed mature trees across 

millions of hectares of pine in western North America. 

Within particular ranges of minimum winter temperatures 

and growing-season degree days, the reproductive cycle 

is synchronized to the seasonal cycle, permitting larvae to 

emerge at the right time to ensure maximum survival and 

therefore epidemic population size. This “adaptive seasonality,” combined with drought-caused and age-related vulnerability of the host species, has brought an abrupt increase 

in mortality of lodgepole pine across the West (Hicke et al. 

2006).

Conceptually, the thresholds are fairly well understood. 

Mathematical models abound, from early work on catastrophe theory and its associated hysteresis to identification of 

approaching thresholds via statistical properties. This modeling has by necessity taken place in simplified (often virtual) 

ecosystems, and a major challenge remains to apply such 

sophistication to real-world systems outside the specific examples chosen by modelers to test their hypotheses. A larger 

challenge will always be the unpredictability of the occurrence of contingent, interacting events that push systems 

across thresholds.

Stress Complexes: From Conceptual to 

Quantitative Models

In the context of the effects of climate change on ecosystems, sensitivity to disturbance interactions is extended to 

environmental drivers not usually identified as disturbances. 

For example, extreme temperatures, drought, and air pollution put forest ecosystems under stress, which may increase 

their vulnerability to “true” disturbances such as fire, insect 

outbreaks, and pathogens. Following McKenzie et al. 

(2009), we refer to interacting stresses as stress complexes and present three brief conceptual examples, from 

California, Alaska (both drawing on McKenzie et al. 2009), 

and the Southeast. 

A striking feature of mixed-conifer forests in southern 

Sierra Nevada and southern California is ambient air pollution (fig. 2.11), particularly elevated ozone, which affects 

plant vigor by reducing net photosynthesis and therefore 

growth (Peterson et al. 1991) and is often concentrated at 

middle and upper elevations (Brace and Peterson 1998). Air 

pollution exacerbates drought stress from warmer temperatures, which amplifies biotic stresses such as insects and 

pathogens (Ferrell 1996). The stress complex for the Sierra 

Nevada is represented conceptually in fig. 2.12; interacting 

disturbances form the core of drivers of ecosystem change, 

modified by climate, management, and air pollution.

The state of Alaska has experienced massive fires in 

the last decade, including the five largest fires in the United 

States. Over 2.5 million ha burned in the interior in 2004. 

Concurrently (1990s), massive outbreaks of the spruce beetle occurred on and near the Kenai Peninsula in south-central 

Alaska (Berg et al. 2006) (fig. 2.13). Although periodic outbreaks have occurred throughout the historical record, both 

in south-central Alaska and the southwestern Yukon, these 

most recent outbreaks may be unprecedented in both extent 

and percentage of mortality (over 90 percent in many places) 

(Berg et al. 2006).

Both of these phenomena, wildfire and bark beetle 

outbreak, are associated with warmer temperatures in recent 

decades (Duffy et al. 2005, Werner et al. 2006). At the same 

time, major hydrological changes are underway from the 

cumulative effects of warming. Permafrost degradation is 

widespread in central Alaska, shifting ecosystems from birch 

Figure 2.11—Air pollution in the Sierra Nevada foothills from 

the Central Valley in California. 

D. McKenzie
39

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 2.12—A conceptual model of stress complexes in Sierra Nevada and southern Californian mixed-conifer forests. The effects 

of insects and fire disturbance regimes (red box) and of fire exclusion are exacerbated by global warming. Stand-replacing fires and 

drought-induced mortality both contribute to species changes and exotic invasions. Modified from McKenzie et al. (2009).

Figure 2.13—Mortality of white spruce from barkbeetle attack on the Kenai Peninsula, Alaska. W. M. Ciesla
40

GENERAL TECHNICAL REPORT PNW-GTR-870

forests to wetland types such as bogs and fens (Jorgensen 

et al. 2001). If broad-scale water balances become increasingly negative, peatlands may begin to support upland forest 

species (Klein et al. 2005). The stress complex for Alaska 

is represented conceptually in fig. 2.14; upland and lowland 

ecosystems may follow parallel but contrasting paths toward 

new structure and species composition.

Much of the forested landscape in the Southeast is 

adapted to frequent fire such that, unlike much of the West 

and Alaska, prescribed fire is a mainstay of ecosystem management. Fire-adapted inland forests overlap geographically 

with coastal areas affected by hurricanes and potentially by

sea level rise (Ross et al. 2009), such that interactions 

between wildfires and hurricanes are synergistic (fig. 2.15). 

For example, dry-season (prescribed) fires may have actually 

been more severe than wet-season (lightning) fires in some 

areas, causing structural damage via cambium kill and subsequent increased vulnerability to hurricane damage (Platt 

et al. 2002). The stress complex for the Southeast is represented conceptually in fig. 2.16, where different disturbances 

“meet” in the outcomes for forest ecosystems. 

Uncertainties

Our broad understanding of multiple stressors is mainly 

qualitative, despite case studies in various ecosystems that 

have measured the effects of interactions and even followed 

them over time (Hicke et al. 2012b). In our three examples, 

the directional effects of warming-induced stressors may be 

clear (e.g., in California, species composition shifts to those 

associated with frequent fire). However, the magnitudes of

these effects are not, nor are the potentially irreversible 

crossings of ecological thresholds. Given the complexity

Figure 2.14—A conceptual model of stress complexes in the interior and coastal forests of Alaska. Rapid increases in the severity of disturbance regimes (insects and fire) are triggered by global warming. Stand-replacing 

fires, massive mortality from insects, and permafrost degradation contribute to species changes and conversion to 

deciduous life forms. Modified from McKenzie et al. (2009).
41

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 2.16—A conceptual model of stress complex in the interior and coastal forests of the Southeast. Increases 

in the severity of hurricanes are triggered by global warming, while sea level rises. Warmer and drier climate in 

uplands leads to longer periods with flammable fuels. Changes in fire and hydrologic regimes, and responses to 

them, lead to species change and altered C dynamics.

Figure 2.15—Interactions between wildfire and hurricanes are synergistic in the Southern United States. 

Figure depicts a longleaf pine/saw palmetto flatwoods 

on the Atlantic coastal plain, 2.5 years after a hurricane 

and with a previous history of prescribed fire. Courtesy of the Fire and Environmental Research Applications team, U.S. Forest Service, Digital Photo Series. 
42

GENERAL TECHNICAL REPORT PNW-GTR-870

and diversity of potential interacting stressors in U.S. forests,

a fruitful way to advance quantitative knowledge may be 

with explicit simulations with models of “intermediate 

complexity” to ascertain the sensitivity of ecosystems to the 

uncertainties associated with key parameters (e.g., the thickness of the arrows in figs. 2.12, 2.14, and 2.16).

Key Findings

• Interactions among ecological disturbance and stressors 

likely cause larger effects on ecosystems than any 

individual disturbance or stressor.

• Warmer temperature will generally exacerbate stress in 

drier forest ecosystems, partly through reduced vigor 

in vegetation but more importantly through increased 

disturbance.

• Climate-induced increases in wildfire occurrence and 

insect outbreaks across large landscapes will potentially 

cause rapid changes in the structure and function of 

forest ecosystems.

Key Information Needs

• Additional empirical data on stress interactions in a wide 

range of forest ecosystems.

• Transition from qualitative to quantitative analyses and 

models of how stressors and disturbances interact to 

affect forest ecosystems.

Effects of Climate Change on Forest 

Processes

Some of the changes to U.S. forests will be directly caused 

by the effects of an altered climate, such as increases in 

atmospheric CO2 and N deposition on tree growth, increases 

in seed production, mortality, and regeneration, and shifting success among species as a result of altered outcomes of 

competition among species. Other changes will be indirectly 

caused by climate-induced changes in disturbances, such 

as droughts, fire, insect outbreaks, pathogens, and storms. 

Potential changes in the duration of snowpack (discussed 

below) will also affect disturbance and forest processes. In 

this section, we provide a synthesis of current knowledge of 

the potential direct and indirect effects of climate change on 

biogeochemical cycling (i.e., C, nutrients, and water) and 

forest tree distributions.

Carbon and Nutrient Cycling 

The United States has 303 million ha of forest land, about 

8 percent of the world’s total. Forest C stocks and uptake 

or loss rates differ greatly with the wide range in environmental conditions, land use and land use history, and current 

human influences. Forests of the conterminous United States 

cover 281 million ha and contain 45,988 Tg of C. Estimates 

of the amount of the Nation’s CO2

 emissions (1500 Tg of 

C in 2009) offset by forests and forest products vary with 

assumptions and accounting methods (e.g., from 10 to 20 

percent) (McKinley et al. 2011), with 13 percent being the 

most recent and commonly used estimate for the United 

States (USEPA 2011). Ninety-four percent of forest C storage comes from growth on current forest lands, with the 

remaining 6 percent from a net positive conversion of other 

land uses to forests. Regional differences in forest C pools 

and storage rates are reported in McKinley et al. (2011), 

Woodbury et al. (2007), and U.S. Environmental Protection 

Agency (USEPA) (2011). Updates of the inventories used to 

estimate these pools and storage rates may be important to 

capture C losses in recent large fires, bark beetle outbreaks, 

and drought mortality. Also, certain components, such as 

dead wood and C in soil, are either sparsely measured or are 

only estimated (Woodbury et al. 2007).

These forest C storage estimates are similar to those 

reported in a global study of forest sinks derived from the 

same sources (Pan et al. 2011). An analysis using eddy covariance flux measurements, satellite observations, and modeling estimated a C storage in the conterminous United States 

of 630 Tg·C·yr -1 (Xiao et al. 2011), largely from forests and 

savannas. However, most agricultural lands either store little 

additional carbon or lose C (USEPA 2011). The large discrepancy between the biometric USEPA estimates and those 

of Xiao et al. (2011) is probably caused by two factors: (1) 

woodland encroachment (McKinley and Blair 2008, Pacala 

et al. 2001, Van Auken 2000) that is not measured by the 

USDA Forest Service Forest Inventory and Analysis used 
43

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

for the USEPA reporting, and (2) the poor performance of 

eddy covariance measurements to estimate ecosystem respiration, consistently leading to ~30 percent overestimates 

of ecosystem C storage (Barford et al. 2001, Bolstad et al. 

2004, Kutsch et al. 2008, Lavigne et al. 1997, Wang et al. 

2010). Other estimates for the conterminous United States 

are 1200 ± 400 Tg·C·yr-1 from inversion analysis (Butler et 

al. 2010) and 500 ± 400 Tg·C ·yr -1 from three-dimensional 

atmospheric CO2 sampling (Crevoisier et al. 2010). 

Response of Forest Carbon Cycling 

Processes to Increased Temperature, 

Changes in Precipitation, Increased 

Carbon Dioxide, Nitrogen Deposition,

and Tropospheric Ozone

Carbon storage in forest ecosystems results from the balance 

between growth of wood, foliage, and roots and their death 

or shedding and subsequent decomposition. Temperature, 

atmospheric CO2 concentration, ecosystem water balance, 

and N cycling all interact to alter photosynthesis and growth. 

The critical issue is the balance among these factors affecting growth. For example, higher temperatures can benefit 

growth, but the most benefit would come with adequate 

nutrition and a positive water balance. Disturbance is the 

largest factor changing the balance between production and 

decomposition, but chronic changes in temperature, precipitation, CO2

, and N deposition over large areas can also alter 

the U.S. forest C balance.

Insights for the U.S. forest carbon balance from experiments and measurements—

Atmospheric concentrations of CO2

, currently about 390 

parts per million (ppm), are expected to rise to 700 to 900 

ppm by 2100, depending on future anthropogenic emissions and any changes in atmospheric uptake by terrestrial 

and aquatic ecosystems. Experimental results continue to 

confirm that the primary direct effect of elevated CO2 on 

forest vegetation is an increase in photosynthesis (Norby et 

al. 2005), but individual studies show that photosynthetic 

enhancement, growth and C storage are moderated by the 

presence of drought or nutrient limitations (Finzi et al. 2006, 

Garten et al. 2011, Johnson 2006, Norby et al. 2010). A recent synthesis of free-air CO2

 enrichment studies (Norby and 

Zak 2011) showed that (1) elevated CO2 does not increase 

the leaf area of forested sites, (2) net primary production is 

enhanced under elevated CO2 only when water and nutrient supplies are abundant, (3) water use is reduced through 

stomatal closure (Leuzinger and Körner 2007, Warren et al. 

2011) (see “Forest Hydrological Processes” below), and (4) 

CO2-promoted increases in photosynthesis and net primary 

productivity do not always increase forest C storage. Despite 

the known limitations on tree response to elevated CO2, a 

19 percent increase in CO2 over the past five decades may 

have increased aspen growth more than 50 percent (Cole et 

al. 2010).

Elevated atmospheric CO2 will likely increase forest productivity, but because of the known limitations and 

uncertainties to the response, we do not know how much. 

Major uncertainties in projecting forest response to elevated 

CO2 include projecting the responses of belowground 

processes such as soil C storage (Lukac et al. 2009), mature 

trees, and wetlands. Elevated CO2 commonly enhances soil 

CO2 efflux, suggesting that some of the additional photosynthesis is rapidly cycled back to the atmosphere (Bernhardt et 

al. 2006). An increase in labile C in soil pools may increase 

decomposition of more recalcitrant soil C and potentially 

reduce soil C storage (Hofmockel et al. 2011). For a mature 

forest, sustained increases in photosynthesis in response 

to elevated CO2 (Bader et al. 2009) did not increase wood 

growth (Körner et al. 2005), soil respiration (Bader and 

Körner 2010), or root or soil C storage (Asshoff et al. 2006, 

Bader et al. 2009). For wetlands, elevated CO2

 can increase 

CO2 and methane efflux (Ellis et al. 2009, Hutchin et al. 

1995), but these fluxes strongly interact with precipitation, 

the water table, and potential species changes (Fenner et al. 

2007). 

Models project annual temperature to increase by 4 to 5

°C by 2070, with high-latitude boreal forests experiencing 

the largest increases in temperature. For temperate and 

boreal forests, modest increases in temperature tend to 

increase growth (Way et al. 2010). Warming will probably 
44

GENERAL TECHNICAL REPORT PNW-GTR-870

enhance upland forest growth for ecosystems with ample 

water, through changes in annual plant development and

a longer growth season (Bronson et al. 2009, Gunderson et 

al. 2012, Hänninen et al. 2007). Growth in water-limited 

ecosystems will probably be reduced (Arend et al. 2011,

Hu et al. 2010), and net C storage may be reduced (Cai et 

al. 2010). Observed changes in growth for these studies 

were not caused by increases in photosynthesis (Bronson 

and Gower 2010, Gunderson et al. 2010). Warming will also 

enhance microbial decomposition and nutrient mineralization in soils (Melillo et al. 2002), increasing plant nutrient 

availability (Melillo et al. 2011) (discussed below), but the 

long-term tradeoff between soil C loss and nutrient enhanced 

productivity is unknown. A longer growing season may 

increase the possibility of damage to trees from late frost 

events (Augspurger 2009, Gu et al. 2008).

Projected precipitation for 2070 to 2100 differs by 

region; the Southwest and areas of the Great Plains, Texas, 

Arkansas, and southern Missouri will receive lower summer 

precipitation, and precipitation in the East will increase in

all seasons except the summer. Eastern forests, particularly 

on deep soils, are well buffered against substantial reductions in precipitation; forest growth, soil C storage, and 

nutrient availability show little effect of a chronic 12-year, 

33-percent reduction in precipitation (Froberg et al. 2008, 

Hanson et al. 2007, Johnson et al. 2008). Western forests, 

particularly those that rely on snowmelt for their water,

will probably show lower growth under drier conditions 

(Boisvenue and Running 2010, Hu et al. 2010). More frequent droughts in the Western United States will reduce tree 

growth and vigor and increase tree mortality (McDowell et 

al. 2008, McDowell 2011). A modeling study suggested that 

the amount of precipitation was more important for forest 

productivity than its frequency and intensity (Gerten et al. 

2008).

Nitrogen deposition may increase in some regions and 

decrease or remain the same in other regions, depending on 

emissions associated with human population trends and the 

effectiveness of regulations to reduce N emissions. In areas 

where N deposition increases, it may enhance ecosystem 

C storage by increasing forest productivity (Churkina et 

al. 2009, de Vries 2009) and decreasing decomposition of 

soil organic matter (Janssens et al. 2010), but those gains 

may be offset by the concurrent release of nitrous oxide, a 

potent greenhouse gas (Zaehle et al. 2011). The potential for 

enhancing C gain would be low in regions where N deposition is already high (e.g., the Northeast) and high in regions 

where N deposition is low (e.g., the Southwest). In addition, 

tree species have a wide range of susceptibility to tropospheric ozone, which also varies regionally, and damage 

caused by ozone is not completely offset by elevated CO2

(Karnosky et al. 2005).

Projections of the U.S. forest carbon balance from 

models—

Experimental manipulations of temperature and precipitation 

are rare for forest ecosystems, and ecological process models 

are needed to project how changes in multiple factors over 

large areas might affect forest C balance. Forests in different 

regions will probably respond differently to climate change 

because of differences in species composition, water and 

nutrient availability, soil depth and texture, and strength of 

other environmental factors such as ozone and N deposition. 

Model projections vary by region, just as projected changes 

in climate vary by region. Different models also produce 

different results. 

Overall, in the Eastern United States, productivity 

or forest C storage is expected to increase with projected 

changes in climate, N, and CO2. This is because the increased precipitation projected for many areas in the Eastern 

United States allows more photosynthesis under increased 

temperature and CO2

. For example, upland oak forests in 

Tennessee are projected to increase their current C storage 

rate by 20 percent for the climate and atmosphere predicted 

for 2100 (CO2 concentration of 770 ppm, ozone concentration 20 ppb higher than today’s level, 4 °C temperature 

increase, and 20 percent more November–March precipitation) (Hanson et al. 2005). Globally, temperate forest and 

grassland net productivity is projected to increase 25 to 28 

percent for CO2 concentration of 550 ppm (Pinsonneault 

et al. 2011), an estimate that includes expected changes in 

climate. Based on a four-model simulation of the effects of 

increased temperature and CO2 and altered precipitation, wet 

sites such as Eastern forests showed large absolute changes 
45

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

in net C storage rates and net ecosystem production (Luo 

et al. 2008). Modeled forest productivity also increased for 

New England forests (Campbell et al. 2009).

For the Western United States, many models project 

lower productivity or C storage for forests. Changes in climate and CO2

 are projected to turn Rocky Mountain forests 

into a C source by 2090 (Biome-BGC model) (Boisvenue 

and Running 2010), and decrease forest C storage for boreal 

aspen (Grant et al. 2006). However, other model studies project increases in C storage for Western U.S. forests 

(CENTURY Model) (Melillo et al. 2011, Pinsonneault et al. 

2011, Smithwick et al. 2009). Carbon in northern bogs, peat 

lands, and permafrost regions may be lost with a warming climate (increasing methane production), depending on 

hydrology and other factors (Heijmans et al. 2008, Ise et al. 

2008, Koven et al. 2011). Both global model simulations of 

climate change and ecosystem productivity (Friend 2010, 

Pinsonneault et al. 2011) projected higher C storage for both 

Eastern and Western U.S. forests, with the larger increase in 

the East. It is important to note that none of these local or 

global simulations include the effects of altered disturbance 

regimes in their projections.

Effects of changes in disturbance rates on the U.S. forest 

carbon balance—

For Western U.S. forests, climate-driven increased fire and 

bark beetle outbreaks are likely to substantially reduce forest 

C storage and storage rate (Metsaranta et al. 2010; Westerling et al. 2006, 2011), jeopardizing the current U.S. forest 

sink. Disturbances, mostly large-scale fires, have already 

turned Arizona and Idaho forests from a C sink into a C 

source (USEPA 2011). Tree mortality has increased

globally, and large-tree mortality from drought and elevated 

temperature has promoted bark beetle outbreaks, with

the consequence of a short-term C loss for Western U.S. 

forests (Allen et al. 2010). Tree mortality not caused by

fire or insect outbreaks has also increased in the West

(van Mantgem et al. 2009). We have no information on

tree mortality trends in the Eastern United States, but tree 

mortality rates there are sensitive to air pollution exposure 

(Dietze and Moorcroft 2011). Tree regeneration after

disturbance is critical for maintaining forest cover and 

associated C stocks (McKinley et al. 2011). Tree regeneration is uncertain for western montane forests, where fire 

intensity exceeds historical patterns (Bonnet et al. 2005, 

Haire and McGarigal 2010). Temperature and precipitation 

extremes are important for initiating disturbances, but the 

mean projections of the many GCMs used (and the individual models in general) do a poor job of predicting extreme 

events.

Effects on Eastern forests where precipitation is currently in excess—

In the next 30 years, projected changes in CO2

, temperature, 

and precipitation are not likely to change forest C storage 

and uptake from current levels or may even increase them,

if tropospheric ozone levels are managed to remain at or 

below current levels (fig. 2.17). Changes in species composition through time will probably remain driven by 

competition between plants and interactions with pests and 

pathogens, except for sites with shallow or coarse textured 

soils that increase the effects of drought. Toward the end of 

this century, net C gain by Eastern U.S. forests will probably be reduced by a warming-induced increase in seasonal 

water deficits, but the effects will not be large. The beneficial 

Figure 2.17—Risk analysis diagram for forest carbon cycle. 

Western forests are considered inherently limited by water demands 

that exceed precipitation supplies during substantial portions of 

the year. Xeric eastern forests include those growing on shallow or 

coarse-textured soils or those present at the western prairie forest 

transition zone that experience water deficits in some years. Mesic 

eastern forests experience severe water deficits only in occasional 

years and for relatively brief periods.
46

GENERAL TECHNICAL REPORT PNW-GTR-870

effects of elevated CO2 and the extended growing season 

length will allow ample opportunity for C gain, even though 

the probability of water stress occurrence in the summer 

months will increase. On coarse-textured or shallow soils, 

the forest will show reduced annual C uptake (fig. 2.17). 

Effects on forests in the Western, Southwestern, Lower 

Central, and Southeastern United States and Alaska—

Changes in temperature and precipitation have already 

increased forest fire. Bark beetle outbreaks have increased 

forest C loss and are likely to continue to do so in the next 

30 years, probably negating the increased productivity from 

warming temperatures and elevated CO2 (fig. 2.17). After 

2050, projected temperature and precipitation changes indicate that the survival of current tree species is uncertain, and 

that tree species change will be accompanied by disturbance 

and major C loss. An increased focus on forest regeneration 

after disturbance, perhaps with species adapted to the new 

climate, will probably be necessary to maintain forest cover 

and C stocks.

Effects on Nutrient Cycling

As noted, C cycling responses to elevated CO2 and warming will be tightly linked to nutrient availability, especially 

N. Climate change is likely to have both direct and indirect 

effects on processes that regulate availability and fluxes. 

Biological processes that convert nutrients held in organic 

matter to available mineral forms are generally temperaturedependent. Experimental soil-warming studies confirm

that N mineralization will increase in response to higher 

temperatures (Melillo et al. 2011), with an average increase 

in net N mineralization of about 50 percent (Rustad et al. 

2001). These effects may be transient, however, because the

supply of mineralizable substrates may not keep pace with 

opportunities for mineralization. Although experimental

soil-warming studies have been critical to show potential 

effects of warming, results from these studies are limited 

by methodological constraints that make it difficult to scale 

results to the ecosystem level or incorporate whole-system 

interactions. Modeling approaches that scale to the ecosystem and incorporate interactions have generally confirmed 

patterns observed in soil-warming experiments (Campbell

et al. 2009). Recent studies have used observed climate variability and corresponding measures of stream N in forested 

watersheds to infer changes in N-cycling processes. For 

example, in the Western United States, Baron et al. (2009) 

suggested that recent warming temperatures have melted 

glacial ice, subsequently flushing N from microbially active 

sediments. In the Eastern United States, Brookshire et al. 

(2011) found that seasonal variation in stream nitrate was 

tightly coupled to recent warming, and they used modeling 

to extrapolate effects of future warming on microbial activity 

and stream nitrate export. Brookshire et al. (2011) suggested 

that the consequences of elevated temperature will increase 

future N export threefold more than will projected changes 

in N deposition.

Effects of biotic disturbance on nutrient cycling in 

forests may also occur when species composition is changed 

because tree species affect belowground processes (Knoepp 

et al. 2011, Lovett et al. 2006). For example, forests afflicted 

with beech bark disease have increased litter decomposition, 

decreased soil C:N ratio, and increased extractable nitrate 

in the soil and nitrate in soil solution (Lovett et al. 2010). In 

eastern hemlock stands infested with hemlock woolly adelgid, litter N is increased, and N mineralization is accelerated 

even before tree mortality is observed (Orwig et al. 2008, 

Stadler et al. 2006). Defoliation also alters N pools and 

fluxes within forests (Lovett et al. 2002). 

Key Findings

• Forest growth and afforestation offset 13 percent of 

fossil fuel CO2

 production in 2009 according to a recent 

analysis by the USEPA.

• In the Western United States, increased fire, bark beetle 

outbreaks, and droughts have likely reduced forest C 

storage, and these reductions will likely be larger in the 

future, slowing or halting the current C sink in the United 

States.

• In the Eastern United States, elevated CO2

 and 

temperature and sufficient water will increase forest 

growth and will likely increase C storage, except on sites 

with shallow soils or areas more subject to drought.

• Warmer temperatures will probably lead to increased 

nutrient cycling, promoting increased forest growth and 

elevated N levels in streams and rivers.
47

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

 Key Information Needs

• More and longer term elevated CO2 experiments in 

forests, especially in mature forests.

• More forest-scale warming experiments.

• More information on multifactor interactions and species 

changes, processes leading to tree mortality and species 

migration, and the cause and potential saturation of the 

current C sink in the United States.

• Analyses of long-term stream chemistry data to provide 

an integrated measure of nutrient cycling responses to 

climate variability, including more analyses across a 

wider range of ecosystems to understand variation in 

controls and response patterns.

Forest Hydrological Processes 

Abundant and clean water are fundamental to the viability of aquatic ecosystems, human welfare, and economic 

growth and development throughout the world (Cech 2005, 

Jackson et al. 2001). The combination of increased demand 

for fresh water, changes in land use and cover, and climate 

change will place even greater demands on forest watersheds 

across the globe to meet the water resource needs of humans 

and aquatic ecosystems (Vörösmarty et al. 2010). Climate 

change will have both indirect and direct effects on forest 

water cycling. Indirect effects are associated with changes in 

atmospheric CO2, increased temperature, altered soil water 

availability, climate-mediated changes in species composition, and changes in disturbance regimes or management and 

policy decisions that alter forest structure and composition. 

Indirect effects of climate change on forest water cycling 

work primarily through effects on forest evapotranspiration 

(ET), the combination of evaporation of water from plant 

and ground surfaces and transpiration. As discussed in the 

“Erosions and Landslides” section, direct effects are associated with more rainfall and more intense storms. These in 

turn increase base flows in streams (particularly intermittent 

streams), increase flood risk, accelerate erosion, and increase 

the potential for both landslides and increased interstorm 

periods and drought, along with climate-related changes in 

infiltration rate owing to extreme wildfire. Indirect and direct 

effects are interdependent. 

Forest Evapotranspiration and Streamflow

Forest ET may be changing in response to changing climate 

(Gedney et al. 2006, Labat et al. 2004, Walter et al. 2004), 

but studies disagree about the direction of the change. Over 

relatively large areas and long temporal scales, streamflow is the balance between rainfall input and ET. Hence, 

the rainfall not used in ET is available for streamflow and 

groundwater recharge, and in many forest ecosystems, ET is 

a major regulator of streamflow and groundwater recharge. 

Walter et al. (2004) concluded that ET has been increasing 

across most of the United States at a rate of 10.4 mm per 

decade (inferred from U.S. Geological Survey records of 

precipitation and river discharge in six major basins in the 

United States). In contrast, river discharge across the globe 

has been increasing at a rate of 4 percent for each 1 °C 

increase in global temperature (Labat et al. 2004), suggesting a reduction in ET. Different response patterns are not 

surprising, because ET is affected by several co-occurring 

variables. For example, the increase in discharge in the study 

by Labat et al. (2004) has been attributed to the physiological effect of CO2

 decreasing ET, and not to the effect of 

changing land use (Gedney et al. 2006). Besides elevated 

atmospheric CO2 concentration, some of the most important 

variables affecting forest ET are temperature, humidity, water availability, and species distributions. Potential effects of 

climate change on these variables and their interactions are 

discussed below, and they will probably result in differing 

patterns of change in ET at local and regional scales. 

Elevated Atmospheric Carbon Dioxide

Over long time scales, higher CO2 concentrations decrease 

stomatal density and aperture, both of which reduce transpiration (Beerling 1996, Ehleringer and Cerling 1995, 

Franks and Beerling 2009, Prentice and Harrison 2009). 

Indeed, both observational and experimental studies confirm 

long-term and large-scale changes in leaf stomatal conductance in response to elevated CO2

 (Lammertsma et al. 2011, 

Warren et al. 2011). As leaf stomatal conductance declines, 

ecosystem ET can also decline; however, any decline will 

depend on factors such as stand age, species composition, 
48

GENERAL TECHNICAL REPORT PNW-GTR-870

and leaf area. Empirical studies linking reduced stomatal 

conductance to reduced stand-level ET have not yet been 

possible, and most researchers have used modeling to make 

that linkage. 

Warren et al. (2011) applied the Forest BGC model to 

data from several elevated CO2 studies and projected that 

ET was reduced by 11 percent in older stands that did not 

experience an increase in leaf area. In younger stands, ET 

increased because of stimulation of leaf area. In a modeling study of deciduous forests in the northeastern United 

States, the estimated effect of elevated CO2 on ET was 

modest, ranging from a 4-percent decrease to an 11-percent 

increase (Ollinger et al. 2008). In Mediterranean forest 

systems, changes in ET are also expected to be modest with 

increased temperature and CO2, ranging from no change to 

a 10-percent decrease (Tague et al. 2009). Studies have not 

yet identified an increase in stand leaf area with elevated 

CO2 (Norby and Zak 2011). Although the effects of elevated 

CO2 on ET remain uncertain, studies agree that the direct effects will be modest (±10 percent) compared to the changes 

expected for other variables that affect ET, such as precipitation variability (Leuzinger and Korner 2010).

Warmer Temperatures and Drought

Although elevated CO2 is likely to decrease ET, the increases in temperature and thus the increases in the vapor 

pressure deficit (VPD) between the inside of the leaf and 

the surrounding air may offset this effect, such that ET is 

affected little or not at all. As the air becomes drier, transpiration typically increases following an exponential saturation curve, with the rate of increase continually slowed by 

reduced stomatal opening. Most studies show that a physiological effect of reduced stomatal conductance in response 

to elevated CO2

 is observed only when the canopy air is 

very humid (low VPD). In a study of six deciduous tree 

species, a 22-percent reduction in transpiration occurred 

under elevated CO2, but only at low VPD (Cech et al. 2003). 

These results support the idea that the physiological effects 

of elevated CO2

 on ecosystem water balance may depend on 

precipitation and atmospheric humidity.

Warming has changed the timing of foliage green-up

and senescence, but the effects of these phenological 

changes on ET are complex and not well understood. 

Warming-induced lengthening of the growth season could 

increase ET and offset the reduction in stomatal conductance 

from elevated CO2, but these effects are difficult to generalize across species and regions (Hänninen and Tanino 2011). 

Although the frost-free season across the United States has 

lengthened by about 2 weeks, resulting in a longer, warmer 

growing season, growth cessation in the autumn might come 

earlier with increasing temperatures for some boreal and 

temperate tree species (Kunkel et al. 2004). For other tree 

species, spring bud burst might be delayed by warmer temperatures (Zhang et al. 2007), perhaps because of not receiving the requisite chilling hours (Schwartz and Hanes 2010). 

In higher latitudes where chilling requirements are still being 

met, green-up is occurring sooner. Thus, springtime ET in 

the lower latitudes could be delayed while ET in the higher 

latitudes could be advanced. 

The potential increase in ET owing to a lengthened 

growing season can be constrained by the water availability 

and drought that often arise late in the growing season (Zhao 

and Running 2010). Water limitations are a direct control 

on ET (lower water availability reduces transpiration), and 

many regions of the United States have experienced more 

frequent precipitation extremes, including droughts, over the 

last 50 years (Easterling et al. 2000b, Groisman et al. 2004, 

Huntington 2006, Solomon et al. 2007). 

Changing Species Composition

Evapotranspiration is affected by the plant and tree species 

that comprise the canopy cover of a forest ecosystem. In 

general, pine forests are much more responsive to climatic 

variation than are deciduous forests (Ford et al. 2011a, Stoy 

et al. 2006). However, even within the same forest, growing 

season transpiration rates among canopy species (adjusted 

for differences in tree size) can vary by as much as fourfold, 

and co-occurring species can differ considerably in their 

responsiveness to climatic variation (Ford et al. 2011a). 

Characteristics of the xylem and sapwood, which vary by 
49

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

species, are among the most important determinants of stand 

transpiration in both observational (Vose and Ford 2011, 

Wullschleger et al. 2001) and theoretical studies (Enquist et 

al. 1998, Meinzer et al. 2005). Therefore, shifts in hydroclimate may be accommodated by changes in canopy leaf area, 

phenology, or species-based hydraulic efficiency. 

Increased drought severity and frequency may contribute to rapid changes in forest species composition through 

two important processes. First, drought plays an important role in tree mortality (Allen et al. 2010); as soil water 

availability declines, forest trees either reduce stomatal 

conductance to reduce water loss (drought avoidance), or 

they experience progressive hydraulic failure (Anderegg 

et al. 2011). Second, some native insect outbreaks, and the 

mortality they cause, are also triggered by drought. Increasing temperatures are also expected to interact with drought. 

As temperatures increase, plant metabolism increases 

exponentially. If high temperature coincides with drought 

stress in forests, C starvation and mortality can occur more 

quickly than if these factors did not coincide (Adams et al. 

2009). For example, Adams et al. (2009) projected a fivefold 

increase in the extent of pinyon pine mortality from an increase of 4.3 °C in temperature, based on historical drought 

frequency. If drought frequency increases as is expected 

under climate change scenarios (described in “Scenarios for 

Projecting Future Climate” section above), the projected 

mortality could be even greater. 

Evapotranspiration will also change with changes in 

canopy density, canopy composition, water demand, and resulting energy partitioning in new communities, which will 

occur in response to species changes that accompany climate 

change, especially if large areas of forests experience mortality (Breshears et al. 2005). Insect and pathogen outbreaks 

and fire will be the likely primary forces behind large-scale 

and rapid changes in forest composition and structure; however, direct studies of these effects on hydrology are limited 

(Tchakerian and Couslon 2011). Potential biogeophysical 

effects from tree-killing biotic disturbances include (1) 

increased surface albedo, which will reduce the absorption 

of solar radiation; (2) decreased transpiration until the new 

forest is reestablished; and (3) decreased surface roughness,

which affects atmospheric drag (Bonan 2008). Despite 

their importance as potential feedbacks to the atmospheric 

system (Adams et al. 2010, Rotenberg and Yakir 2010), little 

is known about how these processes have been altered by 

insect and pathogen outbreaks. After outbreaks that cause 

widespread tree mortality, streamflow increases, the annual 

hydrograph advances, and low flows increase (Potts 1984); 

at the same time, snow accumulation increases and snowmelt is more rapid after needle drop (Boon 2012, Pugh and 

Small 2011). According to one evaluation of radiative forcing effects from mountain pine beetle infestations, the cooling associated with increased albedo exceeded the warming 

associated with increased atmospheric CO2, leading to a net 

cooling in the first 14 years after attack (O’Halloran et al. 

2012). Increased surface albedo was especially pronounced 

in winter, when needle loss following tree mortality exposed 

more of the reflective snow surface. These studies show 

strong, but mostly indirect, evidence that large-scale forest 

mortality will alter water cycling processes; however, the 

magnitude and duration of responses will differ among species and across regions.

Snowmelt

Because of climate warming, snow cover in North America 

has shown a general reduction in duration, extent, and depth 

over the last few decades, with increased interannual variability (Barnett et al. 2008, Luce and Holden 2009, Mote et

al. 2005, Pagano and Garen 2005, Regonda et al. 2005). A 

reduction in snowpack depth, persistence, and duration has 

significant effects on forest ecosystems, including water 

stress, disturbance, erosion, and biogeochemical cycling. 

In arid and semiarid systems, early and reduced snowmelt 

has led to increased water stress in the late growing season and increases in fire frequency and magnitude and the 

susceptibility of forest stands to infestation (Adams et al. 

2011, Breshears et al. 2005, Holden et al. 2011b, Westerling et al. 2011). The rapid flush of water to the soil in 

spring snowmelt can release mobile solutes that have been 

slowly accumulating as a result of subnival biogeochemical cycling (e.g., Williams et al. 2009). These spring pulses 
50

GENERAL TECHNICAL REPORT PNW-GTR-870

can provide the major input of nutrients to aquatic ecosystems. Reductions in the spring flush, and increased rain in 

winter and early spring, can change the timing of N release 

from northern forests so that they resemble more southern 

ecosystems that lack the distinct seasonality of stream water 

N concentrations and export. Greater frequency and magnitude of rain-on-snow events may also increase soil erosion, 

sedimentation, and landslides. 

Soil Infiltration, Ground water Recharge, 

and Lateral Redistribution

Forest ecosystems typically support high infiltration capacities because of large soil pores developed by root systems 

and soil fauna, so surface runoff is not common compared 

to other land cover. However, high-intensity precipitation or 

snowmelt events can rapidly move water in the soil to the 

unsaturated zone or ground water, or into the local stream, 

particularly in steep terrain (Brooks et al. 2011, Laio et 

al. 2001, Troch et al. 2009).7 Increases in storm intensity 

projected for the future may increase peak streamflow and 

flooding through this process.

Carbon and Water Tradeoffs

Expanding C sequestration or wood-based bioenergy 

markets to offset fossil fuel emissions may affect water 

resources (Jackson et al. 2005), and these effects will depend 

on both the specific management activity and the scale of 

implementation. Planting fast-growing species for bioenergy 

production (or C sequestration) may reduce water resources 

(Jackson et al. 2005), but these reductions may be localized 

and minor if the planting area is small relative to the watershed size. In favoring certain species, the choice of species 

and the regional climate may influence overall effects. In 

wetter regions, where interception represents a higher proportion of ET, evergreen species may have a greater effect 

on site water balance. In drier regions, where transpiration 

represents the greatest proportion of ET, high water-use

species such as Populus or Eucalyptus may have greater

7

 Hwang, T.; Band, L.E.; Vose, J.M. [N.d.]. Ecosystem processes at 

the watershed scale: hydrologic vegetation gradient as an indicator 

for lateral hydrologic connectivity of headwater catchments. On file 

with: Taehee Hwang, University of North Carolina, Institute for the 

Environment, Campus Box 1105, Chapel Hill, NC 27599-1105.

effects (Farley et al. 2005). Shortening rotation length might 

increase streamflow because the proportion of time that the 

stand is at canopy closure (when leaf area index is maximum 

and streamflow is lowest) will be reduced. In an analysis 

of forest plantations across the globe, Jackson et al. (2005) 

found the largest reductions in streamflow in 15- to 20-yearold plantations. If these plantations become widespread, 

irrigation of short-rotation forests would increase water use. 

A primary concern in the Western United States would be 

survival of plantations under drought.

Key Findings

• Effects of elevated CO2 on transpiration will likely 

be modest (±10 percent), compared to the effects of 

precipitation variability on transpiration.

• Large-scale disturbances such as fire, bark beetle 

outbreaks, and defoliating insects will likely increase 

runoff.

• Increased temperatures have recently decreased snow 

cover depth, duration and extent and have advanced 

the timing of runoff. These effects will likely intensify 

as temperatures warm further. Fast-growth forests, if 

widely applied, may reduce streamflow. 

Key Information Needs

• More information on interactions among hydrology, 

climate change, disturbance, and changing species 

composition and phenology. 

• Better projections of future effects on hydrologic 

processes and water resources, which will require 

improved hydrologic models that can account for 

variation in species and stand structures, yet can be 

readily scaled to larger and more complex landscapes 

with mixed land uses.

• Planting fast-growing trees is an option to offset C 

emissions and increase sequestration, but studies are 

needed to rigorously evaluate the potential effects on 

water resources.

Forest Tree Species Distributions 

The ranges of plant species have always shifted through time 

(Davis and Shaw 2001, Davis and Zabinski 1992, Webb 
51

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

1992), but in recent decades, evidence is building that species are moving faster than in historical times (Chen et al. 

2010, Dobrowski et al. 2011, Parmesan and Yohe 2003).

For example, in a meta-analysis of 764 species range changes (mostly insects and no tree species), the average rate of 

northward migration was 16.9 km per decade (Chen et al. 

2011). In contrast, an earlier meta-analysis, using 99 species 

of birds, butterflies, and alpine herbs, reported a northward 

migration of 6.1 km per decade (Parmesan and Yohe 2003). 

There is evidence of upward elevational migration of tree 

species (Beckage et al. 2008, Holzinger et al. 2008, Lenoir et 

al. 2008). However, to our knowledge, no study has documented northward latitudinal migration for trees in response 

to recent changes. Woodall et al. (2009) used forest inventory data to investigate surrogates for migration among 40 

Eastern United States tree species. They used a comparison 

of the mean latitude of biomass of larger trees (>2.5 cm 

diameter at breast height [dbh]) relative to the mean latitude 

of density of seedlings (<2.5 cm dbh) across each species’ 

range of latitude to detect possible future trends in distribution. For many of the species, this analysis indicated higher 

regeneration success at the northern edge of their ranges. 

Compared to mean latitude of tree biomass, mean latitude 

of seedlings was significantly farther north (>20 km) for the 

northern study species, southern species showed no shift, 

and general species showed southern expansion. Density 

of seedlings relative to tree biomass of northern tree species was nearly 10 times higher in northern latitudes than in 

southern latitudes. These results suggest that the process of 

northward tree migration in the Eastern United States is currently underway, with rates approaching 100 km per century 

for many species. 

Pollen records suggest migration rates for some tree 

species of 2 to 2.5 km per decade during the period of 

roughly 5,000 to 6,000 years ago (Davis 1989), a time when 

species were not slowed by forest fragmentation (Iverson et 

al. 2004a, 2004b; Schwartz 1993). If the same rates applied 

today, these slow tree migration rates would make measuring 

species migration to current climate change logistically difficult. Instead, researchers must rely on modeling to project 

the potential effects of climate change on tree migration 

processes. 

Two types of predictive models of vegetation change 

exist: (1) empirical, species distribution models that

establish statistical relationships between species or life 

forms and (often numerous) predictor variables, and (2) 

process-based models, which simulate vegetation dynamics 

at the taxonomic resolution of species or life forms. There 

are well-recognized tradeoffs between using these different models to assess potential changes in species habitats 

resulting from forecasts of environmental change (Thuiller 

et al. 2008), and both approaches are widely used. When 

both approaches yield similar results for a particular area, 

confidence in model projections is improved. Other modeling approaches are used to inform these two approaches. 

Demography studies inform species distribution models 

(SDMs), and migration models are used with process-based 

models. 

Species Distribution Models

Species distribution models are used to extrapolate species 

distributions in space and time, based on statistical models 

of habitat suitability (Franklin 2009). Species distribution 

models are built with observations of species occurrences 

along with environmental variables thought to influence 

habitat suitability and equilibrium species distribution. 

Predictive mapping of suitable habitat (but not whether 

a species will reach those habitats) in space and time are 

therefore possible by extension of these models. The SDMs 

have limitations, which include the assumptions that (1) the 

selected variables reflect the niche requirements of a species, 

(2) species are in equilibrium with their suitable habitat, (3) 

species will be able to disperse to their suitable locations, (4) 

projections can be made for novel climates and land covers, (5) the effects of adaptation and evolution are minimal, 

and (6) the effects of biotic interactions (including human 

interactions) are minimal (Ibanez et al. 2006, Pearson et al. 

2006). However, SDMs can provide glimpses of probable 

futures useful for incorporating future conditions into conservation and management practices and decisions.

Species distribution models project a northward movement of tree species habitat in North America from 400 

to 800 km by 2100 depending on the assumptions used in 

projecting future climate (Iverson et al. 2008, McKenney 
52

GENERAL TECHNICAL REPORT PNW-GTR-870

et al. 2011). Species distribution model projections also 

differ based on future scenarios and with time. For example, 

under a scenario of high greenhouse gas emissions (Hadley 

A1F1), about 66 species would gain and 54 species would 

lose at least 10 percent of their suitable habitat under climate 

change. A lower emission pathway would result in both 

fewer losers and gainers. Sugar maple (Acer saccharum

Marsh.) would lose a large proportion of its habitat under 

the warmest scenario (fig. 2.18) (Iverson et al. 2008, Lovett 

and Mitchell 2004), but it would still maintain a presence of 

habitat in most areas. When multiple species were compiled 

together to create forest types, models project a severe loss 

of suitable habitat for spruce-fir (Picea-Abies), white-redjack pine (Pinus strobus L., P. resinosa Aiton, P. banksiana

Lamb.), and aspen-birch (Populus-Betula) suitable habitat, 

but a wide expansion of suitable habitat for oak-hickory 

(Quercus-Carya) (fig. 2.19) (Iverson and Prasad 2001, Iverson et al. 2008). 

Process Models

To model species composition changes, a fully processdriven approach might be preferable to isolate mechanisms 

and to create “what-if scenarios.” However, such an approach is presently impossible because of (1) the necessity 

of detailed parameterization of species life histories and 

physiologies for more than 100 species, (2) the complexity 

of many interacting disturbance factors, and (3) the necessary high-resolution modeling over very large areas (Lawler 

et al. 2006). Dynamic global vegetation models (DGVM) 

operate at scales from regional (hundreds of kilometers) to 

global; these models can aggregate species into life forms or 

plant functional types (PFTs), using structural or functional 

attributes such as needleleaf vs. broadleaf and evergreen vs. 

deciduous (Bachelet et al. 2003, Bonan et al. 2003, Neilson 

et al. 2005). Most of these models project shifts to more 

drought-tolerant and disturbance-tolerant species or PFTs 

for future climates. This general shift in vegetation may be 

offset by physiological changes induced by CO2

 fertilization, as suggested by a DGVM (MC-1) that links water-use 

efficiency to CO2-simulated expansion of forests into areas 

whose climate is currently too dry (Bachelet et al. 2003). 

This particular issue deserves further study to resolve the 

extent and duration of such mitigating effects of CO2; these 

effects could change substantially depending on the outcome 

of climate-change projections. Ravenscroft et al. (2010) 

used the LANDIS model to simulate the potential effects 

of climate change to 2095 and found that mesic birch–aspen–spruce–fir and jack pine–black spruce (Picea mariana

[Mill.] Britton, Sterns & Poggenb.) forest types would be 

substantially altered because of the loss of northerly species 

and the expansion of red (Acer rubrum L.) and sugar maple 

(fig. 2.20). Another promising modeling system that also 

includes climate variables is the Regional Hydro-Ecologic 

Simulation System (RHESSys) (Tague and Band 2004). Using this model in a Sierra Nevada mountain system, Christiansen et al. (2008) found significant elevational differences 

in vegetation water use and sensitivity to climate, both of 

which will probably be critical to controlling responses and 

vulnerability of similar ecosystems under climate change. 

Transpiration at the lowest elevations was consistent across 

years because of topographically controlled high moistures, 

the mid-elevation transpiration rates were controlled primarily by precipitation, and the high-elevation transpiration 

rates were controlled primarily by temperature (fig. 2.21).

Demography Studies

Demography studies track individuals over time, rather than 

use periodic plot-level inventories, to fully understand the 

role of climate relative to other risk factors like competition, 

variation in physiology and function, and vulnerability to 

insects and pathogens. Such demography data sets are rare, 

but one study has tracked more than 27,000 individuals of 

40 species over 6 to 11 years to address these interactions 

over a portion of the southeastern United States (Clark et al. 

2011). This study found that the primary climatic controls 

are spring temperature (regulating species fecundity) and 

growing season moisture, particularly for species of Pinus, 

Ulmus, Magnolia, and Fagus. Pitch pine (Pinus rigida Mill.) 

tracked both spring temperature and summer drought, yellow poplar (Liriodendron tulipifera L.) tracked neither, and 

sweetgum (Liquidambar styraciflua L.) tracked summer 

drought but not spring temperature (Clark et al. 2011).

Overall, the effect of competition on growth and mortality 

exceeded the effects of climate variation for most species. 
53

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 2.18—Maps of current and 

potential future suitable habitat for 

sugar maple show the expected 

northward migration of habitat 

with climate warming by 2100. The 

map includes the current inventory 

estimate of abundance from Forest 

Inventory and Analysis (FIA-current) 

sampling, the modeled current 

distribution (RF-current), and four 

model projections for future climate: 

(1) a low emissions scenario (B1) 

using the average of three global 

climate models (GCM3 Avg lo); (2) 

a low emissions scenario (B1) using 

the National Center for Atmospheric 

Research’s Parallel Climate Model 

(PCM lo); (3) a high emissions 

scenario (A1F1) using the average of 

three global climate models (GCM3 

Avg hi); (4) a high emissions scenario (A1F1) using the HadleyCM3 

model (Hadley hi). Data from Prasad 

and Iverson (1999–ongoing).
54

GENERAL TECHNICAL REPORT PNW-GTR-870

Figure 2.19—Maps of current and 

potential future suitable habitat for 

USDA Forest Service forest types 

(named according to dominant spe-

cies) also show the major forests 

types moving northward with climate 

warming by 2100. The map includes 

the current inventory estimate of 

abundance from Forest Inventory and 

Analysis (FIA-current) sampling, 

the modeled current distribution (RF 

current) and four model projections 

for future climate: (1) a low emission scenario (B1) using the National 

Center for Atmospheric Research’s 

Parallel Climate Model (PCM lo); (2) 

a low emission scenario (B1) usingthe 

average of three global climate models 

(GCM3 Avg lo); (3) a high emission 

scenario (A1F1) using the average of 

three global climate models (GCM3 

Avg hi); (4) a high emission scenario

(A1F1) using the HadleyCM3 model

(Hadley hi). From Iverson et al. 

(2008).
55

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 2.20—Maps of northern Minnesota forest types at year 2095 for nine scenarios (three emission levels × three management 

intensities). Sugar and red maple dominate the landscape by 2095 regardless of the emission scenario or management. Cells are 

classified to the forest type (named according to dominant species) with the highest total biomass, as generated via the LANDIS 

vegetation model. Climate simulations are done with the Hadley C3 global climate model with the A2 (high) and B1 (low) emission scenarios. From Ravenscroft et al. (2010).
56

GENERAL TECHNICAL REPORT PNW-GTR-870

Figure 2.21—Maps of Upper Merced River watershed, Yosemite Valley, California, showing areas with greatest differences in transpiration between (a) warmest and coldest simulation years, (b) wet vs. average precipitation year, and (c) dry vs. average precipitation year. 

Greatest decreases in transpiration are shown in red; increases between years are shown in green. From Christensen et al. (2008).
57

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Thus, the extra effort of demographic tracking can determine 

the vulnerability of individual trees to risk factors, including 

climate change over time, variation in abiotic variables over 

space, and competition (Clark et al. 2011). 

Dispersal and Migration Models

To colonize suitable habitat resulting from a changing 

climate, each affected species will need to either migrate 

or be moved. Approaches used to model migration include 

reaction-diffusion models, phenomenological models, 

mechanistic models, and simulation models (Clark et al. 

2003, Hardy 2005, Katul et al. 2005, Levin et al. 2003, Nathan et al. 2011). Recent advances in digital computation and 

more reliable data from seed dispersal studies have allowed 

improvement of these models so that they can begin to 

project the parameter values of seed dispersal curves as well 

as the seed distributions. For example, Nathan et al. (2011) 

modeled 12 North American wind-dispersed tree species 

for current and projected future spread according to 10 key 

dispersal, demographic, and environmental factors affecting 

population spread. They found a very low likelihood of the 

ability of any of the 12 species to spread 300 to 500 m yr -1, 

the rate of change expected under climate change (Loarie 

et al. 2009). In contrast, the SHIFT model uses historical 

migration rates along with the strengths of the seed sources 

(its abundance within its current range) and potential future 

sinks (abundance of potential suitable habitat), rather than 

using poorly understood life history parameters (Iverson et 

al. 2004a, 2004b; Schwartz et al. 2001). When SHIFT model 

outputs of colonization potentials were combined with a 

species distribution model (DISTRIB) simulation of suitable 

habitat for five species—common persimmon (Diospyros 

virginiana L.), sweetgum, sourwood (Oxydendrum arboreum [L.] DC), loblolly pine, and southern red oak (Quercus 

falcata Michx.)—only 15 percent of the newly suitable habitat had any likelihood of being colonized by those species 

within 100 years (Iverson et al. 2004a, 2004b). These results 

both suggest that a serious lag will occur before species 

migration into the new habitat. 

Assisted Migration

Many species will be unable to migrate to suitable habitat 

within 100 years (Iverson et al. 2004a, 2004b) and may face 

serious consequences if they cannot adapt to new climatic 

conditions. Assisted migration may help mitigate climate 

change by intentionally moving species to climatically suitable locations outside their natural range (Hoegh-Guldberg 

et al. 2008, McLachlan et al. 2007). Assisted migration has 

been controversial, with some advocating for it (Minteer and 

Collins 2010, Vitt et al. 2010) and some against (Ricciardi 

and Simberloff 2009). Proponents state that these drastic 

measures are needed to save certain species that cannot 

adapt or disperse fast enough in an era of unprecedented 

global change. The main concern of opponents is that the 

placement of species outside their range may disturb native 

species and ecosystems when these ‘‘climate refugees’’ 

establish themselves in new environments. The uncertainty 

of climate in the future and the complexity and contingency 

associated with ecosystem response also argue against assisted migration.

One way to resolve the debate is to subdivide assisted 

migration into “rescue-assisted migration” and “forestryassisted migration.” As the names imply, rescue assisted 

migration moves species to rescue them from extinction in 

the face of climate change, and this type is the source of 

most of the controversy. Forestry-assisted migration is aimed 

more at maintaining high levels of productivity and diversity 

in widespread tree species that are commercially, socially, 

culturally, or ecologically valuable (Gray et al. 2011, Kreyling et al. 2011). With forestry-assisted migration, maintaining forest productivity and ecosystem services are the most 

obvious desired outcomes. Given the broad distribution of 

most tree species, and the relatively short distances proposed 

for tree seed migration, forestry-assisted migration typically 

involves transfers within or just beyond current range limits 

to locations where a population’s bioclimatic envelope is 

expected to reside within the lifetime of the planted population (Gray et al. 2011). The introduction of genotypes to 

climatically appropriate locations may also contribute to 

overall forest health by establishing vigorous plantations 
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GENERAL TECHNICAL REPORT PNW-GTR-870

across the landscape that are less susceptible to forest pests 

and pathogens (Wu et al. 2005). If realized, such an outcome 

would help ensure the continued flow of the many ecosystem 

services provided by forests, such as wildlife habitat, erosion 

prevention, and C uptake (Kreyling et al. 2011). If practiced 

in a manner where genotypes are transferred within or just 

beyond current range limits, forestry-assisted migration may 

be a viable tool for adaptation to climate change, especially 

if limited to current intensively managed plantations. Turning extensive areas of forest in the United States from lightly 

managed forests into managed plantations would likely be 

unpopular.

Key Findings

• Models project that species habitat for most species 

will move up in elevation and northward in latitude and 

be reduced or disappear from current habitats in lower 

elevations and lower latitudes.

• Habitats will probably move faster than tree species can 

disperse, creating uncertainty about the future vegetation 

composition of these new habitats.

Key Information Needs

• Studies on the mitigating effects of elevated CO2 on 

drought stress and subsequent effects on projections of 

future habitat suitability. 

Effects of Altered Forest Processes 

and Functions on Ecosystem 

Services

Ecosystem services link the effects of altered forest processes, conditions, and disturbance regimes to human well-being 

(World Resources Institute 2005). A broad range of utility 

and values derive from four types of ecosystem services: 

(1) provisioning or products from ecosystems, (2) regulation 

of ecosystem processes, (3) cultural or nonmaterial benefits, 

and (4) supporting services required for the production of 

all other ecosystem services (Joyce et al. 2008) (fig. 2.22). 

Anticipated climate changes portend changes in all types 

of ecosystem services derived from forests. Because the 

assessment endpoint for ecosystem services is human 

well-being, we are ultimately concerned about the potential 

effects of climate change on the ecosystem services that 

forests provide. This subsection explores these changes and 

provides a linkage between climate change effects on biophysical processes and human well-being.

Ecosystem services differ across temporal and spatial scales but are most often assessed and recognized for 

watersheds and regions (or subregions). However, they can 

also be meaningful at the forest stand or national scales. 

Disturbances (natural and human) and stressors can control 

delivery of ecosystem services across variable timeframes. 

Ecosystem services occur in forests not as a single service 

but rather as a suite or bundle of services. The bundle of 

services changes with time and in response to disturbance 

regimes and stressors.

The vulnerability of ecosystem services to climate 

change will vary widely, depending not only on the service 

of concern (e.g., wood products or flood regulation) and 

location (defined by region, such as the Southwest versus 

the northeastern), but also on the location in reference to 

human condition, such as rural versus urban settings. The 

value of the affected service multiplied by the likelihood of 

effect defines the risk to ecosystem services and provides a 

framework for understanding potential consequences and 

prioritizing actions. 

Climate-related mechanisms of change in the Nation’s 

forests could alter ecosystem services in ways that are not 

yet fully understood, and estimating these effects introduces 

another layer of uncertainty. That is, climate regulates forest 

processes that control future forest conditions that determine 

future ecosystem services. Still, the potential effects of climate change on forest ecosystems could have profound and 

mostly disruptive consequences for ecosystem services with 

important implications for human well-being. Ecosystem 

services also depend on the interactions with land use, human 

demographics and economies, which may simultaneously 

adjust to climate stimuli (see chapter 3). 

Forests in the United States consist of both managed 

(active) and unmanaged (passive) ecosystems (Ryan et al. 

2008) held in public and private ownerships. Some public 

forests or wildlands are withdrawn from active management 

(national parks, state parks, wilderness areas and wild and 
59

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 2.22—Linkages between ecosystem services and human well-being (World Resources Institute 2005).

scenic rivers), but most of them are managed for multipleuse goals (e.g., most national forests and Bureau of Land 

Management lands). Public land management in the United 

States is largely focused on nonmarket ecosystem services, 

including recreation, aesthetic values, and water purification. 

Most forest management for timber production occurs on 

private forest lands in the United States (Smith et al. 2009), 

using both capital-intensive (short- rotation, pine plantation silviculture) and land-extensive approaches (occasional 

harvesting followed by natural regeneration). Private lands 

also provide the full spectrum of ecosystem services, either 

by design through conservation easements, or as a byproduct 

of other management objectives (see Butler et al. 2007). In 

many cases, private forest lands provide ecosystem services 

that accrue to broader social (human) well-being without equitable financial compensation. Public ownership dominates 

Western forests, and private ownership dominates Eastern 

forests. 

The bundle of ecosystem services provided under current climate conditions differs across the assortment of public and private and managed and unmanaged forests. As a 

consequence of the regional distribution of anticipated future 

climate change, the provision of ecosystem services from 

these lands could also change and be modified by adaptation 

and mitigation strategies. Social perception of the risks to 

ecosystem services will be determined by the rate of change 

in these services (flows) as well as by an understanding of 

the adaptation and mitigation strategies applied in response 
60

GENERAL TECHNICAL REPORT PNW-GTR-870

to climate change. Social systems will adapt to climate 

change and affect the condition of forests in the United 

States and throughout the world.

Several mechanisms of change in forest ecosystems 

described in the preceding subsections hold implications 

for ecosystem services. First, climate change could alter the 

amount and distribution of forest biomass in forests, either 

through shifts in productivity associated with atmospheric C 

concentrations or through altered forest disturbance regimes. 

Changes in forest biomass directly influence the supply of 

wood products from lumber to fuel for electricity production (provisioning services), and they alter the amount of C 

stored in forest pools (a regulating service). Future productivity and disturbance effects would probably be focused in 

the Rocky Mountain and Intermountain West and Alaska, 

where only a small portion of U.S. timber production occurs, 

suggesting declines in timber supply from these regions. 

Declines would be small in the context of national markets, 

but they could represent substantial shares of local rural economic activity in these regions. The scale of timber effects 

would likely be local.

Changes in tree cover will affect microclimate conditions (e.g., the cooling of urban heat islands), whereas shifts 

in C stocks through accumulation of biomass could affect 

changes in global climate trajectories. Projections of accelerated emissions related to elevated insect epidemics and fire 

activity could represent a substantial effect on the Nation’s 

forest C reservoirs. The scale of C storage effects could shift 

U.S. forests from net C sinks to net C emitters (Wear et al. 

2012).

Because productivity effects related to atmospheric 

changes are ambiguous and highly uncertain, their influence 

on timber markets or C stocks are generally unknown. However, if forest productivity were to increase in the Eastern 

United States and decrease in the Western United States, this 

could accelerate the shift in timber production from West to 

East, and especially to the Southeast.

