Effects of Drought on Forests and 

Rangelands in the United States: 

A Comprehensive Science Synthesis

United States Department of Agriculture

Forest Service Gen. Tech. Report WO-93a November 2015

EXECUTIVE SUMMARY
Cover Photographs (from top to bottom):

J.S. Quick, Colorado State University, Bugwood.org 

Ronald F. Billings, Texas Forest Service, Bugwood.org

U.S. Geological Survey
J.M. Vose is a Research Ecologist, U.S. Department of Agriculture, Forest Service, 

Southern Research Station (SRS), Center for Integrated Forest Science (CIFS), North 

Carolina State University, Department of Forestry and Environmental Resources, Raleigh, 

NC 27695.

J.S. Clark is the Nicholas Professor of Environmental Science and Professor of Statistical 

Science, Nicholas School of the Environment, Duke University, Durham, NC 27708-0338.

C.H. Luce is a Research Hydrologist, U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Research Station, Boise, ID 83702.

T. Patel-Weynand is Director of the Forest Management Sciences’ (SFMR) staff, 

U.S. Department of Agriculture, Forest Service, Research and Development (R&D), 

Washington, DC 20024.

Effects of Drought on Forests and 

Rangelands in the United States:

A Comprehensive Science Synthesis

James M. Vose

James S. Clark 

Charles H. Luce

Toral Patel-Weynand

EXECUTIVE SUMMARY

1

EXECUTIVE SUMMARY

Overview and Purpose 

This assessment provides input to the reauthorized National 

Integrated Drought Information System (NIDIS) and the National 

Climate Assessment (NCA); it also establishes the scientific 

foundation needed to manage for drought resilience and adaptation. The 

NIDIS Act1 was signed into law in 2006 and reauthorized by Congress 

in 2014.2 NIDIS will be implemented through a network of agencies 

and partners to integrate drought monitoring and forecasting systems 

at multiple levels (Federal, State, and local). It will support research 

that focuses on drought risk assessment, forecasting, and monitoring. 

Produced every 4 years, the NCA evaluates the effects of global climate 

change on forests, agriculture, rangelands, land and water resources, 

human health and welfare, and biological diversity, and it projects major 

trends. The NCA is based on technical information produced by public 

agencies and nongovernmental organizations. 

As drought regimes change, the ability to quantify and predict 

the impacts on forests and rangelands is critical to developing 

and implementing management actions to increase resiliency and 

adaptation. The U.S. Department of Agriculture (USDA), Forest 

Service, Research and Development scientists in partnership with 

Duke University authored this assessment, entitled, Effects of Drought 

on Forests and Rangelands in the United States: A Comprehensive 

Science Synthesis. The assessment is a collaborative effort authored 

by 77 scientists from the Forest Service and other Federal agencies, 

research institutions, and various universities across the United States. 

The authors identified key issues from a series of virtual workshops 

involving scientists and stakeholders. Focal areas in the assessment 

include drought characterization; drought impacts on forest processes 

and disturbances such as insect outbreaks and wildfire; and the 

consequences on forest and rangeland values. The assessment closely 

follows the Intergovernmental Panel on Climate Change (IPCC) process, 

which is organized with convening authors, lead chapter authors, and 

contributing authors. The convening authors for the assessment had 

the chapters individually peer reviewed, and the lead and contributing 

authors revised the text in response to reviewer comments. 

1 National Integrated Drought Information System Act of 2006. P.L. 109-430 (December 20, 2006). 

15 U.S.C. § 313d.

2 National Integrated Drought Information System Act of 2014. P.L. 113-86 (March 6, 2014). 

This assessment establishes 

the scientific foundation 

needed to manage forests 

and rangelands for resilience 

and adaption to drought. 

Photo credits (from top to bottom):

Craig D. Allen, U.S. Geological Survey

J.S. Quick, Colorado State University, Bugwood.org
2

Effects of Drought on Forests and Rangelands in the United States: A Comprehensive Science Synthesis

Topics Addressed in This Assessment 

l Characterizing Drought for Forested Landscapes 

and Streams

l Physiological Responses of Forests to Future Drought

l Impacts of Drought on Forest Dynamics, Structure, 

Diversity, and Management

l Forest Biogeochemistry in Response to Drought

l Insect and Pathogen Responses to Drought

l Fire and Drought

l Rangeland Drought: Effects, Restoration, and Adaptation

l Detecting and Monitoring Large-Scale Drought Effects 

on Forests: Toward an Integrated Approach

l Ecohydrological Implications of Drought

l Economics and Societal Considerations of Drought in 

Forests and Rangelands

This assessment focuses on 

drought characterization, the 

impacts on forest processes 

and disturbances, and the 

consequences on forest and 

rangeland values.