Disturbance regimes affect other ecosystem services 

as well. Estimates of the economic consequences of insect 

and pathogen outbreaks focus either on timber market effects (e.g., southern pine beetle, [Pye et al. 2011]) or on the 

influence of tree mortality on property values (e.g., hemlock 

woolly adelgid, [Holmes et al. 2010]). These measures of 

market effects for price-based services address one element 

of a complex of values affected by forest disturbances. In the 

case of forest insects, management decisions already account 

for a certain level of expected tree mortality, so the more 

relevant question is whether effects significantly exceed the 

“background” losses associated with endemic insects and 

pathogens.8 Property values define the effect of disturbance 

and related mortality on the ecosystem services delivered to 

private property owners, but they cannot capture the “public 

good” aspects of changes to forest aesthetics for people who 

view forests. To illustrate, widespread tree mortality related 

to pine beetle epidemics on national forests can reduce the 

aesthetic values for millions of people. These quality of life 

effects represent real value losses, but they are difficult to 

estimate and may even be transitory as regrowth occurs and 

society adjusts expectations regarding what constitutes a 

natural or aesthetically appealing condition.

Climate change could alter the complex of interactions 

between forest conditions and waterflow and quality. Forest 

cover and condition constitute only one element of a complex system, so effects may be difficult to isolate, but forest 

condition appears to be strongly related to both flood protection (a regulating service) and water quantity and quality (a 

provisioning service). More variable precipitation patterns 

(stronger drought and extreme rainfall events) both increase 

the service value of forests in protecting against flooding and 

landslides, but they also change forest conditions in ways 

that reduce soil-protecting qualities. This negative feedback 

suggests potential for accelerated losses of flood protection 

services of forests in many places. Reduced supplies of these 

services would coincide with strong growth in the demand 

for water services caused by population growth and associated water needs for personal and commercial uses.

The longer term and less certain effects of climate 

change on forest conditions discussed above suggest a 

growing area of forests in a state of disequilibrium with 

8

 Studies of economic effects of endemic pests such as 

Pye et al. (2011) may best be viewed as providing estimates of background losses (i.e., the “business as usual” 

case). 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

disjoint species-climate associations. The notion of “novel” 

conditions suggests “unknowable” implications, especially 

regarding the supply and demands for ecosystem services 

and the reactions of private landowners and government to 

increasing scarcity of important services. However, economic factors will likely drive responses. It is also likely that the 

risks of climate change to forests may open public dialogue 

regarding the costs and benefits of providing ecosystem 

services as well as how changes in forest policy may better 

align the producers and consumers of these services on both 

private and public forest lands (e.g., providing compensation 

for private landowners’ provision of scarce ecosystem services, as anticipated by the 2008 Farm Bill [Food, Conservation, and Energy Act of 2008]).

Adaption and mitigation strategies for forests will alter 

the provisioning of ecosystem services and involve explicit 

tradeoffs between services. For example, thinning and fuel 

treatment to reduce the vulnerability of forests to disturbance 

regimes and stressors defines a specific tradeoff between 

short-term changes in C stocks to enhance the long-term 

sequestration of C. 

Adaptation strategies in forests can build resistance to 

climate-related stressors, increase ecosystem resilience by 

minimizing the severity of climate change effects, or facilitate large-scale ecological transitions in response to changing environmental conditions. Resistance, resilience, and 

transitions are tiered to increasing levels of environmental 

change and time scale. Each one of these adaptation strategies will result in a changing bundle of ecosystem services.

Because effects on ecosystem services connect climate 

change to human well-being, they provide a key metric of 

costs to society. The scarcity of ecosystem services may 

provide an impetus for adaptive actions, either through 

individual or company decisions where private goods are 

affected, or through government intervention where ecosystem services represent public goods. Future provision of 

forest-related ecosystem services will develop not only from 

climate-induced changes in forested ecosystems but also 

from the response of these social choice systems to perceived scarcity of ecosystem services. Furthermore, policy 

choices designed to mitigate climate change imply important 

effects on forest conditions and the provision of ecosystem 

services. For example, increased demands for wood-based 

bioenergy would alter forest biomass, C, and species composition and would change the distribution of rural land uses. 

The interaction between climate change and social systems 

in determining the future of forest ecosystem services is the 

topic of chapter 3.

Conclusions

As documented in the U.S. Climate Change Science Program Synthesis and Assessment Product 4.3 (Backlund et 

al. 2008), climate change is occurring and we are observing 

many effects on forests. Some of the most notable observed 

effects occur in the Western United States and include an 

increase in the size and intensity of forest fires, bark beetle 

outbreaks killing trees over enormous areas, accelerated tree 

mortality from drought, and earlier snowmelt and runoff. 

Predictions of the effects of climate change on forests have 

changed little since the 2008 report, but additional research 

has provided wider documentation of the effects of climate 

change and better tools for projecting future effects.

If climate change proceeds as simulated by models, 

the United States will be warmer, changes in precipitation 

will differ by region, and precipitation will become more 

variable. Global climate model simulations predict that the 

average annual temperature in the United States will increase 2.5 to 5.3 °C by 2100 above average temperatures in 

1971 through 2000, with the largest temperature increase in 

the northern and interior regions. By 2100, precipitation will 

likely decrease 6 to 12 percent in the Southwest and increase 

6 to 10 percent in Northern states, and droughts will become 

more common. Sea level will rise between 0.2 and 2.0 m by 

2100, depending on the emissions scenario and model. Days 

with temperature greater than 35 °C will also increase.

The projected changes will likely lead to even more disturbance from insect outbreaks, forest fire, and drought, and 

the tree mortality from these disturbances may switch the 

United States from a current C sink (offsetting 13 percent of 

U.S. fossil fuel greenhouse gas emissions) to a source. Such 

a switch would provide a positive feedback by accelerating 

atmospheric CO2

 concentrations and climate warming. Carbon losses from tree mortality caused by disturbance may be 
62

GENERAL TECHNICAL REPORT PNW-GTR-870

partially offset by increased growth in Eastern U.S. forests, 

where water is sufficient and elevated atmospheric CO2 and 

N deposition promote tree growth. Fire suppression efforts 

will likely be more costly as climate warms and fires become 

larger and more frequent, and as more houses are built in the 

wildland-urban interface. Interactions among disturbances 

are currently difficult to project, but will likely increase 

overall disturbance. 

Habitat for species will likely move northward and 

upward in elevation, and the movement of suitable habitat 

may be faster than species can disperse to the new habitats. 

Climate change will likely accelerate the establishment of 

invasive species in forests, with perhaps the highest risk in 

mountainous ecosystems.

Direct and indirect effects of climate change will affect 

the hydrological cycle. The effects of elevated CO2 on transpiration will likely be less than ± 10 percent, a relatively 

small change compared to the effects of precipitation variability on transpiration. More frequent droughts will reduce 

streamflow, and concentrating precipitation in intense storms 

will likely increase the risk of erosion and landslides. Tree 

mortality from disturbances will likely increase runoff, and 

elevated temperatures will probably decrease snow cover 

depth, duration, and extent and advance the timing of runoff.

Predictions for many other ecosystem effects remain 

much less certain than those presented above. Some effects, 

like the response of mature trees to elevated CO2 or the combined effects of temperature and drought, currently lack the 

science to make good projections. Other projections, such as 

the movement of individual species, the location and timing 

of insect and pathogen outbreaks, and the success of specific 

invasive species, result from complex interacting factors 

and contingencies—perhaps too complex to ever be predictable. Local predictions for future climate remain uncertain, 

which makes local projections of effects on ecosystems also 

uncertain. 

Despite some uncertainty about how future ecosystems 

will evolve, what their specific species complement will be, 

and the timing and exact frequency of future disturbance, 

climate change will bring more disturbance to forests and 

with it, significant management challenges. Approaches 

to dealing with many of these management challenges are 

presented in chapter 4 of this assessment.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

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Chapter 3

Climate Change, Human Communities, and Forests in Rural, Urban, 

and Wildland-Urban Interface Environments

David N. Wear and Linda A Joyce1

Contributors 

Brett J. Butler, Cassandra Y. Johnson, Susan I. Stewart, and David J. Nowak2

Introduction

Human concerns about the effects of climate change on 

forests are related to the values that forests provide to human 

populations, that is, to the effects on ecosystem services 

derived from forests. Service values include the consumption of timber products, the regulation of climate and water 

quality, and aesthetic and spiritual values. Effects of climate 

change on ecological systems are expected to change service 

flows, people’s perception of value, and their decisions 

regarding land and resource uses. Thus, social systems will 

adapt to climate changes, producing secondary and tertiary 

effects on the condition of forests throughout the world. This 

chapter explores how social systems might interact with 

changing climate conditions in determining the future of 

forested ecosystems in the United States.

Forests and derivative ecosystem services are produced 

and consumed in three types of environments. Most forested 

lands are in rural settings, where human population densities are low and forest cover dominates. In contrast, human 

1 David N. Wear is a research forest economist, U.S. Department 

of Agriculture, Forest Service, Southern Research Station, Center 

for Integrated Forest Science and Synthesis at North Carolina State 

University, Department of Forestry and Environmental Resources, 

Campus Box 8008, Raleigh, NC 27695; Linda A. Joyce is a research quantitative ecologist, U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station, 240 West Prospect, Fort 

Collins, CO 80526.

2 Brett J. Butler is a research forester, U.S. Department of Agriculture, Forest Service, Northern Research Station, 160 Holdsworth 

Way, Amherst, MA 01003; Cassandra Y. Johnson is a research 

social scientist, U.S. Department of Agriculture, Forest Service, 

Southern Research Station, 320 Green Street, Athens, GA 30602; 

Susan I. Stewart is a research social scientist, U.S. Department 

of Agriculture, Forest Service, Northern Research Station, 1033 

University Place, Suite 360, Evanston, IL 60201; David J. Nowak is 

a research forester, U.S. Department of Agriculture, Forest Service, 

Northern Research Station, c/o SUNY ESF, 5 Moon Library, Syracuse, NY 13210.

populations dominate urban settings, where forests and trees 

may be scarce but their relative value, measured as direct 

ecosystem services, may be high. In urban areas within 

grassland biomes or in arid zones, tree cover may be highest 

where people live. Transition zones between rural and urban 

settings contain the wildland-urban interface (WUI), where 

forest settings comingle with human populations. These 

three settings pose different challenges for climate changerelated resource management and policy, and each defines a 

unique set of opportunities to affect changes in forest conditions and service flows.

This chapter explores the interactions among forest 

condition, human value, policy, management, and other 

institutions, and the potential effects of these interactions on 

human well-being. We examine (1) the socioeconomic context (ownership structure, how value is derived, institutional 

context), (2) interactions between land use changes and 

climate change that affect forest ecosystems, and (3) social 

interactions with forests under climate change (climate factors, community structure, social vulnerability). In addition, 

forests have the potential to mitigate climate change through 

carbon (C) sequestration and through bioenergy production 

to substitute for fossil fuel energy. Hence, we also examine 

the potential influence of C mitigation on forest production, 

the forest economic sector, and forest land use.

Socioeconomic Context: Ownership, 

Values, and Institutions

In the United States, forest conditions and the flow of ecosystem services from forest land strongly reflect a long history of use and restoration as well as the influence of policy 

affecting both public and private forests (Williams 1989). 

Future forest management and policy, including responses to 
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climate change, require an understanding of socioeconomic 

interactions with forests and how they might determine future conditions under different climate futures. Three key elements of the socioeconomic context of forests in the United 

States are (1) ownership patterns that define the institutional 

context of management, (2) forest contributions to human 

well-being through provision of various ecosystem services, 

and (3) the institutional settings that shape decisionmaking 

processes.

Forest Ownership Patterns in the United 

States

Forest owners, those who own and manage the land, comprise the individuals and groups most directly affected by, 

and most capable of mitigating, the potential impacts of 

global climate change on forests. Working within social and 

biophysical constraints, the owners ultimately decide the fate 

of the forest: whether it will remain forested, and whether 

and how it will be actively managed. Of the 304 million ha 

of forest land in the United States, 56 percent is privately 

owned by individuals, families, corporations, Native American tribes, and other private groups (fig. 3.1) (Butler 2008). 

The remaining forest land is publicly owned and controlled 

by federal, state, and local government agencies. 

Ownership patterns differ significantly across the 

United States (fig. 3.2). In the East, where 51 percent of the 

Figure 3.1—Forest ownership in the United States, 2006 (Butler 

2008).

Nation’s forests are located, the extent of private ownership 

is much higher (81 percent) than in the West and in some 

states is as high as 94 percent. In contrast, the West is dominated by public, primarily federal, ownership (70 percent), 

with public forest ownership in some Western States as high 

as 98 percent (Butler 2008).

Public agencies have acquired land through various 

methods and manage them for diverse objectives. The 

federal government owns 33 percent of all forest land, with 

ownership dominated by the U.S. Forest Service (59 million 

ha) and the Bureau of Land Management (19 million ha). 

Other federal agencies with forest land holdings include the 

U.S. Fish and Wildlife Service, the National Park Service, 

and the Department of Defense. Public forests often have 

multiple uses, although one use may dominate at local 

scales (e.g., water protection, timber production, wildlife 

habitat, preservation of unique places, buffers for military 

exercises).

State agencies control 9 percent of all U.S. forest land, 

and county and municipal governments control 2 percent. 

Many state-owned forest lands are managed by forestry, 

wildlife, and park agencies. Other than military uses, most 

state and local uses mirror federal uses. Common objectives 

of many local land management agencies are water protection, recreation, and open space preservation.

Of the major forest ownership categories, families and 

individuals own a plurality (35 percent, 106 million ha) of 

the forest land in the United States. There are over 10 million of these ownerships, collectively called family forest 

ownerships.3

 The characteristics of their holdings differ, as 

do their reasons for owning them. Although most (61 percent) family forest ownerships are small (0.4 to 3.6 ha), 53 

percent of the land in these ownerships is owned by those 

with 41 ha or more (fig. 3.3).

Most family forest ownerships own forest land for its 

amenity values, such as its beauty, legacy for future generations, and privacy. Financial motivations are not usually 

rated as important, although for a significant number of

3

 Defined by the U.S. Forest Service as families, individuals, trusts, 

estates, family partnerships, and other unincorporated groups of 

individuals who own forest land. The minimum forest holding size 

is 0.4 ha.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 3.2—Distribution of public and private forest ownership in the United States.

Figure 3.3—Family forest ownerships in the 

United States by size of forest holdings, 2006. 

(Butler 2008).
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ownerships, especially with larger forest holdings, timber 

production and land investment are important. 

Although timber production is not a primary ownership 

objective of most family forest owners, 27 percent of the 

family forest ownerships, owning 58 percent of the family 

forest land, have harvested trees. Few family forest owners 

have a written management plan (4 percent of family forest 

ownerships; 17 percent of family forest land), have participated in a cost-share program (6 percent; 21 percent), have 

their land green-certified (1 percent; 4 percent), or have a 

conservation easement on their land (2 percent; 4 percent) 

(Butler 2008). Nevertheless, evidence from landowner surveys indicates that most family forest owners have a strong 

land ethic and are conservation-minded (Butler et al. 2007).

Most other private forest land is controlled by corporations (56 million ha; 18 percent of all forest land). These 

include traditional forest industry and forest management 

companies, timber investment management organizations 

(TIMOs), and real estate investment trusts (REITs). Many 

other corporations also own forest land but do not have forest management as their primary ownership objective (e.g., 

utilities, mining companies, and those that happen to have 

forest acreage associated with a property, such as a manufacturing plant). 

Native American tribes, nongovernmental organizations, 

clubs, and unincorporated partnerships control 8.5 million ha 

(3 percent) of the Nation’s forest land. Some ownerships are 

explicitly for forest conservation (e.g., land trusts), others 

are largely for recreation (e.g., hunting clubs), and there are 

many other proposes. 

From 1977 to 2007, U.S. forest land increased a net 

8.9 million ha (4 percent) (Smith et al. 2009) (fig. 3.4). This 

increase occurred mostly in public, and in particular state, 

ownership. From 1997 to 2007, however, private forest land 

decreased a net 0.4 million ha. Over the next 50 years, U.S. 

forest land is projected to have a net loss of 9.3 million ha 

(Alig et al. 2003), mostly on private lands owing largely to 

urbanization.

Since the 1980s, the types of corporations that own forest land have undergone a major change. Traditionally, most 

corporate forest land was owned by vertically integrated 

forest industry companies, which owned both forest land and 

the facilities to process the wood. Beginning in the 1980s 

and accelerating in the 1990s, most of these vertically integrated companies separated their forest holdings from their 

other assets, and many began to divest themselves of land. 

This decrease was paralleled by an increase in TIMOs and 

REITs. The vertically integrated companies were, at least in 

theory, more long-term-oriented and interested in supplying 

their mills from their lands. Conversely, TIMOs and REITs 

often have shorter investment time horizons and no need to 

supply mills, and hence they have different objectives.

Family forests have been undergoing parcelization, 

the dividing of larger parcels of land into smaller ones. If 

parcelization is accompanied by new houses, roads, or

other changes, then forest fragmentation will increase,

which in turn can harm ecosystem functions. Twenty percent of current family forest landowners are at least 75 years 

old, suggesting that a large amount of land will soon change 

hands. It is at this point of transfer that parcelization will 

probably occur, along with other changes in forest ownership objectives. 

These forest ownership patterns have important implications for global climate change. It is especially notable that 

more than one-half of the forest land in the United States is 

Figure 3.4—Trends in U.S. forest ownerships, 1977 through 2007 

(Smith et al. 2009).
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

currently owned by private landowners, thus these landowners could play a critical role in mitigating climate change 

effects. These ownership patterns, as well as the dynamics 

of change in forest land ownership, suggest the importance 

of engaging in a dialogue with landowners on the role of 

forest land management with respect to changes in both 

climate and land use. Such discussions might include the 

level of management necessary to sustain a suite of ecosystem services from forests (e.g., assisted migration of species, 

management of fire regimes) and to enhance the resilience of 

existing forest ecosystems. Policies that aim to mitigate the 

effects of climate change on forests must take into account 

the needs, desires, and resources of the owners.

Economic Contributions of Forests

Forest landowners have many reasons for owning forests, 

and forests deliver values in many forms to private landowners. Forests also provide a suite of ecosystem services that 

accrue to broader social well-being. For example, aesthetic 

values are usually not identified by the landowners as a monetary benefit of forest ownership. Likewise, forest owners 

may enhance wildlife habitat and use forest cover to protect 

watersheds without receiving financial returns. 

In rural settings, forest cover can generally be equated 

with forest land use, because forests are a consequence of 

a decision either to dedicate land to growing trees, where 

other potential uses are not viable, or to allow land to return 

to a fallow condition. Rural forest ownership may provide 

direct returns, consumptive values, and monetary returns. 

Direct returns can accrue either through extractive activities 

(mainly commercial timber harvesting) or to the in situ value 

of forests (e.g., hunting leases, conservation easements). 

Consumptive values may accrue through direct use of forests 

for recreation, existence value, and aesthetics. Most monetary returns are generally confined to timber production, with 

some additional returns from recreation leases, conservation 

easements, and payments for other ecosystem services (generally through government programs such as state wetland 

mitigation programs guided by U.S. Environmental Protection Agency (USEPA) requirements under the Clean Water 

Act of 1977).

The United States produces more timber by volume than 

any other nation, and timber represents a significant source 

of value for forest landowners. Although the volume of 

roundwood used for industry and fuelwood nearly doubled 

between 1945 and the late 1980s, production since then has 

leveled off and declined (fig. 3.5). In 2006, the year before 

the latest recession, total timber production stood at about 90 

percent of its peak value in 1988. The economic contribution of harvested timber has also declined. Between 1997 

and 2006, the total value of shipments (the sum of net selling 

values of freight on board of all products shipped by the 

sector) fell by 7 percent, from a peak of $334 billion to $309 

billion (Howard et al. 2010b).4 Nearly the entire decline in 

the value of shipments over this period is explained by declines in the paper products sector. In 2006, production from 

the Eastern United States dominated this sector, representing 82 percent of the value of shipments in paper products 

industries and 74 percent of the value of shipments in wood 

products industries (79 percent of the total).

In contrast to declining production of wood products in 

the United States, consumption has been growing, implying 

increasing reliance on imported wood products. Consumption expanded from 0.37 billion m3

 in 1988 to about 0.57 

billion m3 in the 1990s and 2000s (Howard et al. 2010a). 

Although per capita consumption has been trending downward since the late 1980s, population growth has continued 

to push total consumption upward in recent years (fig. 3.6). 

Between 1957 and 2006, U.S. per capita consumption of 

wood products averaged 2 m3 per person, peaking in the 

late 1980s (2.26 to 2.32 m3

 per person) and falling in the 

2000s (1.95 m3 per person). Nearly all the reduction over 

this period is explained by reductions in the consumption of 

fuelwood, leading to the conclusion that consumption levels 

of total wood and paper products in the United States have 

risen in direct proportion to population growth (Howard et 

al. 2010b).

4

 All dollar values in this section are measured as real 

2005 dollars defined by the gross domesitic product price 

deflator.
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Figure 3.5—U.S. roundwood production, 1957 through 2006 (Howard et al. 

2010a).

Figure 3.6—U.S. per capita apparent 

roundwood consumption, 1957 through 

2006 (Howard et al. 2010b).
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 3.7—Value of shipments for forest products industries (NAICS codes 321, 322, and 

337), 1997 through 2006 (Source: Howard et al. 2010a).

The value of U.S. timber production returned to forest 

landowners is difficult to assess because of data limitations. 

One estimate (USDA Forest Service 2011) puts this value at 

$22 billion in 1997, with 89 percent returned to private landowners. This is roughly 7 percent of the value of shipments 

for the wood products sectors (fig. 3.7). In 2006, the value 

of all wild-harvested nontimber resources was about $0.5 

billion, and direct payments to landowners for forest-based 

ecosystem services was about $2 billion (USDA Forest 

Service 2011). Most payments for ecosystem services come 

from returns from conservation easements, hunting leases, 

and wetland mitigation banks. Total revenue to private forest landowners in 2006 was $20 million (about $119 ha-1), 

representing an average capitalized value (at a discount rate 

of 5 percent) of about $2,347 ha-1 for all private forest land 

in the United States.

In rural settings, many ecosystem services from forest 

land provide benefits of forest ownership and use for which 

landowners are not compensated. For example, cultural 

values associated with forest areas—such as aesthetics, 

dispersed recreation, and spiritual needs—rarely lead to 

monetary compensation; nor do the benefits of protection of 

water quality and regulation of climate and flooding. Current 

policy initiatives (e.g., the 2008 Farm Bill [Food, Conservation, and Energy Act of 2008]) focus on providing payments, 

often through constructed markets, to compensate landowners for ecosystem services. An emerging area of engagement 

involves compensation from municipalities to landowners in 

municipal watersheds for activities that enhance or protect 

water quality (Brauman et al. 2007, Greenwalt and McGrath 

2009). In urban settings, tree cover can affect environmental 

and aesthetic services for many people. Urban trees remove 

pollution, store C, and cool microclimates. Urban parks provide important recreation sites in the midst of human settlements. The forested area of the WUI is seen as an attractive 

environment in which to live, near rural or small-town 

settings. Here the extractive value of trees depends upon 

the size of the land and the landowner’s preferences, but the 

trees in the WUI typically have little extractive value other 

than as fuel wood. The environmental and aesthetic services 

in the WUI differ from those of rural forests, because these 

environments are greatly influenced by the human activities 

in them. 

Policy Context of Forest Management in 

Response to Climate Change

An institution is any rule or organization that governs the 

behavior of humans. In the context of forest management 

activities in response to climate change, relevant institutions 

include the structures of public and private land ownership, 
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GENERAL TECHNICAL REPORT PNW-GTR-870

nongovernmental organizations addressing aspects of forest 

values, and policy instruments, such as forest management 

laws and taxes that influence land management decisions. 

Human behavior, expressed through land use decisions, is 

the dominant cause of landscape change; thus, institutions 

are crucial control mechanisms in determining future forest 

conditions and responses to climate change.

Forest management in the United States derives from 

the interaction of the two dominant institutional structures 

of private and public ownership. Private ownership affords 

extensive property rights held by autonomous landowners but is constrained by tax and regulatory policy. Under 

the right conditions, enlightened self-interest should guide 

landowners to allocate land to the highest-valued uses and, 

in the process, to effectively produce marketable goods and 

services. However, nonmarket ecosystem services, which 

deliver considerable value to society, are not likely to be 

fully valued in private transactions. On the other hand, the 

production of nonmarket goods is a primary rationale for 

public ownership of forest land (e.g., Krutilla and Haigh 

1978). In theory, public land management aims to provide 

the “right mix” of all important goods and services.

Public ownership is not the only mechanism for providing nonmarket goods. Government can direct the actions of 

individual landowners toward producing other nonmarket 

benefits, by altering incentives (e.g., reforestation subsidies 

and severance taxes) and selectively restricting property 

rights (e.g., through forestry practice regulations). Nongovernmental organizations can directly affect changes in land 

use and resource allocation, through outright purchases of 

land or purchase of development or other rights using conservation easements. The use of all policy tools has its costs, 

including costs of both administration and of forgone market 

benefits. Balancing these regulatory costs against public 

benefits is a critical part of policy design.

Private and public forest ownerships offer up very different models of response to climate changes. For example, 

private forest owners might be expected to alter their 

management plans more rapidly in response to events such 

as observed or anticipated climate impacts, altered market 

signals (prices), or policy instruments that might affect the 

provision of nonmarket, ecosystem services. The extent of 

such a response is ultimately governed by the preference 

structure of private forest owners. Butler et al. (2007) found 

structural dissimilarities both between the objectives of 

corporate and family forest owners and among subgroups of 

the family forest owners. Still, the private forest sector has 

shown high responsiveness to market signals in harvesting 

timber and investing in future timber production, especially 

in the southeastern United States (Wear and Prestemon 

2004). For example, the area of intensively managed pine 

plantations more than doubled in the South between 1990 

and 2010 (Wear and Greis 2011) as production shifted from 

western to southern regions. 

Overall, private forests in the United States have lower 

levels of forest inventory (reflecting a “younger” forest age 

class distribution), are generally more accessible, and are 

much more likely to be harvested or receive other forms of 

forest management. On the other hand, public management 

can attempt to maximize broad public welfare derived from 

forests and thus produce benefits not ordinarily provided by 

markets. By design, public forest management in the United 

States, especially at the federal level, can be slow to adjust, 

given the interplay between technical tradeoff analysis and 

an adversarial public process of resource planning (e.g., 

Wilkensen and Anderson 1987, Yaffee 1994). Overall, compared to private forests in the United States, public forests 

carry higher levels of forest inventory, are typically more 

remote, and are less likely to receive active management. 

Future responses to climate signals, and especially to 

programs designed to mitigate greenhouse gas emissions, 

would probably be much larger on private lands, where market signals and direct policy instruments (incentives and disincentives) are readily translated into management actions. 

These responses could be in the form of increased harvesting 

(resulting from introduction of new product markets such as 

biofuels/bioenergy) and altered forest management (responding to demands for forest-based C storage), but they could 

also occur as forest area decreases or increases, depending 

on the comparative returns to land from forest and agricultural uses. As a result, we expect faster and larger policy impacts on greenhouse gas mitigation management activities to 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

occur in the Eastern United States, where private ownership 

dominates and transportation and processing infrastructure 

for wood products are more extensive.

In the forest sector, policy responses to climate change 

have focused largely on mitigation actions that reduce either 

the use of fossil fuel, through bioenergy products, or the 

amount of carbon dioxide (CO2) in the atmosphere, through 

C sequestration in forests. The use of woody biomass for the 

provision of energy could offset fossil fuels, substituting 

either ethanol for oil in transportation fuels or wood for coal 

in electricity production. In addition, C cap and trade initiatives focus on sequestering additional C through growth in 

forests and consumption of durable wood products. Potentially more significant, policies outside the forest sector 

could have secondary impacts on forest area and conditions, 

largely through land use changes. Federal agricultural policies, such as crop price support programs, likely affect the 

total area of cropland in production and therefore the area of 

forests. Local policies regarding land uses affect the rate at 

which forest land is converted to developed uses. 

Interactions Among Forests, Land 

Use Change, and Climate Change

Land use changes are influenced by choices of landowners, 

market forces, and economic and environmental policies. 

These forces differ across the three types of forested environments (rural, urban, WUI). In rural environments, market 

forces influence shifts between agricultural uses and forest 

uses. Urban expansion converts forest land, with loss of 

some trees, and intensification of urban areas often leads to 

the loss of most trees. In the WUI, conversion of large forest 

tracts to residential areas is driven by home buyers who value the amenities of living in or near forests and are willing 

to pay more to do so. Such land use-induced changes in land 

cover can have local effects on climate, both temperature 

and precipitation (Fall et al. 2009). Hence, land use changes 

may interact with changes resulting from greenhouse gases 

and together strongly influence forest dynamics. This section 

focuses on understanding the nature of land use change and 

the role of landowner choices and institutional policies in 

retaining forest land cover under a changing climate. 

Interactions Among Forests, Agricultural 

Land Use, and Climate Change in Rural 

Environments

United States forests produce more timber products than any 

other nation (United Nations Food and Agricultural Organization 2000), and U.S. agriculture produces 41 percent 

of the world’s corn and 38 percent of the world’s soybeans 

(Schlenker and Roberts 2009). History demonstrates a 

tradeoff between agricultural and forest uses in the United 

States, based on shifting advantages and returns between 

these two types of use. Both climate change and programs 

designed to subsidize land-based products may affect land 

use choices and the extent of forest area in the United States. 

Where people occupy rural areas, these areas will be affected 

not just by the dynamics influencing forest and agricultural 

use, but also by population growth. 

Climate (e.g., precipitation amount and variability, air 

temperature, solar insolation, snow cover) is a key driver of 

agricultural productivity. Climate change could influence 

not only the returns to agriculture but also land use switching between crops, pasture, forest, and other uses. Modeling 

studies indicate that crop productivity is negatively related to 

temperature increases (for all seasons except fall) and positively related to nonfall precipitation (e.g., Mendelsohn et al. 

1994). Climatic variability may also affect crop productivity 

(Mendelsohn et al. 2007). Estimates of potential climatic 

effects on productivity are influenced by how the model is 

specified. Schlenker and Roberts (2009) investigated nonlinear relationships between key climate variables and crop 

productivity that lead to critical thresholds in these relationships; for example, beyond a certain maximum temperature, 

small increases in temperature are related to large declines 

in crop productivity. Based on climate scenarios generated 

by the Intergovernmental Panel on Climate Change (IPCC), 

precipitous declines in productivity are projected for important crops in the United States, especially in the latter part of 

the 21st century (Schlenker and Roberts 2009).

Compared to assessments of climate effects on agriculture, estimates of its effects on forest productivity have 

been less definitive and emphatic. Unlike annual crops,

forest ecosystems are long living and complex, and this may 
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GENERAL TECHNICAL REPORT PNW-GTR-870

buffer some effects of climate variation. However, compared 

to agricultural crops, regeneration and mortality phases in 

the forest ecosystem are not well understood in relation to 

climate, and these are critical phases in forest establishment. 

Furthermore, disturbances such as wildfire, hurricanes, and 

intense rainfall and flood events can result in immediate 

changes to the forest ecosystem, including extensive mortality and erosion (for a full discussion, see chapter 2).

The history of land use in the United States indicates 

flexibility at the margins between agriculture and forests, but 

mainly in the East, where many states experienced agricultural abandonment and the recovery of forest cover through 

the 20th century (Ramankutty et al. 2010, Waisanen and Bliss 

2002). At the same time, cropland expanded strongly in the 

Corn Belt and Northern Plains (Illinois, Iowa, Minnesota, 

North Dakota, and Montana) and in Florida. These trends are 

consistent with reduced transportation costs and increased 

integration of markets across and between regions, leading 

to consolidation of agricultural production in a few subregions (e.g., cereal crops in the Corn Belt, vegetable crops in 

Florida and California). 

Changes in crop prices have affected changes in 

cropland allocation as well. In the 1970s, soybean markets 

drove conversion of forest land to cropland, especially in 

the Mississippi Alluvial Valley (Lubowski et al. 2006). 

Between 2000 and 2009, ethanol demands expanded U.S. 

corn production by 10 percent (2.91 million ha), and corn 

prices increased by about 75 percent (Congressional Budget 

Office 2009, Wallander et al. 2011). Expanded corn planting 

resulted largely from shifts out of soybeans, but with compensatory shifts toward soybeans and among other crop and 

hay land uses (Wallander et al. 2011). Although these crop 

substitutions moderated the push toward corn ethanol, they 

placed upward pressure on farm commodity and food prices 

in the United States and elsewhere (roughly 20 percent of 

increased prices were attributed to corn ethanol production) 

(Congressional Budget Office 2009).

Land use changes are not driven just by market factors 

such as price and transportation costs, but they may also be 

directly influenced by economic and environmental policies. United States agriculture is perhaps the most heavily subsidized sector of the U.S. economy, and changes in 

support prices and other programs could affect changes in 

land use allocation. Some policies directly encourage land 

use switching, as in the case of the Conservation Reserve 

Program (CRP). Established in the 1985 Farm Bill (Food 

Security Act of 1985), the CRP pays landowners to retire 

erodible cropland to natural cover; in February 2010, about 

12.6 million ha were in the program (Hellerstein 2010). The 

most recent decline in cropland area in the United States 

coincides with the establishment of this program.

Future rural land uses are likely to adjust in response 

to a combination of three factors: population-driven urbanization, the comparative returns to agriculture and forestry, 

and policies that influence the expression of the first two 

factors. The recent Resources Planning Act (RPA) assessment (USDA FS 2012, Wear 2011) forecasts an increase in 

developed uses from about 30 million ha in 1997 to 54 to 65 

million ha in 2060 (a gain of 24 to 35 million ha), based on 

alternative projections of population and income linked to 

IIPCC scenarios. The RPA assessment incorporates changes 

in relative returns to forests but holds agricultural returns 

constant over the forecast period.

Comparative returns to agriculture and forestry could 

be altered directly and indirectly by climate change. Direct 

effects derive from potential shifts in productivity, as examined above. At the margin, shifts in agricultural productivity would lead to land use switching between forests and 

crops. At a broader market scale, increased scarcity of crop 

output would drive up prices and overall demands for land 

in crops. Stronger shifts in comparative returns to forestry 

and agriculture would probably result from policy changes, 

especially those designed to encourage bioenergy production. The degree to which a bioenergy sector favors agricultural feedstocks, such as corn, or cellulosic feedstocks from 

forests will determine the comparative position of forest and 

crop returns to land use, and therefore land use allocations. 

The allocation among feedstock sources depends on energy 

policies at both federal and state levels, which could differentially affect rural land uses. For example, federal policy 

to date has subsidized corn ethanol production, but the 2008 

Farm Bill and some state-level Renewable Portfolio Standard policies encourage use of wood in electricity generation. These policies would likely add to rather than supplant 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

current emphasis on agricultural feedstocks. Policy initiatives to mitigate climate change through bioenergy and C 

sequestration may have more direct and immediate impacts 

on land use and the forest area than the impacts of climate 

change itself.

Interactions Between Trees and 

Climate in Urban Environments 

Although it is common to distinguish between forest and developed land cover types, trees within developed areas may 

provide a disproportionately higher value of ecosystem services because of their proximity to human habitation. Trees 

in urban environments both influence and are influenced by 

climate change. As the area of urban use expands, the extent 

and importance of urban trees will increase. Climate change 

will likewise have important effects on these trees, and urban 

trees may be especially well positioned to provide critical 

services in moderating climate in urban environments.

In 2000, urban areas occupied 24 million ha (3.1 percent) of the conterminous United States and contained over 

80 percent of the country’s population (Nowak et al. 2005), 

and urban and community lands occupied 41 million ha (5.3 

percent) (Nowak and Greenfield 2012). The definition of 

urban is based on population density using the U.S. Census 

Bureau’s definition (2007): all territory, population, and 

housing units located within urbanized areas or urban clusters. The definition of community is based on jurisdictional 

or political boundaries delimited by U.S. Census Bureau 

definitions of incorporated or designated places (U.S. Census 

Bureau 2007). Community lands are places of established 

human settlement that may include all, some, or no urban 

land within their boundaries. Because urban land reveals 

the more heavily populated areas (population density-based 

definition) and community land has varying amounts of 

urban land that are recognized by their geopolitical boundaries (political definition), the category of “urban/community” 

was created to classify the union of these two geographically 

overlapping definitions where most people live. 

Between 1990 and 2000, urban areas increased from 

2.5 percent to 3.1 percent of U.S. land areas (an increase 

about the combined size of Vermont and New Hampshire), 

and they are projected to increase to around 8.1 percent in 

2050 (an increase in area larger than Montana) (Nowak et al. 

2005, Nowak and Walton 2005). Given a projected increase 

in urban land of 38.8 million ha between 2000 and 2050 

and a concomitant conversion of about 11.8 million ha of 

forest to urban land (Nowak and Walton 2005), the current 

20.8 billion Mg of C stored in U.S. urban forests (above and 

belowground biomass) nationally is projected to decrease 

to 20.1 billion Mg by 2050.5 In addition, based on various 

climate change/development scenarios, percentage of tree 

cover nationally is projected to decrease by 1.1 to 1.6 percent between 2000 and 2060 (USDA FS 2012).

Urbanization can either increase or decrease tree cover 

depending where the urbanization occurs. The percentage of 

tree cover in urban/community areas tends to be significantly 

higher than in rural areas (i.e., lands outside of urban/community areas) in several predominantly grassland states, with 

the greatest difference in Kansas (17.3 percent) (Nowak and 

Greenfield 2012). In some cases, urban forest stewardship 

activities, both public and private, have helped to significantly increase and maintain forest area within cities. Urban/

community land in most states in more forested regions had 

lower tree cover compared to rural lands, with the greatest 

difference in Kentucky (-37.9 percent). Within urban areas 

of the conterminous United States, percentage tree cover 

is declining at a rate of about 0.03 percent per year, which 

equates to 7900 ha or 4.0 million trees per year (Nowak and 

Greenfield 2012) out of an estimated 3.8 billion urban trees 

(Nowak et al. 2001). Analysis of 18 moderate- to large-sized 

U.S. cities reveals that percentage tree cover has declined, 

on average, by about 0.27 percent of city area per year (0.9 

percent of tree cover) for these more densely populated

areas (Nowak and Greenfield 2012). In urban/community 

areas of the conterminous United States, tree cover averages 

35.1 percent (14.6 million ha), which is close to the national 

average (34.2 percent) (Nowak and Greenfield 2012). Cities 

developed in naturally forested regions typically have a

5

 Gavier-Pizarro, G.I.; Radeloff, V.C.; Stewart, S.I. [et 

al.]. [N.d.]. Seventy-year legacies of housing and road 

patterns are related to non-native invasive plant patterns 

in the forests of the Baraboo Hills, Wisconsin, USA. 

Ecosystems. Manuscript in preparation.
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GENERAL TECHNICAL REPORT PNW-GTR-870

higher percentage of tree cover than cities developed in 

grassland or desert areas (Nowak and Greenfield 2012; 

Nowak et al. 1996, 2001).

The structural value of the urban trees (e.g., cost of 

replacement or compensation for loss of trees) in the United 

States is estimated at $2.4 trillion (Nowak et al. 2002b). 

Urban trees provide many additional benefits, such as air 

pollution removal and C storage and sequestration. Annual 

pollution removal (fine particulates, ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide) by U.S. urban trees 

is estimated at 783 000 Mg ($3.8 billion value) (Nowak 

et al. 2006). Thus, as climates change, not only will these 

urban forests and their associated benefits be affected, but 

these forests will also help to mitigate the effects of climate 

change and reduce CO2 emissions emanating from urban 

areas. For example, U.S. urban trees are estimated to store 

771 million Mg of C ($14.3 billion value; based on a price 

of $20.3 per Mg of C), with a gross C sequestration rate 

of 25.1 million Tg·C ·yr -1 ($460 million yr -1) (Nowak and 

Crane 2002).

Effects of Climate Change on Urban Trees 

The greatest effects of climate change on urban trees and 

forests will likely be caused by warmer air temperature, altered precipitation, strengthening wind patterns, and extreme 

weather events, including droughts, storms, and heat waves. 

These changes, along with higher levels of CO2, are likely 

to have significant implications for urban forests and their 

management. 

In addition to regional and global climate changes, the 

urban environment creates local climatic changes that will 

affect urban forests. At the local scale, urban surfaces and 

activities (e.g., buildings, vegetation, emissions) influence 

local meteorological variables such as air temperature, precipitation, and windspeed. Urban areas often create what is 

known as the “urban heat island,” where urban surface and 

air temperatures are higher than in surrounding rural areas. 

These urban heat islands can vary in intensity, size, and location and can lead to increases in temperatures of 1 to 6 o

C 

(US EPA 2008). Urban areas also affect local precipitation 

percent (Shepherd 2005). For example, in some areas in the 

southeastern United States, monthly rainfall rates increase, 

on average, by about 28 percent (about 0.8 mm hr- 1) within 

30 to 60 km downwind of city areas (maximum downwind 

increase of 51 percent), with a modest increase (5.6 percent) 

over the city area (Shepherd et al. 2002). 

These environmental changes caused by the interaction of climate change and urbanization are likely to affect 

urban tree populations (Nowak 2010). Potential effects on 

urban tree populations include changes in (1) tree stress and 

decline in some species from changes in air temperature, 

precipitation, storm frequency and intensity, CO2 levels and 

associated changes in air pollution, (2) changes in species composition owing to both changes in climate (e.g., 

Iverson and Prasad 2001) and human actions and invasive 

plant characteristics that are influenced by climate change, 

(3) insect and disease compositions and prevalence, and (4) 

management and maintenance activities focused on offsetting tree health and species compositional changes (Nowak 

2000). Management activities to sustain healthy tree cover 

will alter C emissions (because of fossil fuel use), species 

composition, and urban forest attributes such as biodiversity, 

wildlife habitat, and human preferences and attitudes toward 

urban vegetation.

Effects of climate change may be accelerated or reduced 

in cities depending on whether managers alter plant populations toward better adapted species or attempt to minimize 

the effect of global climate change through enhanced maintenance activities (e.g., watering, fertilizing). The degree to 

which urban tree populations are affected by climate change 

will depend on actual changes in air temperatures, precipitation, and length of growing season, as well as on human activities in urban areas that affect outcomes such as pollution 

and CO2

 concentrations, disturbance patterns, and decisions 

related to vegetation maintenance, design, selection, plantings, and removals.

Effects of Urban Trees on Climate Change

Climate change can have both positive and negative effects 

on the urban forest. Management activities can produce 

healthy and sustainable urban forests to help offset impacts 

of climate change. Nowak (2000) proposed four main ways 

that urban forests affect global climate change: 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Removing and storing carbon dioxide—

Trees, through their growth process, remove CO2 from the 

atmosphere and sequester the carbon within their biomass 

(McKinley et al. 2011). The net C sequestered from afforestation or reforestation programs is mostly the C sequestered 

by the first generation of trees. Future generations of trees 

sequester back the C lost through decomposition of previous 

generations. Thus, the net C storage in a given area with a 

given tree composition will cycle through time as the population grows and declines. When forest growth (C accumulation) is larger than decomposition, net C storage increases. 

Some C from previous generations can also be locked up 

in soils. Management activities can enhance long-term C 

storage in several ways: with large, long-lived species that 

are adapted to local site conditions, minimized use of fossil 

fuels to manage vegetation, vegetation designs to reduce air 

temperature and energy use, and use of urban tree biomass in 

long-term products (or limits on wood decomposition after 

removal) and energy production (Nowak et al. 2002a).

Emitting atmospheric chemicals through vegetation 

maintenance practices—

Urban tree management often uses relatively large amounts 

of energy, primarily from fossil fuels, to maintain the 

vegetation structure. Thus, to determine the net effect of 

urban forests on global climate change, the emissions from 

maintenance/management activities need to be considered. 

For example, equipment used to plant, maintain, and remove 

vegetation in cities includes vehicles for transport or maintenance, chain saws, back hoes, leaf blowers, chippers, and 

shredders.

Altering urban microclimates—

Trees are part of the urban structure so they affect the local 

urban microclimate by cooling the air through transpiration, 

blocking winds, shading surfaces, and helping to mitigate 

heat island effects.

Altering energy use in buildings and consequently 

emissions from powerplants, by planting trees in 

energy-conserving locations around buildings—

Urban trees can reduce energy use in summer through shade 

and reduced air temperatures, and they can either increase or 

decrease winter energy use (Heisler 1986), depending on 

tree location around the building (e.g., providing shade, 

blocking of winter winds).

Interactions Between Climate 

Change and the Wildland-Urban 

Interface

The WUI is where homes and associated developments 

co-occur with wildland vegetation (Radeloff et al. 2005). 

This WUI zone is delineated under wildland fire policy in 

the United States, because the risk of wildland fire affecting homes and other structures is greatest here. However, 

the WUI has perhaps even greater significance beyond fire 

management and policy, in that it encompasses where people 

live in direct contact with forests and other wildlands, and 

where development of forested lands for residential and 

commercial uses has direct, ongoing effects on the forest. 

Key changes driven by climate change, population growth, 

and markets for land uses are especially concentrated in this 

zone. 

Over time, these impacts are expected to increase, 

because growth in the WUI has outpaced growth outside 

the WUI, a trend expected to continue in coming decades, 

particularly in Western States. (Hammer et al. 2009). Theobald and Romme (2007) reached a similar conclusion; they 

estimated that Arizona, Colorado, Idaho, Montana, Nevada, 

and Utah will see the strongest WUI growth in the decades 

to come, and estimate at least 10 percent WUI growth in the 

United States by 2030. Analysis of historical growth patterns 

and projected growth rates in the United States generated estimates of 17 percent growth within 50 km of national parks, 

national forests, and wilderness areas by 2030 (Radeloff 

et al. 2010). The primary reason for continuing expansion 

of the WUI is that it reflects the affinity of many American 

home buyers for a house near or in the woods, and in a rural 

or small-town setting (McGranahan 1999). Forests are considered amenities, and home buyers prefer and pay more for 

home sites in or near the forest to maximize privacy, aesthetics, and recreational access. 

Expected WUI growth differs across the United States. 

Population growth is a strong predictor of housing growth 
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GENERAL TECHNICAL REPORT PNW-GTR-870

(Liu et al. 2003), and areas where population is expected to 

grow are concentrated in the West and South (Hammer et al. 

2009). Population growth is not the only factor driving housing growth; affluence, declining household size, and ownership of multiple homes also contribute to an expanding 

housing stock (Hammer et al. 2009). Although the housing 

market has changed dramatically since 2009, the downturn in home construction has been modest relative to the 

decades-long expansion of housing stock. Between 1940 and 

2000, although nationwide population doubled, the number 

of housing units more than tripled (Hammer et al. 2009). 

Not all rural areas are equally attractive (Johnson and 

Beale 2002, McGrannahan 1999); natural resources and 

other amenities add value, and protected area status is an 

added attraction to buyers because it guarantees that changes 

to the landscape (e.g., to species mix or forest age, conversion from forest cover to commercial, residential, or other 

use) will be modest. In studies that isolate just protected 

areas (i.e., the areas protected from development under 

various laws), the highest housing growth has occurred in 

proximity to protected areas (Radeloff et al. 2010, Wade and 

Theobald 2010). These lands have protected status in part to 

ensure that plant and animal species will be sustained, which 

makes their attractiveness for housing growth troubling from 

an ecological perspective (Gimmi et al. 2011).

Disturbances in the Wildland-Urban 

Interface

The WUI allows extensive contact between people and 

forests, through both simple proximity and intentional activity. Consequently, people in the WUI are more likely to be 

aware of fire and other forest disturbances that might be 

exacerbated by climate change, such as insect and disease 

outbreaks, severe weather damage, drought, and the spread 

of invasive plants. When forests are part of the residential setting, even less obvious changes (e.g., how early the 

maples leaf out, when migratory birds return) are more 

likely to be noticed, because the WUI resident sees the forest 

both daily and over long periods.

Among the best examples of how humans respond to 

disturbance and risk in the WUI is wildland fire, because it 

potentially threatens homes, typically a family’s largest single investment of capital. Awareness and acceptance of the 

need to prepare for wildland fire has grown with the WUI. 

Since 2002, the Firewise program has enlisted communities 

across the country to develop and maintain their residential 

areas in ways that minimize fire risk (NFPA 2011). This 

program formalized ideas that had been developing over 

preceding decades in response to both growing losses in the 

WUI and empirical evidence regarding wildland fire safety 

(e.g., the safest configuration of vegetation surrounding the 

home, building materials, and home and yard maintenance) 

(Cohen 2000, NFPA 2011). 

Ideally, entire communities would be “fire adapted,” 

where fire should be able to pass through a community without causing extensive damage. The creation of fire-adapted 

human communities is now being based on an interagency 

cohesive wildland fire strategy that uses a risk-based approach and is grounded in scientific research (Calkin et al. 

2011). The process of enlisting, encouraging, reminding, and 

assisting homeowners and communities in fire safety programs is far from simple. Basic research on psychology of 

risk, which measures and quantifies factors that affect how 

people judge risk, provides a basis for changing perceptions 

of risk and encouraging action to reduce risk (Slovik 1987), 

and some specific aspects of risk perception and response 

related to wildland fire have verified and extended this work 

(e.g., Cova et al. 2009). However, the same body of research 

also cautions that people are seldom willing to limit their 

frame of reference for a given risk as strictly as scientists 

might do and as policymakers might prefer. For example, 

even if asked by a scientist to focus on a specific source 

of risk without considering its context, most responses 

are shaped by additional factors (Slovik 1987). This phenomenon can be seen in forest management situations, for 

example when willingness to remove vegetation around the 

home is met with resistance, not because home owners do 

not understand risk, but rather, because they do not believe 

that removing vegetation would reduce their risk (McCaffrey 

2009). What the manager considers relevant to the situation 

is not the same as what the homeowner considers relevant 

(McCaffrey and Winter 2011), and this lesson has implications for managing forests under climate change. 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Climate change, like wildland fire, presents many challenges for the ability of people to understand, judge, and 

act on new information. Changes occur slowly and many 

threats are anticipated in the distant future, both of which 

attenuate the urgency of a response. In the broader cultural 

context, climate change beliefs are partisan and polarized 

and connect with deeply held convictions, such as whether 

humankind is the source of or a solution to problems, how to 

balance public good and private property rights, and whether 

science should be trusted or suspected. Long-term problems, 

large-scale solutions, and divisive underlying issues dramatically hurt chances for galvanizing public support for bold 

change. In contrast, specific, observable changes in forest 

resources, particularly in familiar and local forests, are best 

able to engage the attention and concern of people outside forest management and research communities. Local, 

place-specific solutions for problems are most likely to find 

support, especially if residents already know and trust local 

resource managers. Because people attach such great value 

to forests, the challenges of making climate change a salient 

issue and finding an engaged constituency are more modest 

than for climate change as a global concern. 

Fire and other forest disturbances (e.g., insect outbreaks) are a source of concern to many homeowners, yet 

living in the WUI is itself the source of many direct and 

indirect forms of disturbance. For example, changes in the 

density and use of road systems have many negative effects 

(Hawbaker et al. 2006, Radeloff et al. 2010), and in the 

yards surrounding homes people modify vegetation for functional and aesthetic purposes (Cook et al. 2011). The plant 

species used in landscaping may include exotic invasive species, which can be evident decades after they are introduced 

by the homeowner (Rogers et al. 2009). Feeding wildlife 

and keeping pets can alter the trophic balance of forested 

ecosystems, particularly when domestic cats and dogs roam 

outdoors (Lepcyzk et al. 2003). Building and landscaping 

disturb soils and change light availability, which can facilitate expansion of highly invasive species, and yard maintenance intentionally changes the distribution of water and 

other nutrients, another source of indirect effects on forests 

(Cook et al. 2011). In short, the overall effect of residential 

land use on forests and their ecosystems is often negative.

Multiple Stressors on Wildlands in the 

Interface

Like climate change itself, development of and activity in 

the WUI results in various changes to the forest. Multiple 

stressors are more problematic than single or sequential 

stressors because they overtax the resilience of the forest. 

Although regulations such as zoning ordinances that limit 

housing density, and neighborhood covenants governing 

property management are intended (in part) to protect the 

environment, forests may still suffer because each individual 

stressor is dealt with as though it occurs alone. Taken together, however, the many small disturbances can overwhelm the 

ability of forests and wildlife to adapt by requiring too much 

change too quickly; this problem is not typically reflected in 

land use and other residential policies. 

Awareness of the harm caused by multiple stressors is 

not apparent in the institutions that govern forests. A sobering possibility is that residential areas in or near forests 

could be well designed and governed under fully enforced 

and effective regulations, yet still sap the resilient capacity 

that the forest needs to adapt to climate change. An example 

would be a development plan that specifies what percentage 

of trees will be retained in a subdivision without accounting 

for their configuration, resulting in a fragmented forest, and 

disrupted wildlife habitat and corridors.

Human perception, unaided by science-based monitoring, will not easily detect the diffuse, slow-to-develop 

problems in the WUI stemming from multiple stressors, nor 

does this suite of stressors lend itself to simple explanation. 

For example, research suggests that housing development 

gives rise to an increase in native bird species richness, 

perhaps owing to the more varied habitat types (open areas, 

forest edges) created in a WUI development, but that higher 

levels of residential development decrease native bird richness (Lepczyk et al. 2003). This phenomenon, observed in 

relation to biodiversity of many species (McKinney 2002), 

illustrates how multiple stressors on the WUI may go largely 

unnoticed by the human communities responsible for them, 

yet have significant consequences. Once known and understood, however, resource management concerns that are 

conveyed effectively can change human behavior. Initial 

case-study research gives reason for optimism; in Fremont 
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GENERAL TECHNICAL REPORT PNW-GTR-870

County, Colorado, WUI residents actively learned from each 

other and were engaged in managing many complex WUI 

resource issues (Larsen et al. 2011). Given expectations 

for continued WUI growth, together with the impacts of 

climate change, such activities will be essential to maintaining enough forest health and resilience to adapt to whatever 

changes occur.

Social Interactions With Forests 

Under Climate Change 

Social interactions with forests extend beyond the definition of forest area and service flows defined by land uses. 

As discussed, human activity can enhance or diminish the 

effects of a changing climate on forested ecosystems. People 

and the actions they take directly alter the capacity of forests 

to sequester carbon and to adapt to a changing climate. By 

reshaping the landscape, people alter the extent of forest and 

with it, forest health, sustainability, and capacity to meet the 

needs of other species. Society relies on forests for products 

and for wide-ranging ecosystem services, from life-sustaining ones (e.g., air and water filtering) to enhancements in 

quality of life (e.g., scenic vistas, recreation). Thus, people 

and societies mediate the relationship between forests and 

climate change both directly (by altering forests) and indirectly (by changing other physical and biological systems 

that in turn alter forests). The interaction between this social 

relationship with forests and climate change potentially 

will alter ecosystem services that people depend upon from 

forests and woodlands.

The relationship between forests and climate change in 

the United States cannot be understood without considering people and the communities in which they live. Some 

communities are embedded within social systems strongly 

linked to the condition and uses of natural resources. These 

natural resource-based communities, where the relationship 

is based on commodities such as timber or amenities such 

as recreation, may be disproportionately affected by interlinked climate and forest ecosystem changes. Another set 

of communities consists of tribal areas, which may become 

especially vulnerable to effects of climate change because of 

the relatively strong links between these communities, their 

economies, and their natural resource base. Unlike other 

sectors, the possibility of adaptation through migration is 

limited because of strong cultural ties to tribal lands. This 

section explores the extent and form of these two types of 

communities and their resilience to changes in interlinked 

climate and forest conditions. Assessing the resilience to 

climate change of both natural resource-based communities 

and tribal communities requires understanding of not only 

the economic and ecological vulnerabilities but also the 

social vulnerability of each. We propose a framework for 

exploring those vulnerabilities in light of climate change. 

Natural Resource-Based Communities 

Natural resource-based communities are closely linked 

with their geographic setting and environmental context. In 

these communities, people with collective, intersecting, and 

competing values interact because they are at the dynamic 

interface of societal and environmental processes (Flint and 

Luloff 2005). These communities also derive economic benefits from the surrounding natural resources and withstand 

their associated natural disturbances, such as wildfires and 

hurricanes. Natural resource-based communities are affected 

by both technological and macroeconomic changes. Using 

six categories, the U.S. Department of Agriculture, Economic Research Service (USDA ERS 2011) classified economic 

dependence by county. Farm dependency has declined 

considerably; in 2000, only 20 percent of nonmetropolitan 

counties were considered farming dependent (Dimitri et al. 

2005); most are now centered in the Great Plains (fig. 3.8).6

Other counties, particularly in the West, depend on federal 

6

 Farm dependent, 1998 through 2000. Source: U.S. Department of 

Agriculture, Economic Research Service, County Typology Codes, 

using data from the U.S. Census Bureau and the Bureau of Economic Analysis. Type of data: Multiyear averages and point-in-time 

census data. Year(s): 1998 through 2000. Definition: Classification 

of counties by measures of farm earnings and employment, where 1 

= farm-dependent county; 0 = all other counties; a county is defined 

as farm dependent if farm earnings accounted for an annual average 

of 15 percent or more of total county earnings during 1998 through 

2000, or farm occupations accounted for 15 percent or more of all 

employed county residents reporting an occupation in 2000.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

or state government and mining. Of the 368 recreation-dominated counties, 91 percent (334) were in rural areas (Lal et 

al. 2010).7

Structural changes in the timber industry have resulted 

in large-scale changes in land tenure, corporate consolidation in the timber industry, and separation of processing

7

 Nonmetropolitan recreation-dependent, 1997–2000. Source: U.S. 

Department of Agriculture, Economic Research Service, County 

Typology Codes, using data from the U.S. Census Bureau and the 

Bureau of Economic Analysis. Type of data: Multiyear averages and 

point-in-time census data. Year(s): 1997–2000. Definition: Classification of nonmetropolitan counties by measures indicating high 

recreational activity, where 1 = recreation-dependent county; 0 = all 

other counties; measures of recreational activity were (1) wage and 

salary employment in entertainment and recreation, accommodations, eating and drinking places, and real estate as a percentage of 

all employment reported in the Census Bureau’s County Business 

Patterns for 1999; (2) percentage of total personal income reported 

for these same categories by the Bureau of Economic Analysis; (3) 

percentage of housing units intended for seasonal or occasional use 

reported in the 2000 Census; and (4) per capita receipts from motels 

and hotels as reported in the 1997 Census of Business.

capacity ownership from timberland ownership (Bliss et 

al. 2010). Many forested areas previously owned by timber 

companies are now owned by timberland investors, who 

differ markedly from the industrial owners in, for example, 

their landholding objectives, time horizons, and management 

capacities. According to Bliss et al. (2010), these changes in 

the timber industry are dynamic, and any predictions about 

future ownership patterns and their implications for smallscale forestry and rural natural resource-based communities 

are highly speculative; however, they suggest three possible 

trajectories for future land uses: intensive timber production 

forestry, “highest and best use” parcelization and conversion, 

and conservation forestry.

Natural resource-based communities, such as those

situated in or near forests or other expansive resources,

often experience the consequences of natural disasters or 

environmental stresses sooner than do farther-removed 

Figure 3.8—Economic dependence (USDA ERS 2011).
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GENERAL TECHNICAL REPORT PNW-GTR-870

populations (Haque and Etkin 2007, Lynn and Gerlitz 2006, 

Lynn et al. 2011). These manifestations are most pronounced 

in developing areas of the world, including West Africa, 

South America, and portions of south Asia (Amisah et al. 

2009, Laurance et al. 1998), where processes such as forest 

fragmentation, deforestation, and conversion of land from 

forests to agricultural uses have accelerated the rate of 

climate change and contributed to erratic rainfall patterns, 

increased temperature and wildfire frequency, and stressed 

water sources.

Although developing regions of the globe contain the 

best examples of forest-dependent communities vulnerable 

to climate change, these relationships also occur in developed regions of the Northern Hemisphere, highlighting 

the unevenness of climate vulnerability within developed 

nations. These vulnerabilities inherently relate to biophysical 

conditions of place, but they also manifest in terms of the 

socioeconomic and political milieu associated with many resource-dependent communities of the Northern Hemisphere. 

For example, individual and community vulnerability can be 

affected by characteristics such as income level, race, ethnicity, health, language, literacy, and land use patterns. Thus, 

the social vulnerability of natural resource-based communities to effects of climate change is important both to understand and to include in discussions of climate vulnerability, 

because the sociology of a given locale can compound or 

exacerbate biophysical vulnerabilities of place.

An analysis of forest-dependent communities in Canada 

suggests that specific social characteristics associated with 

forest-based communities increase climate change risks 

for such communities (Davidson et al. 2003). For example, 

human capital development is typically lower with respect 

to educational attainment in these areas, and there is a 

concentration in a specific skill set that makes it difficult for 

laborers to transfer skills to other occupations or contexts. 

The politicization of deforestation’s role in climate change 

has also, in some cases, created a larger populace (often removed from place) that is unsympathetic to the labor dilemmas facing communities dependent on traditional forestry 

activities. Further, uncertainty about the exact nature of climate changes, coupled with the long-term planning horizon 

necessary for forest management, elevates risks associated 

with investments in forest-based industries, making such 

investments less appealing to potential investors. The result 

could lead to under-investments in communities primarily 

dependent upon a single sector economy. Moreover, climate 

change may not be perceived as such by local residents or 

key decisionmakers in forest-based communities, resulting 

in reluctance by communities to devise adaptive strategies to 

help mitigate current and future environmental stresses and 

hazards. Finally, methods used to assess climate risks may 

be inadequate in situations where climate change is occurring alongside other isolated environmental events.

Similar to biophysical vulnerabilities, social vulnerabilities differ spatially and are more prominent for certain 

sociodemographic groups such as racial and ethnic minorities, women, the elderly, the very young, and for people 

in specific geographical contexts such as forest-proximate 

communities. The fourth IPCC assessment (Pachauri and 

Resinger 2007, Solomon et al. 2007) addresses the spatiality 

of climate vulnerability: “There are sharp differences across 

regions and those in the weakest economic position are often 

the most vulnerable to climate change and are frequently the 

most susceptible to climate-related damages.... There is increasing evidence of greater vulnerability of specific groups 

such as the poor and elderly not only in developing but also 

in developed countries.”

Tribal Forests

American Indians and Alaska Natives rely on reservation 

lands and access to traditional territories beyond the bounds 

of reservations for economic, cultural, and spiritual well 

being. Tribes have unique rights, including treaties with the 

federal government that reserved rights to water, hunting, 

fishing, gathering, and cultural practices (Lynn et al. 2011, 

Pevar 1992). We focus on the forests and woodlands on Indian reservations, how climate change will affect these lands, 

and the tribal communities that depend on these ecosystems. 

Indian reservations contain 7.2 million ha of forest land, 

of which 3.1 million ha are classified as timberland and 2.3 

million ha as commercial timberland (Gordon et al. (2003). 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

These forests are diverse, ranging from productive conifer 

forest in the Pacific Northwest to dry pine forest and juniper 

woodland in the Southwest, mixed hardwood-conifer forest 

in the Lake States, and spruce forest in the southern Appalachians (Gordon et al. 2003) (fig. 3.9). Of the 7.2 million 

ha of forest land, 4.1 million ha were classified as woodland 

(defined as less than 5 percent canopy cover of commercial 

timber species but at least 10 percent total canopy cover), of 

which 1.4 million ha are commercial woodland.

As part of a 10-year assessment of Indian forest management, surveys were conducted to identify the Indian 

vision for tribal forests. Gordon et al. (2003) described the 

Indian vision in terms of the major themes expressed by 

Native people: (1) natural, healthy, beautiful places; (2) 

integrated management; (3) self-governance and trust responsibility; (4) communication, tribal public involvement, 

and education. The same report also described resource 

management of tribal forests as moving close to attaining 

Figure 3.9—Reservations with significant timberland resources. Numbers 1 through 41 have over 4047 ha of commercial timberland, 

or over 2360 m3

 allowable cut, per reservation. Numbers 41 through 83 have less area in timberland, but what they have is economically viable. (1) Jicrailla, (2) Mescalero Apache, (3) Southern Ute, (4) Blackfeet, (5) Crow, (6) Northern Cheyenne, (7) Eastern Band 

of Cherokee, (8) Mississippi Choctaw, (9) Passamaquoddy, (10) Penobscot, (11) Bad River, (12) Bois Fort, (13) Grand Portage, (14) 

Lac Courte Oreilles, (15) Lac du Flambeau, (16) Leech Lake, (17) Menominee, (18) Red Lake, (19) Stockbridge/Munsee, (20) White 

Earth, (21) Navajo, (22) White Mtn. Apache, (23) Hualapai, (24) San Carlos, (25) Uintah and Ouray, (26) Annette Islands, (27) Cour 

d’Alene, (28) Colville, (29) Flathead, (30) Grand Ronde, (31) Makah, (32) Nez Perce, (33) Quinault, (34) Siletz, (35) Spokane, (36) 

Tulalip, (37) Umatilla, (38) Warm Springs, (39) Yakama, (40) Hoopa Valley, (41) Tule River, (42) Omaha, (43) Pine Ridge, (44) 

Rosebud, (45) Turtle Mountain, (46) Winnebago, (47) Acoma, (48) Isleta, (49) Jemez, (50) Laguna, (51) Picuris, (52) Santa Clara, 

(53) Zuni, (54) Alabama-Coushatta, (55) Fort Belknap, (56) Rocky Boy’s, (57) Wind River, (58) Big Cypress, (59) Narragansett, 

(60) Pequot, (61) Fond du Lac, (62) L’Anse, (63) Mille Lacs, (64) Potawatomi, (65) Red Cliff, (66) Cherokee, (67) Chickasaw, (68) 

Choctaw, (69) Chehalis, (70) Fort Hall, (71) Kalispel, (72) Lummi, (73) Muckleshoot, (74) Nisqually, (75) Port Gamble, (76) Port 

Madison, (77) Quileute, (78) Skokomish, (79) Squaxin Island, (80) Swinomish, (81) Fort Bidwell, (82) Round Valley, (83) Yurok, 

(84) Alaska trust properties (Chugachmiut, Metlakatla, Tanana Chiefs Conference). Adapted, with permission, from the Intertribal 

Timber Council (IFMAT 2003).
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GENERAL TECHNICAL REPORT PNW-GTR-870

this vision. Gordon et al. (2003) noted that the ecological 

condition and management of tribal forests has improved 

since the previous assessment (IFMAT 1993). Increasingly 

complex ecological approaches are being implemented, as 

well as increased fire management activities. In 2008, the 

U.S. Department of the Interior, Bureau of Indian Affairs 

(BIA) reported that an estimated 91 percent of the forest area 

had a forest management plan or an integrated resource management plan with forest management provisions. 

In 2001, the total allowable annual cut on timberlands 

was reported as 1 840 000 m3 and for woodlands was 230 

000 m3, with harvest volume at 1 430 000 m3. Harvest value 

was estimated at $65.9 million, a 27 percent decline from 

the 1991 harvest value of $117.4 million (numbers adjusted 

for inflation, Gordon et al. 2003). The Northwest and the 

Lake States accounted for the greatest harvest volume and 

stumpage revenue in 2001. Tribal forestry faces challenges 

common to forestry, limited wood processing capacity, 

and, in the Western United States, poor markets for small 

wood products. In 2008, the BIA reported the effects of the 

continuing decline in the housing construction market on 

forestry-related products from tribal lands as well as the 

effects of rising fuel costs on transporting forestry-related 

products. The Mescalero Apaches had no market for small 

merchantable logs, severely hindering their forest management program. In the Northwest, traditionally stable tribal 

sawmills were having difficulties paying their bills. As 

with other forests in the United States, tribal forests face 

new challenges from invasive species, pest outbreaks, and 

large-scale fires initiating on tribal lands as well as spreading 

from adjacent forest lands. The BIA (2008) also reported the 

beginning of a projected decline in the number of professional foresters. 

Tribal forests and woodlands provide jobs and revenue 

from timber production, nontimber forest products, grazing, and fishing and hunting. They also provide recreation 

opportunities, energy resources, and material for shelter, 

clothing, medicines, food, as well as places for religious 

ceremonies and solitude. In addition to the broader effects 

on forests discussed earlier, climate change effects on tribal 

forest and woodland ecosystems will have implications for 

treaty rights if culturally significant plant, animal, and fungi 

species ranges move outside reservation boundaries. Water 

resources and tribal water rights may be especially affected 

by climate change (Curry et al. 2011, Karl et al. 2009). 

Adaptation responses may be challenging, given fragmented 

tribal lands and the small size of some reservations. Lynn 

et al. (2011) also describe current adaptation approaches on 

tribal lands, including watershed management surrounding 

sacred waters, natural hazard management, and legislation 

to foster green jobs, such as farmers’ markets to small-scale 

energy projects. Some tribes have begun to explore options 

to manage their forest lands for C sequestration. The fixed 

location of tribal lands defines important limits, however, to 

the adaptive capacity of tribal communities with regard to 

climate change.

Social Vulnerability and Climate Change 

Generally, socially vulnerable populations are understood 

as marginal groups, in terms of material well being, which 

renders them relatively unable to anticipate, cope with, or 

recover from environmental stresses that occur within a 

geographically defined setting (Kelly and Adger 2000). A 

common conceptualization of vulnerability is informed by 

the widely held idea that interprets vulnerability not just 

in terms of susceptibility or sensitivity to loss arising from 

hazard exposure, but also as a function of three primary 

contributors: hazard exposure, sensitivity, and resilience or 

adaptive capacity (Brooks 2003, Polsky et al. 2007, Smit and 

Wandel 2006): 

social vulnerability = f (exposure, sensitivity,

adaptive capacity).

Exposure is understood as proximity to a physical 

hazard or stressor. Sensitivity is the susceptibility of humans 

in sociodemographic terms to physical hazard, which can 

also include sensitivities of the built environment, such as 

geography or land use change. Adaptive capacity is any 

mitigation and adaptation to hazard via sociodemographic 

factors or other means. 

Birkmann (2006) identified at least 25 conceptualizations of vulnerability in terms of human populations. 

Definitions differ by disciplinary area and underlying 

assumptions concerning the nature of risk, disaster, and 

exposure. However, these variant understandings of vulner-
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

ability can be viewed analytically as either the “outcome” or 

“contextual” framing of vulnerability (Brooks 2003, Kelly 

and Adger 2000, O’Brien et al. 2007). 

Outcome framing describes vulnerability as a resultant 

state that occurs after an exposure unit (e.g., individuals, 

communities) has experienced and adapted to an environmental stressor, such as more incremental changes in 

climate (Watson 2001). This understanding focuses attention on estimating or projecting a “future,” an endpoint 

of vulnerability that comes about as a consequence of 

climate-changing emissions (e.g., greenhouse gases) and 

resultant climate scenarios. Biophysical impacts to humans 

or physical systems are then predicted from given scenarios, 

and finally adaptations to projected impacts are formulated. 

Implicit here is that vulnerabilities are not considered to be 

an inherent quality of place or community; rather, vulnerabilities arise after exposure to climate-altering processes or 

events. The definition of vulnerability above (Watson 2001) 

is an example of an outcome framing of vulnerability. The 

outcome perspective is also assumed in projects such as the 

U.S. Forest Service Forest Futures analysis, which examines 

the effect of future climate scenarios on forest resources. 

Most social vulnerability research related to forests has also 

used outcome framing. 

Contextual vulnerability differs from outcome vulnerability in that it analyzes current vulnerabilities within the 

current social structure of a given place. An analysis of contextual vulnerability (e.g., economic reliance on river-based 

tourism) focuses on the relationships among political actors 

(elected officials), institutions (rules for concessionaires), 

socioeconomic well-being (workforce education level), and 

culture to identify how goods and information are distributed 

across society. From this evidence, the analysis predicts 

response to a future threat (e.g., whether guides will be able 

to maintain their concession for river rafting as in-stream 

flows decline). This approach assumes that human vulnerability to natural events depends entirely on the capabilities 

already existing in a social system. The efficacy with which 

communities cope with a range of current environmental and 

societal stressors determines how well they will respond to 

future stressors. The contextual vulnerability approach generates management implications; it suggests that currently 

vulnerable communities can be identified and management 

action taken to improve current adaptive capacity. 

Contextual assessments are appealing because of the 

clarity of their implications, yet few such assessments have 

been undertaken. The most vulnerable human populations 

are often difficult to identify, and understanding the values and perceptions of risk that community members hold 

requires more than a review of existing social and economic 

conditions. For example, using data from the U.S. Census 

Bureau to identify forest-dependent communities based on 

low income or high unemployment would not suffice and 

could misclassify communities, though this approach holds 

obvious appeal to assessment teams with little time and few 

resources.

Providing better guidance for conducting applied vulnerability assessments was one goal of a workshop co-sponsored by the U.S. Forest Service and University of Montana, 

and attended by social scientists and resource managers 

from federal agencies and universities. The group developed 

an initial template for socioeconomic vulnerability assessments (SEVAs), which begins with a review of secondary 

data from Census Bureau and similar sources. Following this 

review, a SEVA will (1) briefly discuss the social history of 

the forest and its human geography, including both communities of place and communities of interest, (2) link current 

and expected biophysical changes to community-relevant 

outcomes, (3) determine stakeholders’ perceptions of values 

at risk (e.g., resources, livelihoods, cultures or places threatened by climate change), and (4) prioritize threats to vulnerable communities and identify those that the landowner or 

land manager, singly or with their partners, can best address. 

This basic outline will need testing and refinement over time 

as land managers elaborate and improve on it, but it represents a first step toward bringing SEVAs within reach of any 

assessment team. 

Conclusions

Although climate change has been identified as an important 

issue for management and policy, it is clear that the interaction among changes in biophysical environments (climate, 
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GENERAL TECHNICAL REPORT PNW-GTR-870

disturbance, and invasive species) and human responses 

to those changes (management and policy) will determine 

outcomes of consequence to people. The ultimate effects on 

people are measured in terms of changes in ecosystem services provided by forested landscapes, including traditional 

timber products and new extractive uses, rural and urban 

recreation, cultural resources, the contributions of urban 

forests to human health, and the protection of water quality. 

Climate change has been linked to bioenergy and C sequestration policy options as mitigation strategies, emphasizing 

the effect of potential climate change-human interactions 

on forests as well as the role of forests in mitigating climate 

change. Any effect of climate change on forests will result 

in a ripple effect of policy and economic response affecting 

economic sectors and human communities in U.S. society. 

The key mechanism of change in human-dominated 

landscapes is choice. Where private ownership dominates, 

choices regarding land use and resource production directly 

and indirectly affect changes in forest conditions and the 

flow of ecosystem services. The choices are directly influenced by shifts in land productivity, the prices of various 

products and ultimately the returns to different land uses. 

Land use shifts in rural areas under climate change could 

involve conversion between forests and agricultural uses,

depending on market conditions. Climate changes are 

expected to alter productivity (local scale) and prices 

(market scale). Land use patterns dictate the availability of 

the full range of ecosystem services from forests and from 

trees within other land uses. Both WUI and urban areas are 

projected to increase, often at the expense of rural forests. 

Anticipated climate changes, coupled with population 

growth, strongly increase the extent and value of urban trees 

in providing ecosystem services and for mitigating climate 

change impacts at fine scales. However, climate change also 

increases the challenge of keeping trees healthy in urban 

environments.

Collective choice, in the form of various policies, also 

holds sway over land use and forest condition outcomes. 

Policies targeting climate mitigation, especially for bioenergy production and C sequestration, directly target forest 

extent and use. Implemented through markets, these policies would yield secondary and tertiary impacts to forest 

composition and structure through direct action and through 

resource input and product substitutions in related sectors. 

These and other policies (e.g., forest management regulations, land use restrictions, property taxes) also set the 

context for and potentially constrain the adaptive choices by 

private landowners.

Human communities living in environments along the 

gradient from urban to rural environments will experience 

changes to forests. Those communities dependent on forests 

for economic, cultural, or spiritual services are likely to see 

the effects of climate change first. The potential for human 

communities to adapt to potential climate changes is linked 

to their exposure to climate change, which differs along 

the rural-to-urban gradient, and also to the nature of the 

social and institutional structures in each environment. One 

can prepare for or mitigate future climate stresses in these 

environments by ensuring that the resilience of human communities in these environments are intact today, because the 

efficacy with which humans are presently able to deal with 

change will determine how well they will be able to respond 

to future stresses. 
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Chapter 4

Adaptation and Mitigation

Constance I. Millar, Kenneth E. Skog, Duncan C. McKinley, Richard A. Birdsey, Christopher W. Swanston, Sarah 

J. Hines, Christopher W. Woodall, Elizabeth D. Reinhardt, David L. Peterson, and James M. Vose1

Strategies for Adapting to Climate 

Change

Forest ecosystems respond to natural climatic variability and 

human-caused climate change in ways that are adverse as 

well as beneficial to the biophysical environment and

to society. Adaptation refers to responses or adjustments 

made—whether passive, reactive, or anticipatory—to 

climatic variability and change (Carter et al. 1994). Many 

adjustments occur whether humans intervene or not; for example, plants and animals shift to favorable habitats resulting in range expansion or contraction, as well as changes in 

gene frequencies for traits that enable persistence in warm 

climates. Here we assess strategies and tactics resource 

managers can use in the process of reducing forest vulnerability and increasing adaptation to changing climates

1 Constance I. Millar is a research paleoecologist, U.S. Department 

of Agriculture, Forest Service, Pacific Southwest Research Station, 

800 Buchanan St, West Annex Building, Albany, CA 94710; Kenneth E. Skog is a research forester, U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory, One Gifford Pinchot 

Drive, Madison, WI 53726; Duncan C. McKinley is an ecologist, U.S. Department of Agriculture, Forest Service, Research and 

Development, 1400 Independence Ave. SW, Washington, DC 20250; 

Richard A. Birdsey is a program manager, U.S. Department of 

Agriculture, Forest Service, Northern Research Station, 11 Campus 

Blvd., Suite 200, Newtown Square, PA 19073; Christopher W. 

Swanston is a research ecologist, U.S. Department of Agriculture, 

Forest Service, Northern Research Station, 410 MacInnes Drive, 

Houghton, MI 49931; Sarah J. Hines is a resource specialist, 

U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station, 240 West Prospect Road, Fort Collins, CO 80526; 

Christopher W. Woodall is a research forester, U.S. Department 

of Agriculture, Forest Service, Northern Research Station, 1992 

Folwell Avenue, St. Paul, MN 55108; Elizabeth D. Reinhardt

is a national program leader for fire research, U.S. Department 

of Agriculture, Forest Service, Research and Development, 1400 

Independence Ave. SW, Washington, DC 20250; David L. Peterson

is a research biological scientist, U.S. Department of Agriculture, 

Forest Service, Pacific Northwest Research Station, Pacific Wildland 

Fire Sciences Laboratory, 400 N, 34th St, Suite 201, Seattle, WA 

98103; James M. Vose is a research ecologist, U.S. Department 

of Agriculture, Forest Service, Southern Research Station, Center 

for Integrated Forest Science and Synthesis at North Carolina State 

University, Department of Forestry and Environmental Resources, 

Campus Box 8008, Raleigh, NC 2769.

(Peterson et al. 2011). Plans and activities range from shortterm, stop-gap measures, such as removing conifers that are 

progressively invading mountain meadows, to long-range, 

proactive commitments, such as fuels management to reduce 

the likelihood of severe wildfire or of beetle-mediated forest 

mortality.

Principles for Forest Climate 

Adaptation

The following principles apply broadly in developing new 

perspectives on forest climate adaptation:

Successful Climate Adaptation Planning 

and Implementation

In the context of this chapter, adaptation strategies, plans, 

and management actions are implicitly tied to broad goals 

of ecosystem sustainability. Restoration, maintenance, and 

promotion of natural ecological processes and ecosystem 

services define the mission of most public land-management 

agencies as well as many private (e.g., nongovernmental 

organizations) forest reserves. These goals often underlie 

economic and utilitarian goals of the forest industry and 

other special-use forest land owners as well. Climate adaptation efforts that benefit and promote goals of ecosystem 

sustainability are considered successful. Successful implementation of climate adaptation plans occurs when projects 

are developed and deployed for specific places with concrete treatments and prescriptions, explicit objectives, and 

for definitive time periods. Successful implementation also 

implies that monitoring and adaptive management schedules 

are integrated in out-year efforts, and are secured with funds 

and capacity needed for completion.

Education and Training

Given the limited inclusion until recently of climate-science 

and climate-effects courses in college curricula for forest 
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managers, a knowledge vacuum exists among practitioners and decisionmakers about basic scientific principles. 

Training for practitioners in the fundamental concepts of 

climatology and physical and ecological sciences related to 

climate change is essential. Such knowledge will increase 

the institutional capacity to understand potential effects of 

climate change and associated irreducible uncertainty, and to 

construct appropriate strategies and actions. A multilevel approach facilitates climate change education and dialogue for 

practitioners. A regional education program in the Northern 

United States incorporated several elements (Peterson et 

al. 2011), including basic education, intensive training, and 

discipline-specific and targeted workshops (fig. 4.1). Short 

(1- to 2-day) basic educational seminars convey fundamental principles of climate change and the effects of climate 

change on ecosystems and generate discussion of how different resources under management consideration can adapt 

to projected changes. Intensive training includes week-long 

courses providing indepth information and detailed explanations of fundamental climate processes and interactions, as 

well as greater detail on mechanisms of forest response to 

climate stressors. Participants have the opportunity to evaluate issues or resources by using available (e.g., online) tools. 

Discipline-specific trainings allow focused presentation 

and discussion of climate change implications for specific 

resource issues (e.g., silviculture, wildlife). 

Science-Management Partnerships

Partnerships between scientists and managers are needed to 

improve understanding of climate science and increase experience in developing adaptation strategies. These collaborations can develop in different forms. For example, science 

information might reside with staff within an agency, but 

in different program areas than those traditionally involved 

with forest management. University extension specialists 

have a long history of spanning boundaries between science and applications (e.g., providing genetic expertise in 

developing seed-transfer rules), and can be brought into 

partnerships. Research scientists with universities and agencies increasingly participate in resource management collaborations. A key element in all collaborations is that they 

maintain interactive dialogue, with managers and scientists 

reciprocally learning from and informing each other about 

relevance.

Risk and Uncertainty

Given the environmental complexities of forest ecosystems, 

and their diverse and often conflicting institutional and 

societal roles, decisionmakers have long confronted challenges of risk and uncertainty. Climate change adds further 

dimensions of uncertainty, increasing the complexity of risk 

analyses. Although trends in climate and ecosystem response 

Figure 4.1—Conceptual diagram of educational and training efforts leading to increased complexity of adaptation planning and activities. These elements are integrated but need not be taken consecutively. Distance learning can be incorporated into all activities. (From Peterson et al. 2011.)
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

usually can be bounded with probabilistic envelopes, these 

are often wide and should be considered as only a guide for 

evaluating local decisions; unexpected conditions and surprises are especially important at local scales. In developing 

forest adaptation strategies, effort should be made to (1) be 

aware of risks, (2) assess vulnerabilities, (3) develop adaptation responses that are realistic yet minimize uncertainties, 

and (4) incorporate new knowledge and learning gained over 

time to modify decisions as appropriate (adaptive management) (Moser and Luers 2008). Adaptation responses to risk 

include (1) no action—continue conventional practices, (2) 

contingency planning—develop a response strategy (e.g., 

to anticipated major disturbance), and (3) anticipatory and 

proactive strategies—curtail or diminish potential impacts 

(e.g., of a major disturbance) while optimizing attainment of 

goals (Joyce et al. 2008). 

Toolkit Approach

Novelty and surprise in climate-change effects, combined 

with a diversity of management objectives and of spatial and 

temporal management scales, mean that no single approach 

will fit all situations. A toolkit approach to adaptation strategies recognizes that, from a wide array of available methods 

in the literature or in practice, the best strategy will require 

selecting appropriate methods for the specific situation. 

Tools include resource management practices, educational 

and reference modules, decision-support aids, and qualitative and quantitative models that address adaptation of natural and cultural resources to climate change (Peterson et al. 

2011). Tools include existing management practices, perhaps 

used in new ways, as well as novel approaches developed 

specifically to meet climate challenges.

No-Regrets Decisionmaking

“No-regrets” decisionmaking refers to actions that result 

in a variety of benefits under multiple scenarios and have 

little or no risk of socially undesired outcomes. This would 

include (1) implementing fuel treatments in dry forests to 

reduce fire hazard and facilitate ecological restoration, while 

creating resilience to increased fire occurrence in a warmer 

climate, and (2) installing new, larger culverts in locations 

where peak flows during flooding are expected to be higher 

in a warmer climate, thus protecting roads and reducing 

maintenance costs. These types of actions benefit resources 

and values regardless of climate-change effects and can 

be implemented in the near term (Swanston and Janowiak 

2012). 

Flexibility and Adaptive Learning

Uncertainty about future climates and ecosystem responses, 

and limited experience to date in developing forest adaptation strategies, imply that flexibility, experimentation, and 

adaptive learning should be incorporated into all efforts to 

develop adaptation strategies. Ideally a formal adaptivemanagement program will be developed in conjunction

with projects implemented, but other approaches to monitoring that enable change of management practices are also 

appropriate.

Mixed-Models Approach

Climate- and ecosystem-response models are proliferating 

rapidly. Regional and locally downscaled climate-change 

scenarios logically seem useful for conducting vulnerability analyses and developing adaptation responses at scales 

relevant to forest sector needs. However, given the many 

processes about which we know very little, output from 

projections should be used cautiously and in conjunction 

with other filters. Models are often useful for examining 

forest-response correlates with recent historical events and 

for attributing influence or causality (e.g., dissecting climatic 

factors that might have influenced large wildfires or insect 

outbreaks). Models are often less useful for forecasting at 

small spatial scales or over long time periods, and in regard 

to complex biological processes. Output from models is 

useful as background information for envisioning a range 

of potential futures rather than to project a single outcome. 

The use of different types of models to address the same area 

and issue is recommended, such as models built with different assumptions, process interactions, and input data. Both 

quantitative (algorithm-based) and qualitative models (e.g., 

flow charts, indices, and verbal tools) should also be considered, and differences and similarities in projected futures 
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can be evaluated. In recent years, it has been suggested that, 

if a model (or several models) hindcasts observed historical 

conditions well, it will also accurately predict future conditions. This is not necessarily true, because a given result 

can be reached via multiple pathways; in other words, a 

model can produce a correct historical reconstruction for the 

wrong reasons (Crook and Forster 2011), which means that 

forecasts could also be wrong. The experience and judgment 

of resource professionals are also important for evaluating 

potential future climate conditions and ecosystem responses. 

A recently developed summary of frequently asked questions 

(Daniels et al. 2012) can guide the effective use of models.

Integration With Other Priorities and 

Demands of Forest Management

Mitigation, involving actions to reduce human influence 

on the climate system, is another fundamental approach 

for addressing climate challenges (Metz et al. 2001), and 

integrating mitigation activities with adaptation strategies is 

important. The best approach is usually to address mitigation and adaptation goals concurrently, although in some 

situations, strategies may conflict, and compromise choices 

may be required. Climate change remains only one of 

many challenges confronting forest management, and other 

priorities must be evaluated at different temporal scales. For 

example, managing under the Endangered Species Act of 

1973 (ESA) can invoke actions that, by regulatory imperative, are required in the short term but make little sense, 

given long-term projections of the effects of climate change. 

For forest lands where ecological sustainability is the central 

goal, ecosystem-based management as practiced in land 

management since the late 1980s (e.g., Kohm and Franklin 

1997, Lackey 1995) provides a foundation for addressing 

most aspects of climate-change effects. Ecosystem-based 

management acknowledges that natural systems change 

continuously and that such dynamics bring high levels of 

uncertainty. Ecosystem-based management concepts are 

therefore appropriate foundational principles in developing 

forest adaptation strategies.

Placing Adaptation in Context 

Forest ecosystems in the United States occur in diverse environmental, institutional, and regulatory contexts. Socially 

beneficial outcomes for climate adaptation depend on matching the best strategy with the context. 

Biogeography and Bioclimate

Composition, structure, and processes of forests are influenced by their location, which determines the continental-tolocal climatic regimes of forest ecosystems, physical context 

(geomorphology, soils, tectonics, topography), biogeographic constraints and opportunities (corridors or barriers 

to movement), ecological legacy (historical and prehistorical 

ecosystems), and a myriad of societal influences, such as 

land ownership, regulatory context, and land use histories. 

Adaptation strategies will differ in detail, if not always 

overall approach, for forest ecosystems in different parts of 

the United States.

Scale

Climate change affects forest ecosystems at many temporal 

and spatial scales, for example, from its influence on timing 

of bud burst to the evolution of ecotypes, and from trophic 

interactions on a rotting log to shifts in biome distribution 

across continents. The longevity of forest trees, combined 

with their significant influence on the physical landscape 

(e.g., soil development, watershed quality) and role as habitat, adds complexity to scale issues. Analysis at the correct 

spatial scales is especially important for assessing trends of 

climate change and ecological response, given that averages 

and trends on broad scales (e.g., continental) can mask variability at fine scales (e.g., watershed).

An adaptation framework based on appropriate temporal 

and spatial scales (e.g., Peterson and Parker 1998) ensures 

that plans and activities address climate effects and responses effectively. Because scales are nested, the best strategies focus on the scale of the relevant project and include 

evaluation of conditions and effects at scales broader than 

the project level, as well as analysis of effects at finer scales 

(tables 4.1 and 4.2). Broad-scale analysis establishes context, 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Factor

 Relevance by spatial scale

Largea Intermediateb Smallc

Availability of information on climate and 

climate change effects

High for future climate and 

general effects on vegetation and water

Moderate for river systems, 

vegetation, and animals

High for resource data, low 

for climate change

Accuracy of predictions of 

climate change effects

High Moderate to high High for temperature and water, low to moderate for other 

resources

a

 More than 10 000 km2 (e.g., basin, multiple national forests). b

 100 to 10 000 km2 (e.g., subbasin, national forest, ranger district). c

 Less than 100 km2 (e.g., watershed).

Source: Modified from Peterson et al. 2011. 

Table 4.1—Factors that affect the relevance of information for assessing vulnerability to climate change of 

large, intermediate, and small spatial scales

Usefulness for specific 

projects

Generally not relevant Relevant for forest density 

management, fuel treatment, wildlife, and fisheries 

Can be useful if confident that 

information can be downscaled accurately 

Usefulness for planning High if collaboration across 

management units is 

effective

High for a wide range of

applications

Low to moderate

Factor

 Relevance by time scale

Largea Intermediateb Smallc

Availability of information on climate and 

climate change effects

High for climate, moderate 

for effects 

High for climate and effects Not relevant for climate 

change and effects predictions 

Accuracy of predictions 

of climate change effects

High for climate and water, 

low to moderate for other 

resources

High for climate and water, 

moderate for other resources

Low

a

 More than 50 years.

b

 5 to 50 years.

c

 Less than 5 years.

Source: Modified from Peterson et al. 2011.

Usefulness for specific 

projects

High for temperature and 

water, low to moderate for 

other resources

High for water, moderate for 

other resources

Low owing to inaccuracy of 

information at this scale

Usefulness for planning High High for water, moderate for 

other resources

Low

Table 4.2—Factors that affect the relevance of information for assessing vulnerability to climate change of 

large, intermediate, and small time scales
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including recognition of processes and effects that manifest 

only at large scales (e.g., species decline, cumulative watershed effects), potential undesired consequences that could 

be alleviated by early action, and the need for large management units and collaboration across ownerships. 

Institutional and Regulatory Contexts

Forests are managed for many goals. Most publically 

administered forest lands are managed for long-term ecological and physical sustainability. Within that broad goal, 

emphasis differs by designation for protection level (parks, 

wilderness, and reserves) and ecosystem services (national 

and state forests, Bureau of Land Management [BLM] 

forest and woodlands, and tribal forest lands). The focus 

on maintenance of ecological and environmental conservation on public lands is subject to strict legal and regulatory 

direction, such as the National Environmental Policy Act 

[NEPA] of 1969, Clean Air Act of 1970, Clean Water Act 

of 1977, Endangered Species Act [ESA] of 1973, and their 

state counterparts. The goals, tactics, and time horizons of 

climate adaptation strategies for lands under these jurisdictions and legal mandates differ considerably from those of 

private forest lands. Adaptation on industrial forest land 

Category Emphasis Reference

Adaptation framework General options for wildlands Millar et al. 2007

Options for protected lands Baron et al. 2008, 2009

Adaptation guidebooks Peterson et al. 2012, Snover et al. 2007,

Swanston and Janowiak 2012

Vulnerability analysis Climate change scenarios Cayan et al. 2008

Scenario exercises Weeks et al. 2011

Forest ecosystems Aubry et al. 2011, Littell et al. 2010

Watershed analysis Furniss et al. 2010

Genetic management Seed transfer guidelines McKenney et al. 2009

Risk assessment Potter and Crane 2010

Assisted migration Framework for translocation McLachlan et al. 2007, Riccardi and Simberloff 2008

Decisionmaking Silvicultural practices Janowiak et al. 2011b

Climate adaptation workbook Janowiak et al. 2011a

Priority setting Climate project screening tool Morelli et al. 2011b

focuses on strategies most effective to sustain productive 

output over the period of economic analysis (Sedjo 2010), 

whereas adaptation on nonindustrial private forest lands differs by the diverse goals and capacities of landowners. Other 

institutional considerations that influence adaptation relate to 

educational and technological capacities, staff resources, and 

funding. Choices also depend on the quality of collaboration, 

because support, trust, and interaction among stakeholders 

influence the type of risk accepted and commitment to novel 

or experimental approaches.

Adaptation Strategies and 

Implementation

Overview of Forest Adaptation Strategies 

The literature on conceptual approaches to forest adaptation 

strategies (Baron et al. 2008, Joyce et al. 2008, Peterson et 

al. 2011, Swanston and Janowiak 2012) (table 4.3) includes 

broad conceptual frameworks, approaches to specific types 

of analyses (e.g., vulnerability assessments, scenario planning, adaptive management), and tools and guidance for sitespecific or issue-specific problems. An umbrella approach 

for addressing adaptation at the highest conceptual level in 

Table 4.3—Climate adaptation guides relevant to the forest sector
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

forest ecosystems focuses on resistance, resilience, response, 

and realignment strategies (Millar et al. 2007) (box 4.1). 

These general principles help in early phases of planning 

to dissect the range and scales of appropriate options at the 

broadest levels (Spittlehouse 2005), and they apply to many 

management and land-ownership contexts, but they do not 

provide guidance for developing site-specific plans. Similarly, broad discussions focus on other fundamental principles 

relative to forest adaptation planning, such as reinterpreting 

the role of historical variability, ecological change over time, 

and use of historic targets in management and restoration 

(Harris et al. 2006, Jackson 2012, Milly et al. 2008). 

Special concerns for adaptation in parks and protected 

areas were developed by Baron et al. (2008, 2009) and 

Stephenson and Millar (2012), who emphasize the need to 

acknowledge that future ecosystems will differ from the 

past, and that fundamental changes in species and their environments will be inevitable. Given anticipated nonanalog 

climates and ecosystem responses, science-based adaptation 

will be essential. Baron et al. (2009) emphasized the need 

to identify resources and processes at risk, define thresholds and reference conditions, establish monitoring and 

assessment programs (adaptive management), and conduct 

scenario planning. They emphasize that preparing for and 

adapting to climate change is as much a cultural and intellectual challenge as an ecological issue. Diverse regulatory 

and value contexts dictate what will be desired for future 

ecosystem conditions, which drive decisions about goals, 

strategies, and actions.

The reality of change and novelty in future forest ecosystems under changing climates underscores the importance of vulnerability assessments in developing adaptation 

strategies (Aubry et al. 2011, Johnstone and Williamson 

2007, Lindner et al. 2010, Littell and Peterson 2005, Littell 

et al. 2010, Nitschke and Innes 2008, Spittlehouse 2005). 

Vulnerability assessments can differ in terms of subject 

matter, geographic focus, level of detail, and quantitative 

rigor, but usually require a science-management partnership 

to ensure that current science is used to evaluate climatechange effects. Regional-scale assessments can be cautiously 

downscaled to smaller management units, recognizing there 

Box 4.1

A general framework for adaptation options suitable for 

conditions of forested ecosystems. Options range from 

short-term, conservative strategic approaches to strategies for long-term, proactive plans. (From Millar et al. 

2007.)

Promote resistance

Actions that enhance the ability of species, ecosystems, 

or environments to resist forces of climate change and 

that maintain values and ecosystem services in their 

present or desired states and conditions.

Increase resilience

Actions that enhance the capacity of ecosystems to withstand or absorb increasing impact without irreversible 

changes in important processes and functionality.

Enable ecosystems to respond

Actions that assist climatically driven transitions to future states by mitigating and minimizing undesired and 

disruptive outcomes.

Realign highly altered ecosystems

Actions that use restoration techniques to enable ecosystem processes and functions (including conditions 

that may or may not have existed in the past) to persist 

through altered climates and in alignment with changing 

conditions.

will be tradeoffs in accuracy. Some of the most detailed approaches to vulnerability assessment in response to climate 

challenges have focused on watersheds (e.g., Furniss et al. 

2010), as described in the examples below. Scenario planning as a tool for vulnerability assessment has been well 

developed for forested ecosystems in U.S. national parks 

(Weeks et al. 2011). Tools developed for setting priorities in 

forest planning and for assessing risks are especially applicable for near-term decisionmaking (Janowiak et al. 2011a, 

Morelli et al. 2011).

Several efforts have taken comprehensive approaches 

to incorporate both conceptual strategies and specific tools 

into integrated guidebooks for developing adaptation strategies in the forest sector (Peterson et al. 2011, Swanston and 

Janowiak 2012). These guidebooks encourage education and 

training in the basic climate sciences and describe how to 

proceed from assessment to on-the-ground practices. 
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Strategic steps for forest climate adaptation—

The following steps represent broad consensus that has 

emerged on developing forest climate adaptation strategies 

(review in Swanston and Janowiak 2012).

Step 1: Define location (spatial extent), management goals 

and objectives, and timeframes—Determining spatial and 

temporal scales and site-specific locations is essential for 

developing appropriate strategies. Management goals and 

objectives (box 4.2) for climate adaptation should be explicit 

and integrated with mitigation and other nonclimate-related 

management goals. This does not necessarily mean that 

goals are stated in narrowly specific quantitative terms; 

indeed, many forest adaptation goals and objectives can be 

defined broadly (e.g., sustaining ecosystem services).

Step 2: Analyze vulnerabilities—Vulnerability to climate 

change can be defined as “the degree to which geophysical, 

biological, and socio-economic systems are susceptible to, 

and unable to cope with, adverse impacts of climate change” 

(Solomon et al. 2007). Vulnerability is a function of the 

degree to which a system is exposed to a change in climatic 

conditions, its sensitivity to that change, and its adaptive 

capacity (Gallopín 2006, IPCC 2001, Solomon et al. 2007). 

Climate-vulnerability assessments are a central step in 

developing adaptation strategies and can take different forms 

(Glick et al. 2011, USGCRP 2011). Whichever approach 

is used, the intent is to determine how climatic variability 

and change might affect resources of concern, and to aid in 

developing appropriate priorities, strategies, and timeframes 

for action.

Step 3: Determine priorities—Priority actions for climate 

adaptation often differ from those for traditional forest 

management contexts. Furthermore, given rapidly changing 

conditions and emerging understanding of trends, priorities 

need to be reassessed regularly. When conditions are urgent 

and resources limited (e.g., a species in rapid decline), triage 

methods can be useful (Joyce et al. 2008); in longer term 

planning, no-regrets assessments (National Research Council 2002, Overpeck and Udall 2010) minimize risk. 

Step 4: Develop options, strategies, and

tactics—Swanston and Janowiak (2012) present a framework approach for developing adaptation plans. This 

process begins at a broad conceptual level and steps down to 

regional and local, site-specific project planning, as reflected 

by the increasing specificity of the following terms (fig. 

4.2). Adaptation options are fundamental concepts and the 

broadest and most widely applicable level in a continuum of 

Box 4.2

Management Goals

Management goals are broad, general statements that express a desired state or process to be achieved. They are often 

not attainable in the short term and provide the context for more specific objectives. Examples of management goals 

include:

• Maintain and improve forest health and vigor

• Maintain wildlife habitat for a variety of species

Management Objectives

Management objectives are concise, time-specific statements of measurable planned results that correspond to preestablished goals in achieving a desired outcome. These objectives include information on resources to be used for planning 

that defines precise steps to achieve identified goals. Examples of management objectives include:

• Regenerate a portion of the oldest aspen forest type through clearcut harvest in the next year to improve forest vigor 

in young aspen (Populus spp.) stands.

• Identify and implement silvicultural treatments within 5 years to increase the oak (Quercus spp.) component of 

selected stands and enhance wildlife habitat.
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Figure 4.2—A continuum of adaptation options to address needs at appropriate scales, and examples of each (shaded boxes). (From 

Janowiak et al. 2011.)

management responses to climate change. Options include 

resistance, resilience, response, and realignment, which 

reflect conservative, short-term categories to proactive,

long-range ones (Millar et al. 2007) (see box 4.1); they

can be general or specific and focused on the local situation. Adaptation strategies illustrate ways that options can 

be used. Similar to options, strategies are broad and can 

be applied in many ways across different forest landscapes 

(table 4.4). Approaches provide greater detail on how forest 

managers can respond, and differences in application among 

specific forest types and management goals become evident. 

Tactics are the most specific adaptation response, providing prescriptive direction in how actions are applied on the 

ground. The culmination of this process is development 

of a plan, such as a NEPA document or other project plan, 

prescription, or treatment description.

Step 5: Implement plans and projects—Implementation of

projects should include replication, randomization, and other 

experimental design elements, as possible, which sets up the 

value of the final step.

Step 6: Monitor, review, adjust—Formal adaptive management is often advocated as a key element in forest climateadaptation planning (Baron et al. 2008, 2009; Joyce et al. 

2008). Adaptive management involves a comprehensive 

set of steps developed in an experimental framework. 

Monitoring is tied to predefined thresholds and other target 

goals. These are developed to test hypotheses about project 

effectiveness and appropriateness, and, if thresholds are 

exceeded, trigger review and adjustment of plans (Joyce et 

al. 2008, 2009; Margoluis and Salafsky 1998; Walters 1986). 

In practice, many constraints exist to implementing the full 

adaptive management cycle in forest ecosystems (Joyce et 

al. 2009). However, informal monitoring keyed to assessing 

treatment effectiveness and enabling adjustment of practices 

is essential because of dynamic conditions driven by climate 

change.
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Tools and Resources for Adaptation 

and Implementation

Until recently, few guides to implementing climate adaptation plans were available, but many active projects now 

exist, including in the forest sector. The examples below are 

not exhaustive, but represent the type of tools available and 

the meta-level databases and Web resources that assist in 

finding relevant tools for specific locations and needs.

Web Sites

Climate Change Resource Center (CCRC)—

Described more below, the CCRC (http://www.fs.fed.us/

ccrc) is a U.S. Forest Service-sponsored portal dedicated to 

compiling comprehensive, credible information and resources relevant to forest resource managers (USDA FS 2011a). 

Climate Adaptation Knowledge Exchange (CAKE)—

This is a joint project of Island Press and EcoAdapt (CAKE 

2011) (http://www.cakex.org). Its main feature is a retrievable knowledge base that can assist in managing natural 

systems in the face of rapid climate change by compiling 

relevant information. The CAKE maintains an interactive 

online platform, creating a directory of practitioners to share 

knowledge and strategies, and identifying data tools and information available from other sites. Case studies, toolboxes, 

and reference materials are relevant to forest sector issues. 

NaturePeopleFuture.org—

This is The Nature Conservancy (TNC) knowledge base for 

climate adaptation (TNC 2011a) (http://conserveonline.org/

workspaces/climateadaptation). The Web site is used to collect input on climate-adaptation projects, summarize relevant 

products and ideas, and communicate about TNC efforts to 

draw together scientific research and innovative conservation projects. Geographically diverse forest ecosystem situations are presented, and adaptation tools and the methods 

discussed are relevant to forest sector issues. 

Tribes and Climate Change—

Developed by the Institute for Tribal Environmental Professionals and Northern Arizona University, Tribes and Climate 

Change (http://www4.nau.edu/tribalclimatechange) summarizes information and resources to help Native people better 

understand climate change and its effects on their communities (NAU 2011). The site provides basic climate science 

information, including climate change scenarios and vulnerability assessment background, profiles of tribes throughout 

the United States that are addressing climate change effects, 

audio files of elders discussing adaptation from traditional 

perspectives, and resources and contacts to develop adaptation strategies. A section is devoted to forest ecosystems. 

Source: Butler et al. 2012.

Table 4.4—Climate change adaptation strategies 

under broad adaptation options

Strategy Resistance Resilience Response

Sustain fundamental 

ecological conditions

X X X

Reduce the impact 

of existing ecological stressors

X X X

Protect forests from 

large-scale fire and 

wind disturbance

X

Maintain or create 

refugia

X

Maintain or enhance 

species and structural diversity

X X

Increase ecosystem 

redundancy across 

the landscape

X X

Promote landscape 

connectivity

X X

Enhance genetic 

diversity

X X

Facilitate community adjustments 

through species 

transitions

X

Plan for and respond 

to disturbance

X
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Tools

Climate Wizard—

Sponsored and developed by TNC, Climate Wizard is a 

Web-based tool that uses select climate projections relevant 

to the time and space resolution of inquiries, enabling users 

to visualize modeled changes at several time and spatial 

scales (TNC 2011b). Used with scenario exercises, the Wizard can assist development of forest adaptation strategies. 

Vegetation Dynamics Development Tool (VDDT)—

Developed by the TNC Southwest Forest Climate Assessment Project, VDDT is a user-friendly computer tool for 

forest resource managers (ESSA 2011). VDDT provides a 

state-and-transition landscape modeling framework for examining the role of various disturbance agents and management actions in vegetation change. It allows users to create 

and test descriptions of vegetation dynamics, simulating 

them at the landscape level. VDDT provides a common platform for specialists from different disciplines to collectively 

define the roles of various processes and agents of change on 

landscape-level vegetation dynamics, and allows for rapid 

gaming and testing of ecosystem sensitivity to alternative 

assumptions. 

Template for Assessing Climate Change Impacts and 

Management Options (TACCIMO)—

This Web-based tool connects forest planning to climatechange science providing access to relevant climate-change 

projections and links to peer-reviewed scientific statements 

describing effects and management adaptation options 

(North Carolina State University 2011). The tool is intended 

for all forest planners with a need for public and private 

land management information. Input is given by the user on 

management conditions and capabilities to address climate 

change, which is linked with available physical and biological information on climate impacts and management options. 

TACCIMO produces a customized report that synthesizes 

user input needs with available science and related planning 

options. 

Climate Project Screening Tool (CPST)—

This verbal interview tool helps resource managers explore 

options for ameliorating the effects of climate in resource 

projects (Morelli et al. 2011b). The CPST also acts as a 

priority-setting tool, allowing managers to assess relative 

vulnerabilities and anticipate effects of different actions. 

It also helps managers identify and assess projects that are 

soon to be implemented but have not benefited from serious 

consideration of climate influence. Through a set of guided 

questions and development of answers based on available 

climate and ecosystem information, the CPST reduces uncertainty by identifying possible effects of both climate change 

and adaptation actions on resources. 

Climate Change Adaptation Workbook—

The Climate Change Adaptation Workbook is designed to 

help forest managers more effectively bring climate change 

considerations to the spatial and temporal scales where 

management decisions are made (Janowiak et al. 2011a). 

The workbook is an analytical process built on a conceptual 

model for adaptation derived from adaptive management 

principles. It draws on regionally specific information, filtering climate and vegetation projections through professional 

judgment and experience. Using a five-step process, the 

workbook can help incorporate climate change in resource 

management at different spatial scales (e.g., stand, large 

ownership) and levels of decisionmaking (e.g., planning, 

problemsolving, implementation). By defining current management goals and objectives in the first step, the process 

integrates climate change adaptation into existing management efforts. It is not intended to provide specific guidance 

or replace other forms of management planning; rather, it 

relies on the expertise of natural resource professionals and 

complements existing management planning and decisionmaking. 

System for Assessing Vulnerability of Species (SAVS)—

This verbal index tool identifies relative vulnerability or 

resilience of vertebrate species to climate change (Bagne 

et al. 2011). Designed for resource managers, SAVS uses 

a questionnaire with 22 predictive criteria to create vulnerability scores. The user scores species attributes relating to 

potential vulnerability or resilience associated with projections for their region. Six scores are produced: (1) an overall 

score denoting level of vulnerability or resilience, (2) four 

categorical scores (habitat, physiology, phenology, and 
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biotic interactions) indicating source of vulnerability, and 

(3) an uncertainty score, which reflects user confidence in 

the predicted response. The SAVS provides a framework for 

integrating new information into climate change assessments 

and developing adaptation plans.

Institutional Responses

President’s Directive

Executive Order 13514 (2009), “Federal Leadership in Environmental, Energy, and Economic Performance,” directs 

each federal agency to evaluate climate change risks and 

vulnerabilities to manage the short- and long-term effects of 

climate change on the agency’s mission and operations. An 

interagency climate change adaptation task force includes 20 

federal agencies and develops recommendations for agency 

actions in support of a national climate change adaptation 

strategy. The task force recommended that federal agencies 

establish climate change adaptation policies, increase agency 

understanding of how climate is changing, apply understanding of climate change to agency mission and operations, 

develop an adaptation plan and implement at least three 

adaptation actions in 2012, and evaluate and share “lessons 

learned” with other agencies.

Some of the more successful adaptation efforts to date 

have involved collaboration among different institutions. 

Collaboration can take many forms, such as between federal 

agencies, between federal and state agencies, between 

various agencies and Native American tribes, and between 

various land management agencies and a wide range of 

stakeholders. There is no standard model, and effective collaborations will differ by landscape and local institutional 

relationships.

U.S. Forest Service

The U.S. Forest Service climate response is led by the climate change advisor’s office, which develops guidance and 

evaluates progress toward climate adaptation. Agency goals 

and actions are described in a strategic framework document 

(USDA FS 2008). Forest Service research and development also has a climate change strategic plan (Solomon et 

al. 2009). These documents state the conceptual visions for 

science-based adaptation on the 175 national forests and 

national grasslands. Of seven key goals in the overall framework, five pertain to climate adaptation:

Science—

Advance understanding of the environmental, economic, and 

social implications of climate change and related adaptation 

activities on forests and grasslands.

Adaptation—

Enhance the capacity of forests and grasslands to adapt to 

the environmental stresses of climate change and maintain 

ecosystem services.

Policy—

Integrate climate change, as appropriate, into policies, 

program guidance, and communications and put in place 

effective mechanisms to coordinate across and within deputy 

areas.

Education—

Advance awareness and understanding regarding principles 

and methods for sustaining forests and grasslands, and sustainable resource consumption, in a changing climate.

Alliances—

Establish, enhance, and retain strong alliances and partnerships with federal agencies, state and local governments, 

tribes, private landowners, nongovernmental organizations, 

and international partners to provide sustainable forests and 

grasslands for present and future generations.

Tactical approaches and implementation are outlined in 

the National Roadmap for Responding to Climate Change 

(USDA FS 2011b). The roadmap identifies 10 steps along 

four major dimensions, namely, agency and organizational 

capacity, partnerships and conservation education, adaptation, and mitigation (fig. 4.3). The process includes (1) 

science-based assessments of risk and vulnerability; (2) 

evaluation of knowledge gaps and management outcomes; 

(3) engagement of staff, collaborators, and partners through 

education, science-based partnerships, and alliances; and (4) 

management of resources via adaptation and mitigation. To 

assist in these tasks, the CCRC (USDA FS 2011a) serves as

a reference Web site with information and tools to address 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

climate change in planning and project implementation.

Climate change coordinators are designated for each Forest 

Service region. Current initiatives from research and management branches of the agency provide climate science, 

develop vulnerability assessments, prepare adaptive monitoring plans, and align planning, policy, and regulations with 

climate challenges (box 4.3). 

The Performance Scorecard (USDA FS 2011c) (table 

4.5) is used annually to document progress of national forests, regions, and research stations on adaptation plans and 

“climate smart” actions. The scorecard also identifies areas 

of weakness, knowledge gaps, and budgetary limitations, 

which the climate change advisor’s office can subsequently 

highlight for attention. 

U.S. Department of the Interior (DOI) 

A U.S. Department of the Interior secretarial order (2009) 

provides a framework to coordinate climate change activities 

among DOI bureaus and to integrate science and management expertise with DOI partners. Climate Science Centers 

and Landscape Conservation Cooperatives form the cornerstones of the framework. Each has a distinct science and 

resource management role, but they share complementary 

capabilities in support of DOI resource managers and of 

integrated climate solutions with federal, state, local, tribal, 

and other stakeholders.

Climate science centers (CSC)—

Climate science centers are seven regional centers in development because of cooperative endeavors between DOI and 

universities to distill and make climate-adaptation information available to users. The CSCs fund the development of 

scientific information, tools, and techniques for resource 

Figure 4.3—Four dimensions of action outlined by the U.S. Forest Service roadmap for responding to climate change. (From USDA FS 

2011b.)
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Box 4.3

U.S. Forest Service initiatives to promote progress toward achieving goals of the national roadmap for responding to 

climate change. (From USDA FS 2011b.)

Furnish predictive information on climate change and variability, both immediate and longer term, building on current research capacity and partnerships with the National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration, U.S. Geological Survey, and other scientific agencies.

• Develop, interpret, and deliver spatially explicit scientific information on recent shifts in temperature and moisture 

regimes, including incidence and frequency of extreme events.

• Provide readily interpretable forecasts at regional and subregional scales.

Develop vulnerability assessments, working through research and management partnerships and collaboratively

with partners.

• Assess the vulnerability of species, ecosystems, communities, and infrastructure and identify potential adaptation 

strategies.

• Assess the impacts of climate change and associated policies on tribes, rural communities, and other resourcedependent communities.

• Collaborate with the U.S. Fish and Wildlife Service and National Marine Fisheries Service to assess the vulnerability 

of threatened and endangered species and to develop potential adaptation measures.

Tailor monitoring to facilitate adaptive responses.

• Expand observation networks, intensify sampling in some cases, and integrate monitoring systems across 

jurisdictions (see, for example, the national climate tower network on the experimental forests and ranges). 

• Monitor the status and trends of key ecosystem characteristics, focusing on threats and stressors that may affect the 

diversity of plant and animal communities and ecological sustainability. Link the results to adaptation and genetic 

conservation efforts.

Align Forest Service policy and direction with the Forest Service strategic response to climate change. 

• Revise National Forest System land management plans using guidance established in the new Planning Rule, which 

requires consideration of climate change and the need to maintain and restore ecosystem and watershed health and 

resilience.

• Review Forest Service Manuals and other policy documents to assess their support for the agency’s strategic climate 

change direction. Evaluate current policy direction for its ability to provide the flexibility and integration needed to 

deal with climate change.

• Develop proposals for addressing critical policy gaps.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Source: Adapted from USDA FS 2011b, 2011c.

Table 4.5—Performance scorecard used by the U.S. Forest Service for annual review of progress and compliance, and to identify deficit areas in implementation of the national roadmap for responding to climate change

Scorecard element Questions to be addressed Yes/no

Organizational capacity:

Employee education Are all employees provided with training on the basics of climate 

change, impacts on forests and grasslands, and the Forest Service 

response? 

Are resource specialists made aware of the potential contribution of their 

own work to climate change response?

Designated climate change 

 coordinators

Is at least one employee assigned to coordinate climate change activities 

and be a resource for climate change questions and issues? 

Is this employee provided with the time, training, and resources to make 

his/her assignment successful?

Program guidance Does the unit have written guidance for progressively integrating climate 

change considerations and activities into unit-level operations?

Engagement:

Science and management 

partnerships

Does the unit actively engage with scientific organizations to improve its 

ability to respond to climate change? 

Other partnerships Have climate change-related considerations and activities been incorporated into existing or new partnerships (other than science partnerships)?

Adaptation:

Assessing vulnerability Has the unit engaged in developing relevant information about the vulnerability of key resources, such as human communities and ecosystem 

elements, to the impacts of climate change?

Adaptation actions Does the unit conduct management actions that reduce the vulnerability 

of resources and places to climate change?

Monitoring Is monitoring being conducted to track climate change impacts and the 

effectiveness of adaptation activities?

Mitigation and sustainable consumption:

Carbon assessment and stewardship Does the unit have a baseline assessment of carbon stocks and an assessment of the influence of disturbance and management activities on 

these stocks? 

Is the unit integrating carbon stewardship with the management of other 

benefits being provided by the unit?

Sustainable operations Is progress being made toward achieving sustainable operations requirements to reduce the environmental footprint of the agency?
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managers to anticipate, monitor, and adapt to climate and 

to develop adaptation responses at multiple scales. Forest 

ecosystems are a primary focus of several CSCs. 

Landscape conservation cooperatives (LCC)—

The LCCs complement existing science and conservation 

efforts of the CSCs and partners by leveraging resources and 

strategically targeting science topics to inform conservation 

decisions and actions (USDI FWS 2011). Each LCC operates within a specific landscape, with 21 geographic areas 

total (fig. 4.4). Partners include federal, state, and local governments, tribes, universities, and other stakeholders. The 

LCCs form a network of resource managers, scientists, and 

public and private organizations that share a common need 

for scientific information and interest in conservation. Land 

conservation cooperatives products include resource assessments, examples demonstrating the application of climate 

models, vulnerability assessments, inventory and monitoring 

protocols, and conservation plans and designs. Adaptation 

products include assessments of climate change effects and 

development of adaptation strategies for wildlife migration 

corridors, wildfire risk and fuel treatments, drought impacts 

and amelioration, detection and control of invasive species, 

and restoration of forest landscapes.

National Park Service (NPS)—

The NPS climate change response strategy (NPS 2010) 

provides direction for addressing effects of climate change 

in NPS-administered park units. The strategy directs NPS to 

adapt natural resources on its lands by using scenario exercises as a central approach, thereby creating flexible plans at 

park scales for dealing with climate effects. The broad goals 

of the strategy include developing effective natural-resource 

adaptation plans and promoting ecosystem resilience. Specifically the strategy requires that units (1) develop adaptive 

capacity for managing natural and cultural resources; (2) 

inventory resources at risk and conduct vulnerability assessments; (3) prioritize and implement actions and monitor the 

results; (4) explore scenarios, associated risks, and possible 

management options; and (5) integrate climate change effects in facilities management. The legacy dictum for NPS 

management has been to preserve and restore natural (usually interpreted as historical) conditions. Ecosystem dynamics associated with climate change have forced rethinking 

of this concept, and new paradigms are emerging in national 

park management for incorporating ecological change in 

adaptation philosophies and managing “beyond naturalness” (Cole and Yung 2010, Stephenson and Millar 2012). 

Emphasis on scenario exploration as a discussion focus is 

intended to promote solutions that address multiple feasible 

future outcomes.

Bureau of Land Management (BLM)—

The BLM focuses on a landscape approach to climate 

change adaptation, working within functional ecosystems 

at large scales and across agency boundaries, and assessing 

natural resource conditions and trends, natural and human 

influences, and opportunities for resource conservation and 

development. The landscape approach consists of (1) rapid 

ecoregional assessments (REA), which synthesize the information about resource conditions and trends within an ecoregion, with emphasis on areas of high ecological value (e.g., 

important wildlife habitats and corridors); (2) ecoregional 

direction, which uses the results of REAs to identify management priorities for public lands in an ecoregion and guide 

adaptation actions; (3) monitoring for adaptive management, 

which relies on monitoring and mapping programs to meet 

information needs and assessment, understand resource conditions and trends, and evaluate and refine implementation 

actions; and (4) science integration, which relies on participation with CSCs to provide science for management needs. 

To date, these have not yielded operational climate-change 

adaptation plans.

Regional Integrated Sciences and 

Assessments (RISA)

Funded by National Oceanographic and Atmospheric Administration’s Climate Program Office, the RISA program 

supports research and stakeholder interaction to improve 

understanding of how climate affects various regions of the 

United States, and to facilitate the use of climate information in decisionmaking. The RISA teams analyze climate 

data; apply, provide, and interpret climatic information for 

resource managers and policymakers in the United States; 

and are a good source of information on climate change and 

regional effects of climate change. 
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Figure 4.4—Twenty-one landscape conservation cooperatives of the U.S. Department of the Interior integrate climateadaptation responses across federal, state, tribal, local, and private interests within geographically coherent regions. (From 

DOI FWS 2011.)
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State and Local Institutions

Climate-adaptation responses of state and local institutions 

are diverse, ranging from minimal action to fully developed 

and formal programs. State responses that focus on forestsector issues include the following.

Washington State climate response strategy—

Beginning in 2009, the Washington Department of Ecology 

in partnership with the departments of Agriculture, Commerce, Fish and Wildlife, Natural Resouces, and Transportation, began developing a strategy to prepare for the effects 

of climate change outlined in the Washington State Climate 

Change Impacts Assessment (McGuire et al. 2009, Washington State Department of Ecology 2012). This collaborative 

effort involving a variety of public and private stakeholders 

sought to develop recommendations for addressing the effects of climate change. The working groups emphasized the 

priority of forest resources in the strategy, and recommendations for climate-adaptation efforts in major forest ecological 

systems have been developed (Helbrecht et al. 2011) (box 

4.4), including for fire management and genetic preservation 

(Jamison et al. 2011). These options emphasize research, assessing vulnerabilities, developing pilot projects, improving 

forest health, avoiding forest conversion, using prescribed 

fire, and using adaptive management in decisionmaking. 

Strategies consistent with adaptation on forest lands include 

(1) preserve and protect Washington’s existing working 

forest, (2) assess how land management decisions help or 

hinder adaptation, (3) foster interagency collaboration, (4) 

promote sociocultural and economic relations between eastern and western Washington to improve collaboration, (5) 

develop options that address major disturbance events, and 

(6) incorporate state decisions with global and local factors 

when adapting to climate change (Washington State Department of Ecology 2012).

Western Governors’ Association (WGA)—

A nonpartisan organization of governors from 19 Western 

States, two Pacific territories, and one commonwealth, the 

WGA addresses the effects of climate on forest health, wildfire, water and watersheds, recreation, and forest products. 

The WGA supports integration of climate adaptation science 

in Western States (WGA 2009) and published a report on 

priorities for climate response in the West (WGA 2010), 

including sharing climate-smart practices for adaptation, 

developing science to be used in decisionmaking, and coordinating with federal entities and other climate adaptation 

initiatives. The WGA is focusing on developing training to 

help states incorporate new protocols and strategies relative 

to climate change (box 4.5), and improving coordination of 

state and federal climate adaptation initiatives. The WGA 

recommends that new state-level programs be designed that 

are relevant for on-the-ground climate change issues and 

also comply with federal regulations.