Photo credits (from top to bottom):

Craig D. Allen, U.S. Geological Survey

John M. Randall, The Nature Conservancy, Bugwood.org

U.S. Geological Survey
3

EXECUTIVE SUMMARY

Key Messages from the Report 

Characterizing and Predicting Future Drought

In simple terms, drought is a lack of water over a given temporal and 

spatial scale. Drought can be a severe natural disaster with substantial 

social and economic consequences. Drought becomes most obvious 

when large-scale changes are observed (i.e., insect outbreaks or 

wildfires) or when water requirements for human or agricultural 

needs are not met; however, even moderate drought can have 

long-lasting impacts on the structure and function of forests and 

rangelands without these obvious large-scale changes. Droughts are 

generally identified as one of four types: (1) meteorological, 

(2) hydrologic, (3) agricultural, or (4) socioeconomic. Meteorological 

and hydrologic droughts relate water availability to a reference 

condition (e.g., long-term mean); agricultural and socioeconomic 

droughts relate to impacts. In agricultural systems producing annual 

crops, characterizing and assessing drought impacts can be fairly 

straight forward; however, in systems with perennial vegetation (both 

natural and agricultural systems), characterizing and assessing drought 

impacts is much more complex, as responses can vary in space, time, 

and among species. 

Historical and paleoclimatic evidence shows that drought has 

always impacted the physical environment and will continue to do 

so. The direction of trends in recent history varies from region to 

region, with the Western United States showing a trend toward dry 

conditions while trends in the East are more variable and complex. 

Much of the variability in how drought is characterized depends on 

definitions for terms, for reference conditions, and for methods of 

averaging short-term weather into climate statistics. Predicting future 

changes in drought frequency and severity has proven difficult using 

General Circulation Models (GCMs), but recent trends are a growing 

global concern. Uncertainty arises primarily from limited capacity to 

predict future precipitation changes, particularly long-term lapses in 

precipitation. Despite this uncertainty, there is growing consensus that 

extreme precipitation events (e.g., lapses in precipitation and more 

intense storms) will increase in frequency, and warmer temperatures 

will exacerbate the impacts of drought on forests and rangelands in the 

future (chapter 2).

Drought becomes obvious 

when large-scale changes 

such as insect outbreaks, 

(top) or wildfires (center) are 

observed and when water 

requirements for human or 

agricultural needs are not 

met (bottom).

Photo credits (from top to bottom):

USDA Forest Service, Bugwood.org

National Interagency Fire Center, Bugwood.org

Gerald Holmes, California Polytechnic State University, 

 Bugwood.org
4

Effects of Drought on Forests and Rangelands in the United States: A Comprehensive Science Synthesis

Understanding the Effects of Drought on Forests and Rangelands

There is a critical need to understand how drought affects forests and 

rangelands, in part because drought severity and drought-associated 

forest disturbances are expected to increase with climatic change. 

Drought affects forest and rangeland systems both directly and 

indirectly. In regions where seasonal droughts are common, forest 

and rangeland ecosystems respond through various physiological and 

morphological adaptations. In regions where drought is less common, 

responses can be substantial because ecosystems are not well adapted 

to drought conditions. 

High evaporative demand, the combination of high temperature and 

low humidity, combines with low soil moisture to induce stress through 

closure of stomata, which can lead to carbon stress, loss of hydraulic 

function, and mortality. Species vary in their vulnerability to drought 

due to differences in their allocation to roots, mycorrhizal associations, 

and xylem anatomy (chapter 3). Large stand-level impacts of drought 

are already underway in the West, but all U.S. forests are vulnerable 

to drought. Changes in climate will continue to stress forests and alter 

suitable habitat. Combined field evidence and models suggest that 

climate change is causing relocation of habitats at rates much faster than 

populations of trees can migrate. Reorganizations of stand structure and 

species composition are expected to lag behind shifts in habitat caused 

by increasing drought and temperature change (chapter 4).