Minnesota State climate response—

The Minnesota Department of Natural Resources is building intellectual and funding capacity to implement policies 

that address climate change and renewable energy issues, 

Box 4.4

Interim recommendations of the Washington State 

Climate Change Response Strategy’s topic advisory 

group on species, habitats, and ecosystems. (From 

Helbrecht et al. 2011.)

Facilitate the resistance, resilience, and response of 

natural systems 

1. Provide for habitat connectivity across a range of 

environmental gradients. 

2. For each habitat type, protect and restore areas most 

likely to be resistant to climate change. 

3. Increase ecosystem resilience to large-scale 

disturbances, including pathogens, invasive species, 

wildfire, flooding, and drought. 

4. Address stressors contributing to increased 

vulnerability to climate change. 

5. Incorporate climate change projections in plans for 

protecting sensitive species. 

Build scientific and institutional readiness to support

effective adaptation 

6. Fill critical information gaps and focus monitoring on 

climate change. 

7. Build climate change into land use planning. 

8. Develop applied tools to assist land managers. 

9. Strengthen collaboration and partnerships. 

10. Conduct outreach on the values provided by natural 

systems at risk from climate change. 
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including vulnerability assessments that identify risks and 

adaptation strategies for forest ecosystems. These efforts 

complement climate adaptation efforts occurring in the state 

through the multi-institutional Northwoods climate change 

response framework (see “Regional Examples”). The Minnesota Forest Resources Council, which includes public and 

private stakeholders from the forestry sector, is developing 

recommendations to the governor and federal, state, county, 

and local governments on policies and practices that result 

in the sustainable management of forest resources. Regional 

landscape committees establish landscape plans that identify 

local issues, desired future forest conditions, and strategies 

to attain these goals (MFRC 2011). The regional landscape 

committees plan to integrate with the Northwoods climate 

change response framework to ensure that climate change is 

integrated in forest management and planning.

North Carolina State climate response—

The North Carolina Department of Environment and Natural 

Resources (DENR) is developing a comprehensive adaptation strategy to identify and address potential effects on 

natural resources, with emphasis on climate-sensitive ecosystems and land use planning and development. The North 

Carolina Natural Heritage Program is evaluating likely 

effects of climate change on state natural resources, including 14 forest ecosystems that are likely to respond to climate 

change in similar ways. The DENR co-hosted a statewide 

climate change adaptation workshop in 2010 and is now 

coordinating with other agencies on an integrated climate 

response and developing a climate change response plan and 

down-scaled climate assessments.

State university and academic responses—

In the Pacific Northwest, the University of Washington 

Climate Impacts Group (CIG) has a strong focus on climate 

science in the public interest. Besides conducting research 

and assessing climate effects on water, forests, salmon, and 

coasts, the CIG applies scientific information in regional 

decisions. The CIG works closely with stakeholders and 

has been a key coordinator for forest climate adaptation 

projects (e.g., Halofsky et al. 2011, Littell et al. 2011). An 

adaptation guidebook developed in collaboration with King 

County, Washington describes an approach for developing 

local, regional, and state action plans (Snover et al. 2007). 

In Alaska, the Alaska Coastal Rainforest Center, based at 

the University of Alaska Southeast, in partnership with the 

University of Alaska Fairbanks and other stakeholders, 

provides educational opportunities, facilitates research, and 

promotes learning about temperate rain forests. The center 

facilitates dialogue on interactions among forest ecosystems, 

communities, and social and economic systems and has 

developed a framework for integrating human and ecosystem adaptation. In Hawaii, the Center for Island Climate 

Adaptation and Policy, based at the University of Hawaii at 

Mānoa, promotes interdisciplinary research and solutions to 

public and private sectors, with a focus on science, planning, 

indigenous knowledge, and policy relative to climate adaptation. Recent projects focus on education, coordinating with 

state natural resource departments on adapting to climate 

change (CICAP 2009), and policy barriers and opportunities 

for adaptation. Forest-related climate issues include effects 

of invasive species, forest growth and decline, migration and 

loss of forest species, and threats to sustainability of water 

resources.

Box 4.5

Goals of the Western Governors’ Association (WGA) 

climate-adaptation initiative on training. (From WGA 

2010.)

The WGA seeks to provide training to its 19 member 

states and collaborators with goals to: 

1. Provide state resource planners with the tools, 

methods, and technical assistance needed to 

incorporate climate change into ongoing planning 

processes.

2. Create a forum to enhance communication and 

dialogue with climate adaptation researchers to help 

set priorities for investment in science and research 

that informs decisionmakers.

3. Identify multistate or cross-boundary climate 

adaptation needs, as well as regional data sharing 

needs, and consider how they may be addressed 

through regional collaboration.

4. Determine how state agencies can collaborate with 

federal and local governments and other partners.

5. Develop a clearinghouse of best practices that state 

agencies and managers may refer to when developing 

their state’s adaptation efforts.
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Industrial Forestry

The response from forest industries in the United States to 

climate change has to date focused mostly on carbon sequestration, energy conservation, the role of biomass, and other 

climate-mitigation issues. Detailed assessments and efforts 

to develop adaptation strategies for the forest-industry sector have mostly been at the global to national scale (Sedjo 

2010; Seppälä et al. 2009a, 2009b). Many forestry corporations promote stewardship forestry focused on adaptability 

of forest ecosystems to environmental challenges, but most 

ongoing adaptation projects are small scale and nascent. 

For example, Sierra Pacific Industries (SPI) in California is 

evaluating the potential for giant sequoia (Sequoiadendron 

giganteum [Lindl.] J. Buchholz) plantations to serve as a 

safeguard against changing climates. Giant sequoia currently 

grows in small groves scattered in the Sierra Nevada. Germplasm would be collected by SPI from the native groves and 

planted in riparian corridors on productive industry land, 

then managed as reserves that would benefit from the resilience of giant sequoia to climatic variability and its ability to 

regenerate after disturbance.

In Australia, the forest industry has been more assertive 

in addressing climate change. For example, the National 

Association of Forest Industries of Australia is working to 

improve the ability of forest industry to reduce the harmful 

effects of, and exploit opportunities from, changing climate. A short-term objective is to promote general awareness of the extent and range of likely climatic impacts and 

vulnerabilities specific to key forest regions, together with 

practical options for adaptation and mitigation given available scientific knowledge. The longer term objective is to 

provide tools and mechanisms to promote incorporation of 

adaptation options in forest-based industries. The Australian 

Commonwealth Scientific and Industrial Research Organization developed an initial assessment of climate risks and 

adaptation strategies for plantation forestry, with specific 

recommendations for planting, germplasm selection, and 

silvicultural actions (Pinkard et al. 2010). 

Native American Tribes and Nations

Many Native American tribes and nations have been actively 

developing detailed forest adaptation plans in response to 

climate change. Overall goals commonly relate to promoting 

ecosystem sustainability and resilience, restoration of forest 

ecosystems, and maintenance of biodiversity, especially of 

elements having historical and legacy significance to tribes. 

Maintenance of cultural tradition within the framework of 

changing times is also inherent in many projects. 

An exceptional example of a tribal response is the 

climate change initiative of the Swinomish Tribe in Washington (SITC 2010) (box 4.6). The Swinomish Reservation (3900 ha) is located in northwestern Washington and 

includes 3000 ha of upland forest. The initiative focuses on 

building understanding among the tribal community about 

climate change effects, including support from tribal elders 

and external partners. A recent scientific assessment summarizes vulnerabilities of forest resources to climate change, 

and outlines potential adaptation options (Rose 2010). A 

report completed in 2009 provides a baseline for adaptation 

planning and states that the tribe’s forest resources are at risk 

from wildfire, which will be addressed in an action plan on 

adaptive response (SITC 2010). 

Tribes have been active partners in collaborative forest 

adaptation plans. An example is the Confederated Tribes 

and Bands of the Yakama Nation, whose reservation occupies 490 000 ha in south-central Washington. Tribal lands 

comprise forest, grazing, and farm lands in watersheds of the 

Cascade Range. The Yakama Nation has extensive experience in managing dry forest ecosystems and implementing 

forest action plans, and belongs to the Tapash Sustainable 

Forest Collaborative, in partnership with the U.S. Forest 

Service, Washington State Departments of Fish and Wildlife and of Natural Resources, and TNC. The collaborative 

encourages coordination among landowners to respond to 

common challenges to natural resources (Tapash Collaborative 2010). Climate change was ranked as a significant threat 

to forest productivity, leading to a proposal to incorporate 

specific adaptation strategies and tactics across the Tapash 

landscape, most of which relate to fire, fuels, and restoration 

in dry forest. 
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Nongovernmental Organizations 

Nongovernmental organizations and professional organizations serve a wide range of special interests, and thus 

respond to climate adaptation challenges in diverse ways. 

Box 4.6

Adaptation framework of the Swinomish Tribe 

Washington) climate change initiative. (From SITC 

2010).

Phase 1 (2007–2009)

• Tribal buy-in leads to issuance of the 2007 Climate 

Change Proclamation

• Secure funding

• Identification of partners, development of advisory 

committee, and identification of roles and responsibilities

• Development of the impact assessment

- Data review and analysis

- Risk zone mapping and inventory

- Vulnerability assessment

- Risk analysis

• Policy and strategy scoping (intergovernmental)

• Community outreach

- Formed tribal outreach group

- Held public meetings

- Conducted personal interviews of tribal

members and elders

- Conducted storytelling workshop with tribal

members

Phase 2 (2010)

Development of the action plan

• Adaptation goals

• Strategy evaluation and priorities

• Action recommendations

• Coordination of funding 

• Other implementation issues

Phase 3 (future work)

• Action plan implementation

• Monitoring and adaptive management

• Update of the impact assessment

Pacific Forest Trust (PFT)—

A nonprofit organization dedicated to conserving and 

sustaining America’s productive forest landscapes, PFT 

provides support, knowledge, and coordination on private 

forest lands in the United States. Through its Working 

Forests, Winning Climate program, PFT has created policy 

and market frameworks to expand conservation stewardship of U.S. forests to help sustain ecosystem services (PFT 

2011). The PFT also supports climate adaptation by working 

with private forest owners to promote stewardship forestry, 

whereby forests are managed to provide goods and services 

that society has come to expect. The PFT currently works 

with stakeholders on working forest lands to call on policymakers to safeguard U.S. forests for their value in adaptation 

and mitigation.

The Nature Conservancy (TNC)—

A science-based conservation organization, TNC has a mission to preserve plants, animals, and natural communities 

by protecting the lands and waters they need to survive. The 

TNC climate change adaptation program seeks to enhance 

the resilience of people and nature to climate change effects 

by protecting and maintaining ecosystems that support biodiversity and deliver ecosystem services. The program promotes ecosystem-based approaches for adaptation through 

partnerships, policy strategies for climate adaptation, tools 

to assist resource managers, and research. The Canyonlands 

Research Center (Monticello, Utah), a TNC initiative that 

conducts research and develops conservation applications 

for resource issues in the Colorado Plateau region, focuses 

on forest-climate concerns such as woodland ecosystem restoration, invasive species, and effects of drought on pinyon 

pine and juniper woodlands. 

Trust for Public Land (TPL)—

A conservation organization that helps agencies and communities conserve land for public use and benefit, TPL uses 

vulnerability assessments, resilience and connectivity data, 

and other tools to realign its conservation planning at different spatial scales. The TPL is also designing and implementing restoration to enhance the climate resilience of protected 

tracts. As a member of the Northern Institute of Applied 
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Climate Science, TPL provides guidance to federal and 

nonfederal partners on strategic planning and on-the-ground 

management.

The Wilderness Society (TWS)—

The Wilderness Society leads efforts to fund natural resource 

adaptation and manage lands so they are more resilient under 

stresses of climate change, and is a leader in the Natural 

Resources Adaptation Coalition, which is focused on maintaining and restoring wildlands that include forest wilderness. Specific TWS goals relative to adaptation in forests 

include (1) restoring native landscapes to increase ecosystem 

resiliency, (2) protecting rural communities and providing 

flexibility in wildland fire management, (3) removing invasive species from ecosystems, and (4) repairing damaged 

watersheds.

Ski Industry

Although not a direct member of the forest sector, the ski 

industry relies on mountainous terrain, usually forested 

land leased from federal landowners, and is concerned 

about reduced snow, rising temperatures, extreme weather 

events, and other consequences of climate change that may 

affect the profitability of ski areas. Adaptation options used 

by the ski industry (Scott and McBoyle 2007) include (1) 

snowmaking to increase the duration of the ski season (Scott 

et al. 2006), (2) optimizing snow retention (slope development and operational practices such as slope contouring, 

vegetation management, and glacier protection), and (3) 

cloud seeding. Forest vigor and stand conditions within and 

adjacent to ski area boundaries are important to ski areas, because forests burned by wildfire or killed by insect outbreaks 

affect snow retention, wind patterns, and aesthetic value. 

Examples of Regional and National 

Responses

Although general guidance and strategic plans about climate 

adaptation exist for many land management agencies, strategies for specific places and resource issues are in the early 

stages. Below we summarize examples for which the intent 

was to explore how forest adaptation strategies could be 

developed for specific locations.

Western United States 

Olympic National Forest/Olympic National Park 

(ONFP), Washington—

This case study in the Northwest was undertaken to represent a large landscape within a geographic mosaic of lands 

managed by federal and state agencies, tribal groups, and 

private landowners (Littell et al. 2011). The ONFP supports 

a diverse set of ecosystem services, including recreation, 

timber, water supply to municipal watersheds, pristine air 

quality, and abundant fish and wildlife. Management of 

Olympic National Forest focuses on “restoration forestry,” 

which emphasizes facilitation of late-successional characteristics, biodiversity, and watershed values in second-growth 

forest. Collaboration with adjoining Olympic National Park, 

which has a forest protection and preservation mission, is 

strong. 

Development of the ONFP adaptation approach employed a science-management partnership, including 

scientific expertise from the CIG, to implement education, 

analysis, and recommendations for action. Analysis focused 

on hydrology and roads, vegetation, wildlife, and fish, which 

were the resources most valued by agency resource managers and most likely to be influenced by climate change. A 

vulnerability assessment workshop for each resource area 

was paired with a workshop to develop adaptation options 

based on the assessment, resulting in adaptation options for 

management issues within each disciplinary topic. Emphasis 

in adaptation was on conserving biodiversity while working 

to restore late-successional forest structure through active 

management. The overall process used in the case study 

has been adopted by local resource managers to incorporate 

climate change issues in forest plans and projects (Halofsky 

et al. 2011) and is currently being used to catalyze climatechange education, vulnerability assessment, and adaptation 

planning across 2.5 million ha in Washington state (North 

Cascadia Adaptation Partnership 2011). 

Inyo National Forest and Devils Postpile National 

Monument, California—

Inyo National Forest (INF) in eastern California contains 

Mediterranean and dry forest ecosystems, grading from alpine through forest to shrub-steppe vegetation. Much of the 
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national forest is wilderness with a high degree of biodiversity. Water on the national forest is scarce, fire and insects 

are important issues, and recreation is the dominant use of 

public lands. Devils Postpile National Monument (DEPO) 

is a small national park unit surrounded by INF lands, and 

collaboration with INF is strong. Ongoing near- and midterm projects of highest concern focus on vulnerability of 

INF resources to climate effects that might affect DEPO, 

and climate adaptation is a high priority in the DEPO general management plan. A science-management partnership 

facilitated sharing of knowledge about climate change and 

effects through targeted workshops (Peterson et al. 2011), 

and assessment reports developed by scientists (Morelli et al. 

2011a) assisted managers to consider climate effects relevant 

to specific resource responsibilities. A scientific technical 

committee (Peterson et al. 2011) helped to meet science 

needs for managers of these units. For INF, the Climate 

Project Screening Tool (Morelli et al. 2011b) was developed, 

providing a screening process to rapidly assess if climate 

change would affect resources in the queue for current-year 

management implementation. Questions about climate-mediated quaking aspen (Populus tremuloides Michx.) decline 

spurred a review of aspen responses to climate and an aspen 

screening tool for the INF (Morelli and Carr 2011). 

For DEPO, where ecosystem protection is prioritized, 

managing the monument as a climate refugium (Joyce et 

al. 2008, Peterson et al. 2011) is being evaluated. Because 

DEPO is at the bottom of a large canyon with cold-air drain

age, it contains high biodiversity, and the potential for coldair drainage to increase in the future may ameliorate the effects of a warmer climate (Daly et al. 2009). In anticipation 

of this, a network of temperature sensors in multiple-elevation transects and a climate monitoring station were recently 

installed to measure ongoing changes in temperature. 

Shoshone National Forest, Wyoming—

Resource managers in Shoshone National Forest worked 

with Forest Service scientists to write a synthesis on climate 

change effects and a vulnerability assessment of key water 

and vegetation resources. The synthesis (Rice et al. 2012) 

describes what is currently understood about local climate 

and the surrounding Greater Yellowstone Ecosystem, including paleoclimate, and how future climate change may affect 

plants, animals, and ecosystems. The assessment highlights 

components of local ecosystems considered most vulnerable to projected changes in climate and will be integrated in 

resource-related decisionmaking processes of forest management through collaborative workshops to train managers.

The Strategic Framework for Science in Support of 

Management in the Southern Sierra Nevada, California 

(SFS)—

The SFS addresses collaborative climate adaptation for the 

southern Sierra Nevada bioregion of California (Nydick 

and Sydoriak 2011), including the southern and western 

slopes of the Sierra Nevada, three national parks, a national 

monument, three national forests, tribal lands, state and local 

public lands, forest industry, and other private lands. This 

landscape spans ecosystems from alpine through diverse 

conifer and hardwood forests to woodland and chaparral. 

The effort is coordinated by a coalition of federal resource 

managers and academic and agency scientists, and was 

launched with a public symposium to review the state of science on climate issues and adaptation options. The framework document (Exline et al. 2009) guides adaptation by 

asking (1) Which ecosystem changes are happening, why are 

they happening, and what does it mean? (2) What is a range 

of plausible futures? (3) What can we do about it?, and (4) 

How can relevant information be made available? Interactions among climate change and habitat fragmentation, 

encroaching urbanization, shifting fire regimes, invasive species, and increasing air pollution are also being considered. 

To date the SFS collaborative has generated a list of ideas to 

provide knowledge and tools regarding agents of change and 

potential responses (box 4.7). An information clearinghouse 

will be established, including data for vulnerability assessments, decision-support tools, and reports. 

Southern United States 

Uwharrie National Forest (UNF) represents a typical 

national forest context in the southeastern United States, 

containing 61 parcels mixed with private land and near 
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metropolitan areas (Joyce et al. 2008). Providing a wide 

range of ecosystem services, the region is undergoing a 

rapid increase in recreational demand. The UNF identified 

drought-related forest mortality, wildfire, insect outbreaks, 

soil erosion, stream sedimentation, and water shortages as 

key issues relative to climate effects. Revision of the forest 

land management plan explicitly considers climate change 

effects. Opportunities for adaptation in UNF focus on reestablishing longleaf pine (Pinus palustris Mill.) through selective forest management (Joyce et al. 2008). Replanting of 

drought-tolerant species could provide increased resistance 

to potential future drought and intense wildfire. Selective 

harvest and prescribed burns also could target restoration of 

longleaf pine savannas, mitigating water stress, fuel loads, 

and wildfire risk anticipated under warming conditions. 

Concerns about soil erosion and stream sedimentation focus 

on increasing the size of stream buffer zones where trees are 

not harvested. Collaboration with surrounding landowners to 

remove fuels in wildland-urban interfaces is a high priority.

Northern United States

The U.S. Forest Service in the northeast and upper Midwest is pursuing a comprehensive program of adaptation to 

climate change (fig. 4.5), including education and training, 

partnership building, vulnerability assessment and synthesis, 

planning and decision support, and implementation of demonstration projects. The Forest Service Northern Research 

Station, Northeastern Area State and Private Forestry, and 

Northern Institute of Applied Climate Science work collectively to respond to climate change needs. The Climate 

Change Response Framework (CCRF) developed by these 

entities augments the institutional capacity of national 

forests to adapt to climate change by providing a model for 

collaborative management and climate change response 

that can accommodate multiple locations, landscapes, and 

organizations (fig. 4.6). As of 2011, three projects were underway in the Northwoods, Central Hardwoods, and Central 

Appalachians (fig. 4.7). 

The projects focus on building science-management 

partnerships, developing vulnerability assessments and 

synthesis of existing information, and establishing a

Box 4.7

Initiatives begun and proposed by the strategic framework for science in support of management in the Southern Sierra 

Nevada cooperative and their alignment with the goals of the strategic framework. (From Nydick and Sydoriak 2011.)

Goal 1: Detection and attribution

• Coordinate regional monitoring strategies—tree population dynamics and fisher (Martes pennanti Erxleben) 

populations

Goal 2: Forecasting future conditions

• Alternative fire management futures

• Comparison and integration of climate adaptation projects

Goal 3: Tools and actions

• Both projects under goal 2 also address goal 3

• Kaweah Watershed coordinated restoration initiative

• Enabling forest restoration goals via ecologically managed biomass generation, including a cost-benefit analysis

Goal 4: Communication

• Information clearinghouse for shared learning

• Education and outreach initiative

Integration across goals

• Reevaluation of invasive plant programs and practices under alternative climate futures

• Investigation of the vulnerability of blue oak woodlands to climate change and development of adaptive 

management guidelines
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Figure 4.5—The U.S. Forest Service Eastern Region approach to climate change response works from 

ecoregional scales down to the stand scale by moving information to action through partnerships, science, 

and communication.

Figure 4.6—The Climate Change Response Framework 

uses an adaptive management approach to help land 

managers understand the potential effects of climate 

change on forest ecosystems and integrate climate 

change considerations into management. (From Swanston et al. 2012.)
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standardized process for considering management plans 

and activities in the context of the assessment. First, an 

ecosystem vulnerability assessment and synthesis evaluates 

ecosystem vulnerabilities and management implications 

under a range of plausible future climates. Second, a shared 

landscape initiative promotes dialogue among stakeholders 

and managers about climate change, ecosystem response, 

and management. Third, a science team encourages rapid 

dissemination of information. Fourth, an adaptation resources document includes relevant strategies and a process for 

managers to devise appropriate tactics. Fifth, demonstration 

projects incorporate project information and tools in adaptation activities. The CCRF emphasizes an all-lands approach, 

including national forests, other agencies, and other landowners and stakeholders. 

The Northwoods CCRF Project covers 26 million ha 

of forest in Michigan, Minnesota, and Wisconsin, including 

six national forests, the Forest Service Northern Research 

Station, state resource agencies, universities, and other 

stakeholders. Products to date focus on northern Wisconsin, 

including a vulnerability assessment (Swanston et al. 2011), 

a forest adaptation resources document (Swanston and 

Janowiak 2012), and initiation of demonstration projects in 

Chequamegon-Nicolet National Forest, where each district 

was asked to integrate climate change considerations into 

forest activities. The Medford-Park Falls District identified two aspen stands where silvicultural prescriptions had 

already been written, but timber had not been marked for 

harvest, then used CCRF information to consider climate 

change effects and devised specific adaptation tactics.

Figure 4.7—The U.S. Forest Service Eastern Region with Climate Change Response Framework (CCRF) projects, identified by 

shading. National forests are core partners in the CCRF projects, but the projects take an all-lands approach with numerous federal 

and nonfederal partners. The goal of the CCRF is to complement science-based management decisions made by multiple organizations, each with their own diverse goals, so that forest ecosystems managed by these organizations can become better adapted to a 

changing climate.
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A demonstration project started in 2011 convenes a variety of landowners, forest managers, and other stakeholders 

to discuss climate change effects on specific forest ecosystems, identify adaptation actions, and coordinate implementation of adaptation activities across multiple ownerships. 

Participants are using CCRF information and tools to devise 

adaptation tactics appropriate to their management goals. 

In addition, the Central Hardwoods CCRF, which covers 17 

million ha of hardwood forest in Missouri, Illinois, and Indiana, has formed a regional coordinating team with partners 

from three national forests, the Northern Research Station, 

and other stakeholders. The Central Appalachians CCRF, 

which covers 11 million ha of central Appalachian forest in 

West Virginia and Ohio, includes partners from two national 

forests and state forestry agencies.

National Example

Watershed vulnerability assessment—

In 2010, a draft watershed vulnerability assessment process 

was tested in 11 national forests (Furniss et al. 2010), with 

the goal of quantifying current and projected future condition of watersheds as affected by climate change. A principal 

objective was to develop a general process that could be 

tailored to local data availability and resource investment 

(box 4.8). National forests were asked to include infrastructure, aquatic species, and water uses in the assessments, with 

analysis areas including at least one “river basin” watershed 

(hydrologic unit code [HUC] 4). Design of useful strategies 

for reducing the effects of climate change on ecosystem 

services requires the ability to (1) identify watersheds of 

highest priority for protecting amenity values, (2) identify 

watersheds in which climate-related risk to those values is 

greatest and least, (3) detect evidence of the magnitudes of 

change as early as possible, and (4) select actions appropriate for reducing effects in particular watersheds (Peterson

et al. 2011).

Hydrologic specialists from participating forests developed an approach for quantifying watershed vulnerability 

within a relatively short period, and four national forests 

completed the process within 8 months. Acquiring suitable 

climate exposure data (the degree, duration, or extent of

deviation in climate that a system experiences), which had 

not been previously used by the participants, was challenging. Threshold values for species and water use differed 

across the forests. For example, brook trout (Salvelinus fontinalis Mitchill) was viewed as a stressor in one forest and a 

valued resource in another. These differences suggest that, 

whereas information on processes and resource conditions 

can be shared among forests, local (forest- and watershedscale) assessments have the greatest value. 

Challenges and Opportunities 

Assessing Adaptation Response 

In recent years, federal agencies responsible for administering forest ecosystems have produced national climate 

change response strategies that define adaptation goals and 

describe a framework for action in field units. These strategies, intended to inform and guide consistent agency-wide 

responses, emphasize (1) staff training and education in 

Box 4.8

Steps defining the watershed vulnerability assessment 

process and the types of questions to be addressed. 

(From Furniss et al. 2010.)

Step 1—Set up the analysis and establish the scope and 

 water resource values that will drive the

 assessment

Step 2—Assess exposure

Step 3—Assess sensitivity

Step 4—Evaluate and categorize vulnerability

Step 5—Recommend responses

Step 6—Critique the vulnerability assessment

Typical questions to be addressed in a watershed

vulnerability assessment:

• Which places are vulnerable?

• Which places are resilient?

• Where are the potential refugia?

• Where will conflicts arise first, and worst?

• Which factors can exacerbate or ameliorate local 

vulnerability to climate change?

• What are the priorities for adaptive efforts?

• How can context-sensitive adaptations be designed?

• What needs tracking and monitoring?
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climate sciences, (2) science-management partnerships, (3) 

assessment of vulnerabilities and risks, (4) maintenance of 

ecosystem sustainability and biodiversity conservation, (5) 

integration of climate challenges with other forest disturbance agents and stressors, (6) integration of adaptation with 

greenhouse-gas mitigation, (7) all-lands and collaborative 

approaches (working with whole ecosystems and across 

jurisdictional borders), (8) recognition of short- and longterm planning perspectives, (9) setting priorities, and (10) 

monitoring and adaptive management.

Adaptation strategies have been advanced unevenly by 

federal agencies at their regional levels and across local units 

(e.g., national forests and national parks). Implementation of 

these strategies is assisted by the presence of local motivated 

leaders, the support and flexibility provided by regional 

directors and supervisors, and the understanding and concurrence of constituencies. Some units have worked with local 

scientists to analyze regional climate projections, develop 

ecosystem vulnerability assessments, and develop intellectual capacity through staff and constituency education. Collaborative partnerships that extend across ownerships and 

jurisdictions have been developed as a foundation for some 

adaptation projects and an aid to communication across 

ownerships with different resource objectives. A few progressive units have implemented climate adaptation projects 

on the ground. Only a few site-specific adaptation projects, 

as described in this chapter, have been implemented across 

a range of resource issues and tiered to local and regional 

strategies. Responses of state governments have also been 

variable, with major forest-sector states in the Western and 

Northern United States taking leading adaptation strategies. 

Similar to the federal situation, concepts and frameworks for 

adaptation are sometimes available, but site-specific project 

implementation is rare. Education, vulnerability assessments, collaborative partnerships, biodiversity protection, 

and adaptive management have been key features in adaptation responses by tribes and nongovernmental organizations. 

Among the groups that actively address forest adaptation, they commonly address climate change as a metadisturbance agent with other ecosystem stressors. Frequently, 

climate adaptation is not identified as the primary reason for 

planning; rather, climate response strategies are subordinate 

to ecosystem sustainability, forest and watershed restoration, 

and biodiversity conservation. Adaptation goals are thus 

commonly met through projects that address high-priority 

management goals, such as management of fuels and fire, 

invasive species, insects and pathogens, and watershed 

condition. 

Implementation of site- and issue-specific adaptation 

plans has been uneven and often superficial across the forest 

sector, and there appears to be a tendency to rely on quantitative climate- and ecosystem-response models without corroboration to local ecosystems (Millar et al. 2007). A subtle 

danger in using complex, downscaled, spatially rendered 

models is that users (e.g., forest managers and planners) may 

accept model output as the single and likely future, rather 

than one among many possible outcomes. Models are better 

used to understand processes and cautiously project future 

climates and ecosystem responses on specific landscapes 

and definitive timescales, allowing adaptation treatments to 

be developed for those outcomes. Better understanding by 

practitioners of how models are built, and what they can and 

cannot do, would improve effective application of model 

output to adaptation.

Existing Constraints

Various organizations have made progress on adaptation in 

forest lands, but implementation has been slow, integration 

across the various sectors (e.g., multiple use, protected area, 

forest industry) unbalanced, awareness generally low, and 

site-specific projects few. Numerous barriers appear to impede development and implementation of plans that would 

promote widespread readiness for American forests to adapt 

to climate change. 

Education, awareness, and empowerment—

Many natural resource science curricula now include 

courses on climate science, ecosystem responses to climate 

change, and implications for resource management. However, education on historical climatology is rare. Without a 

clear understanding of mechanisms of climatic dynamics, 
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use of concepts like “100-year floods” or “restoration to historic conditions,” which rest on assumptions of stationary 

long-term conditions, may lead to inappropriate interpretations and management actions (Milly et al. 2008).

Lack of experience and understanding of climate science by resource managers can lead to low confidence in 

taking management action in response to climate threats; 

similar limitations through the chain of supervision and decisionmaking appear to constrain appropriate efforts. Inconsistent support for climate readiness and action extending from 

executive levels can impede regional planning, which in turn 

sets up barriers to local implementation. Even if resource 

managers are trained and competent in climate science, they 

may lack support from their superiors to implement adaptation strategies and projects.

Lack of public awareness of how climate change affects natural resources influences the level and nature of 

adaptation by public institutions. Despite widespread public 

engagement in land management over the past 30 years, 

pressure to act on climate change has not been as prominent 

as for other resource issues. On the one hand, little support 

exists for implementing projects directed to adaptation; on 

the other hand, there is often strong opposition to projects 

that address indirect effects of climate, such as forest thinning, postfire logging, herbicide treatments to encourage regeneration, and road improvements for watershed protection. 

Nonetheless, public pressure can result in climate issues 

being addressed in resource evaluations and plans.

In some cases, scientific expertise may be unavailable 

even when science-based strategies are recognized as essential. Scientific institutions have suffered budget reductions, 

and only some scientists have the interest and capacity to 

work in management contexts. Even experienced scientists 

may need to learn the culture, issues, expectations, and 

scientific focus of management organizations. The demand 

for scientific participation in on-the-ground adaptation will 

likely continue to exceed supply as more adaptation programs evolve.

Policy, planning, and regulatory constraints—

Both public and private lands are subject to policy, planning, 

and regulatory direction. Federal agencies are constrained 

by hierarchies of laws and internal policy and direction, 

whereas private forest landowners have greater flexibility 

to determine actions and timing on their land, but remain 

bound by local, state, and federal laws. In federal agencies, 

site-specific projects are tiered to levels of planning at higher 

levels in the organization. 

In national forests, site-specific projects tier to each 

forest’s land management plan. These plans guide resource 

management activities on a forest to ensure that sustainable 

management considers the broader landscape and values for 

various resources. The U.S. Forest Service has developed 

procedures through a new national Planning Rule (Federal Register vol. 76, no. 30; 36 CFR Part 219) to amend, 

revise, and develop land management plans for 176 units 

in the National Forest System (NFS). The Planning Rule 

gives the Forest Service the ability to complete plan revisions more quickly and reduce costs, while using current 

science, collaboration, and an all-lands approach to produce 

better outcomes for federal lands and local communities. 

The Planning Rule addresses management in the context of 

climate change and changing environmental conditions and 

stressors, requiring plans to include components that address 

maintenance and restoration of ecosystem and watershed 

health and resilience, protection of key resources (e.g., 

water, air, and soil), and protection and restoration of water 

quality and riparian areas.

All forest management agencies face the challenge of 

working at spatial and temporal scales compatible with climate change. This demands integration of goals and projects 

from small to large scales, a reality that often clashes with 

the mix of ownerships and regulations, making collaboration across multiple organizations essential. As noted above, 

progress has been made by recent collaborative efforts that 

recognized that different regulatory and policy environments 

(e.g., federal versus private) were not necessarily a barrier. Even at small scales, such as a single national forest or 

national park, traditional planning approaches dissect lands 

into discrete units. Thus, harvest units, wilderness areas, 

developed recreation zones, and endangered species reserves 

are delimited, subject to standards and guidelines developed 

for the management zone. This area-constrained approach 

is static and inflexible, incompatible with the dynamism of 

climate and climate-related changes and responses. 
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Environmental laws developed over the past four 

decades were conceived primarily with an assumption of 

climatic stationarity, and thus many lack capacity (or legal 

authorization) to accommodate dynamics of climate-related 

changes. For example, endangered species laws are often 

interpreted as indicating native species ranges as they were 

in presettlement times (e.g., before 1900). Climate changes 

since then are catalyzing range shifts that sometimes define 

new native ranges. Enforced maintenance of species in the 

prior range could prove to be counter adaptive. The National 

Forest Management Act (NFMA) of 1976 implies maintenance of the status quo based on historical conditions, 

usually defined, like above, as presettlement (19th-century) 

ranges. For example, the NFMA “diversity clause” requires 

that a similar mix of species be reforested on national forest 

lands after harvest as was present before. Because regeneration is the most effective period for changing forest trajectories, planting nontraditional mixes of similar species or 

introducing new species might be a defensible adaptation 

response (Joyce et al. 2008).

Monitoring and adaptive management—

Future climates and environmental conditions will likely

be nonanalog relative to the past. Compounding this situation is the imprint of human land use that has fragmented, 

restructured, and altered forest ecosystems over the past 

century. Forest adaptation practices must meet this challenge 

of novelty and surprise with equally innovative approaches 

informed by monitoring and adaptive management (“learn 

as you go”). However, adaptive management in public 

agencies and other institutions has had minimal success and 

been implemented slowly, owing in part to lack of funding 

commitment and lack of analyst capacity. Reorientation 

of programs, expectations, and interactions with public 

constituents will be required for monitoring and adaptive 

management to become a successful partner to adaptation.

Budget and fiscal barriers—

Significant additional funding will be needed for a full 

national response to forest climate adaptation. Education and 

training, development of science-management partnerships, 

vulnerability assessments, and development of adaptation 

strategies are critical components of the adaptation process 

and can be integrated with other aspects of management, but 

effective consideration of climate requires additional time 

and effort. Collaboration across management units and organizations, leveraging of institutional capacities, and other 

innovative solutions will be needed to address this budget 

challenge.

Vision for the Future

Vision—

Facilitating long-term sustainability of ecosystem function is 

the foundation of climate change adaptation. Just as there is 

no single approach to sustainable forestry, effective climate 

change adaptation will differ by ecosystem, management 

goals, human community, and regional climate. If adaptation is addressed in a piecemeal fashion (ecological, geographic, and social), large areas and numerous communities 

within the forest sector may suffer the consequences of poor 

preparation, slow response, and inefficiencies. The preceding sections describe principles, policies, approaches, and 

examples of addressing the challenges of climate adaptation. 

Here we offer a vision of successful adaptation across U.S. 

forests within the next 20 years: “A proactive forest sector 

makes the necessary investments to work across institutional 

and ownership boundaries to sustain ecosystem services by 

developing, sharing, and implementing effective adaptation 

approaches.” This broad vision incorporates several critical 

concepts, each embodied by its own vision.

Investment—

Sufficient investment is allocated to successfully achieve 

visions of development, sharing, and implementation. This 

includes (1) investment in basic and applied research; (2) 

support of adequate staffing to accommodate increased planning, monitoring complexity, and interaction with partners; 

and (3) concerted effort to communicate to the general 

public the dynamic nature of climate and forests. Monitoring 

and data sharing are critical to adaptation and adaptive management, and are jointly supported across multiple agencies 

and land ownerships. Climate change resource centers, instructional courses, and professional meetings are supported 

to encourage rapid communication and amplify learning in 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

adaptation management and science. Planning, decisionmaking, and contracting processes that support implementation 

of ground-level activities are adequately funded so that lands 

in need of adaptive treatments can be reached before ecosystem function is jeopardized. 

Development—

Research—Research into all aspects of forest ecosystem 

sciences continues to provide valuable insights into forest 

responses to climate change. Research into effectiveness of 

climate-adaptation strategies guides adaptive policy responses.

Assessment—Credible information is regularly produced 

and updated at scales relevant to management decisions 

that (1) assess vulnerability of ecosystem components, (2) 

incorporate a range of climate projections, (3) use multiple 

modeling approaches to project ecosystem response, and 

(4) incorporate skills and experience of scientists and land 

managers.

Learning—Active learning occurs through traditional research and other pathways: (1) formal adaptive management 

trials continually produce information to evaluate adaptation 

techniques; (2) working forests, especially national forests, 

serve as “living laboratories” with adequate support to pursue adaptive management including adaptation techniques; 

and (3) management on federal lands is sufficiently documented and monitored to identify broad landscape trends 

and efficacy of adaptation efforts.

Sharing—

Transparency—Management goals are clearly stated in 

forest planning documents, and explicit options for sustaining ecosystem function under a range of plausible future 

climates provide a preview of potential choices in meeting 

those goals.

Communication—Vulnerable ecosystems and ecosystem 

components are identified in vulnerability assessments and 

noted in management plans. Existing or conditional decisions to pursue different adaptation options are explicit in 

management plans, and associated risks to ecosystem services are addressed. This information is proactively shared 

and discussed with the general public.

Ownerships—Increased investment in local programs that 

facilitate forest stewardship assists small landowners in 

managing sustainably. Outreach to consulting foresters and 

professional associations creates an informed base of private 

landowners. Information about management activities is 

shared across boundaries of all public and private lands to 

enable the forest sector to take advantage of biological and 

management diversity across large landscapes. Collaboration 

to manage across administrative and ownership boundaries 

is commonplace.

Partnerships—Adaptation across landscapes is addressed 

by engaging in productive partnerships, spanning boundaries 

of agency, ownership, and discipline. Science-management 

partnerships provide critical information and perspective to 

members of each discipline and form strong communities of 

practice.

Implementation—

Planning—Climate change is incorporated in all planning 

activities, and on-the-ground prescriptions are adjusted to include adaptation where necessary. Planning is developed for 

explicit locations with attention to appropriate scale. Public 

land managers and forestry consultants are well versed in 

finding and interpreting climate and vegetation projections, 

and in adjusting plans to accommodate a range of plausible 

future climates. Open avenues of discussion provide the 

scientific community with feedback on the relevance and 

clarity of tools, information, and research directions. 

Monitoring—Monitoring is integrated across multiple scales 

and coordinated across institutions. Monitored indicators are 

sensitive to changes in key ecosystem components. Monitoring data and summaries are freely available. Monitoring 

data, clear thresholds, and transparent processes for interpretation of data are incorporated in processes for decisionmaking and changes in management practices (the adaptive 

management cycle).
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Flexibility—Management plans acknowledge the increased 

potential of extreme events, novel climates, and unanticipated ecosystem responses. Decisionmaking structures 

have adequate flexibility to accommodate multiple potential 

futures, and to adopt alternative goals if prior goals are no 

longer feasible in vulnerable systems. Likewise, if adaptation approaches are ineffective, they are redirected. Lessons 

are shared with the management and scientific communities 

to encourage transparency and flexibility. 

Implementation—New information and lessons are rapidly 

incorporated in management activities. Active management 

is used to promote resistance in short terms and long-term 

resilience where appropriate, and the backlog from previous 

decades of lands in need of treatment is diminishing. Some 

forests are managed to “soften the landing” as they transition 

to new species assemblages and forest structures, such that 

ecosystem processes and ecosystem services are maintained. 

Forests affected by extreme events are rapidly restored, with 

due consideration of future climatic effects on species composition and the long-term function of the recovering forest. 

Path to the Vision

The U.S. forest sector can make significant progress toward 

a vision of sustained forest ecosystem function in the face of 

climate change by doing the following: 

• Embrace education. Widespread understanding of the 

central role of climatic dynamics in ecosystem processes 

and services is fundamental; therefore, training and 

educational programs need to be deployed for resource 

professionals in agencies and for other organizations 

and the general public. Partnerships with universities 

can enhance scientific support to science-management 

partnerships for both adaptation and education. 

• Ensure accountability and infuse climate into all 

organizational efforts. The responsibility for ensuring 

that resource management plans, projects, and decisions 

are “climate smart” rests on every professional within 

agencies and other organizations. Knowledge about 

climate is not an independent staff area but a context 

through which resource issues can be evaluated. 

Implementation of this knowledge is the responsibility of 

personnel across all resource disciplines. 

• Live the all-lands approach and make collaboration 

the norm. Effective collaboration across administrative, 

political, and ownership boundaries, and across diverse 

cultural and social perspectives is difficult but necessary, 

often requiring focused effort over an extended period. 

“Early adapter” collaborations show how regulations, 

traditions, cultures, and organizational legacies can be 

navigated successfully. These collaborations need crossagency and cross-sector support to catalyze effective 

partnerships.

• Streamline planning and put projects on the ground.

Nimbleness and flexibility to implement changes are 

essential ingredients for successful adaptive responses 

to climatic challenges. Much of the current planning 

and project implementation process in public agencies 

contains bureaucratic requirements that detract from 

actual resource work. Planning processes that prioritize 

project implementation, including uncertainty, risk, 

and provisions for experimentation will have the most 

success. For resource managers, emphasizing education 

and resource projects rather than administrative tasks 

will expedite timely adaptation accomplishments. 

The challenge of climate change adaptation will require 

creativity by future generations of forest resource managers. No one agency or organization can fully meet the 

challenge, but this task is within reach of the forest sector 

if willing partners work collaboratively toward sustainable 

management grounded in knowledge of climate science and 

dynamic ecosystems.

Carbon Management

Sequestering more carbon (C) in forests and offsetting C 

emissions with use of wood for energy and products are two 

of a range of objectives in managing forests. Increasing C 

storage and C offsets across a range of C pools and emission 

sources contributes to stabilizing atmospheric concentrations 

of carbon dioxide (CO2). One time period of interest for the 

effect of an increase in C emissions on radiative forcing is 

100 years (Pachauri and Reisinger 2007). Another, although 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

less well defined, is the time period required to achieve 

“... stabilization of greenhouse gas (GHG) concentrations 

in the atmosphere at a level that would prevent dangerous 

anthropogenic interference with the climate system” (United 

Nations 1992). In addition, management activities that 

would be most desirable would contribute co-benefits, avoid 

adverse impacts, and sustainably provide needed goods, 

services, and values.

The historical and current conditions of U.S. forests, 

forest management practices, and use of forest products have 

resulted in net C additions to forests and to harvested wood 

products stocks (tables 4.6, 4.7). However, recent forest 

sector projection scenarios for the 2010 Resource Planning 

Act assessment (USDA FS 2012a) suggest that annual C 

additions could decline more rapidly and U.S. forests could 

become a net C emitter of 10s to 100s of Tg·C·yr -1 within a 

few decades. This possibility highlights the urgency in identifying the most effective C management strategies given the 

complexity of factors that drive broader trends on the forest 

C cycle, and the broad variety of goods, services, and values 

forests provide.

Understanding biophysical and social influences on the 

forest C cycle is critical to developing management strategies that can be used to effectively manage forest C stocks 

and offset C emissions with minimal risks of failure and 

adverse environmental effects (tradeoffs). With sufficient 

knowledge of social processes (e.g., landowner or wood-user 

response to incentives and markets), policies and incentives 

may be chosen to support strategies with maximum effect. 

For example, if forest C stocks are expected to decline owing to decreasing land area caused by land use change (e.g., 

exurban development), policies or incentives to avoid deforestation in those areas may be especially effective. Also, if 

forest C stocks are expected to decline owing to the effects 

of changing climate (e.g., prolonged periods of drought), 

thinning might be especially effective in those areas by 

protecting C stocks or ensuring some level of continued 

productivity. Thinning might also reduce impacts on water 

availability (mainly in arid and semiarid environments) and 

help increase forest resilience to various stressors (Jackson 

et al. 2005, Millar et al. 2007, Reinhardt et al. 2008). Protecting old-growth forests and other forests containing high 

C stocks may be more effective than strategies that would 

seek to attain C offsets associated with wood use, especially 

if those forests would recover C very slowly or would not 

recover in an altered climate. 

Sometimes, even if harvest treatment strategies are effective, there may be tradeoffs (losses) judged to be greater 

than the offset benefits, such as loss of biodiversity in 

sensitive areas. Alternately, if climate change is expected to 

increase potential productivity on a given area over a long 

period of time, increasing forest C stocks through intensive 

management and forest products, including biomass energy, 

may be especially effective. Equally important, knowing 

which strategies to avoid for specific areas will prevent 

excessive risks and tradeoffs that could make strategies 

unsustainable. No widely accepted evaluation framework 

exists to aid decisionmaking on alternative C management 

strategies designed to maximize C storage while minimizing 

risks and tradeoffs.

This section discusses (1) current details and trends 

on where forest C is stored in the United States, (2) issues 

concerning how to measure progress and effectiveness in 

averting emissions, (3) current knowledge on the effectiveness of various management strategies in reducing atmospheric GHGs, and (4) effects of incentives, regulations, and 

institutional arrangements in implementing C management 

strategies. 

Status and Trends in Forest-Related 

Carbon

Net annual C additions to forests and harvested wood 

products account for the vast majority of total annual GHG 

sequestration among all land uses in the United States (fig. 

4.8). Within forests, the two largest C components are 

aboveground biomass and soil organic C (fig. 4.9). Because 

aboveground biomass accumulates, then shifts to dead wood, 

litter, or wood products in a matter of decades, there is an 

opportunity for forest management and land use activities to 

affect aboveground biomass accumulation and its disposition 

over decadal time (i.e., management modifications can result 

in higher C accumulation and emission offsets).

The change in forest C stocks over time is determined 

by change in forest area and the change in forest C per unit 
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Carbon pool 1990 2000 2005 2009

– – Teragrams of carbon per year – – 

Forest:

Live, aboveground -98.2 -78.3 -122.1 -122.1

Live, belowground -19.3 -15.7 -24.1 -24.1

Dead wood -8.6 -3.5 -8.4 -9.1

Litter -8.8 7.5 -11.4 -11.4

Soil organic carbon -14.9 17.6 -53.8 -53.8

 Total forest -149.8 -72.4 -219.9 -220.6

Harvested wood products:

Products in use -17.7 -12.8 -12.4 1.9

Products in solid waste disposal sites -18.3 -18.0 -16.3 -16.7

Total harvested wood products: -35.9 -30.8 -28.7 -14.8

Total net flux -185.7 -103.2 -248.6 -235.4

Carbon pool 1990 2000 2005 2010

– – – – Teragrams of carbon – – – –

Forest: 

Live, aboveground 15,072 16,024 16,536 17,147

Live, belowground 2,995 3,183 3,285 3,405

Dead wood 2,960 3,031 3,060 3,105

Litter 4,791 4,845 4,862 4,919

Soil organic carbon 16,965 17,025 17,143 17,412

 Total forest 42,783 44,108 44,886 45,988

Harvested wood products:

Products in use 1,231 1,382 1,436 1,474

Products in solid waste disposal sites 628 805 890 974

Total harvested wood products: 1,859 2,187 2,325 2,449

Total carbon stock 44,643 46,296 47,211 48,437

Table 4.6—Net annual changes in carbon stocks in forest and 

harvested wood pools, 1990–2009

Table 4.7—Carbon stocks in forest and harvested wood pools, 

1990–2010

Source: USEPA 2011.

Source: USEPA 2011.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure 4.8—Contribution of land areas to net annual 

carbon sequestration, percentage by land type, 2009.

Figure 4.9—Forest carbon pools, share of carbon stored 

in 2009.
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area (C density). Since the 1950s, timberland area nationwide has been stable (fig. 4.10) while the C per unit area has 

been increasing (i.e., increasing C density). In recent years, 

the annual increase per unit area has been increasing. The 

slow accumulation of forests is primarily the result of largescale reforestation of the United States since the early 1900s. 

The increasing rate of annual sequestration is a result of 

gross growth per year continuing to increase, while mortality has increased slowly and harvest removals have stablized 

(fig. 4.10). Although there are national trends of stable forest 

area and increasing annual additions of C to forests, it is 

likely that there are local areas where mortality plus harvest 

exceeds growth. 

Aboveground biomass C stocks are largely found in 

the Pacific coastal region, Appalachian Mountains, Rocky 

Mountains, Lake States, and central hardwoods (fig. 4.11). 

Despite the gradual net increase in forest land area and 

increased C stocks per unit area, there can be higher variation in net annual C sequestration at smaller spatial scales. 

A forest can easily become a net emitter of C on account of 

local disturbances such as wildfire. For most counties, it is 

estimated that C stocks have been increasing in recent years 

(fig. 4.12), although uncertainty in annual net sequestration 

estimates increase greatly as the scale decreases. Given the 

low density of forest plots that are remeasured each year, 

estimates of interannual variation in forest C stocks for a 

local area may only be detectable after major changes such 

as those occurring after large disturbance events (e.g., large 

wildfire). 

Monitoring and Evaluating Effects of 

Carbon Management

Figure 4.13 shows C storage and emission processes that 

can be affected by management of C in forests and wood 

products. Carbon changes are evaluated by tracking C flows 

across the system boundaries over time. The boundary 

around the “forest sector” includes forest, wood products, 

and wood energy processes. The system boundary includes a 

defined forest area. A system can be defined to include only 

C fluxes to and from forests or wood products, or it may 

include C fluxes from equipment used to manage forests and 

make and transport wood products, nonwood products, and 

fossil fuel feedstocks. The effectiveness of C management 

activities for mitigating GHG emissions is based on forest 

removal (and retention) of CO2 from the atmosphere. 

Forest management can also affect GHG emissions beyond the “forest sector.” System boundaries can be expanded 

to include processes to make energy from fossil fuels where 

wood energy can substitute, or to include GHG emitting processes to make nonwood products where wood products can 

substitute. System boundaries can also be expanded beyond 

the defined forest area to nonforest areas where actions may 

cause indirect land use change and associated GHG emissions. System boundaries also include a definition of the 

time period over which C storage or emissions are evaluated. The choice of system boundaries affects the overall 

assessment, and defining an objective to alter C management 

strategy, store C, or alter emissions cannot be done without 

clearly defining boundaries, processes, and time period. Currently, no standard approach exists for doing this to evaluate 

forest biomass as a replacement for fossil fuels. 

Evaluation of C management strategies associated with 

forests requires (at a minimum) (1) monitoring C stock 

changes and emissions over time, and (2) evaluating the effects of altered activities that affect in-forest C (in situ) and 

associated C storage or emissions outside forests (ex situ).

The first accounting framework (type A) determines how C 

fluxes in terrestrial systems and harvested wood products 

have actually changed for a current or past period because 

of management actions and other factors such as natural 

disturbances. The second accounting framework (type B) 

determines the degree to which a change in management 

under various mitigation strategies could increase C storage 

and decrease emissions. 

This accounting compares mitigation activities to a 

baseline to determine the magnitude of additional C offsets 

compared to the baseline. A baseline is the level of C stock, 

C stock change, level of emissions or emissions change as 

the result of a given set of land conditions and activities 

(e.g., forest management, timber harvest, and disturbances) 

and off-land activities (e.g., substitution for fossil emissions, 

as defined by the accounting system and boundaries at a 

point in time or over a period of time). A baseline can be 

defined by a past set of conditions or an envisioned future set 
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Figure 4.10—Growing stock carbon change owing to growth, mortality, and removals, along with timberland area, 1953–2007.

Figure 4.11—Aboveground live biomass in forests.
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Figure 4.12—Aboveground live forest carbon (C) change.

Figure 4.13—Forest sector and nonforest sector greenhouse gas emissions and stock changes that are influenced by 

forest management.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

of conditions. The effectiveness of a new strategy, such as 

an incentive to increase wood use for energy, is determined 

by changes in landowner behavior. For example, with high 

energy use (high price) some landowners may convert nonforest land to wood plantations and accumulate more C as 

well as gain benefit from substituting wood for fossil fuel. In 

addition, an increase in wood prices could cause pulpwood 

to be used for energy and decrease oriented strandboard 

panel production and resultant C storage in panels. 

A specific accounting framework for evaluating C management must include, explicitly or implicitly, a specification of the type of accounting framework (A or B) and of the 

system boundaries for the processes included (e.g., forest 

sector, service sector, nonforest land use, specific forest area, 

time period, wood C only, and other GHG emissions from 

processes).

A “common” type A accounting framework for monitoring is to define system boundaries to include current annual 

C exchange with the atmosphere from forest ecosystems at 

a given geographic scale, plus C additions and emissions for 

harvested wood products from those forests (fig. 4.13). This 

framework can be used to answer this management question for a given forest area: “Are forests and forest products 

continuing to (collectively) withdraw and store C from the 

atmosphere?” The framework is also the basis for reporting 

GHG emissions and sinks in many accounting systems such 

as that used in annual reports to the United Nations Framework Convention on Climate Change.

This framework is not intended for evaluating the full 

effects on atmospheric CO2

 of a change in strategy, which 

would require a system boundary that includes changes in 

nonwood C emissions and C emissions or storage outside 

the forest. Some excluded changes may include altered fossil fuel use, other land use emissions, and altered nonwood 

product emissions (fig. 4.13). Evaluating strategy changes 

requires a framework that includes all processes that significantly change atmospheric CO2

. If changes in emissions 

occur over many years, the framework must evaluate CO2

fluxes over many years. For example, a strategy to increase 

use of wood for heat, electric power, or biofuels via incentives at a national level would change CO2

 flux estimates 

compared to a given baseline over an extended time from 

(1) wood for energy, (2) fossil fuels for energy, (3) land use 

change (crops to plantation, or forest to intensive plantation), 

and (4) flux from forests where wood is removed (including 

regrowth after removal). The accounting system needs to 

include all processes noted in fig 4.13.

“Leakage” is a term used to recognize certain C effects 

when evaluating the effects of a policy or management 

change by using a type B accounting framework. Leakage is 

the C effects of a program change that are outside the system 

boundaries defined by a limited set of processes, (e,g., C 

changes for a specific forest area). Leakage, which includes 

C changes on land outside of a system boundary (e.g., 

caused by changes in harvest or land use) (Gan and McCarl 

2007, Murray et al. 2004, Pachauri and Reisinger 2007, 

Schwarze et al. 2002, Sohngen et al. 1999), differs depending on the mitigation activity and can be quite high (Gan 

and McCarl 2007, Murray et al. 2004). In the United States, 

leakage estimates associated with activities on a given land 

area range from less than 10 percent to greater than 90 percent (proportion of C benefit lost), depending on the activity 

and region (Murray et al. 2004). Globally, leakage estimates 

range between 42 and 95 percent (Gan and McCarl 2007). 

Leakage tends to be highest where programs constrain 

the supply of forest products (e.g., no harvest is allowed) 

or constrain land use change (e.g., forest land conversion to 

agriculture) (Aukland et al. 2003; Depro et al. 2008; Murray et al. 2004; Sohngen et al. 1999, 2008; Sohngen and 

Brown 2008). In contrast, the indirect effects of a program 

can increase C benefits outside of a system boundary, a 

phenomenon termed “spillover” (Magnani et al. 2009). For 

example, spillover can occur if an increase in plantation 

forestry reduces C losses from established forests by increasing C flows in cheaper forest products (Magnani et al. 2009). 

Defining system boundaries to include indirect effects on C 

(e.g., multinational programs) or otherwise accounting for 

leakage ensures program integrity. 

Carbon storage strategies may be ineffective because of 

flaws in incentive structures or policies, and not caused by 

the biophysical attributes of the strategy itself. For example, 

an incentive program might favor harvesting large trees 

that produce lumber, assuming that lumber would replace 
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building materials that emit more C in manufacturing. If this 

incentive strategy were implemented, the lumber could go to 

nonbuilding uses, or an increase in harvest by one landowner 

could be offset by a decrease by another. This is a flaw of the 

incentive system, not of the underlying wood substitution 

strategy. If there were incentives for builders to use wood in 

structures rather than alternate materials, the strategy could 

be effective in reducing overall emissions from manufacturing; however, the effectiveness depends on the assumed 

changes in forest management.

As described above, the focus of evaluating C management strategies was on understanding how altered management influenced C on a given land area. It is also possible to 

evaluate strategies by focusing on the change in C storage 

or emissions associated with producing one unit of wood 

energy or one unit of wood product, by using life cycle 

assessment (LCA). An “attributional LCA” is similar to a 

type A accounting framework and includes specification of 

processes that include forest growth, harvest, manufacturing, 

end use, disposal, and reuse, with the objective of estimating storage and emissions over the life cycle of one unit of 

product. Attributional LCAs are used to monitor inputs and 

emissions associated with production and do not include all 

process that would be affected by a change in production or 

in processes. A “consequential LCA” also specifies a unit 

of product and system boundary, but is similar to the type B 

framework noted above because the objective is to estimate 

the change in emissions associated with a one-unit change in 

product production or some change in processes over the life 

cycle. Consequential LCAs are typically used to analyze the 

potential response of a change to a system, such as a change 

in policy, and can include the effect of changing demand 

levels for products on production and emissions from other 

products across many sectors.

Different C management strategies are often evaluated 

by using different system boundaries, accounting frameworks, models, assumptions, functional units (land area vs. 

product units), and assumed incentives. Therefore, it can be 

difficult to compare the effectiveness of different strategies. 

However, it is possible to describe the effects of strategies 

on changing particular processes, uncertainty in attaining the 

effects, and timing of the effects. 

Carbon Mitigation Strategies

Carbon mitigation through forest management focuses on 

(1) land use change to increase forest area (afforestation) or 

to avoid deforestation, or both; (2) C management in existing forests; and (3) use of wood as biomass energy, in place 

of fossil fuel or in wood products for C storage and in place 

of other building materials. Estimates of the amount of the 

Nation’s CO2 emissions offset by forests and forest products 

(using the type A framework) vary with assumptions and

accounting methods (e.g., from 10 to 20 percent) (McKinley 

et al. 2011), with 13 percent (about 221 Tg·C·yr -1) being 

the most recent estimate for the United States (USEPA 

2011). The first two strategies aim to maintain or increase 

forest C stocks (using the type B framework with a boundary around forest area and other land capable of growing 

forests). The last strategy focuses on increasing C storage or 

reducing fossil fuel emissions, including C fluxes associated 

with forests and products removed from the forest (using 

the type B framework with a boundary around the forest 

sector, services, and nonforest land processes [fig. 4.13]. The 

mitigation potential of these strategies differs in timing and 

magnitude (table 4.8). 

Land use change: afforestation, avoiding deforestation, 

and urban forestry–

Afforestation—In the United States, estimates of the potential for afforestation (active establishment or planting of forests) to sequester C vary from 1 to 225 Tg·C·yr -1 for 2010 to 

2110 (U.S. Climate Change Science Program 2007, USEPA 

2005). Afforestation can be done on land that has not been 

forested for some time (usually more than 20 years), such as 

some agricultural lands, or on lands than have not historically supported forests, such as grasslands. Reforestation refers 

to establishing forests on land that has been in nonforest use 

for less than the specified time period. Mitigation potentials, 

cobenefits, and environmental tradeoffs depend on where 

afforestation and reforestation efforts are implemented 

(table 4.8).

The mitigation potential of afforestation and reforestation on former forest land is significant and generally has the 

greatest cobenefits, lowest risk, and fewest tradeoffs. Forest 

regrowth on abandoned cropland comprises about half of the 
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Table 4.8—Mitigation strategies, timing of impacts, uncertainty in attaining carbon (C) effects, cobenefits and tradeoffsa

Mitigation strategy

Timing of maximium impact

Uncertainty about strategy (biophysical risks)

Uncertainty about strategy (structural risks)b Cobenefits Tradeoffs

Land use change:

Afforestation (on former 

forest land)

Delayed Low Leakage Erosion control; improved water quality; 

increased biodiversity and wildlife habitat

Lost revenue from agriculture 

Afforestation (on 

nonforest land)

Delayed Moderate Leakage Biodiversity Erosion; lower streamflow; decreased biodiversity 

and wildlife habitat; increased nitrous oxide emissions; competition for agricultural water; local warming from lower albedo

Avoided deforestation Immediate Low Leakage Watershed protection; maintain biodiversity 

and wildlife habitat, some recreational activities

Lost economic opportunities affecting farmers or 

developer directly

Urban forestry Delayed High Reduced energy use for cooling; increased 

wildlife habitat; possible recreational opportunities

High maintenance might be required in terms of 

water, energy, and nutrients; possible damage to infrastructure 

In situ forest C management:

Decreasing C outputs Immediate Moderate Leakage Increased old-growth seral stage; increased 

structural and species diversity, and wildlife habitat; effects on benefit depend on 

landscape condition

Displaced economic opportunities affecting forest 

owners, forest industry, and employees

Increasing forest growth Delayed Low Leakage Higher wood production, potential for 

quicker adaptation to climate change

Lower streamflow, loss of biodiversity, release of 

nitrous oxide, greater impact of disturbance on C storage

Fuel treatments Delayed High Lower risk from fire and insects; increased 

economic activity; possible additional offsets from use of wood; climate change 

adaptation tool

Lost economic opportunities to firefighting business 

and employees; lower C on site; site damage caused by treatment

Ex situ forest C management:

Product substitution Part immediate, 

part delayed

Moderate Leakage Increased economic activity in forest product 

industries

Active forest management on larger area; low C 

storage in forests

Biomass energy Immediate 

to delayed depending on source

Moderate/high Leakage Increased economic activity in forest product 

industries; could reduce cost of forest restoration efforts

Intensive management on large area; lower C storage 

in forests

a Uncertainty as defined here is the extent to which an outcome is not known. All mitigation strategies have a risk of leakage and reversal, which could compromise C benefits. Timing of maximum impact 

is adapted from IPCC (2007) and uncertainty, cobenefits, and tradeoffs from McKinley et al. (2011).

b The potential degree of leakage or other structural risk for a strategy depends on the incentives, regulations, or policy used to implement it. For example, if an incentive program to increase forest growth 

is in only one region, then growth may be decreased in other regions. If the incentive is nationwide, there is little leakage within the United States, but may be leakage to other countries. Other structural risks can come from improper selection of locations to implement the strategy. For example, fire hazard reduction treatments could be done on land areas where fire-risk offsets are insufficiently long lasting or emission mitigating in comparison to expected avoided emissions from fire. There can also be risk in selecting wood for fuel (e.g., from older forests) where C recovery will be very slow. 
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U.S. C sink (Pacala et al. 2001). One study estimated that 

sequestering the equivalent of 10 percent of U.S. fossil fuel 

emissions (160 Tg of C) would require that 44 million ha, or 

one-third of U.S. croplands, be converted to tree plantations 

(Jackson and Schlesinger 2004). Another report estimates 

that 262 000 to 1 133 000 ha are needed to sequester 1 Tg 

of C annually (USEPA 2005). Given potential global food 

shortages and high value of many crops, forest establishment 

on productive croplands is not likely tenable and may cause 

project leakage (Murray et al. 2004). However, establishing 

forest plantations on marginal agricultural land or abandoned 

agricultural land is more feasible, because potential interference with food production is lower. Where climatic and soil 

conditions favor forest growth (over crops), irrigation and 

fertilization inputs would be low relative to gains in C storage. Cobenefits may include erosion control, improved water 

quality, higher species diversity, and wildlife habitat. The 

cobenefits of afforestation are enhanced where native species 

comprise a substantial proportion of the regenerated forest. 

Monocultures of nonnative or native improved-growth species may yield high C storage rates and have a low risk for 

unintended results, but may also provide fewer cobenefits.

Afforestation on lands that do not naturally support 

forests may require more human intervention and environmental tradeoffs. Carbon storage in tree and shrub encroachment into grasslands, rangelands, and savannas is estimated 

to be 120 Tg·C·yr -1, a C sink that could be equivalent to 

more than half of what existing U.S. forests sequester annually, although this estimate is highly uncertain (U.S. Climate 

Change Science Program 2007). This C sequestration shows 

the potential (unintentional) effects of land use change and 

other human activities (Van Auken 2000). Planting trees 

where they were not present historically can sometimes alter 

species diversity, lower the water table, cause soil erosion 

on hill slopes, and absorb more solar energy compared 

with the native ecosystem (Farley et al. 2008, Jackson et al. 

2008, Jobbagy and Jackson 2004, McKinley and Blair 2008, 

Schwaiger and Bird 2010). Irrigation and fertilization would 

likely be needed in many areas, particularly in arid and semiarid regions, which might compete with agricultural water 

supply and other uses. Afforestation also has the potential to 

reduce streamflow because some species of trees use more 

water than grasses or crops (Farley et al. 2005, Jackson et al. 

2005). Use of nitrogen (N) fertilizers may increase nitrous 

oxide emissions, a GHG with roughly 300 times more global 

warming potential than CO2. This type of afforestation has 

more risks compared with afforestation on lands that naturally support forests.

Avoiding deforestation—Avoiding the loss of forested 

land can prevent a significant loss of C to the atmosphere. 

Currently, global deforestation results in the gross annual 

loss of nearly 90 000 km2, or 0.2 percent of all forests (FAO 

2007, Pachauri and Reisinger 2007), which is estimated to 

release 1400 to 2000 Tg·C·yr -1, with about two-thirds of the 

deforestation occurring in tropical forests in South America, 

Africa, and Southeast Asia (Houghton 2005, Pachauri and 

Reisinger 2007). Over a recent 150-year period, global land 

use change released 156 000 Tg of C to the atmosphere, 

mostly from deforestation (Houghton 2005). In contrast, 

forested area in the United States increased at a net rate 

of about 340 000 ha·yr -1 in a recent 5-year period (2002 

to 2007). Increases in forested area and forest regrowth 

are largely responsible for the current U.S. forest C sink of 

211 Tg·C·yr -1 (USEPA 2011). However, these dynamics 

will change, with future land use expected to decrease total 

forested area by more than 9 million ha by 2050 (Alig et al. 

2003). Development and conversion of forest to pasture or 

agricultural land are responsible for much of the current and 

expected loss of forests. In addition, increased area burned 

by fire may result in the conversion of some forests to shrublands and meadows (Westerling et al. 2011), or a permanent reduction in C stocks on existing forests if fire-return 

intervals are reduced (Balshi et al. 2009, Harden et al. 2000). 

Potential C mitigation estimates through avoided deforestation are not available for the United States. 

Avoided deforestation protects existing forest C stocks 

and has many cobenefits and low risk (table 4.8). Cobenefits 

include maintaining ecosystem properties and processes, 

such as watersheds, biodiversity, wildlife habitat, and some 

recreational activities (McKinley et al. 2011). Risks include 

incentives to avoid deforestation in one area that may increase removal of forest in other areas, with little net lowering of atmospheric CO2

. Avoided deforestation may decrease 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

economic opportunities for timber, agriculture, pasture, or 

urban development (Meyfroidt et al. 2010). Leakage can be 

large for avoided deforestation, particularly if harvest is not 

allowed (Murray et al. 2004). Regenerating forests after severe wildfires may be important for avoiding conversion of 

forest to meadow or shrubland (Donato et al. 2009, Keyser 

et al. 2008). 

Urban forestry—Urban forestry, the planting and management of trees in and around human settlements, offers limited potential to store additional C, but urban trees provide 

some indirect ways to reduce fossil fuel emissions and have 

many cobenefits. Although U.S. urban C stocks are surprisingly large (Churkina et al. 2010), the potential for urban 

forestry to help offset GHG emissions is limited for two 

reasons: (1) urban areas make up only a small fraction of 

the U.S. landscape (3.5 percent) (Nowak and Crane 2002), 

and (2) urban trees generally require intensive management. 

Urban forests have important indirect effects on climate by 

cooling with shading and transpiration, potentially reducing fossil fuel emissions associated with air conditioning 

(Akbari 2002). When urban forests are planted over very 

large regions, the climate effects are less certain, because 

trees have both warming effects (low albedo) and cooling 

effects, and may result in complex patterns of convection 

that can alter air circulation and cloud formation (Jackson et 

al. 2008). However, urban trees can have high mortality rates 

in all regions (Nowak et al. 2004), and they require ongoing 

maintenance, particularly in cities that are in arid regions; 

risks increase when irrigation, fertilization, and other forms 

of maintenance are necessary (Pataki et al. 2006).

In Situ Forest Carbon Management

Carbon mitigation through forest management focuses on 

efforts to increase forest C stock by either decreasing C 

outputs in the form of harvest and disturbance, or increasing 

C inputs through active management. Potential C mitigation 

for a combined effort including increased harvest intervals, 

increased growth, and preserved establishment could remove 

105 Tg·C·yr -1, although achieving these results would 

require large land areas. It is estimated that between 479 000 

and 707 000 ha of manageable forest land is needed to store 

1 Tg·C·yr -1 (USEPA 2005). 

Increasing forest carbon by decreasing harvest and 

protecting large carbon stocks—

Forest management can increase the average forest C stock 

by increasing the interval between harvests or decreasing 

harvest intensity (Balboa-Murias et al. 2006, Harmon and 

Marks 2002, Harmon et al. 2009, Jiang et al. 2002, Kaipainen et al. 2004, Liski et al. 2001, Schroeder 1992, Seely 

et al. 2002, Thornley and Cannell 2000). Increasing harvest 

intervals would have the biggest effect on forests harvested 

at ages before peak rates of growth begin to decline (culmination of mean annual increment [CMAI]), such as some 

Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) forests 

in the northwestern United States. Increasing rotation age for 

forests with low CMAI, such as southern pine species that 

are already harvested near CMAI, would yield a decreasing 

benefit per year of extended rotation. 

Harvesting forests with high biomass and planting a 

new forest reduces overall C stocks more in the near term 

than if the forest were retained, even counting the C storage 

in harvested wood products (Harmon et al. 1996, 2009). For 

example, some old-growth forests in Oregon store as much 

as 0.0011 Tg·C·ha-1 (Smithwick et al. 2002), which would 

require centuries to regain if these stocks were liquidated 

and replaced, even with fast-growing trees (McKinley et al. 

2011). Low intensity or partial harvests, including leaving 

dead wood on site, maintain higher C stocks compared to 

clearcuts (Harmon et al. 2009), while possibly reducing the 

risk of disturbance, such as fire and damaging storms, and 

concurrently allowing forests to be used for wood products 

or biomass energy. However, although thinning increases 

the size and vigor of residual trees, it generally reduces 

net C storage rates and C storage at the stand scale (Dore 

et al. 2010, Schonau and Coetzee 1989). Studies evaluating the harvest effects on soil C provide mixed conclusions 

(Johnson and Curtis 2001, Nave et al. 2010). Decreasing 

removal of C from forests through longer harvest intervals 

or less intense harvests increases forest C stocks. Benefits 

of decreased outputs include an increase in structural and 
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species diversity (table 4.8). Risks include C loss owing to 

disturbance and reduced substitution of wood for materials 

that emit more C in manufacturing. 

Managing forest carbon with fuel treatments—

Since 1990, CO2 emissions from wildland forest fires in the 

conterminous United States have averaged 67 Tg·C·yr -1

(USEPA 2009a, 2010). The possibility that fuel treatments, 

although reducing onsite C stocks, may contribute to mitigation by providing a source for biomass energy and avoiding 

future wildfire emissions, is attractive, especially because 

fuel treatments may play an important role in climate change 

adaptation. Fuel treatments have other important benefits, including their potential to protect property and restore forest 

conditions more resilient to periodic wildfire. It is unlikely 

that fuel treatments would be implemented solely to manage 

C stocks.

Fuel treatments are a widespread forest management 

practice in the Western United States. (Battaglia et al. 2010), 

and are designed to alter fuel conditions to reduce wildfire 

intensity, crown fires, tree mortality, and suppression difficulty (Reinhardt et al. 2008, Scott and Reinhardt 2001). 

Fuel treatment to reduce crown fire hazard can be done by 

reducing surface fuels, ladder fuels (small trees), and canopy 

fuels (Peterson et al. 2005). All of these remove C from the 

site, whether through harvest or prescribed fire (Reinhardt et 

al. 2010, Stephens et al. 2009), and alter subsequent forest C 

dynamics by modifying the residual stand.

Crown fires often result in near-total tree mortality, 

whereas many trees can survive surface fires. This contrast 

in survival has led to the notion that fuel treatments may 

offer a C benefit by removing some C from the forest to 

protect the remaining C (Dore et al. 2010, Finkral and Evans 

2008, Hurteau et al. 2008, Mitchell et al. 2009, Stephens et 

al. 2009). Thinned stands that burn in a surface fire typically have much higher tree survival and lower C losses 

than similar, unthinned stands that burn in a crown fire (e.g., 

Finkral and Evans 2008, Hurteau and North 2009, Hurteau 

et al. 2008, Stephens et al. 2009), although the net effect of 

fuel treatment C removal and surface fire emissions may 

exceed that from crown fire alone, even when materials from 

fuel treatments are used for wood products (Reinhardt et al. 

2010). Because fuel treatment benefits are transient, they 

may lapse before a wildfire occurs, in which case the C removed by the fuel treatment is not offset by reduced wildfire 

emissions. 

Modeling studies suggest that fuel treatments in most 

landscapes will result in a net decrease in landscape C over 

time (Ager et al. 2010, Harmon et al. 2009, Mitchell et al. 

2009), because the savings in wildfire emissions is gained 

only on the small fraction of the landscape where fire occurs 

each year. For treatments to yield a substantial C benefit, the 

following conditions would be required: (1) relatively light 

C removal would substantially reduce emissions, (2) fire 

occurrence is high in the near term (while fuel treatments 

are still effective), and (3) thinnings can provide wood for 

energy or long-lived products that yield substitution benefits. 

If fuel treatments are implemented, it is advantageous from 

a C management standpoint to use removed fuels for energy 

production or wood products, rather than burning them 

onsite (Coleman et al. 2010, Jones et al. 2010). Feasibility 

and energy implications depend in part on hauling distance 

(Jones et al. 2010). An intriguing alternative to hauling 

bulky biomass to conversion facilities is in situ pyrolysis 

to produce energy-dense liquid fuel and biochar which can 

remain onsite to enhance soil productivity and sequester C 

(Coleman et al. 2010). 

Increasing forest carbon stocks by increasing forest 

growth—

Increasing growth rates in existing or new forests could 

increase C storage on the landscape and increase the supply 

of forest products or biomass energy. Practices that increase 

forest growth include fertilization, irrigation, use of fastgrowing planting stock, and control of weeds, pathogens, 

and insects (Albaugh et al. 1998, 2003, 2004; Allen 2008; 

Amishev and Fox 2006; Borders et al. 2004; Nilsson and 

Allen 2003). The potential associated with increasing forest 

growth differs by site and depends on the specific climate, 

soil, tree species, and management. 

Increased yields from these practices can be impressive. 

In pine forests in the Southern United States, tree breeding has improved wood growth by 10 to 30 percent (Fox 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

et al. 2007b), and has increased insect and stress resistance 

(McKeand et al. 2006.) In this region, pine plantations using improved seedlings, control of competing vegetation, 

and fertilization grow wood four times faster than naturally 

regenerated second-growth pine forests without competition control (Carter and Foster 2006). Tree breeding and 

intensive management could also provide an opportunity to 

plant species and genotypes that are better adapted to future 

climates. 

Many U.S. forests are N limited and would likely 

respond to fertilization (Reich et al. 1997). Nitrogen and 

phosphorus fertilizers have been used in about 6.5 million ha 

of managed forests in the southeast to increase wood production (Albaugh et al. 2007, Fox et al. 2007a, Liski et al. 2001, 

Seely et al. 2002). Fertilization can produce 100 percent 

gains for wood growth (Albaugh et al. 1998, 2004), although 

the benefits of fertilization for growth and C increase would 

need to be balanced by the high emissions associated with 

fertilizer production and potential emissions from eutrophication in aquatic systems (Carpenter et al. 1998) (table 4.8). 

Other risks include reduced water yield (faster growth uses 

more water), which is more pronounced in arid and semiarid 

forests, and a loss of biodiversity if faster growth is done by 

replacing multispecies forests with monocultures (limited 

diversity can make some forests vulnerable to insects and 

pathogens). In some areas, increasing the genetic and species 

diversity of trees and increasing C stocks could be compatible goals (Woodall et al. 2011). 

Markets for current or new forest products can provide 

revenue to invest in growth-enhancing forest management. 

For example, expectation of revenue from the eventual 

sale of high-value timber products would support investment in treatments or tree planting to increase growth rate. 

Taxation or other government incentives may also support 

growth-enhancing management. To the extent that incentives 

to alter growth also alter timber harvest and wood product 

use, evaluation will require type B accounting with system 

boundaries that include forest sector, services sector, and 

possibly nonforest land. 

Ex Situ Forest Carbon Management 

Carbon is removed from the forest for a variety of uses, and 

those uses can have different effects on C balances. Depending on the forest product stream, C can be stored in wood 

products for a variable length of time, oxidized to produce 

heat or electrical energy, or converted to liquid transportation 

fuels and chemicals that would otherwise come from fossil 

fuels (fig. 4.14). In addition, there can be a substitution effect 

when wood products are used in place of other products that 

emit more GHG in manufacturing (Lippke et al. 2011). 

Strategies that would add to storage in long-lived 

wood products, increase use life, and increase use of wood 

products in place of higher emitting alternate products can 

complement strategies aimed at increasing forest C stocks. 

Risk and uncertainty in attaining benefits need to be considered when comparing strategies for increasing forest C with 

strategies for attaining wood product C offsets. Strategies 

need to ensure energy offsets are attained in an acceptable 

period of time and that substitution effects are attained. 

Carbon in forest products—

Wood and paper continue to store C when in use and also in 

landfills (fig. 4.14). Rates of net C accumulation depend on 

rates of additions, disposal, combustion, and landfill decay. 

The half-life for single-family homes made of wood built 

after 1920 is about 80 years (Skog 2008, USEPA 2008), 

whereas the half-life of paper and paperboard products is 

less than 3 years (Skog 2008). About two-thirds of discarded 

wood and one-third of discarded paper go into landfills 

(Skog 2008). Decay in landfills is typically anaerobic and 

very slow (Barlaz 1998), and 77 percent of the C in solid 

wood products and 44 percent in paper products remain in 

landfills for decades (Chen et al. 2008, Skog 2008). However, current rates of methane release and capture can eliminate 

this C storage benefit for certain low lignin paper products 

(Skog 2008). About 2,500 Tg of C was accumulated in wood 

products and landfills in the United States from 1910 to 2005 

(Skog 2008), with about 700 Tg of C (in 2001) in singleand multifamily homes (Skog 2008). In 2007, net additions 

to products in use and those in landfills combined were 27 

Tg·C·yr -1 (USEPA 2009b), with about 19 Tg·C·yr -1 from 

products in use (Skog 2008).
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Product substitution—

Net C emissions associated with production and use of forest products can be substantially less than those associated 

with steel and concrete. Use of 1 Mg of C in wood materials in construction in place of steel or concrete can result 

in 2 Mg of lower C emission (Sathre and O’Connor 2008, 

Schlamadinger and Marland 1996). Sometimes, using wood 

from faster-growing forests for substitution can be more 

effective in lowering atmospheric CO2 than storing C in the 

forest where increased wood production is sustainable (Baral 

and Guha 2004, Marland and Marland 1992, Marland et 

al. 1997) (fig. 4.14a). On the other hand, harvesting forests 

with very high C stocks that have accumulated over many 

Figure 4.14—Carbon (C) balance from two hypothetical management 

projects with different initial ecosystem C stocks and growth rates. 

Cumulative C stocks in forest, C removed from forest for use in wood 

projects (long [L]- and short-lived [S]), substitution, and biomass 

energy are shown on land that (A) has been replanted or afforested, 

or (B) has an established forest with high C stocks. The heavy black 

line represents the trajectory of forest C stocks if no harvest occurred. 

Actual C pathways vary by project. Carbon stocks for trees, litter, and 

soils are net C stocks only. The scenario is harvested in x-year intervals, which in the United States could be as short as 15 years or longer 

than 100 years. This diagram assumes that all harvested biomass will 

be used and does not account for logging emissions. Carbon is sequestered by (1) increasing the average ecosystem C stock (tree biomass) 

by afforestation, or (2) accounting for C stored in wood products in 

use and in landfills, as well as preventing the release of fossil fuel C 

through product substitution or biomass energy. The product-substitution effect is assumed to be 2:1 on average. Biomass is assumed to 

be a 1:1 substitute for fossil fuels in terms of C, but this is not likely 

for many wood-to-energy options. This scenario represents a theoretical maximum C benefit, given this composition of forest products and 

management practices. Carbon “debt” is any period of time at which 

the composition of forest products and remaining forest C stocks after 

harvest is lower than estimated C stocks under a no-harvest scenario. 

(Adapted from McKinley et al. [2011], Pachauri and Reisinger [2007], 

and Solomon et al. [2007]).

decades may result in a large deficit of biological C storage that could take many decades to more than a century to 

restore (McKinley et al. 2011) (fig. 4.14b). Opportunities for 

substitution in the United States are largely in nonresidential 

buildings (McKeever et al. 2006, Upton et al. 2008) because 

most houses are already built with wood, although opportunities to increase the substitution effect in residential buildings exist, for example, by using wood for walls in houses 

(Lippke and Edmonds 2006). Attaining the substitution 

effect requires incentives that avoid or reduce type 1 risks 

by encouraging increased use of wood (box 4.9). Incentives 

focused on landowners to harvest wood for products may not 

provide as many substitution effects because of leakage. In 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

addition, incentives would help avoid type 1 risks in which 

wood may come from forest conditions where C recovery is 

slow (fig. 4.14a) and instead comes from forest conditions 

where C recovery is fast (e.g., fig. 4.14b). 

Biomass energy—

Biomass energy could prevent the release of an estimated 

130 to 190 Tg·C·yr -1 from fossil fuels (Perlack et al. 2005, 

Zerbe 2006). Biomass energy comprises 28 percent of 

renewable energy supply and 2 percent of total energy use 

in the United States; the latter amount has the potential to 

increase to 10 percent (Zerbe 2006). Currently, wood is used 

in the form of chips, pellets, and briquettes to produce heat 

or combined heat and generation of electricity (Saracoglu 

and Gunduz 2009). These basic energy carriers can be further transformed, using advanced conversion technologies, 

into liquid transportation fuels, and gases (e.g., methane and 

hydrogen) (Bessou et al. 2011, Demirbas 2007). Conversion processes for these fuels are still largely experimental 

and require further development to improve efficiency and 

commercial viability. The GHG balances for simple energy 

carriers (e.g., wood chips and pellets) for producing heat 

and electricity are more certain than for advanced energy 

carriers. In addition, the potential exists to create high-value 

chemicals and other bioproducts from wood that would 

otherwise be made from fossil fuels, resulting in reduced 

emissions compared to use of fossil fuels (Hajny 1981, 

USDOE 2004). 

Most biomass for energy is a byproduct of conventional forest product streams, such as milling residues (Gan 

and Smith 2006a), with some use of trees killed by insects, 

disease, and natural disturbance (Peng et al. 2010, Tumuluru 

et al. 2010). However, most of these residues, mainly sawdust and bark, are already used for direct heating in milling 

operations or used for other wood products, such as particle board (Ackom et al. 2010, Mälkki and Virtanen 2003, 

Nilsson et al. 2011); obtaining higher quantities of biomass 

feedstock would require using other residues. A number of 

currently unused residues have been identified, including 

residues from logging, hazardous fuel reduction treatments, 

precommercial thinning, urban areas, insect kill, and other 

sources (Ackom et al. 2010; Gan 2007, Gan and Smith 

2006b; Mälkki and Virtanen 2003; Perlack et al. 2005, 2011; 

Repo et al. 2011; Smeets and Faaij 2007). 

If forest harvesting is expanded to meet the demand for 

biomass energy, roundwood from standing trees will increasingly be used for energy. For example, short-rotation plantations devoted to biomass feedstock production have been 

proposed (Fantozzi and Buratti 2010, Tuskan 1998). If prices 

for biomass energy increase, short-rotation forest crops 

such as poplars could become a significant feedstock source 

(Solomon et al. 2007). Carbon emissions from increased 

use of roundwood for energy may be offset over time by a 

subsequent increase in forest C. This can be done through 

increased forest growth on land where the roundwood is 

harvested. The amount and speed of the offset are influenced 

by the time period considered, forest growth rate, initial 

stand C density, and the efficiency with which wood offsets 

Box 4.9

Each strategy has risks and uncertainties in attaining 

carbon (C) impacts as well as non-C impacts—cobenefits 

and tradeoffs. In this section, we describe two general 

sources of risk that may prevent a strategy from attaining 

its C mitigating potential in terms of magnitude or timing, or both, or possibly resulting in reversal. Type 1 risk 

refers to the failure of incentives or regulations. This risk 

stems from the constructs of the policy or incentive structure, which might not have the intended effect on human 

behaviors. This might include, for example, lower than 

expected participation in markets or unintended negative 

economic distortions, such as supply-side diversions, that 

alter forest management or forest product use. 

Type 1 risks are “structural risk.” One “structural 

risk” if not accounted for is, for example, “leakage” in 

the form of shifting of harvest or land use. Type 2 risk or 

“biophysical risk” refers to failures caused by unpredictable or greater than expected biophysical events, such as 

natural disturbance (e.g., wildfire, insects). Disturbance 

can be a cause of “risk of reversal” or failure to attain 

“permanence.” Type 3 risk or “tradeoffs” is the intensification of negative non-C impacts. Uncertainty may also 

result in greater than expected mitigation. For example, 

changing climate and atmospheric chemistry (e.g., 

increasing carbon dioxide or nitrogen deposition) may 

result in faster accumulation of C in forests than expected 

for some period of time. 
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fossil fuel emissions (Schlamadinger et al. 1995). The offset 

can also be done through increased landowner investment 

in forestry. The investment can include converting nonforest 

land to forest, retaining land in forest that would otherwise 

be converted to nonforest, or planting land in faster growing pulpwood or short-rotation plantations. Forest inventory 

and C projections for the United States indicate that for 

scenarios with higher wood energy use (versus those with 

lower wood energy use) there will be more land retained in 

forest and more land in plantations for the Southern United 

States (USDA FS 2012b). The effect on forest C of retaining 

land in forest is greater than the effect of increasing plantation area. Landowner investment in revenue for biomass is 

expected to be low for most of the United States.

Reductions in GHG emissions from wood-to-energy 

pathways depend, in part, on how efficiently wood substitutes for fossil fuels. The energy value of wood (energy 

content per unit mass) is lower than for fossil fuels (Demirbas 2005, Patzek and Pimentel 2005), a difference that is 

most pronounced when wood substitutes for fossil fuels with 

high energy values (e.g., natural gas). The risk of not attaining various levels of offset from use of wood for energy 

differs, depending on whether biomass is from residues or 

from greater use of roundwood (Schlamadinger et al. 1995, 

Zanchi et al. 2010). Risks for using residues are relatively 

small, especially if forests and supply chains are well managed. Risks associated with using roundwood differ by forest 

conditions, treatments, and degree of landowner response 

by investment in more intensive forest management. Large 

increases in demand could cause loss of C if natural forest 

with high C density were converted to forest plantations or 

agricultural biomass plantations with lower C density.

Recent research has provided contrasting conclusions 

regarding the potential C mitigation benefits from using 

wood for energy. A number of studies report that using 

biomass instead of fossil fuels can significantly reduce net 

C emissions (Boman and Turnbull 1997, Cherubini et al. 

2009, Jones et al. 2010, Malmsheimer et al. 2011, Mann and 

Spath 2001, Spath and Mann 2000). Other studies report 

that the postharvest regrowth period during which forest C is 

initially low negates the benefits of wood energy (Bracmort 

2011, Cardellichio and Walker 2010, Fargione et al. 2008, 

Manomet Center for Conservation Sciences 2010, Mathews 

and Tan 2009, McKechnie et al. 2011, Melamu and von 

Blottnitz 2011, Melillo et al. 2009, Pimentel et al. 2008, 

Repo et al. 2011, Schlamadinger et al. 1995, Searchinger et 

al. 2009). Studies that used life cycle assessments (LCAs) 

with both biomass pathways and forest C dynamics over 

time calculated lower reductions in CO2 emissions than similar LCAs without forest C dynamics. For some cases and 

time periods, LCAs with biomass pathways and forest C dynamics indicate biomass emissions can be higher than fossil 

emissions (Johnson 2009, Manomet Center for Conservation 

Sciences 2010, McKechnie et al. 2011, Pimentel et al. 2008, 

Searchinger et al. 2008).

These conflicting conclusions are caused by differing 

assumptions and methods used in the LCAs (Cherubini et al. 

2009, 2012; Matthews and Tan 2009). Emerging C accounting methods are increasingly focused on the effect of emissions on the atmosphere and climate over an extended time 

period, rather than assuming C neutrality (Cherubini et al. 

2012). Continuing efforts are needed to provide evaluation 

frameworks that are adequate to evaluate the overall C and 

climate effects of specific combinations of forest management and wood energy use.

Mitigation Strategies: Markets, 

Regulations, Taxes, and Incentives

Forests currently comprise about a third of the land area 

in the United States, but fragmentation and conversion of 

forest to other land uses is increasing, especially in the East 

(Drummond and Loveland 2010). Various mechanisms exist 

at national, regional, and local scales that can enhance mitigation efforts while providing incentives to keep forests intact. National forests are not eligible for incentive programs 

or market-based payments for C sequestration or other 

ecosystem services, but markets and incentive programs can 

potentially play a role in ecosystem-enhancing mitigation on 

private and nonfederally owned land. Markets and incentive 

programs can provide a means for landowners to be financially compensated for voluntary restoration activities that 

improve ecosystem services. Some of these mechanisms, 

such as C markets, are designed to encourage mitigation, 

while other mechanisms help maintain or augment C stores 

as an ancillary benefit. 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Markets, registries, and protocols for forest-based 

carbon projects—

Carbon markets are an emissions trading mechanism and 

are typically designed to create a multisector approach that 

encourages reductions and often (but not always) enhances 

sequestration of GHG emissions (measured in megagrams 

of CO2 equivalent, or CO2e) in an economically efficient 

manner. Registries exist to track and account for the C, and 

protocols outline the specific methodologies that are a prerequisite to creating legitimate C offsets. 

The United States does not have a national-level regulatory market, but several mandatory regional efforts and 

voluntary over-the-counter markets provide limited opportunities for mitigation through forest-based C projects. Offsets 

generated from these projects can compensate for emissions 

generated elsewhere. Forest C projects generally take the 

following form:

Avoided emissions

• Avoided deforestation (or avoided conversion): projects 

that avoid emissions by keeping forests threatened with 

conversion to nonforest intact.

Enhanced sequestration

• Afforestation/reforestation: projects that reforest areas 

that are currently nonforested, but may have been 

forested historically.

• Improved forest management: projects that offer 

enhanced C mitigation through better or more sustainable 

management techniques. These projects are compatible 

with sustainable levels of timber harvest.

• Urban forestry: projects that plant trees in urban areas. 

Only sequestered C is eligible (avoided C emissions that 

result from energy savings are not eligible for credit).

The Regional Greenhouse Gas Initiative (RGGI) is a 

mandatory multistate effort in New England and the MidAtlantic that allows offset credits to be generated through 

afforestation projects within RGGI member states. The 

Climate Action Reserve is another mandatory initiative 

that is based in California but accepts forest projects from 

throughout the country. In addition, protocols created by the 

American Carbon Registry, Verified Carbon Standard provide quality assurance to domestic and international forest C 

projects that may be sold on the voluntary market (Kollmuss 

et al. 2010, Peters-Stanley et al. 2011). In 2009, 5.1 Mg of 

CO2

e, or 38 percent of the global share of forest-based C offsets, was generated in North America (Hamilton et al. 2010). 

However, factors such as substantial startup and transaction 

costs and restrictions on the long-term use and stewardship 

of forest land enrolled in C projects often serve as barriers 

to engagement for many private forest landowners in the 

United States (Diaz et al. 2009).

Tax and incentive programs—

Some states offer reduced taxes on forest land, as long as 

certain requirements are met. These tax incentives may be 

crafted to maintain a viable timber industry and achieve 

open space objectives, but have the added benefit of helping 

to maintain or enhance forest C stores. For example, private 

forest landowners enrolled in Wisconsin’s Managed Forest 

Law Program receive an 80 to 95 percent tax reduction on 

land that is at least 80 percent forested and is managed for 

the sustainable production of timber resources. Vermont’s 

Use Value Appraisal Program is similar. Carbon benefits 

from these programs must be evaluated based on specific 

circumstances; younger, rapidly growing forests have 

higher rates of C uptake, whereas older stands may have 

lower C uptake but higher overall storage (Harmon 2001, 

Malmsheimer et al. 2008). A “no harvest” unmanaged forest 

scenario may produce more or less C benefit than a sustainably managed forest, but much depends on current C stocks, 

the likelihood of disturbance, and whether and how the 

harvested timber products are used (Ingerson 2007, Nunnery 

and Keeton 2010). The timeframe of expected C benefits 

therefore depends on both forest management regimes and 

forest product pathways (long-term vs. short-term products) 

(McKinley et al. 2011).

Several federal programs administered by the USDA 

Natural Resources Conservation Service, Forest Service, and 

Farm Service Agency provide cost-share and rental payment 

incentives for good farm, forest, watershed, and wildlife 

habitat stewardship. As an ancillary benefit, these programs 
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GENERAL TECHNICAL REPORT PNW-GTR-870

may also help maintain or enhance C stores, but this is currently not an explicit goal of any of these programs. The area 

enrolled in each program fluctuates annually and depends 

on commodity prices, program funding, and authorization 

levels, as sanctioned in the Farm Bill. In 2010, 13 million ha 

of United States farmland were enrolled in the Conservation 

Reserve Program, down from 15 million ha in 2005 (Claassen et al. 2008, USDA Farm Service Agency 2010). A brief 

description of relevant programs is shown in table 4.9.

If policy favored land management that would decrease 

the buildup of atmospheric CO2, it might be possible to either fine tune existing incentive programs to more explicitly 

support C mitigation strategies, and develop an alternative 

incentive program that prioritizes C management. In the case 

of the former, the explicit objective of the program could 

remain as is (to determine general eligibility), but the financial incentives for enrollment could be related to estimated 

average C benefit per hectare, rather than being calculated 

based only on hectares enrolled. Carbon benefit per hectare 

could be estimated at a county or regional scale based on a 

combination of factors, including geographic location, land 

use, species planted, and overall landscape connectivity. 

This may help to ensure that priority lands for C management receive the highest potential benefits. Alternatively, 

a specific forest C incentive program could complement 

current incentive programs by targeting small family forest 

owners and providing financial incentives that may be sufficient to ensure that forests remain as forests. Best management practices could be made available (e.g., for artificial 

regeneration, thinning, and insect control) (table 4.10), and 

financial incentives could be based on estimated C benefits 

(Pinchot Institute for Conservation 2011). These estimated 

Program Agency Land area Purpose

Millions of hectares

Conservation Reserve 

Program and Continuous 

Conservation Reserve 

Program

Farm Service 

Agency

~13 Reduce erosion, increase wildlife habitat, 

improve water quality, and increase 

forested acres

Environmental Quality Incentives Program (EQIP)

Natural Resources 

Conservation 

Service (NRCS)

~6.9 Forest management practices including 

timber stand improvement, site preparation 

for planting, culverts, stream crossings, 

water bars, planting, prescribed burns, 

hazard reduction, fire breaks, silvopasture, 

fence, grade stabilization, plan preparation

Conservation Stewardship 

Program (CSP)

NRCS n/a Incentives for sustainable forest management 

and conservation activities

Wildlife Habitat Incentive 

Program (WHIP)

NRCS 0.26 Assistance/incentives to develop or improve 

fish and wildlife habitat, including prairie 

and savanna restoration, in-stream fish 

structures, livestock exclusion, and tree 

planting

Forest Legacy Program Forest Service 

(FS)

~0.8 Incentives to preserve privately-owned 

working forest land through conservation 

easements and fee acquisitions

Stewardship Program FS ~14 Encourages private landowners to create and 

implement stewardship plans on their land

n/a = not applicable.

Table 4.9—Programs that influence carbon mitigation
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Table 4.10—Tools and processes to inform forest management

Organization Relevant content Internet site

U.S. Forest Service Forest Inventory 

and Analysis

Forest statistics by state, including 

carbon (C) estimates

Sample plot and tree data

Forest inventory methods and basic 

definitions

http://fia.fs.fed.us

U.S. Forest Service Forest Health 

Monitoring

Forest health status

Regional data on soils, dead wood 

stocks 

Forest health monitoring methods 

http://www.fhm.fs.fed.us

U.S. Department of Agriculture 

(USDA) Greenhouse Gas Inventory

State-by-state forest C estimates http://www.usda.gov/oce/global_change/

gg_inventory.htm

United Nations Framework Convention on Climate Change and Intergovernmental Panel on Climate ChangeInternational guidance on C accounting and estimationhttp://unfccc.int

http://www.ipcc.ch

USDA Natural Resources Conservation ServiceSoil Data Mart—access to a variety 

of soil data

http://soildatamart.nrcs.usda.gov

U.S. Forest Service, Northern Research 

Station

Accounting, reporting procedures, 

and software tools for C estimation

http://www.nrs.fs.fed.us/carbon/tools

U.S. Energy Information Administration, Voluntary GHG Reporting Methods and information for calculating sequestration and emissions 

from forestry

http://www.eia.gov/oiaf/1605/gdlins.html

U.S. Environmental Protection Agency Methods and estimates for GHG 

emissions and sequestration

http://www.epa.gov/climatechange/

emissions/usinventoryreport.html

benefits would require only a statistically robust verification 

of practices rather than annual site monitoring. 

The Role of Public Lands in Mitigation

Public lands encompass large areas of forests and rangelands, about 37 percent of the land area of the United States, 

with federally managed lands occupying 76 percent of the 

total area managed by all public entities. A decision to manage these lands for C benefits would involve a complex set 

of interacting forces and multiple jurisdictions, and would 

be governed by laws mandating multiple uses of land in 

the public domain. The Council on Environmental Quality 

(CEQ) has the responsibility of overseeing environmental 

policy across the federal government. The CEQ has developed draft guidelines for all agencies describing how federal 

agencies can improve their consideration of the effects of 

GHG emissions and climate change when evaluating proposals for federal actions under NEPA (Sutley 2010). Another 

recent policy that affects all federal agencies is Executive 

Order 13,514 (2009), which requires agencies to set targets that focus on sustainability, energy efficiency, reduced 

fossil fuel use, and increased water efficiency. In addition, 

the order requires agencies to measure, report, and reduce 

GHG emissions from direct and indirect activities, including 

federal land management practices. The CEQ guidance and 

these orders are being considered by land management agencies, but it is unclear how effective they will be in reducing 

GHGs, given the many other uses of federal lands. It should 

be noted that large areas of forest land protected by conservation organizations (e.g., The Nature Conservancy) across 

the United States are being managed for public benefits but 

may not be subject to some of the regulatory issues cited 

above.
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Managing Forests in Response to Climate 

Change

Managing forests in response to climate change is just one 

component of the broad and complex task of sustainable

natural resource management. Climatic variability (year 

to year, and decade to decade differences in climate) has 

always been a factor in forest management, but now resource managers must begin to address directional trends 

in human-caused climate change in the context of increased 

variability and movement away from historical averages. 

Climate change provides a context to be considered in 

management, but it is rarely appropriate to focus on climate change exclusive of other issues that affect forest 

resources. An increasing number of potential strategies and 

forest management options are now available for addressing 

climate change. However, these strategies and options are 

rarely institutionalized. Implementing these approaches, or 

at least a thorough consideration thereof, through planning 

and management processes on public and private lands is a 

major organizational and social challenge.

If projected changes in temperature, precipitation, and 

extreme weather events are realized, management activities 

that facilitate adaptation to climate change can be realistically viewed as providing additional time until biological and 

social systems adjust to a new climate. The sooner action is 

implemented, the more options will be available to prepare 

forest systems for a new climate. Two major institutional 

shifts are needed for successful adaptation in U.S. forests. 

First, scientists and resource managers need to agree that 

static and equilibrium concepts relative to ecosystem function and management (historical range of variation, restoration of “presettlement” conditions, climax vegetation, etc.) 

will be less relevant in the future. Ecosystems that exist in 

nonanalog climates with increased disturbance, new species, and invasive species will rarely be in equilibrium with 

climate or other environmental factors, and it will not be 

possible to preserve them intact in a specific location over 

time. Second, natural resource management organizations 

will need to consider climate effects as part of normal

business operations. If ongoing management protocols and 

projects include the role of climatic variability and change, 

then accomplishment targets and on-the-ground practices 

can be adjusted as needed. This will minimize surprises and 

lead to realistic long-term planning objectives. If climate effects are not considered, rapid changes in ecosystem dynamics will challenge their ability to manage forest resources 

sustainably. 

As noted in the adaptation section above, adapting to 

climate change is a viable option for most natural resource 

management organizations if viewed as adaptive management in the context of climatic variability and change. Currently, most public and private institutions need considerable 

input from the scientific community to help interpret climate 

science and model output, and to project the effects of 

climate change on natural resources at different spatial and 

temporal scales. Successful science-management partnerships have typically required 2 to 5 years to make substantial 

progress on science-based solutions to climate challenges. 

To sustainably manage the Nation’s forests, natural resource 

management organizations will need to make climate change 

a mainstream issue (much as “ecosystem management,” 

and “ecological restoration” did previously) that can be 

addressed without continuous high-level, external scientific 

input.

Multi-institutional collaboration is required, both now 

and in the future, to apply consistent strategies and tactics 

across large landscapes. Cooperation among agencies and 

other organizations in addressing natural resource issues has 

often been challenging. However, recent efforts between the 

U.S. Forest Service and National Park Service to collaborate 

on climate change adaptation, and nascent efforts by U.S. 

Fish and Wildlife Service Landscape Conservation Cooperatives to instill an all-lands approach in conservation issues, 

including climate change, provide hope that collaboration 

will become more common. Perhaps more challenging 

are the barriers of “paralysis by analysis” within agencies, 

external litigation, and appeals, which delay timely implementation of projects that can facilitate adaptation. It will be 

difficult to break the gridlock that seems to envelop public 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

land management in some regions until engagement of 

stakeholders and consistent, open communication of climate 

science with the public, policymakers, and land managers 

becomes commonplace. Climate change is at the forefront of 

many policy and management discussions on private lands 

as well (e.g., the Southern Forest Futures Project, http://

www.srs.fs.usda.gov/futures), with similar concerns about 

the effects of climate change on forest lands and potential management options for adaptation and mitigation. In 

regions dominated by private lands such as the southeast 

and northeast, dealing with complex ownership patterns and 

a wide range of management objectives will be critical for 

successful climate-smart management across large landscapes. Given multiple management objectives and limited 

funding and staff for implementation, it will be necessary to 

optimize long-term strategies on a regional to subregional 

basis by considering where the most benefit can be gained. 

Projections of climate change effects are relatively 

certain for some components of forest ecosystems, and less 

certain for others, especially beyond the mid-21st century. 

Developing effective management options to address uncertain, dynamic, and novel conditions will require ongoing 

monitoring to identify ecosystems at risk, detect change, 

and evaluate the success or failure of management activities. Now more than ever, land managers will need detailed 

information on forest conditions to inform management 

decisions and help adapt to changing conditions. The U.S. 

Forest Service Forest Inventory and Analysis program and 

Forest Service Forest Health Monitoring network provide 

information on changes in forest growth and condition over 

most of the Nation. In addition, the Forest Service operates 

80 experimental forests and ranges that are critical assets for 

change detection, climate-change experiments, and management demonstrations. Combined with other large networks 

such as the National Ecological Observatory Network (20 

core sites to be established in representative ecoclimate 

domains), the National Science Foundation Long-Term Ecological Research Program (27 sites located across the United 

States, Puerto Rico, and Antarctica), National Weather Service weather stations, and numerous U.S. Geological Survey 

gauging stations, many variables are monitored across a 

broad geographic area. In most cases, these networks operate 

independently, and although some lack central data storage, data management protocols, and easy access, efforts 

are underway to increase data access for many core data 

sets. These monitoring networks can help detect changes in 

climate and forest condition, but they are not a substitute for 

on-the-ground monitoring that will be required to assess the 

effectiveness of specific management activities. This will 

require a larger investment by land management agencies, 

although improved efficiency and coordination can, in some 

cases, compensate for insufficient funding. 

In the near term, it is logical to pursue management 

strategies that are relatively low cost, have few barriers, and 

will produce near-term results. For adaptation, this would 

include reducing co-occurring stressors in forests (e.g., air 

pollution, exotic pathogens), implementing fuel reduction 

where feasible and effective, and reducing stand densities 

where feasible and appropriate (resistance and resilience 

strategies). For C management, this would include reducing 

deforestation, increasing afforestation, reducing wildfire severity where feasible, increasing growth, and increasing use 

of wood-based bioenergy where economically justified. 

In the long term, specific adaptation strategies will need 

to be considered in light of emerging scientific evidence on 

climate change effects and assessments of the effectiveness 

of various management actions on the ground. Resilience 

strategies in the face of increasing large-scale disturbances 

often include standard management practices (e.g., forest 

thinning). Specific strategies for C management will need 

to be guided by emerging scientific evidence on how to 

concurrently manage forests in situ for products, energy, and 

other ecosystem services. Strategies will differ by location, 

inherent forest productivity, and local management objectives. The mandate for productivity on commercial private 

lands contrasts with objectives on public lands, but both 

private and public perspectives need to be accommodated in 

order to manage C across broad spatial scales, meet multiple 

management objectives, and benefit local economies.
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It will also be important to consider how adaptation 

and mitigation can be coordinated to optimize implementation across specific landscapes. For example, fuel reduction 

treatments can reduce the prevalence of crown fires in dry 

forests, while also providing material for local bioenergy use 

(the long-term effect on C dynamics is site-specific based on 

current evidence). The interaction of adaptation and mitigation has been poorly assessed to date, and successful models 

of both strategic and tactical approaches to this interaction 

are needed. This topic may provide opportunities for 

coalitions among partners who would not normally collaborate on other natural resource issues. In the near term, 

we anticipate that federal land management agencies will 

continue to lead the development of science-management 

partnerships and collaborative approaches to adaptation and 

C management across public lands. Successful adaptation 

and C management will accelerate across larger landscapes 

if and when community-based partnerships become more 

engaged with climate change as a component of sustainable 

resource stewardship. 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

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Chapter 5

Improving Scientific Knowledge

James M. Vose and David L. Peterson1

Scientific literature on the effects of climatic variability and 

change on forest ecosystems has increased significantly 

over the past decade, providing a foundation for establishing forest-climate relationships and projecting the effects of 

continued warming on a wide range of forest resources and 

ecosystem services. In addition, certainty about the nature of 

some of these effects and understanding of risk to biosocial 

values has increased as more evidence has been accrued.

The recent expansion in scientific analysis of the effects 

of climate on ecological disturbance has provided empirical 

data on how wildfire and insects respond to warmer climatic 

periods. However, more information is needed on the interaction of ecological disturbances and other environmental 

stressors, especially for large spatial and temporal scales. 

Thresholds for climatic triggers of environmental change 

are generally poorly understood relative to fire, insects, 

interactions, and functionality of forest ecosystems. Moreover, simulation modeling can suggest how and when those 

thresholds might be exceeded, additional empirical data on 

thresholds will be more definitive, and more process-level 

research is required to improve current or the next generation of predictive models. In general, our understanding of 

stress complexes in forest ecosystems needs to be expanded 

to more ecosystems and transitioned from qualitative to 

quantitative descriptions.

Despite a century of ecological research on humanaltered landscapes, our ability to interpret ecological change 

in the context of human land use and social values is far 

from complete. We especially need to improve our ability to

1 James M. Vose is a research ecologist, U.S. Department of 

Agriculture, Forest Service, Southern Research Station, Center for 

Integrated Forest Science and Synthesis at North Carolina State 

University, Department of Forestry and Environmental Resources, 

Campus Box 8008, Raleigh, NC 27695. David L. Peterson is a 

research biological scientist, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station, Pacific Wildland 

Fire Sciences Laboratory, 400 N 34th Street, Suite 201, Seattle, WA 

98103.

quantify climate-ecosystem relationships in the context of 

land use change at larger spatial and temporal scales. Inferences about climate change effects will be more relevant 

if various land uses, including evaluation of future alternatives, are considered in a context that incorporates humans, 

rather than excluding them or considering their actions to 

be “unnatural” or negative. This leads to the broader need 

to develop a framework for quantifying ecosystem services 

that is transportable across different institutions and that will 

include a wide range of biosocial values.

Some general scientific issues need additional focus. 

First, the value and interpretation of empirical (statistical) 

models versus process (mechanistic) models warrants a rich 

discussion within the scientific community. Conceived from 

different first principles (e.g., assumed equilibrium [empirical] vs. dynamic [process] climate-species relationships), 

the output from these models often differs considerably or 

is difficult to reconcile because of different assumptions, 

spatial resolution, and hierarchical levels (e.g., species vs. 

life form) between the models. This disparity needs to be 

resolved so that resource managers can understand and apply 

model output appropriately. Second, the direct effects of 

elevated carbon dioxide (CO2) on forest ecosystems need to 

be clarified. Most existing evidence is based on experimental 

treatments on seedlings and small trees, and on simulation models that assume certain types of growth responses. 

Assuming CO2 stimulation (or not) can drive the output of 

vegetation effects models to such an extent that it greatly 

modifies simulated response to climate. A unified effort by 

scientists to resolve the significant challenges in scaling and 

interpreting data on CO2 effects is needed to provide accurate 

projections of vegetation change. Third, effects models that 

can explore multicentennial patterns of vegetation distribution, disturbance, and biogeochemical cycling dynamics 

would provide more realistic scenarios for planning and 

policy decisions. Most projections of climate change effects 

extend to only 2100, the limit of projections for most global 

climate models, and a relatively short time for robust evaluations of ecosystem dynamics.
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Some specific research priorities for forest ecosystems 

include:

• Develop and implement long-term studies on the effects 

of elevated CO2 in mature forests. This may involve 

whole forest stands or physiological measurements of 

individual trees within stands. Studies in disparate forest 

ecosystems would provide a broad perspective on this 

topic.

• Develop a standard approach for tracking carbon 

dynamics in different forest ecosystems over space and 

time. This will improve ecological knowledge, as well as 

input to carbon accounting systems. It will be especially 

useful if it can be applied in a straightforward manner by 

resource managers.

• Identify the appropriate uses and limitations of remote 

sensing imagery for detecting the effects of climatic 

variability and change in forest ecosystems. A great 

deal of remote sensing data are available, but they are 

accessible to only a few specialists. If resource managers 

were provided tools to access, analyze, and help interpret 

the most reliable and relevant data, it would provide 

timely feedback on forest stress and other characteristics 

on a routine basis.

• Determine which ongoing and long-term forest 

measurements are useful or could be modified for 

tracking the effects of climate change. Building on 

existing infrastructure for monitoring will be efficient 

and extend time series of measurements taken with 

established protocols.

• Identify standard approaches for evaluating uncertainty 

and risk in vulnerability assessments and adaptation 

planning. Straightforward qualitative and quantitative 

frameworks will advance the decisionmaking process on 

both public and private lands.

• Evaluate recently developed processes and tools for 

vulnerability assessment and adaptation planning to 

identify which ones are most effective for “climate 

smart” management on public and private lands. The 

availability of straightforward social and logistic 

protocols for eliciting and reviewing scientific 

information and stakeholder input will make climate 

change engagement more effective and timely.

It will be especially important to frame the above topics 

at the appropriate spatial and temporal scales in order to 

provide relevant input for different climate change issues. 

In addition, climatic data at different spatial scales needs to 

be matched with applications at different spatial scales to be 

relevant for climate smart management. Despite the urgency 

to provide downscaled climatic and effects data, the appropriate grain and extent of these data differ by resource (hydrology vs. vegetation vs. wildlife) and resource use (timber 

management vs. water supply vs. access for recreation). 

Sharing of information and experience within and among 

organizations involved in climate change will accelerate the 

incorporation of proven methods and applications across any 

particular landscape.
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Chapter 6

Future Assessment Activities

Toral Patel-Weynand1

Introduction

Climate change science has progressed significantly since 

the first National Climate Assessment (NCA) was produced 

(National Assessment Synthesis Team 2001). The ability to 

project climatic regimes and effects on forest ecosystems has 

increased through improved models and scaling techniques, 

as well as a combination of experimental studies and field 

observations that have either validated expected responses or 

challenged conventional thinking. However, as noted in previous chapters, critical information gaps exist in our ability 

to project how forest ecosystems will respond to the direct 

and indirect effects of climate change. Ongoing research 

is addressing many of these knowledge gaps, although the 

complexity of some scientific issues makes it clear that 

management and policy decisions over the next several years 

will continue to be made based on imperfect information. 

By as early as the mid-21st century, a warmer and more 

variable climate, along with interacting stressors, will challenge the ability of public and private land managers to manage forest resources. In response, large-scale adaptation and 

mitigation strategies and tactics will need to be developed 

and applied across the United States to ensure the sustainability of ecosystem services in a changing climate. Engagement will be required in both biophysical and socioeconomic 

research to make viable options available to manage 

re-sources that are being affected by climatic variability and 

change at various spatial and temporal scales. 

The 2013 NCA and the technical products that federal 

agencies and others are providing to the NCA are taking the 

first steps to help improve nationwide climate assessment 

capabilities in an integrated fashion. In the forest sector, a 

number of issues have emerged that need attention from 

national stakeholders and federal and state agencies, as well

1

 Toral Patel-Weynand is a national coordinator for bioclimatology, U.S. Department of Agriculture, Forest Service, Research and 

Development, 1601 N Kent Street, RPC 4, Arlington, VA 22209.

as from a resource management perspective. The current 

NCA approach is more focused than past climate assessments in supporting the Nation’s activities in adaptation and 

mitigation, and in evaluating the current state of scientific 

knowledge relative to climatic effects and trends. It advocates a long-term, consistent process for evaluating climatic 

risks and opportunities and for providing information to 

support decisionmaking processes. The U.S. Global Change 

Research Program and NCA are working toward establishing 

a permanent assessment capacity both inside and outside of 

the federal government. 

The NCA plans to have assessment activities draw upon 

the work of stakeholders and scientists across the country as 

an ongoing and continuous process. Assessment activities 

will support the capacity to conduct ongoing evaluations of 

vulnerability to climate stressors, observe and project effects of climate change within regions and sectors, allow for 

the production of a set of reports and Web-based products 

that are relevant for decisionmaking at multiple levels, and 

develop consistent indicators of progress in adaptation and 
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mitigation. The NCA is working to make products of this 

process useful within management and policy contexts. It is 

expected that an ongoing NCA process will be established 

and sustained through a cooperative community-wide effort 

that incorporates federal, state, and local governmental agencies, nongovernmental organizations, academia, and tribal 

and private interests. The long-term objective of the process 

is to enhance coordination of climate assessment efforts and 

facilitate communication between stakeholders and data 

providers.

Regional Issues

Several large-scale emerging issues identified by the forest 

sector will need attention beyond the 2013 NCA effort. 

Ecological disturbance, invasive species, urban forests, 

forest conversion to other uses, and fragmentation will need 

ongoing research and monitoring so adaptation options can 

be developed and evaluated. Additional effort is also needed 

to better understand how climate change will affect ecosystem services, human health, water and watersheds, energy 

and bioenergy, carbon (C) sequestration, and forest industry 

viability. Many organizations are working to identify potential vulnerabilities and effects, along with adaptation options 

to address them, but few analyses and interventions are set 

within a risk-based framework. Developing a risk-based 

framework to assess climate-related changes is therefore a 

critical need for the future.

Developing a Risk-Based Framework 

The NCA provided a simple set of guidelines on how to use 

a risk-based framework for technical input products for the 

2013 report (Yohe and Leichenko 2009). The guidelines are 

based on the risk and uncertainty framework developed by 

the Intergovernmental Panel on Climate Change (Moss and 

Schneider 2000). Risk assessment is also being incorporated 

in other national and state climate change management efforts. For example, all four National Research Council panel 

reports of “America’s Climate Choices” incorporate this 

framework, as does the draft Adaptation Plan for the United 

States. Although incorporating risk throughout a technical 

input product is challenging, the NCA recommends that this 

framework be incorporated at least for key vulnerabilities. 

Key vulnerabilities are those with a large magnitude, early 

timing, high persistence and irreversibility, wide distributional aspects, high likelihood, and high importance (based 

on human perceptions) (Schneider et al. 2007). All characteristics do not necessarily apply to a key vulnerability, and 

findings that may have a low likelihood but high consequence are still of interest to the NCA audience because of 

their high risk. 

A risk management framework for natural resources 

identifies risks and quantifies the magnitude and likelihood 

of environmental and other effects to the extent possible. 

Although risk management frameworks have been used 

(often informally) in natural resource management for many 

years, it is a new approach for projecting climate-change 

effects, and some time may be needed for both scientists and 

resource managers to feel comfortable with this approach. 

Risk assessment for climate change should be specific to a 

particular region and time period, and needs to be modified 

by an estimate of the confidence in the projections being 

made. Further work is needed to refine and expand existing risk management frameworks to better address climate 

change vulnerabilities and potential effects.

Social Issues

The complexities of human behavior and social vulnerability, value and significance of forests, and their joint sensitivity to climate change argue for social science research and 

community involvement when planning for, managing, and 

communicating about climate change in the forest sector. 

Research indicates that place matters, the planning or decisionmaking process matters, and original, specific, and local 

solutions may be best. However, a consistent and logical 

framework is needed to quantify ecosystem services across 

different forested landscapes, communities, and management 

institutions and to incorporate a wide range of biophysical 

and social values. 
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Management Options

The scientific literature is growing, and on-the-ground 

activities are underway, but no standard evaluation framework exists to aid decisions about effective management 

approaches—encompassing both biophysical and social 

processes—for adapting to or mitigating climate change. A 

framework is needed that will incorporate elements such as 

local forest productivity, management objectives, and socioeconomic conditions.

Identifying areas where forests are most vulnerable to 

change (i.e., have low resistance and resilience) and where 

the effects of change on ecosystem services will be greatest 

is a significant challenge for resource managers. One would 

expect forest ecosystems and species near the limits of their 

biophysical requirements to be vulnerable, but the complexities of fragmented landscapes and multiple stressors are 

likely to change response thresholds in many forest ecosystems. Under these conditions, traditional approaches to forest management are likely to fail. Management approaches 

that anticipate and respond to change by guiding development and adaptation of forest ecosystem structure and function will be needed to sustain desired ecosystem services and 

values across large landscapes and multiple decades. Land 

managers who are currently managing forest ecosystems in 

a sustainable manner are probably already using “climate 

smart” practices, and implementation of climate smart 

management at all spatial scales and by a variety of organizations (federal agencies, private land owners, conservation 

groups) can affect long-term resilience and sustainability. 

However, a systematic effort to communicate and implement 

these experiences more extensively is needed.

Ecological Disturbances and 

Extreme Events

Climatic variability is a driver of regionally episodic fires 

and endemic insect outbreaks; therefore, “new” science on 

climate and ecological disturbances is principally concerned with quantifying the mechanisms and variability in 

relationships between climate and ecological disturbance. 

Relationships between pathogens and climate change are not 

as well understood, but it is plausible that higher stress in 

tree species will reduce forest vigor and increase mortality. 

From an ecosystem perspective, thresholds can be reached 

either through cumulative effects of individual disturbances 

over time or one large event, and can lead to new forest 

composition, land cover, and landscape patterns. However, 

more information is needed on the interaction of ecological 

disturbances and other environmental stressors, especially 

for large spatial and temporal scales.

Climate affects forests through extreme events (e.g., 

hurricanes, wildfire, etc.) and through enabling conditions (e.g., long warm or cool periods, and long wet or dry 

periods). These events or enabling conditions can have 

short-term effects on forests, after which there is a transition 

or recovery, followed by long-term outcomes. Management 

can primarily respond to the enabling conditions by building 

resilience, as well as facilitating the transition or recovery. 

Are we prepared to confront and respond to climate-related 

forest changes within the context of forest management? 

The answer lies in our ability to recognize potential loss, 

quantify risk, examine options, identify tradeoffs, anticipate 

rare but high-consequence events, and invest commensurate 

with risk. The challenge before us will require new tools, 

information, and technology, as well as the experience of 

resource managers. 

Coordination With Other 

Assessment Activities 

Evaluating the future of forests requires understanding 

of human behavior in the context of a changing climate. 

Recently, the U.S. Forest Service conducted an assessment 

of current and future forests and rangelands, as required by 

the Forest and Rangeland Renewable Resources Planning 

Act (RPA) of 1974, which mandates that current conditions, 

trends, and forecasts for the next 50 years be assessed. Recent changes to the assessment mandated by the RPA include 

(1) presenting conditions, trends, and forecasts in a global 

context, (2) utilizing global climate models (three were used) 

and emission scenarios (e.g., A1B, A2, and B2), and (3) integrating the analysis with socioeconomic factors (e.g., wood 

product markets and the price of timber, and agricultural 
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markets and the future of crop prices). The RPA assessment 

indicates that forest area in the United States peaked in 2010 

at 253 million ha and will likely decline through 2060 to 

between 243 and 247 million ha. Product markets, population, income, and climate all interact to determine future 

forest area, biomass, and forest C. Climate will influence the 

outcomes, and although significant variation exists across 

potential climate futures, it is still small relative to human 

factors in the short run. 