Droughts are predicted to accelerate the pace of invasion by some 

nonnative plant species into rangelands and grasslands. Drought can also 

promote plant invasion indirectly by modifying the environment to favor 

nonnative species. For example, opportunities for invasion are created 

when drought kills native plants leaving open niches and bare ground 

(chapter 8). Drought is also an important contributor to the invasive 

annual grass-wildfire loop that threatens ecosystems not adapted to fire 

(e.g., cheatgrass’ positive feedback with fire in parts of western North 

America’s sagebrush biome). In this self-perpetuating cheatgrass-fire 

loop, drought increases the frequency of wildfires, and nonnative plants 

(especially annual grasses) are likely to invade burned sites. 

Drought alters ecosystem processes such as nutrient, carbon, and 

water cycling in ways that are not yet well understood (chapter 

5). Drought tends to slow nutrient uptake by plants and reduce retranslocation of foliar nutrients with premature leaf senescence. 

Dieback that results from combinations of drought and natural enemies 

Ecosystem structure and 

processes are altered by 

drought. Cheatgrass (top) 

is a nonnative plant that 

invades burned sites. 

Dieback (bottom) can 

have global impacts on 

carbon cycles. 

Photo credits (from top to bottom):

Craig D. Allen, U.S. Geological Survey

Chris Evans, Illinois Wildlife Action Plan, Bugwood.org
5

EXECUTIVE SUMMARY

can severely reduce carbon exchange between atmosphere and 

biosphere. Recent large diebacks have had global impacts on carbon 

cycles, including release from biomass and reductions in carbon uptake 

from the atmosphere, although impacts may be offset by vegetation 

regrowth in some regions. Multi-year or severe droughts can have 

substantial impacts on hydrological and stream biogeochemical 

processes. 

Indirect effects of drought on forests can be widespread and 

devastating. Notable recent examples include insect and pathogen 

outbreaks (chapter 6) and increased wildfire risk (chapter 7). Available 

evidence suggests a nonlinear relationship between drought intensity 

and bark beetle outbreaks; moderate drought reduces outbreaks 

whereas long, intense drought can increase it. As a consequence of 

long-term drought and warming in the Western United States, bark 

beetles are currently the most important biotic agent of tree mortality. 

Multiple large outbreaks have killed hundreds of millions of trees 

in recent decades. Host trees weakened by drought allow beetle 

populations to build. Warming facilitates northward range expansion. 

In contrast, there is little current evidence for a role of drought in bark 

beetle outbreaks in coniferous forests of the Eastern United States. 

Fungal pathogens are poorly understood, but available evidence 

suggest reduced pathogen performance and host impacts in response 

to drought for primary pathogens and pathogens whose lifecycle 

depends directly on moisture. In comparison, secondary pathogens that 

depend on stressed hosts for colonization are anticipated to respond to 

drought with greater performance and host impacts. 

Historical and pre-settlement relationships between drought and 

wildfire have been well documented in much of North America, with 

forest fire occurrence and area burned clearly increasing in response 

to drought. This body of evidence indicates that the role of drought in 

historical and likely future fire regimes is an important contingency that 

creates anomalously high potential for ignition, fire spread, and large fire 

events. However, drought is only one aspect of a broader set of controls 

on fire regimes, and by itself is insufficient to predict fire dynamics or 

effects. Whereas the relationships between fire occurrence or area 

burned and drought are well documented, the relationship between 

drought and fire severity can be complex. For example, north-facing 

slopes might offer some degree of local protection during mild 

droughts, but even they become dry under extreme conditions, 

reducing fine-scale heterogeneity in vegetation consequences. 

Drought can increase 

the intensity of insect 

and pathogen outbreaks 

including bark beetles (top), 

Dothistroma needle blight 

(center), and hemlock woolly 

adelgid (bottom).

Photo credits (from top to bottom):

Ronald F. Billings, Texas Forest Service, Bugwood.org

Robert L. James, USDA Forest Service, Bugwood.org

Elizabeth Willhite, USDA Forest Service, Bugwood.org
6

Effects of Drought on Forests and Rangelands in the United States: A Comprehensive Science Synthesis

Streamflow and groundwater recharge respond directly to drought 

through reductions in precipitation (rain and/or snowfall), and they 

respond indirectly via evapotranspiration responses to changing 

evaporative energy and water availability. Hydrologic responses to 

drought can be either mitigated or exacerbated by forest vegetation, 

depending on vegetation water use and how drought affects forest 

population dynamics (chapter 10). Drought affects water quality both 

directly and indirectly. Direct impacts are primarily physical, as reduced 

streamflow concentrates nutrients and sediment, and warms more 

quickly. Indirect effects include a combination of terrestrial, riparian, 

and in-stream processes that impact sediment and nutrient 

concentrations and fluxes.