Effects on Tribal Lands

American Indian and Alaska Native tribes face disproportionate risks from climate change. Tribes have unique 

rights, cultures, economies, and vulnerabilities to climate 

change effects. For indigenous peoples, the effects of 

climate change and the proposed solutions may affect tribal 

subsistence, land rights, cultural survivability, and financial 

resources. Tribes recognized a critical need for coordination among public agencies and organizations in accessing 

climate change resources and information, and in 2009, the 

Tribal Climate Change Project (University of Oregon 2012) 

was established to determine the needs, lessons learned, 

and opportunities American Indian tribes have in planning 

for the effects of climate change. Key research areas for the 

project are (1) increased understanding of tribal adaptation 

and mitigation planning for the physical effects of climate 

change, (2) increased understanding of management of offreservation resources, and (3) government-to-government 

relationships in addressing climate change through consultation, cross-landscape assessments, and tribal involvement 

in federal and state climate change plans. These types of 

research efforts will continue to be important for filling 

critical knowledge gaps.

Carbon Estimation

Through the U.S. Forest Service Forest Inventory and 

Analysis (FIA) program, C accounting is accomplished by 

first estimating land use and then estimating forest biomass. 

Forest C flux is approximated as a change in forest stocks 

over time. The FIA is primarily a large-scale inventory, and 

climate change would need to have a significant effect on net 

forest growth for FIA data to detect it. Accounting for C and 

managing ecosystems raises significant questions because 

of the uncertainty in how C pools will change with climate. 

Thus, management will require an integrated approach to 

mitigation and adaptation at large spatial scales. Avoiding 

deforestation and increasing afforestation can be recommended in the near term, whereas application of improved 

forest management across various regions may cause C 

losses in some locations and gains in others. Applying 

climate smart management at the stand scale is important but 

will be more effective in a broad landscape context.

Carbon mitigation can also be assessed through life 

cycle analysis. A recent synthesis of findings about the 

mitigation effectiveness of alternate forest management and 

wood use options concluded: “In the long term, sustainable 

forest management strategy aimed at maintaining or increasing forest carbon stocks, while producing an annual yield 

of timber, fiber, or energy from the forest, will generate the 

largest sustained mitigation benefit” (Metz et al. 2007). This 

raises the questions: “Which forest management and wood 

use strategies yield the greatest offset, in the near term and 

long term?” and “How confident are we in gains from those 

actions?” Afforestation and avoided deforestation are approaches with the highest confidence (lowest uncertainty) for 

providing C mitigation. Other approaches for which moderate uncertainty exists about effective mitigation include 

decreasing harvest, increasing forest growth, reducing hazardous fuels, using wood for energy, and substituting wood 

for nonwood products. Additional investigation is needed 

for all these topics. Life cycle evaluations of management 

and wood use options suggest more intensive approaches to 

wood production, harvest, and use to maximize C mitigation. The nature of future C markets, especially a regulated 

C market versus a voluntary market, will affect participation and influence wood product markets. Participation by 

private land owners may depend on management objectives 

and type of ownership (e.g., small vs. large properties). The 

motivation of private corporate entities relative to wood and 

C management, if surveyed accurately, will also provide 

important insights.
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Conclusions

Abundant data exist on the climatic, physical, ecological, 

and social aspects of how forests and forestry may respond 

to climate change, but the synthesis and integration of these 

for identifying adaptation options and making decisions 

are limited by (1) an inability to respond rapidly to new 

information such as projections of future climate; (2) social, 

political, and economic forces that affect the structure and 

function of forest ecosystems and their management; and (3) 

inadequate resources for synthesis and integration, particularly for adaptation and mitigation options and consequences. Stakeholders provided diverse recommendations 

for preparation of the forest sector technical report for the 

NCA, and most stakeholders emphasized that connections 

among various biophysical and social factors are not well 

understood or easily modeled. For example, the effects of 

climate change on wildfire can be partially mediated by fuels 

management, which in turn has a set of cascading effects on 

other forest processes and values, depending on the efficacy 

and intensity of management.

A number of periodic assessments by the Forest Sector are relevant to the NCA request for delivery of interim 

products between the 2013 and the 2017 NCA reports. For 

example, required sustained efforts such as the RPA assessment and periodic efforts such as the National Sustainability 

Report (USDA FS 2011) can provide integrated nationalscale information pertinent to the issues discussed here. 

However, a concerted effort will be necessary to identify 

these products and make them available to resource

managers and decisionmakers.

Literature Cited

Forest and Rangeland Renewable Resources Planning 

Act of 1974 (RPA); 16 U.S.C. 1601 (note).

Metz, B.; Davidson, O.R.; Bosch, P.R., eds. 2007. Mitigation of climate change. Contribution of Working Group 

III to the fourth assessment report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom: Cambridge University Press. 752 p.

Moss, R.H.; Schneider, S.H. 2000. Uncertainties in the 

IPCC TAR: recommendations to lead authors for more 

consistent assessment and reporting. In: Pachauri, R.; 

Taniguchi, T.; Tanaka, K., eds. Guidance papers on the 

cross cutting issues of the Third Assessment Report of 

the IPCC. Geneva, Switzerland: World Meteorological 

Organization: 31–51.

National Assessment Synthesis Team. 2001. Climate 

change impacts on the United States: the potential consequences of climate variability and change: foundation 

report. Report for the U.S. Global Change Research Program. 2nd ed. Cambridge, United Kingdom; New York: 

Cambridge University Press. 618 p.

Schneider, S.H.; Semenov, S.; Patwardhan, A. 2007.

Assessing key vulnerabilities and the risk from climate 

change. In: Parry, M.L.; Canziani, J.P.; Palutikof, P.J. [et 

al.], eds. Impacts, adaptation and vulnerability. Contribution of Working Group II to the fourth assessment report 

of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom: Cambridge University Press: 

779–810.

University of Oregon. 2012. Tribal climate change project. 

http://tribalclimate.uoregon.edu. (23 July 2012).

U.S. Department of Agriculture, Forest Service [USDA 

FS]. 2011. National report on sustainable forests—2010. 

FS-979. Washington, DC.

Yohe, G.; Leichenko, R. 2009. Adopting a risk-based approach. In: Climate change adaptation in New York City: 

building a risk management response. New York: New 

York City Climate Change Adaptation Task Force, Annals 

of the New York Academy of Sciences: 29–40.
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Chapter 7

Conclusions

David L. Peterson and James M. Vose1

Introduction

Forest ecosystems in the United States in the year 2100 will 

differ from those of today as a result of a changing climate. 

Those differences will be superimposed on the human imprint of forest management and the legacies of other land 

use activities, stressors, and disturbances of the 19th and 20th

centuries. Future changes in forest ecosystems will occur 

across both public and private lands and will challenge 

our ability to manage forests sustainably, especially as the 

human population continues to grow, demands for ecosystem services increase, and fossil fuel supplies decrease. We 

summarize below the most important inferences from the 

preceding chapters, with emphasis on issues most relevant

to land managers.

Forest Disturbance

Although increases in temperature, changes in precipitation 

magnitude and seasonality, higher atmospheric carbon dioxide (CO2

) concentrations, and higher nitrogen (N) deposition 

may over time modify ecosystem structure and function, 

the fastest and most significant effects on forest ecosystems 

will be caused by altered disturbance regimes. A warmer 

climate will increase the area burned by wildfire and the area 

affected by bark beetles and other insects. These two factors, 

individually, in combination, and as components of broader 

stress complexes, may lead to permanently altered species 

composition, distribution of forest age and structure, and 

spatial patterns across large landscapes.

1 David L. Peterson is a research biological scientist, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research 

Station, Pacific Wildland Fire Sciences Laboratory, 400 N 34th

Street, Suite 201, Seattle, WA 98103; James M. Vose is a research 

ecologist, U.S. Department of Agriculture, Forest Service, Southern 

Research Station, Center for Integrated Forest Science and Synthesis 

at North Carolina State University, Department of Forestry and 

Environmental Resources, Campus Box 8008, Raleigh, NC 27695. 

An increase in wildfire throughout the United States, 

which will likely include at least a doubling of area burned 

by the mid-21st century, will challenge government agencies 

and social institutions. Fire directly affects human communities near wildlands, but it is also stretching the ability 

of federal and state agencies to pay for fire suppression. 

Expanded efforts to reduce hazardous fuels can reduce the 

severity of wildfire on a local basis, but if the current investment in reducing stand densities and fuels does not increase 

significantly, it will be impossible to mitigate the effects of 

increasing crown fires.

The current advance of bark beetles in forests throughout the Western United States and Canada is unprecedented 

and often affects more land area than wildfire on an annual 

basis. Similar to wildfire, insects cause a rapid change in forest structure and function but with a slower return of carbon 

(C) to the atmosphere. Insects appear to affect fire severity 

in some cases and are a component of stress complexes that 

include prolonged drought. The prospect of bark beetles 

affecting higher elevations and different tree species than in 

the past portends major changes in forest ecosystems previously considered unaffected by beetles. Reducing stand densities and improving stand vigor can reduce impacts at the 

local scale, but it will be challenging to implement effective 

mitigation across large landscapes. A strategy of modifying 

the spatial pattern of age and structure in forests affected by 

beetles may provide some hope for controlling the spread of 

insects in the long term.

Invasive plants are another important component of 

stress complexes throughout the United States, and although 

the exact trajectory of this stressor in forest ecosystems is 

difficult to project, invasive plant species will likely become 

more numerous and widespread in the future. Many invasive 

species are more competitive in a warmer environment with 

elevated CO2

. However, increased disturbance from fire, 

insects, and land use change are among the most important 

factors facilitating their dispersal and population growth. 

This risk may be highest in mountain ecosystems, where 

cooler temperatures have historically limited the spread of 

invasives.
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Geomorphic disturbance will also increase if storms 

become more intense as is projected by many climate models. Concentrating precipitation in shorter periods of time 

increases erosion and mass wasting during and following 

storms, and increases the duration and intensity of low soil 

moisture (drought) during the rest of the year. Pulses of erosion and movement of sediment into streams are difficult to 

predict, but if they do indeed increase, they will affect decisions about management of roads and other infrastructure, as 

well as access for users of forest land. Increased drought will 

exacerbate stress complexes with insects and fire, leading to 

increased tree mortality, slow regeneration, and changes in 

species assemblages at some forest ecotones. 

Forest Processes

Stabilizing greenhouse gas concentrations in the atmosphere 

is a major goal for slowing global warming and buying 

time for implementation of alternative energy strategies 

and adaptation of forest ecosystems and human institutions 

to climate change. Forest growth and afforestation in the 

United States currently account for a net gain in C storage 

and offset approximately 13 percent of the Nation’s fossil 

fuel CO2 production. Overall, forest area has been stable 

since 1950, while C density (C per unit area) has increased. 

This assimilation of C is a function primarily of forest 

regrowth following timber harvest and land clearing in the 

previous two centuries and is projected to continue to around 

2040, at which point U.S. forests could become a net emitter 

of C. The majority of this C is in live aboveground biomass 

and soil organic C, so anything that affects these two components will significantly affect total C storage. During the 

next few decades, Eastern forests are expected to continue to 

sequester C through favorable response to elevated CO2

 and 

higher temperature, while Western forests may begin to emit 

C through expanded fire and insect disturbance. At large 

spatial and temporal scales, reduced in forest land cover may 

offset some of the C gains expected in Eastern forests.

No standard evaluation framework exists to aid decisions about which management approaches—encompassing 

both biological and social processes—would be most effective in maximizing C storage (reducing emissions) while 

minimizing risks. However, five approaches guide strategic 

and tactical management of forest C: (1) increase forest area 

and avoid deforestation, (2) manage C in existing forests,

(3) use wood as biomass energy, (4) use wood in place of 

other building materials, and (5) use wood products for C 

storage. These approaches differ considerably based on local 

forest productivity, management objectives, and economic 

conditions.

No-regrets strategies for enhancing C storage include 

preventing conversion of forest land to other uses and extending the life cycle of wood products. Avoided deforestation protects existing forest C stocks with low risk and has 

many co-benefits, although incentives to avoid deforestation 

in one area may increase removal of forest in other areas, as 

well as decrease economic opportunities for timber, agriculture, and urban development. Evidence for the benefits 

of fuel treatments (thinning plus surface fuel treatment) for 

C storage is equivocal, and the value of C offsets would be 

higher if thinning material had higher commercial value as 

long-lived products that yield substitution benefits and not 

just as bioenergy. The benefit of stored C in wood products 

is multiplied when wood is used in place of materials that 

require much higher C emissions to produce (e.g., concrete 

and steel). Careful management of forest products has potential for C mitigation that accrues over time and complements strategies for increasing forest C stocks, but effective 

strategies need to ensure that energy offsets are attained in 

an acceptable period of time and that substitution effects are 

attained. 

The effects of climate change on water resources and 

biogeochemical cycling will differ by forest ecosystem and 

local climatic conditions, as mediated by local management 

actions. Large-scale disturbances such as fire, bark beetle 

outbreaks, and defoliating insects will reduce water uptake, 

causing a near-term increase in runoff and potentially erosion. In systems with a long regeneration time, as in lowelevation forests and woodlands of the Southwest, erosion 

may be high for years to decades following disturbance. 

Increased temperature during the past few decades has decreased snow cover depth, duration, and extent, a trend that 

will likely continue with further warming. Decreased snow 

cover will alter the seasonal timing of runoff and exacerbate 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

soil moisture deficit in some forests, which may decrease 

tree vigor and increase susceptibility to insects and pathogens. In addition, fuels may remain dry and flammable for 

a longer period of time, leading to higher fire hazard and a 

longer period of time during which wildfires will burn. Less 

snow and drier fuels may also extend the time during which 

prescribed burning can be conducted, a potential benefit to 

resource managers.

Elevated CO2 may increase the water use efficiency of 

some tree species, thus reducing evapotranspiration, but the 

effect on hydrologic dynamics will likely be modest. Warmer temperature may also modify tree phenology, although the 

effects on evapotranspiration are uncertain. If species and 

genotypes that grow fast are widely planted in the future, 

their demand for soil water could reduce streamflow in some 

locations. Warmer temperature may also accelerate the rate 

of nutrient cycling in some systems, promoting increased 

forest growth and elevated N levels in streams.

Species Distributions

It has been difficult to infer if changes in forest species distribution and abundance are occurring in response to climate 

change, partly because of the lack of long, high-quality time 

series on species distribution, and partly because the legacy 

of widespread land use actions are so persistent in most 

landscapes. Most models predict that suitable habitat for 

many species will move upward in elevation and northward 

in latitude and be reduced or disappear from current habitats 

in lower elevations and lower latitudes. This is supported 

by both process-based and empirical modeling, although 

the different assumptions and resolutions of the models lead 

to rather different spatial and temporal inferences about 

habitats and species. It is possible that new climatic conditions will “move” faster in some locations than tree species 

can disperse, creating uncertainty about the future vegetation composition of these new habitats. It is also possible 

that topographic diversity, and thus microclimatic diversity, 

in mountainous regions will be sufficient to support most 

current species but with different spatial distributions and 

abundances. Despite the uncertainty of current modeling, 

the paleoecological literature suggests that major changes 

in species distribution and abundance, often mediated by 

disturbance, are possible with small but persistent changes in 

temperature and precipitation. 

Risk and Social Context

The Intergovernmental Panel on Climate Change (IPCC) and 

U.S. Global Change Research Program (USGCRP) currently 

emphasize that risk and uncertainty should be clearly articulated in order to provide a realistic context for interpreting 

scientific data and inferences. Risk assessment considers 

both the magnitude of a particular climate-change effect and 

the likelihood that it will occur. A risk management framework for natural resources means that risks are identified and 

that magnitude and likelihood of effects are quantified to the 

extent possible. Although risk management has been used 

(often informally) in natural resource management for many 

years, it is a new approach for projecting climate-change 

effects, and some time may be needed for both scientists and 

resource managers to feel comfortable with it. Risk assessment for climate change should be specific to a particular 

region and time period, and needs to be modified by an 

estimate of confidence in the projections being made.

The IPCC and USGCRP also emphasize that climatechange effects need to be considered in light of ecosystem 

services provided to local communities and human enterprises. Climate-change effects in forests are likely to reduce 

ecosystem services in some areas and increase them in others. Some areas may be particularly vulnerable because current infrastructure and resource production are based on past 

climate and steady-state conditions. For example, increased 

fire and insect attacks will, at least temporarily, reduce 

productivity, economic benefits from timber harvest, and C 

storage, and, in some cases, will increase surface runoff and 

erosion. In this case, potential losses of resource value and 

economic value are large, exclusive of the huge economic 

cost of fire suppression that may be required. Any change in 

forest ecosystems that affects water resources will result in a 

significant loss of ecosystem services.
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GENERAL TECHNICAL REPORT PNW-GTR-870

Preparing for Climate Change

Federal agencies, and the U.S. Forest Service in particular, 

have made significant progress in developing scientifically 

based principles and tools for adapting to climate change. 

Adaptation builds on a sequence of activities that starts with 

education, continues with an assessment of vulnerability of 

natural resources to climate change, and culminates in development of adaptation strategies and tactics. This process is 

most effectively conducted through a science-management 

partnership in which scientists lead the education and vulnerability assessment phases, and resource managers provide 

most of the input for adaptation. Tools and techniques 

available to facilitate this process are readily available in 

recent materials developed by the Forest Service, and can be 

applied to both public and private lands. In addition, several 

case studies of adaptation for national forests and national 

parks, individually and in collaboration with other stakeholders, are now available and can be emulated by other land 

management organizations. Collaboration across multiple 

land ownerships over large landscapes will ultimately lead to 

the most effective adaptation strategies and plans.

Although uncertainty exists about the magnitude and 

timing of climate change effects on forest ecosystems, 

sufficient scientific information is available to begin taking action now. However, on-the-ground implementation of 

adaptation plans and carbon management are rare in both 

public and private forest sectors. This is due to a perceived 

lack of urgency, a limited number of personnel trained in 

climate change science, inadequate guidelines and protocols, 

and inadequate resources to implement another “unfunded 

mandate.” 

Fortunately, land managers who are currently managing 

forest ecosystems in a sustainable manner are often already 

using “climate smart” practices. For example, thinning 

and fuel treatments implemented to reduce fire hazard also 

reduce intertree competition and increase resilience in a 

warmer climate. Increasing culvert size under roads reduces 

the risk of damage to roads and downstream resources that 

may occur in response to higher flood frequency and magnitude. Building on practices compatible with adapting to 

climate change will provide early successes and experience 

for resource managers who may want to start the adaptation 

process but do not have sufficient money, time, or personnel 

to initiate a major effort. We anticipate that climate change 

will be a standard component of sustainable resource management by the end of the decade, and that C management 

and adaptation will be fully embraced by forest management organizations. Building the foundation for this new 

management context as soon as possible will ensure that a 

broad range of options will be available for managing forest 

resources sustainably.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Introduction

Alaskan forests cover one-third of the state’s 52 million ha 

of land (Parson et al. 2001), and are regionally and globally 

significant. Ninety percent of Alaskan forests are classified as boreal, representing 4 percent of the world’s boreal 

forests, and are located throughout interior and south-central 

Alaska (fig. A1-1). The remaining 10 percent of Alaskan 

forests are classified as coastal-temperate, representing 19 

percent of the world’s coastal-temperate forests (National 

Synthesis Assessment Team 2003), and are located in southeast Alaska (fig. A1-1). Regional changes in the disturbance 

regimes of Alaskan forests (Wolken et al. 2011) directly 

affect the global climate system through greenhouse gas 

emissions (Tan et al. 2007) and altered surface energy budgets (Chapin et al. 2000, Randerson et al. 2006). Climaterelated changes in Alaskan forests also have regional societal 

consequences, because some forests are in proximity to 

communities (both urban and rural) and provide a diversity 

of ecosystem services (Reid et al. 2005, Wolken et al. 2011).

Interior Alaska

In interior Alaska, the most important biophysical factors responding to changes in climate are permafrost thaw 

and changes in fire regime. The region is characterized by 

discontinuous permafrost, defined as ground (soil or rock) 

that remains at or below 0 °C for at least 2 years (Harris 

et al. 1988). Thawing permafrost may substantially alter 

surface hydrology, resulting in poorly drained wetlands and 

thaw lakes (Smith et al. 2005) or well-drained ecosystems on 

substrates with better drainage. Permafrost thaw may occur 

directly as a result of changes in regional and global climate, 

but it is particularly significant following disturbance to

1 Jane M. Wolken is a postdoctoral researcher, Scenarios Network 

for Alaska and Arctic Planning, University of Alaska, 3352 College 

Road, Fairbanks, AK 99709; Teresa N. Hollingsworth is a research 

ecologist, U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Research Station, P.O. Box 756780, Fairbanks, AK 99775.

the organic soil layer by wildfire (fig. A1-2). As permafrost 

thaws, large pools of stored carbon (C) in frozen ground are 

susceptible to increased decomposition, which will have not 

only regional effects on gross primary productivity (Vogel et 

al. 2009) and species composition (Schuur et al. 2007) but 

also feedbacks to the global C system (Schuur et al. 2008). 

The observed warmer air and permafrost temperatures have 

important societal impacts, because transportation, water 

and sewer, and other public infrastructures may be damaged 

(Larsen et al. 2008, Nelson et al. 2002).

Recent changes in the fire regime in interior Alaska are 

linked to climate. The annual area burned in the interior has 

doubled in the last decade compared to any decade since 

1970, with three of the largest wildfire years on record

(fig. A1-2) also occurring during this time (Kasischke et al. 

2010). Black spruce forests, the dominant forest type in the 

interior, historically burned in low-severity, stand-replacing 

Figure A1-1—The boreal (interior and south-central) and coastaltemperate (southeast) forest regions of Alaska. The boreal forest 

ecoregions include the Alaska Range transition, coastal mountains 

transition, Pacific mountains transition, Kenai boreal, and intermontane boreal; the coastal-temperate forest includes the coastal rain 

forests ecoregion (Nowacki et al. 2001).

Appendix 1: Regional Summaries

Alaska

Jane M. Wolken and Teresa N. Hollingsworth1
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GENERAL TECHNICAL REPORT PNW-GTR-870

fires every 70 to 130 years (Johnstone et al. 2010a). However, postfire succession of black spruce (Picea mariana

[Mill.] Britton, Sterns & Poggenb.) forests has recently shifted toward deciduous-dominated forests with the increase 

in wildfire severity (Johnstone and Chapin 2006, Johnstone 

and Kasischke 2005, Kasischke and Johnstone 2005) and 

the reduction in fire-return interval (Bernhardt et al. 2011; 

Johnstone et al. 2010a, 2010b). With continued warming, 

changes in the fire regime will increase the risk to life and 

property for interior Alaskan residents (Chapin et al. 2008).

South-Central Alaska

South-central Alaska may be particularly sensitive to climate 

changes because of its confluence of human population 

growth and changing disturbance regimes (e.g., insects, 

wildfire, invasive species). Warmer temperatures have 

contributed to recent spruce beetle (Dendroctonus rufipennis Kirby) outbreaks in this region by reducing the beetle 

life cycle from 2 years to 1 year (Berg et al. 2006, Werner 

et al. 2006). Higher fuel loads resulting from beetle-caused 

tree mortality are expected to increase the frequency and 

severity of wildfires (Berg et al. 2006), which raises societal 

concerns of increased risks to life and property (Flint 2006). 

Most goods are shipped to Alaska via ports in south-central 

Alaska, so invasive plant species will probably become an 

increasingly important risk factor. Several invasive plant 

species in Alaska have already spread aggressively into 

Figure A1-2—In 2004, Alaska’s largest wildfire season on 

record, the Boundary Fire, burned 217 000 ha of forest in 

interior Alaska.

burned areas (e.g., Siberian peashrub [Caragana arborescens Lam.], narrowleaf hawksbeard [Crepis tectorum L.], 

and white sweetclover [Melilotus alba Medik.]) (CortésBurns et al. 2008, Lapina and Carlson 2004), and these could 

proliferate further with the increase in wildfire potential. 

Changes in surface hydrology in south-central Alaska have 

also been linked to warmer temperatures. In the Kenai lowlands, a subregion of south-central Alaska (fig. A1-1), many 

water bodies have shrunk in response to warming since 

the 1950s and have subsequently been invaded by woody 

vegetation (Klein et al. 2005). Recently, the rate of woody 

invasion has accelerated as a result of a 56-percent decline 

in water balance since 1968 (Berg et al. 2009). As a result 

of these combined effects of wetland drying and vegetation 

succession, wetlands are becoming weak C sources rather 

than strong C sinks, which has important consequences for 

the global climate system. 

Southeast Alaska 

In southeast Alaska, climatic warming has affected forest 

ecosystems primarily through effects on precipitation (i.e., 

snow versus rain). Historically, this region has average 

winter temperatures close to 0 °C and long growing seasons, 

so even moderate warming could increase rain and reduce 

snow. Many glaciers extending from Glacier Bay and the 

Juneau ice field have receded since 1750, with observed State of Alaska, Division of Forestry.
207

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

reductions in snow (Larsen et al. 2005, Motyka et al. 2002). 

Continued warming and corresponding reductions in snow 

precipitation will influence the hydrologic cycle and thus 

alter fish and mammal habitat, organic matter decomposition, and the C cycle.

For the past 100 years, the culturally and economically 

important Alaska cedar (Callitropsis nootkatensis [D. Don] 

Oerst. ex. D.P. Little), also known as yellow-cedar, has been 

dying throughout southeast Alaska (Hennon et al. 2006). The 

onset of this decline in 1880 (Hennon et al. 1990) is attributed to warmer winters and reduced snow, combined with 

early spring freezing events (Beier et al. 2008). The decline 

in Alaska cedar also has societal consequences because it is 

the highest valued commercial timber species exported from 

the region (Robertson and Brooks 2001). Native Alaskans 

also value this tree for ceremonial carvings; subsistence 

uses include fuel, clothing, baskets, bows, tea, and medicine 

(Pojar and MacKinnon 1994, Schroeder and Kookesh 1990). 

If cedar decline continues, it will alter the structure and function of forest ecosystems, as well as the lifeways of people 

in this region.

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Berg, E.E.; Henry, J.D.; Fastie, C.L. [et al.]. 2006. Spruce 

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Alaska. Journal of Vegetation Science. 22: 32–44.

Chapin, F.S.; McGuire, A.D.; Randerson, J. [et al.]. 2000.

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Biology. 6: 211–223.

Chapin, F.S., III; Trainor, S.F.; Huntington, O. [et al.]. 

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Bureau of Land Management, Alaska State Office. 162 p.

Flint, C.G. 2006. Community perspectives on spruce beetle 

impacts on the Kenai Peninsula, Alaska. Forest Ecology 

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Hennon, P.E.; Shaw, C.G.; Hansen, EM. 1990. Dating 

decline and mortality of Chamaecyparis nootkatensis in 

southeast Alaska. Forest Science. 36: 502–515.

Johnstone, J.F.; Chapin, F.S. 2006. Effects of soil burn 

severity on post-fire tree recruitment in boreal forest. 

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Johnstone, J.F.; Chapin, F.S.; Hollingsworth, T.N. [et 

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Johnstone, J.F.; Hollingsworth, T.N.; Chapin, F.S.; Mack, 

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Johnstone, J.F.; Kasischke, E.S. 2005. Stand-level effects of soil burn severity on post-fire regeneration in a 

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Kasischke, E.S.; Johnstone, J.F. 2005. Variation in groundlayer surface fuel consumption and its effects on site 

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interior Alaska. Canadian Journal of Forest Research. 35: 

2164–2177.

Kasischke, E.S.; Verbyla, D.L.; Rupp, T.S. [et al.]. 2010. 

Alaska’s changing fire regime—implications for the vulnerability of its boreal forests. Canadian Journal of Forest 

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Klein, E.; Berg, E.E.; Dial, R. 2005. Wetland drying and 

succession across the Kenai Peninsula lowlands, southcentral Alaska. Canadian Journal of Forest Research. 35: 

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Lapina, I.; Carlson, M.L. 2004. Non-native plant species 

of Susitna, Matanuska, and Copper River basins: summary of survey findings and recommendations for control 

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of Agriculture, Forest Service, State and Private Forestry. 

64 p.

Larsen, C.F.; Motyka, R.J.; Freymueller, J.T. [et al.]. 

2005. Rapid viscoelastic uplift in southeast Alaska caused 

by post-Little Ice Age glacial retreat. Earth and Planetary 

Science Letters. 237: 548–560.

Larsen, P.H.; Goldsmith, S.; Smith, O. [et al.]. 2008.

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risk from climate change. Global Environmental Change. 

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Motyka, R.J.; O’Neel, S.; Conner, C.L.; Echelmeyer, 

K.A. 2002. Twentieth century thinning of Mendenhall 

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35: 93–112.

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Service, Pacific Northwest Research Station. 29 p.

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and use of fish and wildlife resources and the effects of 

forest management in Hoonah, Alaska. Tech. Paper 142. 

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2008. Vulnerability of permafrost carbon to climate 

change: implications for the global carbon cycle. Bioscience. 58: 701–714.

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M.C. 2007. Plant species composition and productivity 

following permafrost thaw and thermokarst in Alaskan 

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L.D. 2005. Disappearing arctic lakes. Science. 308: 

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of carbon emissions from 2004 wildfires across Alaskan 

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Hawaii and the U.S.-Affiliated Pacific Islands

Christian P. Giardina1

Hawaii increased by 0.3 oC per decade, exceeding the global 

average rate (Giambelluca et al. 2008a). Streamflow decreased by 10 percent during the period 1973 to 2002 compared to 1913 to 1972 (Oki 2004), which is similar to what 

is suggested by simulation modeling for a warmer climate 

(Safeeq and Fares 2011). Preliminary climatic downscaling 

for the Hawaiian Islands projects that continued warming 

and drying will be coupled with more intense rain events 

separated by more dry days (Chu and Chen 2005, Chu et al. 

2010, Norton et al. 2011). This appears to be accurate for the 

central and western Pacific (Mimura et al. 2007), and at least 

for Hawaii, climatic forecasting suggests that this pattern 

will be more pronounced in drier areas of the state.

The direct effects of climate change on forests will be 

variable and strongly dependent on interactions with other 

disturbances, especially novel fire regimes that are expanding into new areas because of invasion by fire-prone exotic 

grass and shrub species (fig. A1-4), such as fountain grass 

(Cenchrus setaceus [Forssk.] Morrone) and common gorse 

(Ulex europaeus L.) in Hawaii and guinea grass (Urochloa 

maxima [Jacq.] R.D. Webster) across the region (D’Antonio 

and Vitousek 1992). Combined with warmer and drier 

conditions, these invasions have the potential to alter or even 

eliminate native forests through conversion of forested systems to open, exotic-dominated grass and shrub lands.

In wet forests, invasive plants can alter hydrologic 

processes by increasing water use by vegetation (Cavaleri 

and Sack 2010), and these effects may be more severe under 

warmer or drier conditions (Giambelluca et al. 2008b). Because invasive species have invaded most native-dominated 

ecosystems (Asner et al. 2005, 2008), anticipated direct 

(higher evapotranspiration) and indirect (increased competitive advantage of high water use plants) effects of climate 

change will modify streamflows and populations of stream 

organisms. Higher temperature will facilitate expansion of 

pathogens into cooler, high-elevation areas and potentially 

reduce native bird populations of Hawaii (Benning et al. 

2002). 

Hawaii and the U.S.-affiliated Pacific islands, including 

Guam, American Samoa, Commonwealth of Northern 

Mariana Islands, Federated States of Micronesia, Republic 

of Palau, and the Marshall Islands (fig. A1-3), contain a 

high diversity of flora, fauna, ecosystems, geographies, and 

cultures, with climates ranging from lowland tropical to 

alpine desert. Forest ecosystems range from equatorial mangrove swamps to subalpine dry forests on high islands, with 

most other forest life zones between. As a result, associated 

climate change effects and potential management strategies 

vary across the region (Mimura et al. 2007). The vulnerability of Pacific islands is caused by the (1) fast rate at which 

climate change is occurring; (2) diversity of climate-related 

threats and drivers of change (sea level rise, precipitation 

changes, invasive species); (3) low financial, technological, and human resource capacities to adapt to or mitigate 

projected effects; (4) pressing economic concerns affecting 

island communities; and (5) uncertainty about the relevance 

of large-scale projections for local scales. However, island 

societies may be somewhat resilient to climate change, 

because cultures are based on traditional knowledge, tools, 

and institutions that have allowed small island communities to persist during historical periods of biosocial change. 

Resilience is also provided by strong, locally based land and 

shore ownerships, subsistence economies, opportunities for 

human migration, and tight linkages among decisionmakers, state-level managers, and landowners (Barnett 2001, 

Mimura et al. 2007).

The distribution and persistence of different forest 

species are largely determined by temperature and precipitation and for coastal forests, sea level rise. Based on known 

historical climate-vegetation relationships, many forests are 

expected to experience significant changes in distribution 

and abundance by the end of the 21st century. Over the past 

30 years, air temperature for mid-elevation ecosystems in 

1 Christian P. Giardina is a research ecologist, U.S. Department 

of Agriculture, Forest Service, Pacific Southwest Research Station, 

60 Novelo Street, Hilo, HI 96720.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure A1-3—Hawaii and the U.S.-affiliated Pacific islands.

Figure A1-4—In Hawaii’s high-elevation forests (shown here) and 

in forests across the Pacific, projected warming and drying will increase invasive plants such as fire-prone grasses, resulting in novel 

fire regimes and conversion of native forests to exotic grasslands. 

For areas already affected in this way, climate change will increase 

the frequency and in some cases intensity of wildfire.

Although Hawaii’s Mauna Loa Observatory has been 

documenting the steady rise in atmospheric carbon dioxide 

(CO2), the direct effects of elevated CO2 in forests of the region have not been examined. However, most forests have at 

least some stimulatory effects from CO2 (Norby et al. 2005), 

especially in younger, faster-growing species. Therefore, the 

effects of climate on fire regimes and streamflow described 

above may be accentuated by rising CO2 through increased 

fuel accumulation and increased competitiveness of invasive 

species; higher water use across the landscape may be partially offset by higher water use efficiency in some species. 

For strand, mangrove, and other coastal forests, anticipated 

sea level rise for the region (about 2 mm·yr -1) (Mimura et 

al. 2007) will have moderate (initial or enhanced inundation 

with expansion to higher elevation) to very large (extirpation 

of forest species in the absence of upland refugia) effects on 

the distribution and persistence of these systems. Enhanced 

storm activity and intensity in the region during some largescale climatic events (e.g., El Niño Southern Oscillation) 

will enhance the effects of storm surges on these coastal 

systems and increase salt water intrusions into the freshwater 

lens that human and natural systems require for existence 

(Mimura et al. 2007). A combination of sea level rise and
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Risk

Confidence 

level

Small islands have characteristics that 

make them especially vulnerable to 

the effects of climate change, sea 

level rise, and extreme events.

Very high

Sea level rise is expected to exacerbate 

inundation, storm surges, erosion, 

and other coastal hazards, thus threatening infrastructure, settlements, and 

facilities that support the livelihood 

of island communities.

Very high

Strong evidence exists that under most 

climate change scenarios, water 

resources in small islands will be 

seriously compromised.

Very high

On some islands, especially those at 

higher latitudes, warming has already 

led to the replacement of some local 

plant species.

High

It is very likely that subsistence and 

commercial agriculture on small 

islands will be adversely affected by 

climate change. 

High

Changes in tropical cyclone tracks are 

closely associated with the El Niño 

Southern Oscillation, so warming 

will increase the risk of more persistent and severe tropical cyclones.

Moderate

Source: Mimura et al. 2007.

Table A1-1—Potential climate change related 

risks, and confidence in projections

increased frequency and severity of storm surges could result 

in extensive loss of forest habitat in low-lying islands.

Mimura et al. (2007) suggest high to moderately high 

confidence for anticipated diverse effects of climate change 

on island ecosystems (table A1-1). These effects will extend 

across federal, state, tribal, and private lands, the most 

vulnerable being coastal systems and human communities. 

Sea level rise, apparent trajectories for storm intensity and 

frequency in the region, and warming and drying trends (for 

Hawaii) are based on robust measurements that suggest high 

confidence in projected ecological changes. Vulnerabilities 

and risks are most relevant in coastal zone forests, but all 

forests of the region are at greater risk of degradation from 

secondary drivers of change, especially fire, invasive species, insects, and pathogens. 

Island systems of the Pacific are home to some of the 

most intact traditional cultures on earth and communities 

that generally are strongly linked to forest resources. Sea 

level rise, increased storm frequency and intensity, and more 

severe droughts will reduce the habitability of atolls, representing a major potential impact in Pacific island countries 

(Barnett and Adger 2003). For low-lying islands of the Pacific, enhanced storm activity and severity and sea level rise 

will cause the relocation of entire communities and even nations; the first climate refugees have already had to relocate 

from homelands in the region (Mimura et al. 2007). Climatedriven reductions in coastal forest area and functionality 

will increase population pressures on already limited natural 

resources, and the combination of inundation and enhanced 

storm damage will damage fragile economies (Mimura et al. 

2007). For high islands, warming and drying in combination 

with expanded cover of invasive species, and in some cases 

increased fire frequency and severity, will alter the hydrological function of forested watersheds, with cascading effects 

on ground-water recharge as well as downstream agriculture, 

urban development, and tourism (Mimura et al. 2007).

Few options are available for managing climate-change 

effects on Pacific island ecosystems. For some very lowlying islands and island systems, such as the Marshall 

Islands where much of the land mass is below anticipated 

future sea levels, climate change will reduce fresh water 

supply and community viability. When fresh water becomes 

contaminated with salt water, the options for persisting in a 

location are logistically challenging and often unsustainable. 

For higher islands, adaptation practices include shoreline 

stabilization through tree planting, reduced tree harvest, 

facilitated upward or inward migration of forest species, and 

shoreline development planning (Mimura 1999). Because 

many Pacific island lands are owned and managed traditionally, adaptation and mitigation can be enhanced at the 

community level through education and outreach focused on 

coastal management and protection, mitigation of sea level 

rise, forest watershed protection, and restoration actions. 
213

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

However, cost-effective prescriptions and examples of effective adaptation strategies are rare. 

Several options for managing climate-change effects 

exist in Hawaii, because adequate financial resources and infrastructure are available. Hawaiian ecological relationships 

differ from those on other islands; for example, mangrove 

forests serve important shoreline conservation functions in 

the U.S.-affiliated Pacific islands, but mangrove species are 

not native to Hawaii and are considered problematic invasives. Land ownership in Hawaii is complex, requiring management for shoreline stabilization to rely on diverse native 

plant species and institutional partnerships. Because Hawaii 

has significant topographic relief, as well as moderately sophisticated management infrastructure, anticipatory planning 

and facilitation of inward species migration is already being 

practiced in some coastal wetlands.

For the majority of Hawaii’s forest systems, sea level 

rise and storm surges are minor threats. Rather, key threats 

to native forest plant biodiversity include climate-driven 

acceleration of invasive species, resulting in displacement of 

native vegetation and in novel fire disturbance. This creates 

the potential for long-term conversion of native forests to 

grass and shrub lands dominated by invasive species. The 

spread of invasive species can be slowed by multifaceted 

management strategies (biocontrol, physical and chemical 

control) and restoration of areas with fire-prone invasives 

(green break planting, native species planting, physical and 

chemical control of weed species). To this end, management 

prescriptions for simultaneously addressing conservation 

objectives and climate change effects are being addressed 

by the Hawaii Department of Land and Natural Resources 

Watershed Initiative, U.S. Fish and Wildlife Service (USFWS) Pacific Island Climate Change Cooperative, Hawaii 

Restoration and Conservation Initiative, and Hawaii Conservation Alliance Effective Conservation Program, as well as 

individual climate change management plans (e.g., USFWS 

Hakalau Forest National Wildlife Refuge Comprehensive 

Conservation Plan). 

The region has lacked resources and expertise for conducting the research required to comprehensively manage 

climate change threats; research needs are particularly acute 

for the U.S.-affiliated Pacific islands. Throughout the region, 

research is needed to identify the thresholds beyond which 

social-ecological systems in atolls will be permanently 

compromised, and the contributions of resource management, behavior, and biophysical factors to pushing systems 

across these thresholds (Barnett and Adger 2003). Stress 

complexes in forest systems affect thresholds; especially 

important are interactions among invasive species, altered 

fire regimes, insects, and pathogens. Silvicultural research is 

needed to understand how to treat extensive forest areas for 

invasive species that appear to use more water than native 

systems. Effective biocontrol agents are also needed to reduce the most damaging invasive species affecting regional 

forests. Expanded research in fire science (fire history, fire 

behavior, fuel characterization) would improve fuel maps 

and understanding about fire ecology and human dimensions 

of wildfire. Conservation genetics research would improve 

understanding of genotypic plasticity and diversity within 

species, restoration needs and adaptation potential, pathogen 

resistance in a changing climate, and locally relevant restoration practices that use genotypes and species suitable for 

future climate.
214

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Literature Cited

Asner, G.P.’ Elmore, A.J.; Hughes, R.F. [et al.]. 2005.

Ecosystem structure along bioclimatic gradients in 

Hawai’i from imaging spectroscopy. Remote Sensing of 

the Environment. 96: 497–508.

Asner, G.P.; Hughes, R.F.; Vitousek, P.M. [et al.]. 2008.

Invasive plants transform the three-dimensional structure 

of rain forests. Proceedings of the National Academy of 

Sciences, USA. 105: 4519–4523.

Barnett, J. 2001. Adapting to climate change in Pacific 

island countries: the problem of uncertainty. World Development. 29: 977–993.

Barnett, J.; Adger, W. 2003. Climate dangers and atoll 

countries. Climatic Change. 61: 321–337.

Benning, T.L.; LaPointe, D.; Atkinson, C.T.; Vitousek, 

P.M. 2002. Interactions of climate change with biological 

invasions and land use in the Hawaiian Islands: modeling 

the fate of endemic birds using a geographic information 

system. Proceedings of the National Academy of Sciences, USA. 99: 14246–14249.

Cavaleri, M.A.; Sack, L. 2010. Comparative water use of 

native and invasive plants at multiple scales: a global 

meta-analysis. Ecology. 91: 2705–2715.

Chu, P.-S.; Chen, H. 2005. Interannual and interdecadal 

rainfall variations in the Hawaiian Islands. Journal of 

Climate. 18: 4796–4813.

Chu, P.-S.; Chen, Y.R.; Schroeder, T.A. 2010. Changes in 

precipitation extremes in the Hawaiian Islands in a warming climate. Journal of Climate. 23: 4881–4900.

D’Antonio, C.M.; Vitousek, P.M. 1992. Biological invasions by exotic grasses, the grass/fire cycle, and global 

change. Annual Review of Ecology and Systematics. 23: 

63–87.

Giambelluca, T.W.; Diaz, H.F.; Luke, S.A. 2008a. Secular 

temperature changes in Hawai’i. Geophysical Research 

Letters. 35, L12702, doi:10.1029/2008GL034377.

Giambelluca, T.; Delay, J.; Asner, G. [et al.]. 2008b.

Stand structural controls on evapotranspiration in native 

and invaded tropical montane cloud forest in Hawai’i. 

[Abstract]. http://adsabs.harvard.edu/abs/2008AGUFM.

B43A0422G. (11 December 2011).

Mimura, N. 1999. Vulnerability of island countries in the 

South Pacific to sea level rise and climate change. Climate Research. 12: 137–143.

Mimura, N.; Nurse, L.; McLean, R.F. [et al.]. 2007. Small 

islands. In: Parry, M.L.; Canziani, O.F.; Palutikof, J.P., 

eds. Climate change 2007: impacts, adaptation and vulnerability. Contribution of Working Group II to the fourth 

assessment report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom: Cambridge 

University Press: 687–716.

Norby, R.; DeLucia, E.; Gielen, B. [et al.]. 2005. Forest response to elevated CO2

 is conserved across a broad range 

of productivity. Proceedings of the National Academy of 

Sciences, USA. 102: 18052–18056.

Norton, C.W.; Chu, P.S.; Schroeder, T.A. 2011. Projecting 

changes in future heavy rainfall events for Oahu, Hawaii: 

a statistical downscaling approach. [Abstract]. Journal of 

Geophysical Research. 116: D17110.

Oki, D.S. 2004. Trends in streamflow characteristics at longterm gaging stations, Hawaii. Scientific Investigations 

Rep. 2004–5080. Denver, CO: U.S. Geological Survey. 

120 p.

Safeeq, M.; Fares, A. 2011. Hydrologic response of a 

Hawaiian watershed to future climate change scenarios. 

Hydrological Processes. doi: 10.1002/hyp.8328.
215

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Northwest

Jeremy S. Littell1

The state of knowledge about climatic effects on forests of 

the Northwest region was recently summarized in a peerreviewed assessment of these effects in Washington (Littell 

et al. 2009, 2010) and a white paper on climatic effects on 

Oregon vegetation (Schafer et al. 2010). Recent PNW and 

West-wide modeling studies provide additional scenarios for 

effects of climate change on wildfire, insects, and dynamic 

vegetation in Oregon and Washington. This summary describes evidence for such effects on climate-sensitive forest 

species and vegetation distribution, fire, insect outbreaks, 

and tree growth.

Based on projections of direct effects of climate change 

on the distribution of Northwest tree species and forest 

biomes, widespread changes in equilibrium vegetation are 

expected. Statistical models of tree species-climate relationships (e.g., McKenzie et al. 2003) show that each tree species has a unique relationship with limiting climatic factors 

(McKenney et al. 2011; McKenzie et al. 2003; Rehfeldt et

al. 2006, 2008). These relationships have been used to project future climate suitability for species in western North 

America (McKenney et al. 2007, 2011; Rehfeldt et al. 2006, 

2009) and in Washington in particular (e.g., Littell et al. 

2010 after Rehfeldt et al. 2006). Climate is projected to become unfavorable for Douglas-fir (Pseudotsuga menziesii

[Mirb.] Franco) over 32 percent of its current range in

Washington, and up to 85 percent of the range of some

pine species may be outside the current climatically suitable 

range (Littell et al. 2010, Rehfeldt 2006). Based on preliminary projections from the global climate model (GCM) 

CCSM2 and the process model 3PG, Coops and Waring 

(2010) projected that the range of lodgepole pine (Pinus 

contorta var. latifolia Engelm. ex S. Watson) will decrease

in the Northwest. Using similar methods, Coops and Waring 

(2011) projected a decline in current climatically suitable 

area for 15 tree species in the Northwest by the 2080s; five

1 Jeremy S. Littell is a research ecologist, U.S. Department 

of the Interior, Geological Survey, Alaska Climate Science 

Center, 4210 University Drive, Anchorage, AK 99508.

of these species would lose less than 20 percent of this 

range, and the range of the other 10 species would decline 

up to 70 percent. 

Various modeling studies project significant changes in 

species distribution in the Northwest, but with considerable 

variation within and between those studies. McKenney et 

al. (2011) summarized responses of tree species to climate 

change across western North America for three emissions 

scenarios. Projected changes in suitable climates for Northwest tree species ranged from near balanced (-5 to +10) to 

greatly altered species distribution at the subregional scale 

(-21 to -38 species), depending on the emissions scenario. 

Modeling results by Shafer et al. (2010) indicate either relatively little change over the 21st century under a moderate 

warming, wetter climate (CSIRO Mk3, B1), or, in western 

Oregon, a nearly complete conversion from maritime to 

evergreen needleleaf forest and subtropical mixed forest 

under a warmer, drier climate (HadCM3, A2). Lenihan et 

al. (2008) concluded that shrublands would be converted to 

woodlands, and woodlands to forest in response to elevated 

carbon dioxide, a trend that would be facilitated by fire

suppression.

Potential changes in fire regimes and area burned have 

major implications for ecosystem function, resource values 

in the wildland-urban interface, and expenditures and policy 

for fire suppression and fuels management. The projected 

effects of climate change on fire in the Northwest generally 

suggest increases in both fire area burned and biomass consumed in forests (Littell et al. 2009, 2010; McKenzie et al. 

2004). Littell et al. (2010) used statistical climate-fire models to project future area burned for the combined area of 

Idaho, Montana, Oregon, and Washington. Median regional 

area burned per year is projected to increase from the current 

0.2 million ha, to 0.3 million ha in the 2020s, 0.5 million ha 

in the 2040s, and 0.8 million ha in the 2080s. Furthermore, 

the area burned compared to the period 1980 through 2006 

is expected to increase, on average, by a factor of 3.8 in forested ecosystems (western and eastern Cascades, Okanogan 

Highlands, Blue Mountains). Rogers et al. (2011) used the 

MC1 dynamic vegetation model to project fire area burned, 

given climate and dynamic vegetation under three GCMs. 

Compared to 1971 to 2000, large increases are predicted by 
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GENERAL TECHNICAL REPORT PNW-GTR-870

2100 in both area burned (76 to 310 percent, depending on 

climate and fire suppression scenario) and burn severities 

(29 to 41 percent). 

Tree vigor and insect populations are both affected by 

temperature: host trees can be more vulnerable because of 

water deficit, and bark beetle outbreaks are correlated with 

high temperature (Powell and Logan 2005) and low precipitation (Berg et al. 2006). Littell et al. (2010) projected 

relationships between climate (vapor pressure deficit) and 

mountain pine beetle (Dendroctonous ponderosae Hopkins) 

(MPB) attack in the late 21st century. They also projected 

potential changes in MPB adaptive seasonality, which suggested that the region of climatic suitability will move higher 

in elevation, eventually reducing the total area of suitability. 

Using future temperature scenarios for the PNW, Bentz et al. 

(2010) simulated changes in adaptive seasonality for MPB 

and single-year offspring survival for spruce bark beetle 

(Dendroctonus rufipennis Kirby) (SBB). The probability 

of MPB adaptive seasonality increases in higher elevation 

areas, particularly in the southern and central Cascade Range 

for the early 21st century and in the north Cascades and central Idaho for the late 21st century. Single-year development 

of SBB offspring also increases at high elevations across the 

region in both the early and late 21st century.

Response of tree growth to climate change will depend on subregional-to-local characteristics that change the 

sensitivity of species along the climatic gradients of their 

ranges (e.g., Chen et al. 2009, Littell et al 2008, Peterson 

and Peterson 2001). Douglas-fir is expected to grow more 

slowly in much of the drier part of its range (Chen et al. 

2009) but may currently be growing faster in many locations 

in the Northwest (Littell et al. 2008). Although no regional 

synthesis of expected trends in tree growth exists, the projected trend toward warmer and possibly drier summers in 

the Northwest (Mote and Salathé 2010) is likely to increase 

growth where trees are energy limited (at higher elevations) 

and decrease growth where trees are water limited (at lowest elevations and in driest areas) (Case and Peterson 2005, 

Holman and Peterson 2006, Littell et al. 2008). Growth at 

middle elevations will depend on summer precipitation (Littell et al. 2008).

The effects of climate change on forest processes in the 

Northwest are expected to be diverse, because the mountainous landscape of the region is complex, and species distribution and growth can differ at small spatial scales. Forest 

cover will change faster via disturbance and subsequent 

regeneration over decades, rather than via gradual readjustment of vegetation to a new climate over a century or more. 

Additional data are needed on interactions between disturbances and on connections between climate-induced changes 

in forests and ecosystem services, including water supply 

and quality, air quality, and wildlife habitat. In addition, 

projected changes in forest distribution, structure, and function need to be synthesized using recent GCM projections, 

including quantification of uncertainties about the effects of 

climate on forest processes. 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Literature Cited

Bentz, B.J.; Régnière, J.; Fettig, C.J. [et al.]. 2010.

Climate change and bark beetles of the Western United 

States and Canada: direct and indirect effects. BioScience. 60: 602–613.

Berg, E.; Davidhenry, J.; Fastie, C. [et al.]. 2006. Spruce 

beetle outbreaks on the Kenai Peninsula, Alaska, and 

Kluane National Park and Reserve, Yukon Territory: relationship to summer temperatures and regional differences 

in disturbance regimes. Forest Ecology and Management. 

227: 219–232.

Case, M.J.; Peterson, D.L. 2005. Fine-scale variability 

in growth-climate relationships of Douglas-fir, North 

Cascade Range, Washington. Canadian Journal of Forest 

Research. 35: 2743–2755.

Chen, P.-Y.; Welsh, C.; Hamann, A. 2010. Geographic 

variation in growth response of Douglas-fir to interannual 

climate variability and projected climate change. Global 

Change Biology. 16: 3374–3385.

Coops, N.C.; Waring, R.H. 2010. A process-based approach to estimate lodgepole pine (Pinus contorta

Dougl.) distribution in the Pacific Northwest under climate change. Climatic Change. 105: 313–328.

Coops, N.C.; Waring, R.H. 2011. Estimating the vulnerability of fifteen tree species under changing climate in 

Northwest North America. Ecological Modelling. 222: 

2119–2129.

Holman, M.L.; Peterson, D.L. 2006. Spatial and temporal 

variability in forest growth in the Olympic Mountains, 

Washington: Sensitivity to climatic variability. Canadian 

Journal of Forest Research. 36: 92–104.

Lenihan, J.; Bachelet, D.; Neilson, R.; Drapek, R. 2008. 

Simulated response of conterminous United States 

ecosystems to climate change at different levels of fire 

suppression, CO2

 emission rate, and growth response to 

CO2. Global and Planetary Change. 64: 16–25.

Littell, J.S.; McKenzie, D.; Peterson, D.L.; Westerling, 

A.L. 2009. Climate and wildfire area burned in western 

U.S. ecoprovinces, 1916–2003. Ecological Applications. 

19: 1003–1021.

Littell, J.S.; Oneil, E.E.; McKenzie, D. [et al.]. 2010. 

Forest ecosystems, disturbance, and climatic change in 

Washington State, USA. Climatic Change. 102: 129–158.

Littell, J.S.; Peterson, D.L.; Tjoelker, M. 2008. Douglasfir growth in mountain ecosystems: water limits tree 

growth from stand to region. Ecological Monographs.

78: 349–368.

McKenney, D.W.; Pedlar, J.H.; Lawrence, K. [et al.].

2007. Potential impacts of climate change on the distribution of North American trees. BioScience. 57: 939–948.

McKenney, D.W.; Pedlar, J.H.; Rood, R.B.; Price, D.

2011. Revisiting projected shifts in the climate envelopes 

of North American trees using updated general circulation 

models. Global Change Biology. 17(8): 2720–2730.

McKenzie, D.; Gedalof, Z.; Peterson, D.L.; Mote, P. 2004. 

Climatic change, wildfire, and conservation. Conservation Biology. 18: 890–902.

McKenzie, D.; Peterson, D.W.; Peterson, D.L.; Thornton, 

P.E. 2003. Climatic and biophysical controls on conifer 

species distributions in mountain forests of Washington 

State, USA. Journal of Biogeography. 30: 1093–1108.

Mote, P.W.; Salathé, E.P. 2010. Future climate in the Pacific Northwest. Climatic Change. 102: 29–50.

Peterson, D.W.; Peterson, D.L. 2001. Mountain hemlock 

growth responds to climatic variability at annual and 

decadal time scales. Ecology. 82: 3330–3345.

Powell, J.; Logan, J. 2005. Insect seasonality: circle map 

analysis of temperature-driven life cycles. Theoretical 

Population Biology. 67: 161–179.
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Rehfeldt, G.E.; Crookston, N.L.; Warwell, M.V.; Evans, 

J.S. 2006. Empirical analyses of plant-climate relationships for the western United States. International Journal 

of Plant Sciences. 167: 1123–1150.

Rehfeldt, G.E.; Ferguson, D.E.; Crookston, N.L. 2008. 

Quantifying the abundance of co-occurring conifers along 

Inland Northwest (USA) climate gradients. Ecology. 89: 

2127–2139.

Rogers, B.M.; Neilson, R.P.; Drapek, R. [et al.]. 2011. Impacts of climate change on fire regimes and carbon stocks 

of the U.S. Pacific Northwest. Journal of Geophysical 

Research. 116: 1–13.

Shafer, S.L.; Harmon, M.E.; Neilson, R.P. [et al.]. 2010. 

The potential effects of climate change on Oregon’s vegetation. In: Dello, K.D.; Mote, P.W., eds. Oregon climate 

assessment report. Corvallis, OR: Oregon State University, College of Oceanic and Atmospheric Sciences, Oregon 

Climate Change Research Institute: 175–210.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Southwest

David L. Peterson1

Dying pinyon pines (Pinus edulis Engelm.) in New Mexico 

and adjacent states in the early 2000s became an iconic 

image of the effects of a warming climate in U.S. forests. 

Several consecutive years of drought reduced the vigor of 

pines, allowing pinyon ips (Ips confusus LeConte) to successfully attack and kill pines across more than 1 million ha 

(Breshears et al. 2005). The pinyon pine dieback was one 

of the most important manifestations of extreme climate in 

North America during the past decade, an indicator that a 

physiological threshold was exceeded because of the effects 

of low soil moisture (Floyd et al. 2009). Although this is not 

direct evidence of the effects of climate change, it demonstrates the effects of severe drought, a phenomenon expected 

more frequently in the future, on large-scale forest structure 

and function in arid environments. 

Aridity dominates forest ecosystems in the Southwest, 

which encompass a wide range of topographic variability 

and Mediterranean, continental, and desert climates. Therefore, disturbance processes that are facilitated by climatic 

extremes, primarily multiyear droughts, dominate the 

potential effects of climatic variability and change on both 

short- and long-term forest dynamics (Allen and Breshears 

1988). Although diebacks in species other than pinyon pine 

have not been widespread, large fires and insect outbreaks 

appear to be increasing in both frequency and spatial extent 

throughout the Southwest. In Arizona and New Mexico, 14 

to 18 percent of the forested area was killed by wildfire and 

bark beetles between 1997 and 2008 (Williams et al. 2010). 

This forest mortality appears to be related to the current 

trend of increasing temperature and decreasing precipitation, 

at least in the southern portion of the region, since the mid 

1970s (Cayan et al. 2010, Weiss et al. 2009). 

1 David L. Peterson is a research biological scientist, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station, Pacific Wildland Fire Sciences Laboratory, 400 N 34th Street, 

Seattle, WA 98103.

In late spring 2011, following a winter with extremely 

low precipitation and a warm spring, the Wallow Fire burned 

217 000 ha of forest and woodland in eastern Arizona and 

western New Mexico, receiving national attention for its 

size and intensity (Incident Information System 2011). The 

Wallow Fire was the largest recorded fire in the conterminous United States, and forced the evacuation of eight communities, cost $109 million to suppress (4,700 firefighters 

involved) and $48 million to implement rehabilitation measures, and resulted in high consumption of organic material 

and extensive overstory mortality across much of the burned 

landscape. A total of 880 000 ha burned in Arizona and New 

Mexico in 2011 (National Interagency Fire Center 2011). 

Large, intense fires illustrate how extreme drought can cause 

rapid, widespread change in forest ecosystems.

Recent large fires may portend future increases in wildfire. Using an empirical analysis of historical fire data on 

federal lands, McKenzie et al. (2004) projected the following increases in annual area burned for these Southwestern 

States, given a temperature increase of 1.5 °C: Arizona, 150 

percent; Colorado, 80 percent; New Mexico, 350 percent; 

and Utah, 300 percent. California and Nevada were projected to be relatively insensitive to temperature, but their data 

included extensive nonforest area. In a more recent analysis, 

Littell et al.2 project the following increases for a 1 °C temperature increase: Arizona, 380 to 470 percent; California, 

310 percent; Colorado, 280 to 660 percent; Nevada, 280 percent; New Mexico, 320 to 380 percent; and Utah, 280 to 470 

percent. Applying the Parallel Climate Model to California, 

Lenihan et al. (2003) projected that area burned will increase 

at least 10 percent per year (compared to historical level) by 

around 2100 (temperature increase of 2.0 °C).

The general increase in fire that is expected in the 

future, and that may already be occurring, will result in 

younger forests, more open structure, increased dominance 

2

 Littell, J.S. Relationships between area burned and climate in the 

Western United States: vegetation-specific historical and future fire. 

Manuscript in preparation. On file with: U.S. Geological Survey, 

Alaska Climate Science Center, 4210 University Drive, Anchorage, 

AK 99508.
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GENERAL TECHNICAL REPORT PNW-GTR-870

Figure A1-5—The effectiveness of 

fuel treatments is seen in this portion 

of the 2011 Wallow Fire near Alpine, 

Arizona. High-intensity crown fire 

was common in this area, but forest that had been thinned and had 

surface fuels removed experienced 

lower fire intensity, and structures in 

the residential area were protected. 

of early successional plant species, and perhaps some invasive species. Because annual accretion of biomass is relatively low in this region, production of live and dead fuels in 

the understory in one year affects the likelihood of fire in the 

next year (Littell et al. 2009). The interaction of climate, fuel 

loading, and fuel moisture will contribute to both future area 

burned and fire severity.

The ongoing expansion of bark beetle outbreaks in 

western North America has been especially prominent in 

Colorado. Since 1996, multiple beetle species have caused 

high forest mortality on 2.7 million ha, of which 1.4 million ha were infested with mountain pine beetle (Dendroctonus ponderosae Hopkins) (USDA FS 2011). Facilitated 

by extended drought and warmer winters, mountain pine 

beetle outbreaks have focused primarily on older (stressed) 

lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. 

Watson) forest. In Arizona and New Mexico, 7.6 to 11.3 percent of forest and woodland area was affected by extensive 

tree mortality owing to bark beetles from 1997 through 2008 

(Williams et al. 2010). As in other areas of the West, bark 

beetles appear to be attacking trees at higher elevations than 

in the past (Gibson et al. 2008).

In a detailed analysis of tree growth data for the United 

States, Williams et al. (2010) found that growth in the Southwest was positively correlated with interannual variability 

in total precipitation and negatively correlated with daily 

maximum temperature during spring through summer, which 

suggests that increased future drought will have a profound 

effect on growth and productivity. Projecting a business-asusual (A2) emission scenario on these growth-climate relationships produced significant growth reductions for forests 

in Arizona, Colorado, and New Mexico after 2050, affecting primarily ponderosa pine (Pinus ponderosa Lawson & 

C. Lawson), Douglas-fir (Pseudotsuga menziesii [Mirb.] 

Franco), and pinyon pine. Projected growth decreases were 

larger than for any other region of the United States (Williams et al. 2010).

Simulation modeling of potential changes in vegetation 

in California suggests that significant changes can be expected by 2100 (Lenihan et al. 2003). Modeling results show that 

mixed evergreen forest will replace evergreen conifer forest 

throughout much of the latter’s historical range. This process 

may include gradual replacement of Douglas-fir–white fir U.S. Forest Service.
221

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Literature Cited

Allen, C.D. 2007. Interactions across spatial scales among 

forest dieback, fire, and erosion in northern New Mexico 

landscapes. Ecosystems. 10: 797–808.

Allen, C.D.; Breshears, D.D. 1998. Drought-induced shift 

of a forest-woodland ecotone: rapid landscape response to 

climate variation. Proceedings of the National Academy 

of Sciences, USA. 95: 14839–14842.

Bostwick, P.; Menakis, J.; Sexton, T. 2011. How fuel treatments saved homes from the 2011 Wallow fire. http://

www.fs.fed.us/fire/management/fuel_treatments.pdf. (19 

December 2011).

Breshears, D.D.; Cobb, N.S.; Rich, P.M. [et al.]. 2005.

Regional vegetation die-off in response to global-change 

type drought. Proceedings of the National Academy of 

Sciences, USA. 102: 15144–15148.

Cayan, D.R.; Das, T.; Pierce, T.P. 2010. Future dryness 

in the southwest US and the hydrology of the early 21st

century drought. Proceedings of the National Academy of 

Sciences, USA. 107: 21271–21276.

Floyd, M.L.; Clifford, M.; Cobb, N.S. [et al.]. 2009.

Relationship of stand characteristics to drought-induced 

mortality in three southwestern piñion-juniper woodlands. 

Ecological Applications. 19: 1223–1230.

Gibson, K.; Skov, K.; Kegley, S. [et al.]. 2008. Mountain 

pine beetle impacts in high-elevation five-needle pines: 

current trends and challenges. R1-08-020. Missoula, MT: 

U.S. Department of Agriculture, Forest Service, Forest 

Health Protection. 32 p.

Incident Information System. 2011. Wallow. http://www.

inciweb.org/incident/2262. (19 December 2011).

Lenihan, J.M.; Drapek, R.; Bachelet, D.; Neilson, R.P. 

2003. Climate change effects on vegetation distribution, 

carbon, and fire in California. Ecological Applications. 

13: 1667–1681.

(Abies concolor [Gordon & Glend.] Lindl. ex Hildebr.)

forest by Douglas-fir–tanoak (Lithocarpus densiflorus

[Hook. & Arn.] Rehd.) forest and the replacement of white 

fir–ponderosa pine forest by ponderosa pine–California 

black oak (Quercus kelloggii Newberry) forest in the

Sierra Nevada. Tanoak–Pacific madrone (Arbutus menziesii

Pursh)–canyon live oak (Q. chrysolepis Liebm.) woodland may replace blue oak (Q. douglasii Hook. & Arn.) 

woodlands, chaparral, and perennial grassland. In general, 

shrubland will replace oak woodland, and grassland will 

replace shrubland throughout the state. Evergreen conifer 

forest will advance into the high–elevation subalpine forest 

in the Sierra Nevada, and species such as Shasta red fir (Abies magnifica A. Murray) and lodgepole pine (Pinus contorta

var. murrayana [Balf.] Engelm.) may become more common in subalpine parklands and meadows. A high degree of 

regional variability in species changes can be expected, and 

large-scale transitions will need to be facilitated through fire 

disturbance that enables regeneration.

Increased disturbance from fire and insects, combined 

with lower forest productivity at most lower elevation 

locations because of a warmer climate, will probably result 

in lower carbon storage in most forest ecosystems. The fireinsect stress complex may keep many low-elevation forests 

in younger age classes in perpetuity. The normal cycle of 

fire followed by high precipitation (in winter in California, 

in early summer in much of the rest of the Southwest) may 

result in increased erosion and downstream sediment delivery (Allen 2007). In a warmer climate, it may be possible 

to reduce fire severity and protect wildland-urban interface 

areas through assertive use of fuel treatments (Peterson et al. 

2011), as shown recently in the Wallow Fire (Bostwick et al. 

2011) (fig. A1-5). It may also be possible to reduce largescale beetle epidemics by maintaining multiple forest age 

classes across the landscape (Li et al. 2005). Significant financial resources and collaboration across different agencies 

and landowners will be necessary to successfully implement 

these adaptive strategies.
222

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Li, C.; Barclay, H.J.; Hawkes, B.C.; Taylor, S.W. 2005.

Lodgepole pine forest age class dynamics and susceptibility to mountain pine beetle attack. Ecological Complexity. 2: 232–239.

Littell, J.S.; McKenzie, D.; Peterson, D.L.; Westerling, 

A.L. 2009. Climate and wildfire area burned in western 

U.S. ecoprovinces, 1916–2003. Ecological Applications. 

19: 1003–1021.

McKenzie, D.; Gedalof, Z.; Peterson, D.L.; Mote, P. 2004.

Climatic change, wildfire, and conservation. Conservation Biology. 18: 890–902.

National Interagency Fire Center. 2011. Statistics: national 

year-to-date report on fires and acres burned by state. 

http://www.nifc.gov/fireInfo/fireInfo_stats_YTD2011.

html. (19 December 2011).

Peterson, D.L.; Halofsky, J.L.; Johnson, M.C. 2011. Managing and adapting to changing fire regimes in a warmer 

climate. In: McKenzie, D.; Miller, C.; Falk, D., eds. The 

landscape ecology of fires. New York: Springer: 249–267.

U.S. Department of Agriculture, Forest Service. 2011.

Rocky Mountain bark beetle. http://www.fs.usda.gov/

main/barkbeetle/home. (19 December 2011).

Weiss, J.L.; Castro, C.L.; Overpeck, J.T. 2009. Distinguishing pronounced droughts in the southwestern United 

States: seasonality and effects of warmer temperatures. 

Journal of Climate. 22: 5918–5932.

Williams, A.P.; Allen, C.D.; Millar, C.I. [et al.]. 2010. 

Forest responses to increasing aridity and warmth in 

southwestern North America. Proceedings of the National 

Academy of Sciences, USA. 107: 21289–21294.
223

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Great Plains

Linda A. Joyce1

Natural vegetation of the Great Plains is primarily grassland 

and shrubland ecosystems with trees occurring in scattered areas along streams and rivers, on planted woodlots, 

as isolated forests such as the Black Hills of South Dakota, 

and near the biogeographic contact with Rocky Mountains 

and eastern deciduous forests. Trees are used in windbreaks 

and shelterbelts for crops and within agroforestry systems, 

extending the tree-covered area considerably (e.g., over 160 

000 ha in Nebraska) (Meneguzzo et al. 2008). Urban areas 

in the Great Plains benefit from trees providing wildlife 

habitat, water storage, recreation, and aesthetic value. The 

Great Plains are divided here into three areas for discussion: 

northern Great Plains (North Dakota, South Dakota, Kansas, 

Nebraska), southern Great Plains (Oklahoma, Texas), and 

western Great Plains (Montana, Wyoming). 

Forests in the northern Great Plains comprise less than 

3 percent of the total land area within each state (Smith et 

al. 2009) (table A1-2). More than half of the forest land in 

South Dakota is in public land ownership in contrast to the 

other three states. Dominant forest types are ponderosa pine 

(Pinus ponderosa Lawson and C. Lawson var. scopulorum

Engelm.), fir-spruce, and western hardwoods. Eastern cottonwood (Populus deltoides Bartr. ex Marsh.) forests are an 

important source of timber in North Dakota (Haugen et al. 

2009) and Nebraska (Meneguzzo et al. 2008). Many cottonwood stands in this region are quite old, and regeneration 

has been minimal owing to infrequent disturbance (Haugen 

et al. 2009, Meneguzzo et al. 2008, Moser et al. 2008, South 

Dakota Resource Conservation and Forestry Division 2007). 

The decline of this species often leads to establishment of 

nonnative species (Haugen et al. 2009) or expansion of natives such as green ash (Fraxinus pennsylvanica Marsh.), 

which is susceptible to the invasive emerald ash borer 

1 Linda A. Joyce is a research quantitative ecologist, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station, 240 West Prospect, Fort Collins, CO 80526.

(Agrilus planipennis Fairmaire). In North Dakota, quaking 

aspen (Populus tremuloides Michx.) forests are generally 

in poor health and have minimal regeneration because of 

fire exclusion (Haugen et al. 2009). In South Dakota, forest 

land is dominated by ponderosa pine forest, which supports 

a local timber industry in the Black Hills area. Management 

concerns include densely stocked stands, high fuel loadings 

and fire hazard, and mountain pine beetle (Dendroctonus 

ponderosae Hopkins) outbreaks. Eastern redcedar (Juniperus 

virginiana L.) is expanding in many states, the result of fire 

exclusion and prolonged drought conditions (Meneguzzo et 

al. 2008, South Dakota Resource Conservation and Forestry 

Division 2007). This presents opportunities for using redcedar for wood products, but also raises concerns about trees 

encroaching into grasslands and altering wildlife habitat 

(Moser et al. 2008). Land use activities that support biofuel 

development, particularly on marginal agricultural land, may 

affect forests in this area (Haugen et al. 2009, Meneguzzo et 

al. 2008).

Forests in the southern Great Plains comprise less than 

17 percent of the land area (table A1-2) (Smith et al. 2009), 

are often fragmented across large areas, and are mostly 

privately owned. In Texas, the forest products industry is one 

of the top 10 manufacturing sectors in the state, with a fiscal 

impact of $33.6 billion on the state economy (Xu 2002). 

Loss of forest to urbanization, oil and gas development, and 

conversion to cropland and grassland has led to a permanent 

reduction in forest cover (Barron 2006, Johnson et al. 2010).

Forests in the western Great Plains comprise less than 

27 percent of the land area (Smith et al. 2009) (table A1-2), 

and most of this land is in public ownership. Montana has 

large contiguous areas of forest, particularly in the western

part of the state where public land, forest industry, and 

private land intermingle. Both Montana and Wyoming have 

forested areas on mountains where the surrounding ecosystems are grassland and shrubland. The three major forest 

types in Montana are also the most commercially important 

species: Douglas fir (Pseudotsuga menziesii [Mirb.] Franco), 

lodgepole pine (Pinus contorta Douglas ex. Loudon var. latifolia Engelm. ex S. Watson), and ponderosa pine (Montana 

Department of Natural Resources and Conservation 2010). 
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GENERAL TECHNICAL REPORT PNW-GTR-870

Total 

land 

area

Total 

forest 

land

Total 

timberland

Planted 

timberland

Natural 

origin

Reserved 

forest 

land

Other 

forest 

land

Other 

land

Thousands of hectares

Northern Great Plains:

Kansas 21 241 852 821 16 805 0 31 20 389

Nebraska 19 913 504 475 14 461 4 25 19 409

North Dakota 17 943 293 216 2 214 10 67 17 650

South Dakota 19 601 681 628 7 621 17 36 18 920

Total Northern

 Great Plains 78 697 2330 2140 39 2101 32 158 76 367

Southern Great Plains:

Oklahoma 17 788 3102 2523 257 2265 18 561 14 686

Texas 67 863 6990 4799 1132 3668 46 2145 60 873

 Total Southern

 Great Plains 85 651 10 092 7322 1389 5933 64 2706 75 559

Western Great Plains:

Montana 37 760 10 123 8009 76 7933 1594 520 27 637

Wyoming 25 116 4632 2427 19 2407 1531 673 20 484

Total Western

 Great Plains 62 876 14 755 10 436 95 10 340 3125 1193 48 121

Total Great Plains 227 244 27 177 79 898 1523 18 374 3221 4057 200 047

Source: Smith et al. 2007.

Fire exclusion has caused higher fire hazard and more mountain pine beetle outbreaks. In recent years, the forest industry 

has been adversely affected by reduced timber supply and 

general economic trends. Wyoming forests are dominated by 

lodgepole pine, followed by spruce-fir and ponderosa pine, 

and land ownership is a mosaic of public, private, and industial. Similar to Montana, the forest industry in Wyoming 

has faced several challenges but continues to be a significant 

component of the state economy (Wyoming State Forestry 

Division 2009). Both Montana and Wyoming have urban 

forests, riparian forests, and windbreaks and shelterbelts 

associated with agriculture. Tree species used in windbreaks 

and shelterbelts, including ponderosa pine and the nonnatives Scots pine (Pinus sylvestris L.) and Austrian pine 

(P. nigra Arnold) are being attacked by mountain pine beetles, and green ash is susceptible to the emerald ash borer. 

Similar to other parts of the Great Plains, some lower elevation riparian forests are in decline, because regeneration has 

been reduced by fire exclusion, water diversions, drought, 

agricultural activities, and urban development. 

Little information is available on the potential effects of 

climate change on Great Plains forests. However, this area 

has been part of continental and national studies (Bachelet 

et al. 2008), and areas such as the Greater Yellowstone 

Ecosystem have a long history of research that has recently 

included climate change. Tree species in the Yellowstone 

area are expected to move to higher elevation in a warmer 

Table A1-2—Land area in the Great Plains 2007
225

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

climate (Bartlein et al. 1997, Koteen 2002, Whitlock et al. 

2003). However, projecting future vegetation distribution is 

complicated by the complex topography of Wyoming, which 

influences the microclimatic environment that controls vegetation distribution. Forests in this area and Montana are currently affected by insect outbreaks and wildfire, and changes 

in these disturbances under climate change could potentially 

disrupt ecosystems across large landscapes. A recent modeling study suggests that a warmer climate will increase the 

frequency and spatial extent of wildfire in the Yellowstone 

area (Westerling et al. 2011).

In a review of the literature on the effects of climate

change in semiarid riparian ecosystems, Perry et al. (2012)

noted that climate-driven changes in streamflow are expected to reduce the abundance of dominant, native, 

early-successional tree species and increase herbaceous, 

drought-tolerant, and late-successional woody species 

(including nonnative species), leading to reduced habitat 

quality for riparian fauna. Riparian systems will be especially important locations on which to focus monitoring for 

the early effects of climate change. 

Reduced tree distribution in the Great Plains will likely 

have a negative effect on agricultural systems, given the 

important role of shelterbelts and windbreaks in reducing 

soil erosion. In these “linear forests,” warmer temperatures 

are expected to reduce aboveground tree biomass and spatial 

variation in biomass at lower elevations, but may increase 

biomass on upland habitats (Guo et al. 2004). Carbon 

sequestration through agroforestry has been suggested as a 

potential mitigation activity (Morgan et al. 2010).

Across the Great Plains, forests are currently exposed 

to many stressors. Common to all states in this region is a 

concern about land use changes that would reduce the total 

area of forests, fragment intact forests, and alter forest dynamics. Current stressors such as insects, fungal pathogens, 

and altered hydrologic dynamics may be exacerbated by a 

warmer climate. The potential for increased wildfire hazard, longer droughts, and increased risk of insect outbreaks, 

individually and in combination, could significantly modify 

Great Plains forest environments. Whereas most studies in 

this region have explored the potential influence of elevated 

carbon dioxide (CO2

)on grassland, Wyckoff and Bowers 

(2010) analyzed the relationship between historical climate 

and tree growth and suggest that the interaction of climate 

change and elevated CO2 could be a potential factor in the 

expansion of forests from the Eastern United States into the 

Great Plains. 
226

GENERAL TECHNICAL REPORT PNW-GTR-870

Literature Cited

Bachelet, D.; Lenihan, J.; Drapek, R.; Neilson, R.P. 2008.

VEMAP vs VINCERA: A DGVM sensitivity to differences 

in climate scenarios. Global and Planetary Change. 64: 

38–48.

Barron, E. 2006. State of the Texas forest 2005. College Station, TX: Texas Forest Service. 37 p.

Bartlein, P.; Whitlock, C.; Shafer, S. 1997. Future climate 

in the Yellowstone National Park region and its potential 

impact on vegetation. Conservation Biology. 11: 782–792.

Guo, Q.; Brandle, J.; Schoeneberger, M.; Buettner, D. 2004.

Simulating the dynamics of linear forests in Great Plains 

agroecosystems under changing climates. Canadian Journal 

of Forest Research. 34: 2564–2572.

Haugen, D.E.; Kangas, M.; Crocker, S.J. [et al.]. 2009. North 

Dakota’s forests 2005. Resour. Bull. NRS-31. Newtown 

Square, PA: U.S. Department of Agriculture, Forest Service, 

Northern Research Station. 82 p.

Johnson, E.; Geissler, G.; Murray, D. 2010. The Oklahoma 

forest resource assessment, 2010: a comprehensive analysis 

of forest-related conditions, trends, threats and opportunities. 

Oklahoma City, OK: Oklahoma Department of Agriculture, 

Food, and Forestry. 163 p.

Koteen, L. 2002. Climate change, whitebark pine, and grizzly 

bears in the Greater Yellowstone Ecosystem. In: Schneider, S.; Root, T., eds. Wildlife response to climate change: 

North American case studies. Washington, DC: Island Press: 

343–414.

Meneguzzo, D.M.; Butler, B.J.; Crocker, S.J. [et al.]. 2008.

Nebraska’s forests, 2005. Resour. Bull. NRS-27. Newtown 

Square, PA: U.S. Department of Agriculture, Forest Service, 

Northern Research Station. 94 p.

Montana Department of Natural Resources and Conservation. 2010. Montana’s state assessment of forest resources: 

base findings and GIS methodology. Missoula, MT: Montana 

Department of Natural Resources and Conservation. 29 p.

Morgan, J.A.; Follett, R.F.; Allen, L.H. [et al.]. 2010. Carbon 

sequestration in agricultural lands in the United States. Journal of Soil and Water Conservation. 65: 6A–13A.

Moser, W.K.; Hansen, M.H.; Atchison, R.L. [et al.]. 2008.

Kansas forests, 2005. Resour. Bull. NRS-26. Newtown 

Square, PA: U.S. Department of Agriculture, Forest Service, 

Northern Research Station. 125 p.

Perry, L.G.; Andersen, D.C.; Reynolds, L.V. [et al.]. 2012. 

Vulnerability of riparian ecosystems to elevated CO2

 and 

climate change in arid and semiarid western North America. 

Global Change Biology. 18: 821–842.

Smith, W.B., tech. coord.; Miles, P.D., data coord.; Perry, 

C.H., map coord.; Pugh, S.A., data CD coord. 2009.

Forest resources of the United States, 2007. Gen Tech. Rep.

GTR-WO-78. Washington, DC: U.S. Department of Agriculture, Forest Service. 336 p.

South Dakota Resource Conservation and Forestry Division. 2007. South Dakota forest stewardship plan, 2007 revision. Pierre, SD: South Dakota Department of Agriculture, 

Resource Conservation and Forestry Division. 35 p.

Westerling, A.L.; Turner, M.G.; Smithwick, E.A.H. [et al.]. 

2011. Continued warming could transform Greater Yellowstone fire regimes by mid-21st century. Proceedings of the 

National Academy of Sciences USA. 108: 13165–13170. 

Whitlock, C.; Shafer, S.; Marlon, J. [et al.]. 2003. The role 

of climate and vegetation change in shaping past and future 

fire regimes in the northwestern U.S. and the implication for 

ecosystem management. Forest Ecology and Management. 

178: 5–21.

Wyckoff, P.H.; Bowers, R. 2010. Response of the prairie–

forest border to climate change: impacts of increasing 

drought may be mitigated by increasing CO2

. Journal of 

Ecology. 98: 197–208. 

Wyoming State Forestry Division. 2009. Wyoming statewide 

assessment. Cheyenne, WY: Wyoming State Forestry. 74 p.

Xu, W. 2002. Economic impact of the Texas forest sector. Texas 

Agric. Exten. Pub. 161. College Station, TX: Texas AgriLife 

Extension Service. 19 p.
227

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Midwest

Christopher W. Swanston and Stephen D. Handler1

Introduction

Forests are a defining landscape feature for much of the 

Midwest, from boreal forests surrounding the northern Great 

Lakes to oak-hickory (Quercus spp., Carya spp.) forests 

blanketing the Ozark Highlands. Savannas and open woodlands mark a major transition between forest and grassland 

biomes in the United States. Forests cover approximately 

28 percent of the area in the eight-state Midwest region and 

help sustain human communities ecologically, economically, 

and culturally. Forest ecosystems are distributed according to patterns of climate, moisture, soils, and disturbance; 

ecoregions capture these broad patterns across the landscape. 

Most of the Midwest is contained within the Laurentian 

Mixed Forest, Eastern Broadleaf Forest (Continental and 

Oceanic), and Prairie Parklands ecoregions (Bailey 1995) 

(fig. A1-6). 

The broad diversity in species composition and structure 

across the Midwest will likely engender higher resilience to 

a changing climate than less diverse biogeographic regions, 

but each ecoregion might be best characterized by a few 

strong vulnerabilities. With this in mind, key vulnerabilities 

related to climate change are summarized below according 

to ecoregions. The term “vulnerability” refers to a decline in 

vigor and productivity, in addition to more severely altered 

community composition or ecosystem function (Swanston 

et al. 2011). In other words, a species or ecosystem may 

be considered vulnerable to climate change by virtue of 

significantly decreased well-being, even if is not projected to 

disappear completely from the landscape.

1 Christopher W. Swanston is a research ecologist and Stephen 

D. Handler is a climate change specialist, U.S. Department of Agriculture, Forest Service, Northern Research Station, 410 MacInnes 

Drive, Houghton, MI 49931.

Laurentian Mixed Forest

Key Vulnerabilities: Decline of Boreal Tree 

Species, Reduced Forested Wetlands

The Laurentian mixed forest spans the northern areas of 

the Great Lakes states (fig. A1-6), typified by a glaciated 

landscape with low relief covered with mesic broadleaf 

deciduous forests, sometimes mixed with conifers, and often 

grading to pure conifers on poor soils. Winters are cold and 

long, often with heavy snowfall, and summers are warm and 

provide much of the annual precipitation. As a transitional 

zone between the boreal forests in the north and the broadleaf forests to the south, the Laurentian forests are often 

dominated by boreal species at the southern edge of their 

suitable habitat range. Many of these species, such as black 

spruce (Picea mariana [Mill.] Britton, Sterns and Poggenb.), 

balsam fir (Abies balsamea [L.] Mill.), paper birch (Betula 

papyrifera Marsh.), and northern whitecedar (Thuja occidentalis L.), are projected to lose suitable habitat through 

much of their current range (Iverson et al. 2008, Walker et 

al. 2002). Associated ecosystems may thus be more likely 

to experience stress and undergo more distinct community 

transitions (Swanston et al. 2011, Xu et al. 2012). Forested 

wetlands, including peatlands, are a major feature of northern Lake States forests, and may be especially susceptible 

to a combination of range shifts and changes in hydrologic 

regimes (e.g., Swanston et al. 2011). These systems store 

a large amount of belowground carbon (Johnson and Kern 

2003) that could be at risk if fire increases in drier conditions. Subboreal species such as sugar maple (Acer saccharum Marsh.) may be less affected than boreal species, but 

any effects may be more apparent aesthetically and economically owing to their prevalence on the landscape (Iverson et 

al. 2008). 

Eastern Broadleaf Forest

Key Vulnerabilities: Accelerated Oak 

Decline, Increased Wildfire, New Invasive 

Species

The Eastern Broadleaf Forest (fig. A1-6) mostly consists 

of the Continental ecoregion, with low rolling hills, some 

glaciation in the north, and the Ozark Highlands to the south. 
228

GENERAL TECHNICAL REPORT PNW-GTR-870

Figure A1-6—Ecoregions within the Midwest, according to Bailey et al. (1995).
229

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Precipitation generally comes during the growing season 

but decreases in the western ecoregion. Oak-hickory forest 

is dominant, grading to maple (Acer spp.), American beech 

(Fagus grandifolia Ehrh.), and American basswood (Tilia 

americana L.) in the north. Oak decline is increasing the 

mortality of oak species throughout the southern half of the 

Midwest and is correlated with drought periods (Wang et al. 

2007). Species in the red oak (Quercus rubra L., Q. coccinea

Münchh., Q. velutina Lam.) group are particularly susceptible to decline and make up a large proportion of upland 

forests in this ecoregion. White oak (Q. alba L.) may also 

be declining on the western margins of its range (Goldblum 

2010), which may be further amplified by higher summer 

temperatures in the future (Iverson et al. 2008). Oak decline 

could worsen if droughts become more frequent or severe, 

and elevated fine and coarse fuels could result from tree 

mortality, thereby increasing wildfire hazard.

Wildfire suppression has gradually favored more mesic 

species such as maple, leaving fire-adapted species like oaks 

and shortleaf pine (Pinus echinata Mill.) at a competitive 

disadvantage (Nowacki and Abrams 2008). With adequate 

moisture and continued fire suppression, these forests are 

likely to persist but may become increasingly susceptible to 

wildfire in a drier climate (Lenihan et al. 2008). A general decline in resilience, in combination with increased 

disturbances such as fire, could make these forests more 

susceptible to invasive species such as kudzu (Pueraria 

lobata [Willd.] Ohwi, an aggressive vine) and Chinese and 

European privet (Ligustrum sinense Lour. and L. vulgare L., 

highly invasive shrubs), that may expand into the Midwest 

as winter minimum temperatures increase (Bradley et al. 

2010). 

Prairie Parklands

Key Vulnerabilities: Increased Wildfire, 

Fragmentation, Loss of Ecosystem 

Function

The Prairie Parklands (fig. A1-6) are predominantly covered 

by agriculture and prairie, with interspersed upland forests 

of oak and hickory. Forest stands are also found near streams 

and on north-facing slopes. Fragmentation (loss of continuity among forested areas) and parcelization (subdivision of 

forest tracts into smaller ownerships) of forest ecosystems 

are more extreme in the Prairie Parklands than in other Midwest ecoregions. For example, over 90 percent of forest land 

in Iowa is currently divided into private holdings averaging 

less than 7 ha (Flickinger 2010). Combined with extensive 

conversion of available land to agricultural monocultures, 

this ecoregion currently exists as a highly fragmented landscape for forest ecosystems, effectively impeding the natural 

migration of tree species. Model simulations indicate that 

factors such as increasing summer temperatures and dryness, 

coupled with inadequate fire suppression, could lead to loss 

of ecosystem function and transition to grasslands or woodland/savanna even under low emissions scenarios (Lenihan 

et al. 2008).

Human Communities and 

Land Use

Key Issues: Parcelization, Poor CrossBoundary Coordination, Ambivalent 

Stewardship

Human communities are an integral part of the landscape 

in the Midwest and have greatly shaped current forests and 

prairie-forest boundaries (Abrams 1992, Mladenoff and 

Pastor 1993). Contemporary land use and ownership patterns provide critical input to policy responses to ecological 

issues, including climate change. Forest ownership patterns differ greatly between the Western and Eastern United 

States, with 68 percent of forests in private ownership in the 

Midwest versus only 21 percent in the West (Butler 2008, 

Nelson et al. 2010). Stewardship of private lands reflects 

diverse values and motivations (Bengston et al. 2011), 

providing a challenge to effective outreach (Kittredge 2004). 

Likewise, a coordinated response to forest ecosystem threats 

is further challenged by parcelization (DeCoster 1998, 

Mehmood and Zhang 2001). Fostering climate preparedness as a component of sustainable land stewardship will 

require significantly increased outreach and coordination to 

communicate relevant and credible information to private 

forest landowners. Conversely, inadequate attention to land 

stewardship will place this forest sector at greater risk of 

avoidable impacts of climate change.
230

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Literature Cited

Abrams, M.D. 1992. Fire and the development of oak forests. 

BioScience. 42: 346–353.

Bailey, R.G. 1995. Description of the ecoregions of the United 

States. Washington, DC: U.S. Department of Agriculture, 

Forest Service. 108 p.

Bengston, D.N.; Asah, S.T.; Butler, B.J. 2011. The diverse 

values and motivations of family forest owners in the United 

States: an analysis of an openended question in the National Woodland Owner Survey. Small-Scale Forestry. 10: 

339–355.

Bradley, B.A.; Wilcove, D.S.; Oppenheimer, M. 2010. Climate change increases risk of plant invasion in the eastern 

United States. Biological Invasions. 12: 1855–1872.

Butler, B.J. 2008. Family forest owners of the United States, 

2006. Gen. Tech. Rep. NRS-27. Newtown Square, PA: 

U.S. Department of Agriculture, Forest Service, Northern 

Research Station. 72 p.

DeCoster, L.A. 1998. The boom in forest owners—a bust for 

forestry? Journal of Forestry. 96: 25–28.

Flickinger, A. 2010. Iowa’s forests today: an assessment of the 

issues and strategies for conserving and maintaining Iowa’s 

forests. In: Miller, E., ed. Des Moines, IA: Iowa Department 

of Natural Resources. http://www.iowadnr.gov/Environment/

Forestry/ForestryLinksPublications/IowaForestActionPlan.

aspx. (26 September 2012).

Goldblum, D. 2010. The geography of white oak’s (Quercus 

alba L.) response to climatic variables in North America and 

speculation on its sensitivity to climate change across its 

range. Dendrochronologia. 28: 73–83.

Iverson, L.R.; Prasad, A.M.; Matthews, S. 2008. Modeling 

potential climate change impacts on the trees of the northeastern United States. Mitigation and Adaptation Strategies 

for Global Change. 13: 487–516.

Johnson, M.G.; Kern, J.S. 2003. Quantifying the organic 

carbon held in forested sols of the United States and Puerto 

Rico. In: Kimble, J.M.; Heath, L.S.; Birdsey, R.A.; Lal, R., 

eds. The potential of U.S. forest soils to sequester carbon 

and mitigate the greehouse effect. New York: CRC Press: 

47–72.

Kittredge, D.B. 2004. Extension/outreach implications for 

Americas family forest owners. Journal of Forestry. 102: 

15–18.

Lenihan, J.M.; Bachelet, D.; Neilson, R.P.; Drapek, R. 2008. 

Simulated response of conterminous United States ecosystems to climate change at different levels of fire suppression, 

CO2

 emission rate, and growth response to CO2. Global and 

Planetary Change. 64: 16–25.

Mehmood, S.R.; Zhang, D. 2001. Forest parcelization in the 

United States: a study of contributing factors. Journal of 

Forestry. 99: 30–34.

Mladenoff, D.J.; Pastor, J. 1993. Sustainable forest ecosystems in the northern hardwood and conifer region: concepts 

and management. In: Aplet, G.H.; Olsen, J.T.; Johnson, N.; 

Sample, V.A., eds. Defining sustainable forestry. Washington, DC: Island Press: 145–180.

Nelson, M.D.; Liknes, G.C.; Butler, B.J. 2010. Map of forest ownership in the conterminous United States. [Scale 

1:7,500,000]. Res. Map NRS-2. Newtown Square, PA: 

U.S. Department of Agriculture, Forest Service, Northern 

Research Station.

Nowacki, G.J.; Abrams, M.D. 2008. The demise of fire and 

“mesophication” of forests in the eastern United States. 

BioScience. 58: 123–138.

Swanston, C.W.; Janowiak, M.; Iverson, L. [et al.]. 2011. 

Ecosystem vulnerability assessment and synthesis: a report 

from the Climate Change Response Framework Project in 

northern Wisconsin, Version 1. Gen. Tech. Rep. NRS-82. 

Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station. 142 p.

Walker, K.V.; Davis, M.B.; Sugita, S. 2002. Climate

change and shifts in potential tree species range limits in 

the Great Lakes region. Journal of Great Lakes Research.

28: 555–567.

Wang, C.; Lu, Z.; Haithcoat, T.L. 2007. Using Landsat 

images to detect oak decline in the Mark Twain National 

Forest, Ozark Highlands. Forest Ecology and Management. 

24: 70–78. 

Xu, C.; Gertner, G.; Scheller, R. 2012. Importance of colonization and competition in forest landscape response to global 

climatic change. Climatic Change. 110: 53–83.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Northeast

Lindsey E. Rustad1

Climate is a key regulator of terrestrial biogeochemical 

processes. For the Northeastern United States, the magnitude of climate change observed during the 20th century and 

expected for the 21st century has had, and will continue to 

have, profound effects on the structure, function, and biodiversity of the region’s forests and the ecosystem services 

they provide. A recent synthesis of climate-change effects 

on forests of the region concluded that changes in climate 

that are already underway will result in changes in forest 

species composition, length of growing season, and forest 

hydrology, which together exert significant controls on forest 

productivity and sustainability (Rustad et al. 2009). 

According to an analysis of climatic data from the 

northeastern United States (Huntington et al. 2009), since 

1900, mean annual temperature has risen by an average of 

0.8 o

C, precipitation has increased by approximately 100 

mm, the onset of spring (based on phenologic indicators) 

has advanced by approximately 4 days, streamflows have 

generally increased, and dates of river and lake ice melt have 

advanced by 1 to 2 weeks. Projections for the 21st century 

(based on climatic models and emission scenarios statistically downscaled for the region) suggest that temperature 

will increase by 2.9 to 5.3 °C, precipitation will increase by 

7 to 14 percent (with little or no change in summer precipitation), the onset of spring will advance by 10 to 14 days, riverflows will increase during winter and spring but decrease 

in summer because of increased frequency of short-term 

droughts, and winter ice and snow will diminish. Variability 

and intensity of weather are also expected to increase, with 

more precipitation during large events with longer intervening dry spells, and more frequent and severe extreme events, 

including hurricanes, winter rain, snow, ice storms, droughts, 

and heat waves.

1 Lindsey E. Rustad is a research ecologist, U.S. Department of 

Agriculture, Forest Service, Northern Research Station, 271 Mast 

Road, Durham, NH 03824.

Forests cover large areas of the land surface in the 

Northeastern United States, from 59 percent in Rhode Island 

to upwards of 89 percent in Maine (National Land Cover 

Database 2001). These forests are currently dominated by 

(1) southern hardwoods (oak [Quercus spp.], hickory [Carya

spp.]) and pines [Pinus spp.] in the southernmost region; 

(2) northern hardwoods (American beech [Fagus grandifolia Ehrh.], paper and yellow birch [Betula papyrifera

Marsh., B. alleghaniensis Britt.], and sugar and red maple 

[Acer saccharum Marsh., A. rubrum L.]) in the central part 

and at lower elevations throughout; and (3) boreal-conifer 

forests in the north and at higher elevations (red and black 

spruce [Picea rubens Sarg., P. mariana {Mill.} Britton, 

Sterns & Poggenb.], and balsam fir [Abies balsamea {L.} 

Mill.]). Eastern hemlock (Tsuga canadensis [L.] Carrière), 

an important shade-tolerant, late-successional species, is 

found throughout the northeast. Paleoecological data from 

the region reveal a strong climate signal in current species 

assemblages and show that tree species have shifted in response to a gradually changing climate over the past 12,000 

years since deglaciation. Projecting how the distribution and 

abundance of species will shift in the future in response to 

climate change is complicated by the longevity of current 

individuals in the existing forest, robustness of the genetic 

pool to accommodate adaptation to new climatic conditions, 

limitations on regeneration and dispersal, and interactions 

with factors such as elevated nitrogen (N) deposition, elevated tropospheric ozone, land use change, habitat fragmentation, and changes in disturbance regimes caused by invasive 

species, pathogens, and fire. 

In lieu of projecting future forest composition, some 

researchers have used “climatic envelopes,” which combine 

information on current species distributions with climatic 

projections for the future, based on an ensemble of earth 

system models and emissions scenarios, to generate maps 

of “suitable habitat” for individual species and assemblages 

of species as forest types. For example, Iverson et al. (2008) 

projected that a warming climate will result in a large contraction of suitable habitat for spruce-fir forest, moderate 

decline in suitable habitat for the maple-birch-beech forest, 
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GENERAL TECHNICAL REPORT PNW-GTR-870

and expansion of suitable habitat for oak-dominated forest (fig. A1-7). Projections of change in suitable habitat for 

individual tree species indicate that, of the 84 most common 

species, 23 to 33 will lose suitable habitat under low- and 

high-emission scenarios, 48 to 50 will gain habitat, and 1 

to 10 will experience no change. Under a high-emission 

scenario, the tree species predicted to have the most affected 

habitat include balsam fir, quaking aspen (Populus tremuloides Michx.), paper birch (80 to 87 percent decrease in 

suitable habitat), and black and white oak (Quercus velutina

Lam., Q. alba L.) (more than 100 percent increase in suitable habitat) (Iverson et al. 2008).

As climate and species composition change, so will 

forest productivity and carbon (C) sequestration. More 

favorable climatic conditions for growth, particularly longer 

growing seasons, are correlated with higher productivity, 

whereas climatic extremes such as droughts, extreme cold 

or heat, and windstorms have been linked with tree diebacks 

and periods of lower productivity (Mohan et al. 2009). At 

Hubbard Brook Experimental Forest (New Hampshire), 

green canopy duration increased by 10 days over a 47-year 

period for a northern hardwood forest, suggesting a future 

longer period for growth and higher productivity (Richardson et al. 2006).

Figure A1-7—Suitable habitat for forest vegetation in New England is expected to shift with changes in climate associated with 

different emissions scenarios. From Mohan et al. (2009).
233

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Model projections indicate that forest productivity 

for hardwood species is likely to be enhanced by future 

warmer temperatures, longer growing seasons, and increased 

concentrations of atmospheric carbon dioxide (CO2). For 

example, Ollinger et al. (2008) used the model PnET-CN to 

project that net primary productivity in deciduous forests 

would increase by 52 to 250 percent by 2100, depending 

on the global model and emissions scenario used. The same 

model projected that current-day spruce forests are likely to 

show a climate-driven decrease in productivity along with 

a contraction of range. The effects of changing tree species 

assemblages and concurrent stress associated with forest 

fragmentation, atmospheric pollution, and invasive plant and 

animal species complicate these projections. 

Changes in climate, hydrology, and forest tree species composition will have cascading effects on associated 

biogeochemical processes in forest ecosystems. Warmer 

temperatures and extended growing seasons will probably 

increase rates of microbial decomposition, N mineralization, 

nitrification, and denitrification, which will provide increased short-term availability of nutrients such as calcium, 

magnesium, and N for forest growth, as well as the potential 

for elevated losses of these same nutrients to surface waters 

(Campbell et al. 2009). Even under a low emission scenario, 

forests may respond to climate change with significant increases in nitrate leaching from soils to surface waters, with 

consequences for downstream water quality and eutrophication (Campbell et al. 2009). Potential accelerated loss of 

calcium and magnesium, especially from areas that have 

already experienced loss of these nutrients owing to decades 

of acidic deposition, may increase soil acidification in the 

region. Warmer temperatures will also probably increase 

rates of root and microbial respiration, with an increased 

release of CO2

 from the soil to the atmosphere. Because soil 

respiration is the second-largest flux in the global C cycle 

(only primary productivity is larger), any increase in soil 

respiration may contribute to further warming. Soil-warming 

experiments in the Northeastern United States have confirmed this relationship (Rustad et al. 2000).

A major unknown in predicting warming-mediated 

biogeochemical responses is the potential interaction with 

projected future short- and long-term droughts, which tend 

to reduce physiological activity or induce dormancy during periods of stress. Current projections suggest that future 

summers will be warmer, and total precipitation will remain 

constant but occur as larger events separated by longer 

dry periods (Huntington 2009). Coupled with potentially 

higher productivity, evaporation and transpiration will likely 

increase, resulting in lower soil moisture during the growing 

season. This has implications for direct effects on biological activity, as well as for wildfire frequency and severity, 

streamflow, and lake levels.

Climate change will affect the distribution and abundance of many wildlife species in the region through 

changes in habitat, food availability, thermal tolerances, 

species interactions such as competition, and susceptibility 

to parasites and pathogens (Rodenhouse et al. 2009). Decades of survey data show that migratory birds are arriving 

earlier and breeding later in response to recent warming, 

with consequences for the annual production of young and 

survival (Rodenhouse et al. 2009). Among 25 species of 

resident birds studied, 15 are increasing in abundance, which 

is consistent with the observation that ranges of these species 

are limited by winter climate. Of the remaining species, five 

are decreasing in abundance, including highly valued species 

such as ruffed grouse (Bonasa umbellus L.), and five show 

no change. Significant range expansions have also been observed, with 27 of 38 species studied expanding their ranges 

in a northward direction (fig. A1-8).

Using a climatic envelope approach, Rodenhouse et al. 

(2009) projected that twice as many resident bird species are 

expected to increase in abundance as decrease; for migrants 

(which comprise more than 85 percent of the avifauna), an 

equal number are expected to increase as decrease. “Winners” (increased abundance) include pileated woodpecker 

(Dryocopus pileatus L.) (+15 to 50 percent), great horned 

owl (Bubo virginianus Gmelin) (+18 to more than 200 

percent), and northern cardinal (Cardinalis cardinalis L.) 

(+20 to 33 percent). “Losers” (decreased abundance) include 

common loon (Gavia immer Brunnich) (-76 to -93 percent), 

winter wren (Troglodytes hiemalis Viellot) (-42 to -73 percent), and rose-breasted grosbeak (Pheucticus ludovicianus 

L.) (-23 to -71 percent). Species such as Bicknell’s thrush 
234

GENERAL TECHNICAL REPORT PNW-GTR-870

Figure A1-8—Climate change is expected to affect bird species richness more intensely in some areas of the Northeastern 

United States than in others. From Rodenhouse et al. (2008).
235

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

(Catharus bicknelli Ridgway), which inhabit high-elevation 

spruce-fir forests, are especially susceptible to climate 

change, because half the suitable habitat available for this 

species is projected to be lost with an increase of only 1 oC 

in mean annual temperature (Rodenhouse et al. 2008). 

Climate-related historical and future projected changes 

in native and introduced insects and pathogens deserve 

special mention because these species contribute heavily to 

disturbance in Northeastern forests, and some species are 

particularly adept at adjusting to changing climatic conditions (see table A1-3). Direct effects of climate change on 

these species are likely to include summer warming-induced 

acceleration of reproductive and development rates, winter 

warming-induced increase in the ability to overwinter, and 

moisture-related changes in survival and fecundity. If minimum winter temperature increases as projected, this may allow the northward migration of many unwanted species. For 

example, hemlock wooly adelgid (Adelges tsugae Annand) 

(HWA) is distributed in areas where minimum winter temperatures stay above -28.8 °C (Skinner et al. 2003). Based 

on recent projections, climatic warming could allow HWA to 

spread unimpeded throughout the range of eastern hemlock 

distribution. The potential effects of widespread hemlock 

mortality include changes in forest composition, structure, 

nutrient cycling, surface water quality, and populations of 

associated wildlife (Dukes et al. 2009). Indirect effects of 

climate change include changes in (1) nutrient supply, availability, and allocation in space and time; (2) distribution, 

life cycles, and phenology of insects, pathogens, and their 

hosts, predator associates, or competitors; (3) mismatches 

between location and optimal conditions, resulting in tree 

stress and increased pathogen susceptibility; and (4) creation 

of novel species assemblages or mismatches between locally 

or regionally coadapted genotypes that could enhance the 

potential for infestation and virulence. 

Climate change is expected to be characterized by an 

increase in the prevalence and severity of extreme events, 

such as heat waves, cold waves, windstorms, floods, and 

droughts (Huntington et al. 2009, Solomon et al. 2007). A 

growing concern exists that these types of events can have a 

larger effect on natural and managed systems than the more 

gradual change in mean climatic conditions. Legacies of 

Table A1-3—Common native and nonnative insects, pathogens, and invasive species of the northeastern 

United States (Dukes et al. 2009)a

Insects Pathogens Invasives

Balsam woolly adelgid

(Adelges piceae Ratzeburg)

Armillaria root rot 

(Armillaria spp.)

Tree-of-heaven

(Ailanthus altissima [P. Mill.] Swingle)

Hemlock woolly adelgid

(Adelges tsugae Annand)

White pine blister rust (Cronartium ribicola A. Dietr.)

Garlic-mustard 

(Alliaria petiolata [M. Bieb.] Cavara & 

Grande)

Emerald ash borer 

(Agrilus planipennis Fairmaire)

Chestnut blight 

(Cryphonectria parasitica [Murrill] 

Barr)

Japanese barberry 

(Berberis thunbergii DC.)

Asian longhorned beetle 

(Anoplophora glabripennis

Motschulsky)

Beech bark disease (Neonectria faginata

[M.L. Lohman, A.M.J. Watson & Ayers] 

Castl. & Rossman)

Japanese stiltgrass 

(Microstegium vimineum [Trin.] A. 

Camus)

Spruce budworm (Choristoneura 

fumiferana Clemens)

Dutch elm disease (Ophiostoma ulmi

[Buisman] Nannf. and O. novo-ulmi

Brasier)

Multiflora rose 

(Rosa multiflora Thunb.)

Gypsy moth 

(Lymantria dispar L.)

White trunk rot 

(Phellinus spp.). 

Wine raspberry

(Rubus phoenicolasius Maxim.)

Forest tent caterpillar 

(Malacosoma disstria Hübner)

Sudden oak death (Phytophthora ramorum S. Werres, A.W.A.M. de Cock)

a

 No relationship is implied from left to right in the table.
236

GENERAL TECHNICAL REPORT PNW-GTR-870

past extreme windstorms and ice storms are apparent across 

the forested landscape of the region. It is imperative for the 

scientific and land management communities to better understand and anticipate the future occurrence and effects of 

these extreme events on forest composition and productivity, 

biogeochemistry, wildlife, insects, pathogens, and invasive 

species.

The 20th century climate of the Northeastern United 

States has changed more rapidly than at any time since 

the last glaciation, and this rate of change is expected to 

continue throughout the 21st century. The direct and indirect 

effects of climate change on Northeastern forests, individually and in combination with other stressors such as acid 

deposition, N and mercury deposition, tropospheric ozone, 

and various land uses, have the potential to cause significant 

changes in ecosystem structure and function. Additional research on indirect and interacting effects of these changes on 

forest ecosystems will be especially valuable for understanding potential effects of climate change, and for developing 

adaptation options that will enhance the sustainability of the 

diverse forests of this region.
237

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Literature Cited

Campbell, J.L.; Rustad, L.E.; Boyer, E.B. [et al.]. 2009.

Consequences of climate change for biogeochemical cycling in forests of northeastern North America. Canadian 

Journal of Forest Research. 39: 264–284.

Dukes, J.S.; Pontius, J.; Orwig, D. 2009. Responses of 

insect pests, pathogens, and invasive plant species to climate change in the forests of northeastern North America: 

What can we predict? Canadian Journal of Forest Research. 39: 231–248.

Huntington, T.G.; Richardson, A.D.; McGuire, K.J.; 

Hayhoe, K. 2009. Climate and hydrological changes in 

the northeastern United States: recent trends and implications for forested and aquatic ecosystems. Canadian 

Journal of Forest Research. 39: 199–212.

Iverson, L.; Prasad, A.; Matthews, S. 2008. Potential 

changes in suitable habitat for 134 tree species in the 

northeastern United States. Mitigation and Adaptation 

Strategies for Global Change. 13: 487–516. 

Mohan, J.E.; Cox, R.M.; Iverson, L.R. 2009. Composition and carbon dynamics of forests in northeastern North 

America in a future, warmer world. Canadian Journal of 

Forest Research. 39: 213–230.

National Land Cover Database. 2001. Multi-resolution 

land characteristics consortium: national land cover database. http://www.mrlc.gov/nlcd01_data.php. (23 December 2011).

Ollinger, S.V.; Goodale, C.L.; Hayhoe, K.; Jenkins, J.P. 

2008. Potential effects of climate change and rising CO2

on ecosystem processes in northeastern U.S. forests. 

Mitigating Adaptation Strategies for Global Change. 13: 

467–485.

Richardson, A.D.; Bailey, A.S.; Denny, E.G. [et al.]. 2006.

Phenology of a northern hardwood forest canopy. Global 

Change Biology. 12: 1174–1188.

Rodenhouse, N.L.; Christenson, L.M.; Parry, D.; Green, 

L.E. 2009. Climate change effects on native fauna of 

northeastern forests. Canadian Journal of Forest Research. 39: 249–263.

Rodenhouse, N.L.; Matthews, S.N.; McFarland, K.P. [et 

al.]. 2008. Potential effects of climate change on birds 

of the Northeast. Mitigation and Adaption Strategies for 

Global Change. 13: 517–540.

Rustad, L.E.; Campbell, J.L.; Cox, R.M. [et al.]. 2009.

NE forests 2100: a synthesis of climate change impacts 

on forests of the northeastern US and eastern Canada. 

2009. Canadian Journal of Forest Research. 39: iii–iv.

Rustad, L.E.; Melillo, J.M.; Mitchell, M.J. [et al.]. 2000.

Effects of soil warming on carbon and nitrogen cycling. 

In: Mickler, R.; Birdsey, R.; Hom, J., eds. Responses 

of northern U.S. forests to environmental change. New 

York: Springer-Verlag: 357–381.

Skinner, M.; Parker, B.L.; Gouli, S.; Ashikaga, T. 2003.

Regional responses of hemlock woolly adelgid (Homoptera: Adelgidae) to low temperatures. Environmental 

Entomology. 32: 523–528.

Solomon, S.; Qin, D.; Manning, M. [et al.], eds. 2007.

Climate change 2007: the physical science basis: contribution of working group I to the fourth assessment 

report of the Intergovernmental Panel on Climate Change. 

Cambridge, United Kingdom; New York: Cambridge 

University Press. 996 p.
238

GENERAL TECHNICAL REPORT PNW-GTR-870

Southeast

Steven G. McNulty1

Forests of the Southeastern United States are a complex 

mixture of private and public land, interspersed with rapidly 

urbanizing areas and agriculture. A long history of active 

forest management, often including intensive management 

such as forest plantations, fertilization, and prescribed fires, 

creates stand conditions and management regimes that differ 

from those in other areas of the United States. For example, 

relative to forests of the Western United States, smaller tracts 

of accessible forest land may be more amenable to management actions that can be used to mitigate carbon (C) emissions or help forests adapt to climate change. On the other 

hand, the large private ownership of relatively small forest 

land holdings makes it challenging to implement uniform or 

coordinated large-scale management activities.

Wildfires, hurricanes, drought, insect outbreaks, and 

pathogen outbreaks have been a driving force for millennia in southeastern forests. However, during the past two 

centuries, the type and magnitude of ecosystem stress and 

disturbance have changed and will likely continue to change 

as the climate warms (Dale et al. 2001). Wind and extreme 

precipitation events associated with hurricanes can have 

significant effects on southeastern forests. A single hurricane 

can reduce total forest C sequestration by 10 percent in the 

year in which it occurs (McNulty 2002), although not all forest species are equally susceptible to wind damage. Longleaf 

pine (Pinus palustris Mill.) shows less damage than does 

loblolly pine (Pinus taeda L.) when exposed to an equal 

level of wind stress (Johnsen et al. 2009), suggesting that 

the former species would be more resistant to an increase in 

windstorms. Extreme precipitation events that accompany 

hurricanes can cause extended submersion of low-lying 

forests, which can kill tree roots by causing anaerobic soil 

conditions (Whitlow and Harris 1979). 

1

 Steven G. McNulty is a research ecologist, U.S. Department of 

Agriculture, Forest Service, Southern Research Station, 920 Main 

Campus Drive, Venture Center 2, Suite 300, Raleigh, NC 27606.

Wildfires are a natural component of ecosystem maintenance and renewal in the southeast, which has more 

area burn annually, with wildfire and prescribed fire, than 

any other region of the United States (except Alaska in 

some years) (Andreu and Hermansen-Baez 2008). However, decades of fire exclusion coupled with increasing air 

temperatures have increased the potential for crown fire in 

some southeastern forests. Future fire potential is expected 

to increase from low to moderate in summer and autumn in 

eastern sections in the South, and from moderate to high in 

western portions of the South (Liu et al. 2010). As fire seasons lengthen in the future, the window for prescribed burning may decrease because of increased fuel flammability, 

thus potentially affecting the management of fuels and C dynamics; fuel treatments with prescribed fire emit 20 percent 

less carbon dioxide (CO2) than wildfires, at least in the short 

term (Wiedinmyer and Hurteau 2010). Historically, longleaf 

pine was a dominant species across the region. It is well 

adapted to drought, with thick bark and fast seedling growth, 

allowing it to thrive in habitats subjected to periodic wildfire 

(Brockway et al. 1997). Most of the longleaf pine was cut 

during the 20th century and replanted with the faster growing 

loblolly pine, which is preferred by the timber industry but 

is less resistant to wildfire damage. Land managers are reassessing the preferential use of loblolly pine, because longleaf 

pine would be more resistant to the increased fire, drought, 

and wind expected with climate change.

Insect and pathogen outbreaks are increasing in southeastern forests (Pye et al. 2011). Higher temperature has 

caused a longer growing season of at least 2 weeks compared to historical lengths, allowing additional time for 

insects and pathogens to find trees that are more susceptible 

and to colonize trees to form new points of spread (Ayres 

and Lombardero 2000). In addition, timing of the predatorprey cycle may be changing. For example, when the growing 

season begins earlier, insects may be hatching and maturing 

before migratory insectivorous bird species return, allowing more insects to reach maturity, speed up the reproductive cycle, and locate susceptible host trees. Finally, higher 

temperature and subsequent soil drying increases stress in 
239

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

trees, reducing their physiological capacity to resist attack 

(McNulty et al. 1998a). If the trend of increasing frequency 

and severity of insect outbreaks continues, the productivity 

and large-scale structure of forest ecosystems will be altered 

significantly.

Some aspects of the exceptionally high biodiversity 

in the Southeast may be susceptible to climate change 

(Thompson et al. 2009), particularly species that are near the 

environmental limit of their range. Red spruce (Picea rubens

Sarg.) and eastern hemlock (Tsuga canadensis [L.] Carrière) 

are well adapted to the cool climates of the last glacial age. 

However, during a period of postglacial warming, the extent 

and dominance of these two species have decreased greatly 

owing to stress complexes that include warmer temperature, 

air pollution, and insects (Elliott and Vose 2011, McNulty 

and Boggs 2010). With further warming, red spruce and 

eastern hemlock are projected to be extirpated from the 

Southern United States before the end of the 21st century 

(Prasad et al. 2007), and small remnant populations of balsam fir (Abies balsamea [L.] Mill.) will also be at risk. Birds 

and other terrestrial vertebrate species that depend on forests 

dominated by these trees for habitat and food must adapt, 

migrate, or face a similar fate. 

Cold water fish species, which are generally confined 

to northern and mountainous areas of the Southeast where 

cooler water (and air) temperatures allow dissolved oxygen 

contents to remain at sufficient levels, will likely face increased stress from higher temperature at the southern limit 

of their range. In addition, rainfall intensity has been increasing for over a century (Karl et al. 1995), which can in turn 

increase soil erosion and stream turbidity (Trimble 2008). 

A combination of higher air temperature and lower water 

quality may significantly reduce trout abundance across the 

southeast during the coming decades (Flebbe et al. 2006). 

The majority of the Nation’s wood and fiber is produced 

in the southeast, but climate change could significantly 

alter productive capacity in the region (Wertin et al. 2010). 

Loblolly pine is the most important commercial species in 

the southeast, and although current air temperature is near 

optimal for growth across much of its range, as temperature 

Figure A1-9—Percentage change in water supply stress owing to climate change, as defined by the water supply stress index 

(WaSSI) for 2050 using the CSIROMK2 B2 climate scenario. WaSSI is calculated by dividing water demand by supply, where 

higher values indicate higher stress on watersheds and water systems. From Lockaby et al. 2011.
240

GENERAL TECHNICAL REPORT PNW-GTR-870

continues to increase, conditions for pine growth may begin 

to deteriorate (McNulty et al. 1998b). Even if regional forest 

productivity remains high, the center of forest productivity 

could shift farther north into North Carolina and Virginia, 

causing significant economic and social effects in those 

areas gaining and losing timber industry jobs (Sohngen et al. 

2001). 

Carbon sequestration is an increasingly valued component of forest productivity. Globally, forests sequester up 

to 16 percent of all the CO2 generated from the burning of 

fossil fuels, and in the United States, much of this storage 

occurs in Southeastern forests (Pan et al. 2011). In addition 

to potentially reducing forest productivity (and therefore C 

uptake), climate change could increase decomposition of 

soil organic matter and CO2 release in the Southeast (Boddy 

1993). When added to the potential for increased wildfires, 

the potential for ecosystem C sequestration may decrease 

in the future, and the ecosystem value of sequestered forest 

C may shift from the Southern to Northern United States 

(Hurteau et al. 2008).

Abundant, year-round rainfall has historically provided 

a sufficient supply of water for industrial, commercial, 

residential, agricultural, and hydro-electric use in the southeast, but several factors may contribute to a shift in water 

abundance. The population of the southeast is increasing 

and much of this increase is centered on metropolitan areas, 

whereas much of the water originates in forested headwaters, often long distances from urban areas. On an average 

annual basis, water supply is approximately 20 times higher 

than demand, although short-term (1 to 3 years) drought can 

significantly increase pressure on available water (Lockaby 

et al. 2011) (fig. A1-9). A combination of increased population, changing land use patterns, and shifts in rainfall 

patterns could further amplify water shortages, and even if 

precipitation rates remain unchanged, higher tree water use 

with higher air temperature, or shifting management regimes 

for new products such as biofuels, could contribute to water 

shortfalls (Lockaby et al. 2011, Sun et al. 2008). The seasonal timing of precipitation within the year could also affect 

water supply. If precipitation occurs in fewer, more intense 

events, then proportionally less water will be retained by forest ecosystems, and more will be lost as runoff, potentially 

causing flooding, soil erosion, and stream sedimentation 

(Trimble 2008). 

The Southeast has diverse year-round recreational opportunities, some of which could be severely affected by 

climate change. Many Southeastern ski areas are marginally 

profitable, and increased winter warming may increase the 

proportion of rain to snow and prevent snow making (Millsaps and Groothuis 2003). Reduced quality or quantity of 

the ski season could force most of the marginal ski areas to 

permanently close. Similarly, cold water fisheries are a major recreational attraction, and revenues from lodging, food, 

and secondary activities are a major economic boost to local 

mountain economies. Therefore, extirpation of trout from 

these areas could significantly harm the recreation industry.
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forest ecosystems in a changing environment. New York: 

Springer-Verlag: 617–634. 

McNulty, S.G.; Vose, J.M.; Swank, W.T. 1998b. Predictions and projections of pine productivity and hydrology 

in response to climate change across the southern United 

States. In: Mickler, R.A.; Fox, S.A., eds. The productivity and sustainability of Southern forest ecosystems in 

a changing environment. New York: Springer-Verlag: 

391–406.

Millsaps, W.; Groothuis, P.A. 2003. The economic

impact of North Carolina ski areas on the economy

of North Carolina: 2002–2003 season. North Carolina

Ski Areas Association. http://www.goskinc.com.

(15 September 2012).

Pan, Y.; Birdsey, R.A.; Fang, J. [et al.]. 2011. A large and 

persistent carbon sink in the world’s forests, 1990–2007. 

Science. 333: 988–993.
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Prasad, A.M.; Iverson, L.R.; Matthews, S.; Peters, M. 

2007-ongoing. A climate change atlas for 134 forest tree 

species of the eastern United States. Delaware, OH: U.S. 

Department of Agriculture, Forest Service, Northern Research Station. [Database] http://www.nrs.fs.fed.us/atlas/

tree. (15 September 2012).

Pye, J.M.; Holmes, T.P.; Prestemon, J.P.; Wear, D.N. 

2011. Economic impacts of the southern pine beetle. In: 

Coulson, R.N.; Klepzig, K.D., eds. Southern pine beetle 

II. Gen. Tech. Rep. SRS-140. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research 

Station: 213–222.

Sohngen, B.; Mendelsohn, R.; Sedjo, R. 2001. A global 

model of climate change impacts on timber markets. 

Journal of Agricultural and Resource Economics. 26: 

326–343.

Sun, G.; McNulty, S.G.; Moore Myers, J.A.; Cohen, E.C. 

2008. Impacts of stresses on water demand and supply 

across the southeastern United States. Journal of the 

American Water Resources Association. 44: 1441–1457.

Thompson, I.; Mackey, B.; McNulty, S.; Mosseler, A. 

2009. Forest resilience, biodiversity, and climate change. 

Technical Series no. 43. Montreal, Canada: Secretariat of 

the Convention on Biological Diversity. 67 p. 

Trimble, S.W. 2008. Man-induced soil erosion on the 

Southern Piedmont: 1700–1970, 2nd ed. Ankeny, IA: Soil 

and Water Conservation Society. 80 p. 

Wertin, T.M.; McGuire, M.A.; Teskey, R.O. 2010. The 

influence of elevated temperature, elevated atmospheric 

CO2 concentration and water stress on net photosynthesis 

of loblolly pine (Pinus taeda L.) at northern, central and 

southern sites in its native range. Global Change Biology. 

16: 2089–2103. 

Whitlow, T.H.; Harris, R.W. 1979. Flood tolerance in 

plants: a state-of-the-art review. Washington, DC: U.S. 

Department of Commerce, National Technical Information Service: 1–161.

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means of reducing forest carbon emissions in the western 

United States. Environmental Science and Technology. 

44: 1926–1932.
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Appendix 2: Risk-Based Framework and Risk Case Studies

Risk-Based Framework for Evaluating Changes in Response Thresholds 

and Vulnerabilities

Dennis S. Ojima, Louis R. Iverson, and Brent L. Sohngen1

What is “risk,” and how can a “risk-based framework” help 

plan for climate change? Risk is described by the likelihood 

of an impact occurring and the magnitude of the consequences of the impact (Yohe 2010) (fig. A2-1). High-magnitude impacts are always risky, even if their probability of 

occurring is low; low-magnitude impacts are not very risky, 

even if their probability of occurring is high. Applying this 

approach to forest management is difficult because both the 

likelihood of occurrence and the magnitude of the effects 

may be difficult to estimate (especially at local scales) and 

often depend on past and current land use, and the timing, 

frequency, duration, and intensity of multiple chronic and 

acute climatic disturbances. 

Despite these challenges, there is much that we do know 

and it is possible to begin thinking about approaches for 

developing a risk-based framework for forests in the context 

of climate change. A risk management framework simply 

means that risks are identified and estimates are made for 

their probability of occurrence and their impact. Where we 

have sufficient knowledge, this framework provides a means 

to quantify what is known, identify where uncertainties exist, and help managers and decisionmakers develop strategies with better knowledge of risks. 

Climate change is likely to affect forest ecosystems, and 

the risk of negative consequences to forests and associated 

socioecological systems will probably increase (Ryan and 

Archer 2008). However, predicting these risks is difficult 

because of uncertainty in almost all aspects of the problem.

1 Dennis S. Ojima is a senior research scientist, Colorado State 

University, NESB B231, Natural Resource Ecology Laboratory, 

Campus Mail 1499, Fort Collins, CO 80523; Louis R. Iverson is 

a research landscape ecologist, U.S. Department of Agriculture, 

Northern Research Station, 359 Main Road, Delaware, OH 43015; 

Brent L. Sohngen is a professor, Ohio State University, Department 

of Agricultural, Environmental, and Development Economics, 103 

Ag Administration, 2120 Fyffe Road, Columbus, OH 43210.

Figure A2-1—A conceptual risk framework used to help identify 

risks associated with climate change and prioritize management 

decisions (Yohe and Leichenko 2010). Colors represent varying 

degrees of risk (red = highest; yellow = lowest). In a qualitative 

definition of consequence, low = climate change is unlikely to have 

a measurable effect on structure, function, or processes within a 

specified timeframe (e.g., 2030s, 2050s, 2090s); medium = climate 

change will cause at least one measurable effect on structure, function, or processes within a specified timeframe; and high = climate 

change will cause multiple or irreversible effects on structure, 

function, or processes within a specified timeframe. In a qualitative 

definition of likelihood, low = climate change impacts are unlikely 

to be measurable within the specified timeframe, medium = climate 

change impacts are likely to be measurable within the specified 

timeframe, and high = climate change impacts are very likely (or 

have already been observed) within or before the specified timeframe.

How can we incorporate uncertainty into an analysis of risks 

and subsequent management decisions?

Regional and local projections of climate change are 

uncertain (Baron et al. 2008, Fagre et al. 2009, Joyce et al. 

2008). Despite these uncertainties, climate science has advanced to provide a set of robust climate change projections: 

the climate is warming, the probability of large precipitation 

events is increasing, seasonal patterns will be altered, and 
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GENERAL TECHNICAL REPORT PNW-GTR-870

extreme events are more likely (Solomon et al. 2007). These 

tendencies are becoming more apparent in observations 

across the United States and will affect forest resources nationwide (Karl et al. 2009). A key challenge is to determine 

how climate change may alter local socioecological systems, 

trigger threshold-dependent events, and create nonlinear interactions across interconnected stressors on forest resources 

(Allen et al. 2010, Fagre et al. 2009), and further, how climate change effects can be addressed by local management 

actions. Forest managers have extensive experience adapting forest management practices to climate variability and 

a wide range of disturbance regimes. For example, conifer 

plantations are often managed in short rotations, which limits exposure to risks from insects, wildfires, and windstorms. 

In mixed-age hardwood forests where management is often 

less intensive (e.g., where partial harvests are the norm), 

managers simultaneously choose trees to remove and trees in 

the understory to release for the next generation of growth. 

Hence, by using silvicultural techniques to select the species, 

density, and age class distribution of the next generation of 

forest, managers can influence susceptibility to a range of 

future threats. 

Given what we do know about climate change, a robust 

decisionmaking approach is needed that acknowledges 

sources of uncertainty, incorporates what is known of system 

vulnerabilities, and evaluates assets critical for making 

sound forest management decisions (Australian Government 2005, Baron et al. 2008, Fagre et al. 2009, Joyce et al. 

2008, Ranger and Garbett-Shiels 2011). A risk management 

approach provides a robust framework for planning management options for climate change, where uncertainties are 

recognized and key elements relative to various management 

objectives and priorities are explicitly addressed (Dessai 

and Wilby 2011, McInerney and Keller 2008, Ranger and 

Garbett-Shiels, 2011, Yohe and Leichenko 2010). This approach incorporates aspects of vulnerability assessments, 

identifies priority actions relative to multiple management 

goals, identifies critical information needs, and provides a 

vision of short- and long-term strategies to enhance the flexibility of management decisions and reduce the probability 

of poor decisions (Australian Government 2005, Peterson et 

al. 2011). This approach also promotes a shift from reactive 

adaptation to proactive adaptation and coping management 

(Ranger and Garbett-Shiels 2011), including the following 

general strategy:

• Identify actions to avoid, that is, avoid choices that lead 

to less flexibility to adjust to changing conditions in the 

future.

• Implement “no regrets” management to cope with 

stresses now and increase resilience to anticipated 

climate-related stresses.

• Make decisions that integrate across landscapes and 

governance and that include all concerned and affected 

stakeholders. 

• Develop activities that have strong links among 

observations, research, and management to understand 

how ecosystems and social systems are changing, help 

make decisions, understand thresholds, and help adjust 

future management and research.

The risk framework must consider the socioecological 

context of the system being evaluated, reflecting the contribution of forest ecosystem services to different communities 

and the capability of forest systems to withstand different 

climate effects. Providing a more thorough consideration of 

sources of uncertainty allows for improved development of 

management strategies, which include key socioeconomic 

properties. This integrated and multisectoral approach will 

incorporate an improved assessment of risk and current 

management capacity, and will identify critical uncertainties 

that may exist under future scenarios if novel consequences 

emerge. 

Case studies using a risk-based framework and concepts 

are discussed in the following sections on carbon, fire, forests, and birds. They are intended as examples, using different approaches to convey risk assessment, and will hopefully 

create interest by scientists and land managers in refining 

risk assessments for the effects of climate change on a wide 

range of forest resources.
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Literature Cited

Allen, C.D.; Macalady, A.K.; Chenchouni, H. [et al.]. 

2010. A global overview of drought and heat-induced 

tree mortality reveals emerging climate change risks for 

forests. Forest Ecology and Management. 259: 660–684.

Australian Government. 2005. Climate change risk and 

vulnerability: promoting an efficient adaptation response 

in Australia. Canberra: Department of the Environment 

and Heritage, Australian Greenhouse Office. 159 p.

Baron, J.S.; Allen, C.D.; Fleishman, E. [et al.]. 2008. National parks. In: Julius, S.H.; West, J.M., eds. Preliminary 

review of adaptation options for climate-sensitive ecosystems and resources: a report by the U.S. Climate Change 

Science Program and the Subcommittee on Climate 

Change Research. Washington, DC: U.S. Environmental 

Protection Agency: 4-1 to 4-68.

Dessai, S.; Wilby, R. 2011. How can developing country

decision makers incorporate uncertainty about climate 

risks into existing planning and policymaking processes?

World Resources Report Uncertainty Series. Washington, 

DC: World Resources Institute.

http://www.worldresourcesreport.org/decision-making-indepth/managing-uncertainty. (15 September 2012).

Fagre, D.B.; Charles, C.W.; Allen, C.D. [et al.]. 2009. 

Thresholds of climate change in ecosystems: a report by 

the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Reston, VA: U.S. 

Department of the Interior, Geological Survey. 70 p.

Joyce, L.A.; Blate, G.M.; Littell, J.S. [et al.]. 2008. National forests. In: Julius, S.H.; West, J.M., eds. Preliminary review of adaptation options for climate-sensitive 

ecosystems and resources: a report by the U.S. Climate 

Change Science Program and the Subcommittee on Climate Change Research. Washington, DC: U.S. Environmental Protection Agency: 3-1 to 3-127. 

Karl, T.R.; Melillo, J.M.; Peterson, T.C.; Hassol, S.J., 

eds. 2009. Global climate change impacts in the United 

States. A report of the U.S. Global Change Research Program. Cambridge, United Kingdom: Cambridge University Press. 192 p.

McInerney, D.; Keller, K. 2008. Economically optimal 

risk reduction strategies in the face of uncertain climate 

thresholds. Climatic Change. 91: 29–41.

Peterson, D.L.; Millar, C.I.; Joyce, L.A. [et al.]. 2011.

Responding to climate change on national forests: a 

guidebook for developing adaptation options. Gen. Tech. 

Rep. PNW-GTR-855. Portland, OR: U.S. Department of 

Agriculture, Forest Service, Pacific Northwest Research 

Station. 109 p.

Ranger, N.; Garbett-Shiels, S-L. 2011. How can decisionmakers in developing countries incorporate uncertainty 

about future climate risks into existing planning and policy-making processes? World Resources Report Uncertainty Series. Washington, DC: World Resources Institute. 

http://www.worldresourcesreport.org/decision-making-indepth/managing-uncertainty. (15 September 2012).

Ryan, M.G.; Archer, S.R. 2008. Land resources: forests 

and arid lands. In: Backlund, P.; Janetos, A.; Hatfield, J. 

[et al.], eds. The effects of climate change on agriculture, 

land resources, water resources, and biodiversity. A report 

by the U.S. Climate Change Science Program and the 

Subcommittee on Global Change Research. Washington, 

DC: U.S. Environmental Protection Agency: 75–120.

Solomon, S.; Quin, D.; Manning, M. [et al.], eds. 2007. 

Climate change 2007: the physical science basis. Contribution of working group I to the fourth assessment 

report of the Intergovernmental Panel on Climate Change. 

Cambridge, United Kingdom: Cambridge University 

Press. 996 p.

Yohe, G. 2010. Risk assessment and risk management for 

infrastructure planning and investment. The Bridge. 

40(3): 14–21.

Yohe, G.; Leichenko, R. 2010. Adopting a risk-based approach. Annals of the New York Academy of Sciences. 

1196: 29–40.
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Forest ecosystems have the ability to reduce the effects of 

climate change through the sequestration of carbon (C) (Pan 

et al. 2011) as well as contribute to net emissions through 

disturbance events such as wildfires and widespread tree 

mortality (Kurz et al. 2008). A conceptual framework for assessing climate-change risks to forest ecosystem C stocks facilitates efficient allocation of efforts to monitor and mitigate 

climate-change effects, and the U.S. National Greenhouse 

Gas Inventory (NGHGI) of forest C stocks (Heath et al. 

2011) may be used as a basis for development of a climatechange risk framework for forest C stocks.2

A forest C stock risk framework incorporates two 

components of risk: consequence and likelihood (fig. A2-2). 

One of the most critical future consequences of climate 

change on forest C stocks is the shift from C sink (net annual 

sequestration) to C source (net annual emission). Although 

global forests currently sequester more carbon than they 

emit on an annual basis (Pan et al. 2011), the ability of forests to continue this trend in the future has been questioned 

(Birdsey et al. 2006, Reich 2011). If the strength of the C 

sink decreases and forests became net emitters of C and 

other greenhouse gasses (GHG) (e.g., methane) a positive 

feedback loop may be created whereby negative climate 

change effects may further exacerbate forest C emissions. 

Likelihood can be phrased as the probability of a C stock 

becoming a net emitter of C. For individual C stocks that are 

least affected over short timespans (e.g., 50 to 100 years), 

likelihoods would be minimal. Taken together, the C risk 

framework hinges on the concepts of both a “status change,” 

1 Christopher W. Woodall and Grant M. Domke are research 

foresters, U.S. Department of Agriculture, Forest Service, Northern 

Research Station, 1992 Folwell Avenue, St. Paul, MN 55108.

2

 Woodall, C.W.; Domke, G.M.; Riley, K.L. [et al.]. Developing a framework for assessing climate change risks to U.S. forest 

carbon stocks across large temporal and spatial scales. Unpublished 

manuscript. On file with: USDA Forest Service, Northern Research 

Station, 1992 Folwell Ave, St. Paul, MN 55108.

Risk Case Study: A Framework for Assessing Climate Change Risks to 

Forest Carbon Stocks

Christopher W. Woodall and Grant M. Domke1

in which forest C stocks transition between C source or sink, 

and a “tipping point”, when forest systems might collapse 

with concomitant emission of C and potential positive feedbacks that may exacerbate climate change.

The consequences of a C stock becoming a net emitter 

of C is postulated as being directly related to its population 

estimate over a region of interest. In this case study, it is 

the C stocks of individual forest pools for the entire United 

States as reported to the Intergovernmental Panel on Climate 

Change to meet United Nations Framework Convention on 

Climate Change requirements (USEPA 2011a, 2011b). If a 

pool is the largest in the United States, then that pool has the 

largest consequence on global climate change if it is entirely 

emitted. All current U.S. forest C stocks represent nearly 

25 years of U.S. GHG emissions at current emission rates 

(Woodall et al. 2011). The pools and estimates (Tg of C) 

of C stocks in 2008 (Heath et al. 2011) are ordered as: soil 

organic carbon (17,136 Tg of C), aboveground live biomass 

(16,854 Tg of C), forest floor (4,925 Tg of C), belowground 

biomass (3,348 Tg of C), and dead wood (3,073 Tg of C).

The likelihood of any individual C stock becoming a 

net emitter of C is an emerging area of research. For the purposes of this risk framework (fig. A2-2), it is proposed that 

the likelihood of a C stock becoming a net emitter is related 

to the empirical variation in the stock across the diverse ecosystems and climates of the United States. If climate change 

occurs such that a mesic boreal forest ecosystem becomes 

a xeric mixed-hardwood shrubland, then the contemporary 

range in variation in C stocks between those systems should 

indicate likelihood of C emission. For example, if forest 

floor C stocks change minimally regardless of climate, then 

in turn climate change should least affect these stocks. As 

an initial appraisal of empirical variation in C stocks across 

the United States, the coefficients of variation (percentage) 

of individual plot-scale measurements of C stocks (Forest 

Inventory and Analysis; Heath et al. 2011) across the United 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

States are ordered as dead wood (126.9 Tg of C), belowground biomass (107.8 Tg of C), aboveground live biomass 

(104.5 Tg of C), forest floor (73.7 Tg of C), and soil organic 

carbon (67.6 Tg of C). Although climate change events can 

alter natural variation in C stocks, when compared to contemporary levels, these estimates of variation can provide a 

starting point for a risk framework.

When the consequences and likelihoods of forest C 

stocks becoming net emitters of C are viewed together, a cohesive approach to monitoring and managing risk emerges. 

Given the magnitude of potential emissions coupled with 

the natural variability in these stocks at the continental scale, 

annual monitoring of dead wood and aboveground live 

biomass C stocks are needed. In addition, strategies to mitigate negative climate change events (e.g., droughts) can be 

undertaken. The major research gap in such an approach is 

how far a pool would move within the risk framework after 

a climate change event (the length and direction of the negative/positive arrows [fig. A2-2]). For example, if forest lands 

convert to grasslands as a result of reduced precipitation and 

lack of tree regeneration, how would the aboveground biomass pool align itself within the risk framework? Despite the 

qualitative nature and research gaps within the forest C stock 

risk framework, this approach provides a conceptual means 

of identifying priority research needs and a decision system 

for mitigating climate change events.

Figure A2-2—Climate change risk matrix for forest ecosystem carbon (C) pools in the United States, in which climate 

change may cause C pools to move in a positive (sink = net annual sequestration) or negative (source = net annual 

emission) direction. Likelihood of change in C stocks is based on the coefficient of variation across the national Forest 

Inventory and Analysis plot network (x-axis). Size of C stocks is based on the U.S. National Greenhouse Gas Inventory (y-axis). Societal response (e.g., immediate adaptive response or periodic monitoring) to climate change events 

depends on the size and relative likelihood of change in stocks. The dead wood pool, a relatively small stock, exhibits 

increasingly high variability across the landscape and therefore may be affected by climate change and disturbance 

events such as wildfire. In contrast, the forest floor is also a relatively small C stock, but has low variability. Potential 

future climate-change effects are not incorporated in the matrix, because they represent many complex feedbacks both 

between C stocks (e.g., live aboveground biomass transitioning to the dead wood pool) and the atmosphere (e.g., forest 

floor decay).
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Literature Cited

Birdsey, R.; Pregitzer, K.; Lucier, A. 2006. Forest carbon 

management in the United States: 1600–2100. Journal of 

Environmental Quality. 35: 1461–1469.

Heath, L.S.; Smith, J.; Skog, K. [et al.]. 2011. Managed 

forest carbon estimates for the U.S. greenhouse gas inventory, 1990–2008. Journal of Forestry. 109: 167–173.

Kurz, W.A.; Stinson, G.; Rampley, G.J., [et al.]. 2008.

Risk of natural disturbances makes future contribution of 

Canada’s forests to the global carbon cycle highly uncertain. Proceedings National Academy of Sciences, USA. 

105: 1551–1555.

Pan, Y.; Birdsey, R.A.; Fang, J. [et al.]. 2011. A large and 

persistent carbon sink in the world’s forests. Science. 

333: 988–993.

Reich, P.B. 2011. Taking stock of forest carbon. Nature 

Climate Change. 1: 346–347.

U.S. Environmental Protection Agency [USEPA]. 2011a.

Inventory of U.S. greenhouse gas emissions and sinks: 

1990–2009. Chapter 7. Land use, land-use change, and 

forestry. Annex 3.12. Methodology for estimating net carbon stock changes in forest land remaining forest lands. 

Washington, DC: #430-R-11-005.

U.S. Environmental Protection Agency [USEPA]. 2011b.

Methodological descriptions for additional source or 

sink categories. Annex 3, section 3.12. In: Inventory of 

U.S. greenhouse gas emissions and sinks: 1990–2009. 

Washington, DC: A254–A284. 

Woodall, C.W.; Skog, K.; Smith, J.E.; Perry, C.H. 2011.

Maintenance of forest contribution to global carbon 

cycles. In: Robertson, G.; Gaulke, P.; McWilliams, R., 

ed. National report on sustainable forests—2010. FS-979. 

Washington, DC: U.S. Department of Agriculture, Forest 

Service: II59-II65. Criterion 5.
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Introduction

Wildfire is one of the two most significant disturbance 

agents (the other being insects) in forest ecosystems of the 

Western United States, and in a warmer climate, will drive 

changes in forest composition, structure, and function (Dale 

et al. 2001, McKenzie et al. 2004). Although wildfire is 

highly stochastic in space and time, sufficient data exist to 

establish clear relationships between some fire characteristics and some climatic parameters. An assessment of wildfire 

risk in response to climate change requires brief definitions 

of the terms “fire hazard” and “fire risk,” which are often 

confused in the scientific literature and other applications 

(Hardy 2005). Fire hazard is the potential for the structure, 

condition, and arrangement of a fuelbed to affect its flammability and energy release. Fire risk is the probability that 

a fire will ignite, spread, and potentially affect one or more 

resources valued by people. The most common means of 

expressing wildfire risk are (1) frequency, (2) a combination 

of intensity (energy release) and severity (effects on forests, 

structures, and other values), and (3) area burned. 

Fire Frequency

Fire frequency, which is the number of fires for a particular 

location and period of time, differs by region as a function 

of both lightning and human ignitions, with the requirement 

that fuels are sufficiently dry and abundant to burn. Lightning ignitions dominate mountainous regions with convective weather patterns (e.g., most of the Rocky Mountains), 

whereas human ignitions dominate regions with little

lightning and high human populations (e.g., southern 

California). Modeling studies (+4.2 oC scenario) (Price 

1

 David L. Peterson is a research biological scientist, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research 

Station, Pacific Wildland Fire Sciences Laboratory, 400 N, 34th

Street, Suite 201, Seattle, WA 98103; Jeremy S. Littell is a research 

ecologist, U.S. Department of the Interior, Geological Survey, 

Alaska Climate Science Center, 4210 University Drive, Anchorage, 

AK 99508.

Risk Assessment for Wildfire in the Western United States

David L. Peterson and Jeremy S. Littell1

and Rind 1994) and empirical studies (+1.0 oC scenario) 

(Reeve and Toumi 1999) suggest that lightning frequency 

will increase up to 40 percent globally in a warmer climate. 

Although no evidence exists to suggest that recent climate 

change has yet caused an increase in lightning or fire frequency in the West, lightning may increase as the temperature continues to rise (Price and Rind 1994, Reeve and 

Toumi 1999). Assuming that human population will increase 

throughout the West, it is reasonable to infer that human ignitions will also increase in most regions. Even if the sources 

and numbers of potential ignitions do not change, a warmer 

climate may facilitate increased drying of fine surface fuels 

(less than 8 cm in diameter) over a longer period (on a daily 

and seasonal basis) than currently exists (Littell and Gwozdz 

2011), allowing more potential ignitions to become actual 

ignitions that will become wildfires. 

Fire Intensity

Fire intensity, or energy released during active burning, is 

directly proportional to fire severity in most forests, and can 

be expressed as effects on vegetation, habitat, and, in some 

cases, human infrastructure. Results of modeling based on 

a doubled carbon dioxide (CO2) emission scenario suggest 

that fire intensity will increase significantly by 2070 in the 

northern Rocky Mountains, Great Basin, and Southwest 

(Brown et al. 2004). Fire severity and biomass consumption 

have increased in boreal forests of Alaska during the past 

10 years (Turetsky et al. 2010), and large, intense fires have 

become more common in California (Miller et al. 2008) and 

the Southwestern United States during the past 20 years. 

However, interannual and longer term variability in climatefire relationships can affect trends, making it difficult to 

infer whether climate change is responsible. Longer time 

series of fire occurrence, when available, will allow better quantification of the influence of multidecadal climatic 

variability (e.g., the Pacific Decadal Oscillation or Atlantic 
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GENERAL TECHNICAL REPORT PNW-GTR-870

Multidecadal Oscillation). Fire intensity and severity are a 

function of both climate and land use history, especially the 

effects of fire exclusion on elevated fuel loads, and forests 

with high fuel loading will continue to be susceptible to 

crown fire in the absence of active management (see below).

Fire Area

Fire area has a stronger relationship with climate in the 

Western United States than does either fire frequency or 

severity/intensity. An empirical analysis of annual area 

burned (1916 to 2003) for federal lands in the West projected 

that, for a temperature increase of 1.6 oC, area burned will 

increase two to three times in most states (McKenzie et al. 

2004). In contrast, a complex, mechanistic model projected 

that, for the same temperature increase, area burned will increase by only 10 percent in California (Lenihan et al. 2003). 

Using the 1977 to 2003 portion of the same data set used 

by McKenzie et al. (2004), Littell et al. (2009) stratified fire 

area data by Bailey’s ecoprovinces (Bailey 1995) to account 

for fire-climate sensitivities. On average, the model explained 66 percent of the variability in historical area burned 

by combinations of seasonal temperature, precipitation, and 

Palmer Drought Severity Index. In most forest ecosystems 

and some woodlands, fire area was primarily associated with 

drought conditions, specifically, increased temperature and 

decreased precipitation in the year of fire and seasons before 

the fire season. In contrast, in arid forests and woodlands 

in the Southwest, fire area was influenced primarily by the 

production of fuels in the year prior to fire and secondarily 

by drought in the year of the fire.

Littell et al.2

 projected the statistical models of Littell et 

al. (2009) forward for a 1 oC temperature increase, calculating median area burned and probabilities that annual fire 

area would exceed the maximum annual area burned in the 

historical record (1950 to 2003). Fire area is projected to 

increase significantly in most ecoprovinces (fig. A2-3);

probability of exceeding the historical maximum annual 

2

 Littell, J.S. [N.d.]. Relationships between area burned and climate 

in the Western United States: Vegetation-specific historical and 

future fire. Manuscript in preparation. On file with: U.S. Department 

of the Interior, Geological Survey, Alaska Climate Science Center, 

4210 University Drive, Anchorage, AK 99508. 

burn area varied greatly by ecoprovince (range 0 to 0.44). 

For the Pacific Northwest, the projected increases in area 

burned from Littell et al. (see footnote 2) are consistent with 

those found by Rogers et al. (2011) using the MC1 simulation model. A weakness of the statistical models is that, if 

the projected increased area burned were sustained over 

several decades, then at some point the large areas burned 

and decreasing fuel loads would result in less area burned 

than projected by the models. Neither statistical nor processbased models can satisfactorily account for the effects of 

extreme fire years and biophysical thresholds that may be 

exceeded in a much warmer climate.

Conclusions

Based on information summarized above and on expert 

judgment of the authors, the effects of climate change on 

fire risk are summarized for fire regimes that occur in forests 

of the Western United States (table A2-1). We estimate risk 

for a 2 oC increase, which is more likely by mid-21st century 

than the more conservative temperature scenarios used by 

McKenzie et al. (2004) and Littell et al. (see footnote 2). All 

fire regimes in forest ecosystems would experience some 

increase in fire risk. Low-severity and mixed-severity fire 

regimes dominate dry forest ecosystems of the West and 

Figure A2-3—Percentage of increase (relative to 1950 to 2003) in 

median area burned for Western United States ecoprovinces for a 

1 °C temperature increase. Color intensity is proportional to the 

magnitude of the projected increase in area burned.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Risk parameter

Fire regime

Low severity Moderate severity High severity Rationale for risk ratings

Frequency:

 Likelihood Moderate Moderate Moderate More fires will occur in all forests because of longer fire 

seasons and higher human population. In low-severity 

systems with low fuel loads, more fires will maintain 

resilience to fire and climate change; in low-severity 

systems with high fuel loads, more fires will cause more 

crown fires. In moderate-severity systems, more fires 

could convert them to low-severity systems. In highseverity systems, even a small increase in fire frequency 

will have a large effect on forest structure, function, and 

carbon dynamics. 

 Magnitude Low Moderate High

Overall risk and 

potential action

Low; no action 

recommended

Moderate; encourage fire prevention 

in high population 

areas

Moderate; 

encourage fire 

prevention in 

high population areas

Intensity/severity:

 Likelihood Moderatec Moderate Low In low-severity and high-severity systems, fire intensity 

and severity will probably be higher because of more 

extreme fire weather and elevated fuel loads for the next 

few decades. In high-severity systems, fuel moisture, 

not quantity, is limiting, so intensity and severity will 

not change much; crown fires are always intense and kill 

much of the overstory.

 Magnitude Moderatec Moderate Low

Overall risk and 

potential action

Moderate; increase fuel treatment area and 

fuel removal

Moderate; increase 

fuel treatment area 

and fuel removal

Low; no action 

recommended

Area burned:

Likelihood High High Moderate All fire regimes will experience more area burned. This 

will be especially prominent in drier, low-severity and 

moderate-severity systems. In high-severity systems, 

more area will burn, but the percentage increase will be 

less than in other systems; this will have significant local 

ecological effects. 

Magnitude High Moderate Moderate

Overall risk and 

potential action

High; greatly 

increase fuel 

treatment area, 

allow some fires 

to burn

Moderate; increase 

fuel treatment area, 

allow some fires 

to burn

Moderate; no 

action recommended

a Risk ratings are qualitative estimates based on information summarized above and on expert judgment of the authors.

b Fire regimes are defined as (1) low severity: 5- to 30-year frequency, less than 20 percent overstory mortality (dry mixed-conifer forests and woodlands); 

(2) mixed severity: 30- to 100-year frequency, patchy and variable overstory mortality (mesic mixed-conifer and drier high-elevation forests); and (3) high 

severity: more than 100-year frequency, more than 80 percent overstory mortality (low-elevation conifer and wetter subalpine forests).

c Fire intensity/severity are expected to increase in the next few decades, but they may decrease if fuel loadings are sufficiently reduced over time.

Table A2-1—Likelihood and magnitude of increased wildfire risk for fire regimes in forests of the Western 

United States, based on a temperature increase of 2 °C a b

would incur the greatest overall risk in terms of land area. 

High-severity regimes cover less land area, so they would 

have less influence on large-scale ecological changes; however, local effects could be significant, particularly where 

high-severity fire regimes occur close to large population 

centers, where socioeconomic exposure could be high even 

if probability of an event were low.

Management of fire risk is a standard component of fire 

management in the Western United States. Fire suppression 

has traditionally been used on both public and private lands 

to reduce fire area and fire severity. Increasing area burned 

will provide significant challenges for federal agencies and 

other organizations that fight fire because of the high cost of 

suppression and difficulty of deploying firefighters to multiple large fires that may burn concurrently and over a longer 

fire season. Fuel treatments in dry forest ecosystems of the 

West can greatly reduce the severity of wildfires (Johnson et 

al. 2011), although funding is available to treat only a small 

percentage of the total area with elevated fuel loadings. Fuel 

treatments that include mechanical thinning and surface fuel 

removal are expensive, especially in the wildland-urban 

interface, and in a warmer climate, more fuel may need to be 
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GENERAL TECHNICAL REPORT PNW-GTR-870

Literature Cited

Bailey, R.G. 1995. Description of the ecoregions of the 

United States [1:7,500,000]. 2nd ed. Misc. Publ.1391. 

Washington, DC: U.S. Department of Agriculture, Forest 

Service.

Brown, T.J.; Hall, B.L.; Westerling, A.L. 2004. The impact of twenty-first century climate change on wildland 

fire danger in the Western United States: an applications 

perspective. Climatic Change. 62: 365–388.

Dale, V.H.; Joyce, L.A.; McNulty, S. [et al.]. 2001. Climate 

change and forest disturbances. BioScience. 51: 723–734.

Hardy, C.C. 2005. Wildland fire hazard and risk: problems, 

definitions, and context. Forest Ecology and Management. 211: 73–82.

Johnson, M.C.; Kennedy, M.C.; Peterson, D.L. 2011.

Simulating fuel treatment effects in dry forests of the 

Western United States: testing the principles of a firesafe forest. Canadian Journal of Forest Research 41: 

1018–1030. 

Lenihan, J.M.; Drapek, R.; Bachelet, D.; Neilson, R.P. 

2003. Climate change effects on vegetation distribution, 

carbon, and fire in California. Ecological Applications. 

13: 1667–1681.

Littell, J.S.; Gwozdz, R.B. 2011. Climatic water balance 

and regional fire years in the Pacific Northwest, USA: 

linking regional climate and fire at landscape scales. In: 

McKenzie, D.; Miller, C.; Falk, D.A., eds. The landscape 

ecology of fire. New York: Springer: 117–139. Chapter 5.

removed to attain the same level of reduction in fire severity as is achieved under current prescriptions (Peterson et 

al. 2011). Allowing more wildfires to burn unsuppressed is 

one way to achieve resource benefits while reducing risk, 

although this approach is often politically unacceptable, 

especially when fire could threaten human infrastructure and 

other values. Managing fire risk will be one of the greatest 

challenges for forest resource managers in the West during 

the next several decades.

Littell, J.S.; McKenzie, D.; Peterson, D.L.; Westerling, 

A.L. 2009. Climate and wildfire area burned in western 

U.S. ecoprovinces, 1916–2003. Ecological Applications. 

19: 1003–1021. 

McKenzie, D.; Gedalof, Z.; Peterson, D.L.; Mote, P. 2004.

Climatic change, wildfire, and conservation. Conservation Biology. 18: 890–902.

Miller, J.D.; Safford, H.D.; Crimmins, M.; Thode, A.E. 

2008. Quantitative evidence for increasing forest fire 

severity in the Sierra Nevada and southern Cascade 

Mountains, California and Nevada, USA. Ecosystems. 

12: 16–32. 

Peterson, D.L.; Halofsky, J.; Johnson, M.C. 2011. Managing and adapting to changing fire regimes in a warmer 

climate. In: McKenzie, D.; Miller, C.; Falk, D., eds. The 

landscape ecology of fire. New York: Springer: 249–267. 

Chapter 10.

Price, C.; Rind, D. 1994. Possible implications of global 

climate change on global lightning distributions and 

frequencies. Journal of Geophysical Research. 99: 

10,823–10,831.

Reeve, N.; Toumi, R. 1999. Lightning activity as an indicator of climate change. Quarterly Journal of the Royal 

Meteorological Society. 125: 893–903.

Rogers, B.M.; Neilson, R.P.; Drapek, R.; Lenihan, 

J.M. [et al.]. 2011. Impacts of climate change on fire 

regimes and carbon stocks of the U.S. Pacific Northwest. Journal of Geophysical Research. 116, G03037, 

doi:10.1029/2011JG001695. 

Turetsky, M.R.; Kane, E.S.; Harden, J.W. [et al.]. 2010. 

Recent acceleration of biomass burning and carbon losses 

in Alaskan forests and peatlands. Nature Geoscience. 4: 

27–31.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

We used a risk matrix to assess risk from climate change for 

multiple forest species by discussing an example that depicts 

a range of risk for three tree species of northern Wisconsin. Risk is defined here as the product of the likelihood of 

an event occurring and the consequences or effects of that 

event. In the context of species habitats, likelihood is related 

to potential changes in suitable habitat at various times in 

the future. Consequences are related to the adaptability of a 

species to cope with the changes, especially the increasing 

intensity or frequency of future disturbance events. Data 

were generated from an atlas of climate change for 134 tree 

species of the Eastern United States (USDA FS 2011). 

A risk matrix allows managers to determine which 

species need adaptation strategies, further evaluation, or 

monitoring programs. For example, a two-dimensional 

framework of likelihood versus consequence was used to 

assess the risk of future flooding on infrastructure in New 

York City (Yohe 2010, Yohe and Leichenko 2010), providing qualitative judgments about the magnitude of vulnerability and the likelihood of flooding exposure at specific 

points in time. This matrix illustrated changes in risk (e.g., 

potential number of buildings damaged or destroyed) over 

time, generated by the implications of sea level rise on the 

return times of what are now considered a 100-year storm 

and a 10-year storm. This matrix was not intended as a basis 

for policy decisions, but rather to help organize individual 

and governmental thinking about near- and long-term risk 

around likelihood and consequence. 

1 Louis R. Iverson is a research landscape ecologist, U.S. Department of Agriculture, Northern Research Station, 359 Main Road, 

Delaware, OH 43015; Stephen N. Matthews is an ecologist, U.S. 

Department of Agriculture, Northern Research Station, 359 Main 

Road, Delaware, OH 43015; Anantha M. Prasad is a research 

ecologist, U.S. Department of Agriculture, Northern Research Station, 359 Main Road, Deleware, OH 43015; Matthew P. Peters is a 

GIS technician, U.S. Department of Agriculture, Northern Research 

Station, 11328 E Quintana Ave, Mesa, AZ 85212; and Gary W. 

Yohe is a professor, Wesleyan University, Economics Department, 

45 Wyllys Avenue, Middletown, CT 06459.

Risk Assessment for Forested Habitats in Northern Wisconsin

Louis R. Iverson, Stephen N. Matthews, Anantha M. Prasad, Matthew P. Peters, and 

Gary W. Yohe1

We adopted the same matrix structure to assess the 

likelihood of exposure and magnitude of vulnerability (or 

consequences) for three tree species in northern Wisconsin 

(fig. A2-4). Much of the climate change literature focuses on 

potential decreases in forest species (“losers”), but increases 

may also pose management challenges, so the matrix was 

modified to include species or forest assemblages that are 

projected to increase in suitable habitat in the future (“gainers”) (fig. A2-4). The risk matrix is demonstrated for black 

ash (Fraxinus nigra Marsh.) (loser), white oak (Quercus 

alba L.) (gainer), and yellow poplar (Lireodendron tulipifera

L.) (new migrant). 

Black ash carries more risk because, among other 

disadvantageous traits, it has low resistance to the emerald 

ash borer (Agrilus planipennis Fairmaire), which currently 

threatens all ash species in North America (Prasad et al. 

2010). White oak is expected to gain habitat in northern 

Wisconsin, because it is well adapted to drier conditions and 

increased disturbance. Relative to other species, projected 

risk over time for this species is relatively low. Yellow 

poplar is not now recorded in northern Wisconsin, according 

to forest inventory information of the U.S. Forest Service. 

As a potential new migrant into the region, this species may 

provide new opportunities for habitat or wood products.

Using methods described in the DISTRIB system 

(Iverson et al. 2008, 2011; Prasad et al. 2009), data for the 

likelihood (x-axis) are based on a series of species distribution models to assess habitat suitability for 134 tree species 

in the Eastern United States, for current and future (2040, 

2070, and 2100) climatic conditions. “Likelihood” in this 

context is, for any point in time, the potential that a section 

of forest within a specified region will have suitable habitat 

for a given species relative to its current suitable habitat. In 

this example, we use emission scenarios of modeled climate 

change, PCMlo and Hadhi, to elicit a range of possible 

risks, from low to high, associated with future climates. The 
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GENERAL TECHNICAL REPORT PNW-GTR-870

Figure A2-4—Risk matrix of potential change in suitable habitat for three tree species in northern Wisconsin that are expected to either lose 

habitat (black ash), gain habitat (white oak), or become a potential new migrant because of newly appearing habitat (yellow poplar). 

matrix shows large variation between the emissions scenarios, with Hadhi causing larger changes in suitable habitat 

for all species. For black ash, which loses habitat, the x-axis 

ranges from 0 (complete loss of habitat over time) to +1 (no 

change in habitat over time). For white oak, which gains 

habitat, the x-axis ranges from +1 to +8. For yellow poplar, 

a species entering new habitat, the range is confined to the 

leftmost column of the graph. These numbers themselves are 

not directly the scale of “likelihood,” but rather are scales 

of future:current importance values, and are plotted only to 

show the quantitative linkages.

Consequences in this context are related to the adaptability of a species or forest assemblage under climate 

change, based on a literature assessment of species biological traits and capacity to respond to disturbances that are 

likely to occur within the 21st century, including how those 

disturbances will be affected by climate change. Data for 

this axis comes from a literature-based scoring system, 

called “modification factors,” to capture species response 

to climate change (Matthews et al. 2011). This approach 

was used to assess the capacity for each species to adapt to 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

12 disturbance types and to assess nine biological characteristics related to species adaptability. Each character was 

scored individually from -3 to +3 as an indication of the 

adaptability of the species to climate change. The mean, 

scaled values for biological and disturbance characteristics 

were each rescaled to 0 to 6 and combined as a hypotenuse 

of a right triangle; the resulting metric (ranging from 0 to 

8.5) was used for the y-axis of the risk matrix (fig. A2-4). 

Because several disturbances (e.g., floods, droughts, insect 

attacks) are expected to increase over time, we also used a 

formula based on modification factors to enhance relevance 

for certain factors from 2040 to 2100. This analysis does not 

include socioeconomic consequences, such as the effects of 

decreased black ash on local basket-making economies of 

Native Americans. 

The risk matrix has a number of useful applications. 

It provides a visual tool for comparing species risks relative to changing habitats associated with climate change. 

Trajectories displayed in the matrix reveal insights about 

species response to climate change and can be considered in 

the development of potential adaptation strategies, although 

they cannot account for nonlinear responses to extreme 

climate and altered disturbance regimes. The risk matrix can 

also help organize “climate change thinking” on a resource 

management team and communicate information to stakeholder groups and the general public. Finally, the risk matrix 

can be used to assess climate change risk for a variety of 

resource disciplines, and although the metrics may not be 

derived from the same methodologies, the capacity to rate 

one species against another, or one location against another, 

will promote a consistent approach to climate change risk 

management.

Literature Cited

Iverson, L.; Prasad, A.M.; Matthews, S.; Peters, M. 

2008. Estimating potential habitat for 134 eastern US tree 

species under six climate scenarios. Forest Ecology and 

Management. 254: 390–406.

Iverson, L.; Prasad, A.M.; Matthews, S.; Peters, M. 2011.

Lessons learned while integrating habitat, dispersal, disturbance, and life-history traits into species habitat models under climate change. Ecosystems. 14: 1005–1020.

Matthews, S.N.; Iverson, L.R.; Prasad, A.M. [et al.]. 

2011. Modifying climate change habitat models using 

tree species-specific assessments of model uncertainty 

and life history factors. Forest Ecology and Management. 

262: 1460–1472.

Prasad, A.; Iverson, L.; Matthews, S.; Peters, M. 2009.

Atlases of tree and bird species habitats for current and 

future climates. Ecological Restoration. 27: 260–263.

Prasad, A.; Iverson, L.; Peters, M. [et al.]. 2010. Modeling the invasive emerald ash borer risk of spread using a 

spatially explicit cellular model. Landscape Ecology. 25: 

353–369.

U.S. Department of Agriculture, Forest Service. 2011. 

Climage change atlas—Northern Research Station. http://

www.nrs.fs.fed.us/atlas. (6 January 2011).

Yohe, G. 2010. Risk assessment and risk management for 

infrastructure planning and investment. The Bridge. 

40(3): 14–21.

Yohe, G.; Leichenko, R. 2010. Adopting a risk-based approach. Annals of the New York Academy of Sciences. 

1196: 29–40.
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GENERAL TECHNICAL REPORT PNW-GTR-870

Species distribution models for 147 bird species have been 

derived using climate, elevation, and distribution of current 

tree species as potential predictors (Matthews et al. 2011). 

In this case study, a risk matrix was developed for two bird 

species (fig. A2-5), with projected change in bird habitat (the 

x axis) based on models of changing suitable habitat resulting from changing climate and tree species habitat. Risk was 

evaluated for three time steps (2040, 2070, 2100) and based 

on two climate models and two emission scenarios (Hadhi 

vs. PCMlo). 

To assess the y-axis of the matrix (fig. A2-5), we used 

the System for Assessing Vulnerability of Species (SAVS) 

(Bagne et al. 2011, Davison et al. 2011) to estimate species 

adaptability to future changes, including disturbances. The 

SAVS tool is based on 22 traits that represent potential areas 

of vulnerability or resilience with respect to future climate 

change. Each trait forms the basis of a question that is scored 

according to predicted effect (reduced, neutral, or increased 

population). By selecting responses for each question, a user 

creates a score that represents relative vulnerability to climate change effects, with higher positive values indicating 

higher vulnerability. Scores are calculated considering all 22 

traits and divided among 4 categories: habitat, physiology, 

phenology, and biotic interactions. To calculate a baseline 

that could be used to compare current versus future vulnerability, we zeroed out individual questions for traits relating 

to exposure to future conditions and calculated a score based 

on the intrinsic characteristics of a species that reflect its 

sensitivity to population declines as a result of stochastic or 

other events.

Northern Wisconsin is near the edge of the distribution 

of the northern cardinal (Cardinalis cardinalis L.) and offers 

relatively limited habitat opportunities owing to the effects 

of current winter climatic conditions. However, with 

1 Megan M. Friggens is a research ecologist, U.S. Department of 

Agriculture, Rocky Mountain Research Station, 333 Broadway SE, 

Suite 115, Albuquerque, NM 87102; Stephen N. Matthews is an 

ecologist, U.S. Department of Agriculture, Forest Service, Northern 

Research Station, 359 Main Road, Delaware, OH 43015.

Risk Assessment for Two Bird Species in Northern Wisconsin

Megan M. Friggens and Stephen N. Matthews1

projected increases in temperatures for northern Wisconsin, 

the habitat for the northern cardinal is projected to double 

by the end of the century (future:current habitat ratio of 2.2). 

The northern cardinal uses habitats ranging from shrublands 

to forests, has a broad diet, and has been shown to be positively associated within an urbanizing landscape (Rodewald 

and Shustack 2008).The SAVS baseline scores indicate less 

vulnerability (-0.91) and that the species does not show 

increased vulnerability risk under climate change (-1.82). 

Characteristics such as adaptability of nesting locations and 

flexibility in reproductive time contribute to the less vulnerable score. 

In contrast, the mourning warbler (Oporornis philadelphia A. Wilson) shows higher risk based on its more 

specialist nature, specificity to breeding habitats, and Neotropical migration life history. These innate traits make the 

mourning warbler more susceptible under current conditions 

(SAVS +3.64) and is also considered at an increased risk of 

exposure to negative effects of climate change (+5.45). The 

mourning warbler is primarily a boreal species and despite 

its use of early successional habitats and a positive response 

to some human disturbances such as timber harvest (Hobson 

and Schieck 1999), its occurrence in northern Wisconsin 

declined over a recent 16-year interval (Howe and Roberts 

2005). Moving beyond contemporary changes, its habitat is 

projected to decrease by two-thirds of its current status by 

the end of the century (future:current ratio as low as 0.13 or 

0.33, depending on climate model). These potential changes 

in habitat are attributed to higher temperatures and loss of 

boreal forest habitat (Iverson et al. 2008). In addition, the 

premontane and montane tropical life zones inhabited by the 

mourning warbler during winter are predicted to be highly 

sensitive to climatic affects (Enquist 2002). Therefore, when 

viewed together, the likelihood and magnitude of projected 

climate change suggest high risk for this species, and an increased opportunity for the northern cardinal, whose habitat 

will expand into northern Wisconsin.
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Figure A2-5—Risk of the effects of climate change on the northern cardinal and mourning warbler, expressed as a combination of likelihood of habitat change (x-axis) and magnitude of adaptability (y-axis). Values are rescaled from calculations that used the approach in the 

System for Assessing Vulnerability of Species (Bagne et al. 2011, Davison et al. 2011).

In this case study, we focused on two species with 

contrasting responses to climate change, but the general 

approach can be applied to a wide range of species, using 

either quantitative information or qualitative logic. The 

empirical statistical models used here provide insights on the 

broad-scale determinants of species distributions, but with 

some limiting assumptions. Models derived from mechanistic relationships that explore processes regulating population 

dynamics also demonstrate the importance of local climatic 

conditions on avian populations (Anders and Post 2006, 

Rodenhouse 1992), but they are available only for a limited 

number of species. The detailed parameterizations of mechanistic models also have important assumptions and can 

be difficult to apply across a broad array of species. Thus, 

more refined inferences on how climate change may affect 

avian populations will require careful consideration of both 

empirical and mechanistic approaches to modeling species 

distributions, especially the influence of ecological disturbances on habitat, as well as threshold values for minimum 

habitat quantity and quality. 
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GENERAL TECHNICAL REPORT PNW-GTR-870

Literature Cited

Anders, A.D.; Post, E. 2006. Distribution-wide effects of 

climate on population densities of a declining migratory 

landbird. Journal of Animal Ecology. 75: 1365–2656.

Bagne, K.E.; Friggens, M.M.; Finch, D.M. 2011. A 

System for Assessing Vulnerability of Species (SAVS) 

to climate change. Gen. Tech. Rep. RMRS-GTR-257. 

Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station. 28 p.

Davison, J.E.; Coe, S.; Finch, D. [et al.]. 2011. Bringing 

indices of species vulnerability to climate change into 

geographic space: an assessment across the Coronado 

National Forest. Biodiversity and Conservation. doi 

10.1007/s10531-011-0175-0.

Enquist, C.A.F. 2002. Predicted regional impacts of climate 

change on the geographical distribution and diversity of 

tropical forest in Costa Rica. Journal of Biogeography. 

29: 519–564.

Hobson, K.A.; Schieck, J. 1999. Change in bird communities in boreal mixedwood forest: harvest and wildfire effects over 30 years. Ecological Applications 9: 849–863.

Howe, R.W.; Roberts, L.J. 2005. Sixteen years of habitatbased bird monitoring in the Nicolet National Forest. In: 

Ralph, C.J.; Rich, T.D., eds. Bird conservation implementation and integration in the Americas: Proceedings of 

the third international Partners in Flight conference. Gen. 

Tech. Rep. PSW-GTR-191. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station: 963–973.

Iverson, L.R.; Prasad, A.M.; Matthews, S.N., Peters; M. 

2008. Estimating potential habitat for 134 eastern US tree 

species under six climate scenarios. Forest Ecology and 

Management. 254: 390–406.

Matthews, S.N.; Iverson, L.R.; Prasad, A.P.; Peters, M.P. 

2011. Changes in potential habitat of 147 North American 

breeding bird species in response to redistribution of trees 

and climate following predicted climate change. Ecography. doi 10.1111/j.1600-0587.2010.06803.x. 

Rodenhouse, N.L. 1992. Potential effects of climatic change 

on migrant landbirds. Conservation Biology. 6: 263–272.

Rodewald, A.D.; Shustack, D.P. 2008. Consumer resource 

matching in urbanizing landscapes: are synanthropic species over-matching? Ecology. 89: 515–521.
259

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Appendix 3: Western Mountain Initiative Synthesis

Response of Western Mountain Ecosystems to Climatic Variability and 

Change: A Synthesis From the Western Mountain Initiative

Crystal L. Raymond1

Introduction

The Western Mountain Initiative (WMI), a consortium of 

research groups in the Western United States, focuses on 

understanding and predicting responses—especially sensitivities, thresholds, resistance, and resilience—of mountain 

ecosystems to climatic variability and change (Peterson et 

al. 2012). The WMI addresses how climatic variability and 

change influence forest processes, disturbance dynamics, 

hydrologic changes, and hydroecological interactions in 

five bioregions: Pacific Northwest, Sierra Nevada, Northern 

Rocky Mountains, Central Rocky Mountains, and Southern Rocky Mountains. A guiding theme of WMI research 

is understanding the linkages among these processes. This 

focus on linkages (e.g., climate change affects disturbance 

regimes, hence vegetation, hence erosion) and the depth and 

breadth of place-based knowledge represented by this work 

contribute to multisite regional comparisons.

Research addresses four key questions: (1) How are 

climatic variability and change likely to affect disturbance 

regimes? (2) How are changing climate and disturbance 

regimes likely to affect the composition, structure, and 

productivity of vegetation? (3) How will climatic variability 

and change affect hydrologic processes in the mountainous 

West? and (4) Which mountain resources and ecosystems 

are likely to be most sensitive to future climatic change, and 

what are possible management responses? Results to date 

have documented how climatic variability and change affect

1 Crystal L. Raymond is a postdoctoral research ecologist, U.S. 

Department of Agriculture, Forest Service, Pacific Northwest 

Research Station, Pacific Wildland Fire Sciences Laboratory, 400 

N, 34th Street, Suite 201, Seattle, WA 98103. Raymond is currently 

located at the U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station, Forestry Sciences Laboratory, 507 25th

Street, Ogden, UT 84401.

several trends: long-term patterns of snow, glaciers, and 

water geochemistry; forest productivity, vigor, and demography; and changing patterns of treeline dynamics and forest 

disturbances. Empirical and simulation modeling indicates 

that major changes in hydrologic function and ecological 

disturbance will occur as the climate continues to warm.

The WMI research has documented trends in temperature, precipitation, and snowpack, and the exceedance of 

biological and ecologically meaningful thresholds of these 

variables in the mountainous West. In the northern Rocky 

Mountains, trends show that extremely cold days (≤ -18 °C) 

end on average 20 days earlier and have declined in number, 

and the number of extremely hot days (≥ 32 °C) has increased over the last 100 years (Pederson et al. 2011). Trends 

in snowpack observations in the northern Rocky Mountains 

indicate declines in snowpack and earlier arrival and melt of 

peak snow water equivalent over the last 40 years (Pederson 

et al. 2011). Although much of this change in snowpack is 

attributed to climatic variability, an extension of this analysis 

to the whole Rocky Mountains region and to the last 800 

years using tree-ring based reconstructions of snowpack 

shows only two periods of sustained low snowpack comparable to those observed in the 20th century (Pederson et al. 

2011). 

Here we focus on WMI results in three areas related to 

forest ecosystem response to climatic variability and change: 

(1) trends in the structure and function of western forest 

ecosystems, (2) the effects of exceeding critical thresholds, 

and (3) the potential for future changes in these systems. 

Changes have been documented in forest demography, 

treeline dynamics, and ecological disturbances and interactions. Although it is not possible to definitively attribute 

recent changes as being caused by climate change, the effects of dominant modes of climatic variability can be used 
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GENERAL TECHNICAL REPORT PNW-GTR-870

with some confidence to infer expected changes in ecosystems in a warmer climate. For example, relationships have 

been documented between the Pacific Decadal Oscillation 

(20- to 30-year cool and warm phases) and temporal variation in regional-scale tree growth (Peterson and Peterson 

2001, Peterson et al. 2002), wildfire (Hessl et al. 2004), and 

long-term drought (Gedalof et al. 2004). These relationships 

suggest that significant changes in ecosystem processes will 

occur as temperature, and extremes in temperature, continue 

to increase.

Forest Die-Off and Demography

Recent broad-scale syntheses have documented climateinduced forest mortality in some locations (Allen 2009, 

Allen et al. 2010). These syntheses reveal diverse patterns 

in forest die-off attributed to drought and heat, including 

localized increases in background mortality and regionalscale forest die-off linked to biotic agents (Allen et al. 2010). 

Drought-induced die-off is commonly observed at the elevation and geographic margins of species ranges and is often 

associated with prolonged periods of moisture stress. Recent 

and ongoing WMI research continues to address key uncertainties in forest mortality processes (Breshears et al. 2009; 

McDowell et al. 2008, 2010). 

Based on demographic trends in long-term plot data, 

mortality rates in old forests have increased in each of three 

subregions (Northwest, California, and interior West) and 

across elevation zones and tree size classes (van Mantgem 

et al. 2009), and tree recruitment rates have not changed, 

contributing to lower stem density and basal area of old 

forests. Since 1955, both temperature (particularly at the 

higher elevations occupied by forests) and climatic water 

deficit (evaporative demand that is not met by available water) increased, and both were positively correlated with tree 

mortality rates (van Mantgem et al. 2009). Warming-induced 

tree mortality is consistent with the apparent role of warming 

in recent forest dieback in western North America (Adams 

et al. 2009) and the positive correlation between short-term 

fluctuations in background tree mortality and water deficits 

in California and Colorado (Bigler et al. 2007, van Mantgem 

and Stephenson 2007). 

Treeline Dynamics

One of the most distinctive features of mountain environments is the transition from subalpine forest to alpine tundra 

(alpine forest-tundra ecotone, AFTE), and climate change 

is likely to affect these environments in the mountainous 

West. Advance of trees into tundra can alter cycling of water, 

carbon, and nutrients and the maintenance of biodiversity. 

Treeline advance and changes in treeline patterns have the 

potential to alter snow retention and hydrology, with implications for local soil moisture and nutrient transport.

Studies of treeline phenomena and limiting factors 

among the five WMI bioregions have demonstrated that 

although ecological dynamics of the AFTE are influenced by 

climate, mechanistic processes that shape the ecotone—seed 

rain, seed germination, seedling establishment, and subsequent tree growth form—also depend on microsite patterns 

(Malanson et al. 2007). In the West, these mechanistic processes are similar among AFTEs, but other processes—prior 

climate, geomorphology, genetics, and historical grazing 

practices—create geographic differences in responses of 

ecotones to climate change. Climate change may affect successful seed dispersal, germination, and survival by modifying the biophysical environment. The three-dimensional 

pattern at treeline is typically patchy, including krummholz 

and dwarf trees, with expansion often facilitated by other 

plants (Resler 2006). The formation of vegetation structures 

that add wind protection, snow collection, and soil development allows subalpine forest species to initiate patches and 

expand in the upper treeline (Smith et al. 2003). Climate and 

variation in geomorphology, geology, and disturbances (e.g., 

snow avalanches) also control mortality at treeline and limit 

treeline elevation.

At regional to continental scales, control of the AFTE 

by temperature is locally modified by moisture (Malanson 

and Butler 2002). Evidence for AFTE response to climatic 

variability suggests upslope advance during warmer conditions, but advance in many sites may be limited by moisture. 

For example, in the 19th century, Glacier National Park experienced upward expansion of the ecotone (Bekker 2005), 

although in the 20th century, density of existing patches 

increased but advance was limited (Klasner and Fagre 2002). 
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Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

Treeline advances in the latter half of the 20th century coincided with the cool (wet) phase of the Pacific Decadal Oscillation, and advance stopped during the warm (dry) phase of 

the 1980s and 1990s (Alftine et al. 2003).

A warmer, wetter climate could alter the structure of 

treeline in all WMI bioregions. In the Pacific Northwest, 

high snowpack inhibits establishment and growth of trees, 

so at the highest treeline elevations, increased snow plus 

increased winter precipitation could limit expansion of 

krummholz higher in the ecotone. The other WMI bioregions are drier, so higher precipitation could improve conditions for tree establishment and growth. For example, in 

Sierra Nevada treelines, tree growth increased during warmer, wetter periods in the 20th century (Millar et al. 2004). At 

all sites, effects are likely to be greatest in the lower AFTE, 

where deeper soils and root zones can use increased water 

(Malanson et al. 2007).

A warmer, drier climate could reduce tree establishment 

in the ecotone of all WMI treeline sites except the Pacific 

Northwest. In the Pacific Northwest, less moisture is unlikely to reduce tree establishment and growth in the AFTE. A 

warmer climate could increase tree growth in the AFTE, and 

less snow could facilitate expansion in the upper ecotone and 

encroachment in meadows in the lower ecotone. Conversely 

for the other WMI bioregions, a drier climate could further 

limit tree establishment, growth, and species diversity in the 

AFTE (Malanson et al. 2007). 

Ecological Disturbance and 

Interaction of Stressors

Empirical and process-based models have been used to estimate the extent and magnitude of future disturbances across 

the West. The indirect effect of climate change on forests 

through changing disturbance regimes is likely to cause 

more rapid changes than the direct effects of higher temperatures on trees (fig. A3-1), and accelerated species turnover 

will occur after severe disturbance because seedlings are less 

resistant to changing climate than are mature individuals 

(McKenzie et al. 2009). In light of the importance of disturbance in Western forests, WMI research has emphasized 

quantifying the effects of climatic variability and change on 

the areal extent and broad-scale spatial patterns of wildfire 

and insect outbreaks (Littell et al. 2010). 

Of particular concern are increases in fire area in a 

warming climate and the effects of extreme wildfire events 

on ecosystems (Gedalof et al. 2005, Littell et al. 2009,

McKenzie and Littell 2011). Strong climatic controls

exist on area burned by wildfire across the West at the

spatial scales of entire states (McKenzie et al. 2004), ecoprovinces (Littell et al. 2009), and sections within ecoprovinces (Littell et al. 2010). In forests across the Northwestern 

United States, climate during the fire season appears to control area burned, whereas in arid mountains and shrublands, 

antecedent climate (e.g., wetter, cooler summers or winters 

preceding the fire season) can increase area burned during 

the fire season by increasing fuel abundance and continuity (Littell et al. 2009, 2010). Research has also quantified 

sediment yields after wildfire in different rainfall regimes 

across the Western United States (Moody and Martin 2008), 

providing a key context for potential climate-mediated watershed changes in postfire runoff and erosion relationships.

Mountain pine beetle (Dendroctonus ponderosae

Hopkins) infestations have historically occurred frequently 

and extensively throughout western North America (Logan 

and Powell 2001). Warming and drought affect development 

rates of beetle life stages, winter mortality, and host tree 

susceptibility (Carroll et al. 2004). Across the West, current 

stand structural conditions make host species susceptible to 

beetle attack (Hicke and Jenkins 2008), and as warming continues, we might expect that forests will become susceptible 

to insect attack more frequently (Raffa et al. 2008). Mountain pine beetle outbreaks are facilitated when the insect’s 

reproductive cycle is very close to one year and when larvae 

emerge at an optimal time for feeding, dispersal, and survival of cold seasons (Logan and Powell 2001). As temperatures increase, the life cycle shortens; therefore, a warmer 

climate is projected to reduce the area of climatic suitability 

for the beetle at low elevations but increase suitability at 

higher elevations (Hicke et al. 2006, Littell et al. 2010). 

Increases in mountain pine beetle outbreaks with climate 
262

GENERAL TECHNICAL REPORT PNW-GTR-870

change will affect the carbon cycle in forest ecosystems and 

have feedbacks to the climate system (Hicke et al. 2012a). 

Research on disturbance interactions and their effects 

on ecosystem processes (Allen 2007, McKenzie and Littell 2011, McKenzie et al. 2009) suggests that synergistic 

interactions between disturbances produce larger effects than 

would occur from an individual disturbance. For example, 

bark beetle outbreaks have been linked to increased likelihood of stand-replacing fire and changes in fire behavior, 

with the nature of the effect depending on the time since 

infestation (Jenkins et al. 2008, Lynch et al. 2006), although 

there is conflicting evidence about whether fire hazard is 

higher following bark beetle outbreaks (Hicke et al. 2012b). 

Combined with increasing climatic stress on tree populations 

and growth, disturbance interactions can alter forest structure and function faster than could be expected from species 

redistribution or disturbance alone (fig. A3-1). Simultaneous 

climatically driven shifts in the locations of species optima, 

ecosystem productivity, disturbance regimes, and interactions between them could reset forest succession over large 

areas and short timeframes.

Figure A3-1—Conceptual model of the relative time scales for disturbance versus climate change alone to alter ecosystems. Times are 

approximate. The focus here is on fire, but much of the same logic applies to insect outbreaks. Adapted from McKenzie et al. (2004).
263

Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector

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