Detecting and Monitoring Future Droughts

The ability to detect drought effects on forest and rangelands over 

broad scales is limited, especially for episodic droughts of moderate 

severity. Compared to agricultural systems, detecting drought impacts 

on trees and other perennial vegetation may require a multi-year 

“memory” of antecedent conditions. With broad-scale monitoring, it is 

often not possible to detect drought effects, as moisture stress is not 

expressed uniformly across vegetation types. 

Remote sensing approaches attempt to observe direct, secondary, 

and longer term effects of drought on vegetation (chapter 9). Remote 

sensing methods integrate across entire mixed-vegetation pixels and 

rarely distinguish the effects of drought on a single species, nor can 

they disentangle drought effects from those caused by various other 

disturbance agents. An integrated data-mining approach may hold the 

most promise for enhancing our ability to resolve drought impacts to 

forests. Efforts to integrate meteorological and remotely sensed data 

streams, together with other ancillary datasets such as vegetation 

type, wildfire occurrence, and pest activity, may help to identify and 

characterize drought effects.

Economic and Social Consequences of Drought

Drought has direct consequences to forest and rangeland production 

(chapter 11). Droughts can negatively impact forest inventories by 

increasing mortality and reducing growth. Drought in rangelands 

reduces forage and water available for livestock grazing. Reduced 

vegetative cover can lead to wind and water erosion. Drought-related 

disturbance, such as wildfire, can have protracted effects that include 

significant timber market losses. 

Drought impacts streams 

(top) by concentrating 

sediments and nutrients. 

In rangelands, drought can 

reduce vegetative cover, 

leading to wind and water 

erosion (bottom). 

Photo credits (from top to bottom):

Carol J. VanDyke, U.S. Geological Survey

Seth Munson, U.S. Geological Survey
7

EXECUTIVE SUMMARY

Reduced water yield from forests and rangelands during extended 

meteorological drought can have substantial impacts on domestic 

and agricultural water supplies, which often results in water markets 

implementing quantity controls. Drought can also have nonmarket 

effects on forests and rangelands. For example, drought affects outdoor 

recreation, where low reservoir levels can reduce availability of fishing, 

recreational boating, swimming, and camping (although some net 

benefit can result from fewer precipitation free days). Low winter snow 

cover reduces economic benefits from skiing and related activities. 

Ongoing drought in the Western United States, where most tribal lands 

exist, is expected to continue to affect tribal health, culture, economies, 

and infrastructure. Competing demands for dwindling water resources 

challenge Federal trust responsibilities. Complicating factors, including 

warming streams and hydrologic cycle changes, affect fish populations 

important to tribal diets and ceremonies. Because of their natural 

resource dependence for income, employment, and cultural practices, 

many tribes are also vulnerable to higher rates of forest and rangeland 

disturbances, including invasive species spread, increased occurrences of 

epidemic pest populations and their associated damages, and wildfires. 

Managing Forests and Rangelands To 

Increase Resiliency and Drought Adaptation

How can forest and rangeland practices adapt to changing drought 

regimes? Frequent low-severity drought may selectively favor more 

drought-tolerant species and create forests and rangelands better 

adapted to future conditions without the need for management 

intervention. By contrast, severe drought (especially in combination with 

insect outbreaks or fire), may threaten large-scale changes that warrant 

substantial management responses. Actions could range from reducing 

vulnerability, facilitating post-drought recovery, or facilitating a transition 

to a new condition.

Management actions can either mitigate or exacerbate the effects of 

drought. A first principal for increasing resilience and adaptation would 

be to avoid management actions that exacerbate the effects of current 

or future drought. Options can include altering structural or functional 

components of vegetation, minimizing drought-mediated disturbance 

such as wildfire or insect outbreaks, and managing for reliable flow 

Drought in rangelands 

reduces forage and water 

available for livestock grazing 

(top). Lower reservoir and 

winter snow levels impact 

outdoor recreation activities 

(bottom).

Photo credits (from top to bottom):

Keith Weller, USDA Agricultural Research Service, Bugwood.org

U.S. Geological Survey
8

Effects of Drought on Forests and Rangelands in the United States: A Comprehensive Science Synthesis

of water. Managers can implement structural changes by thinning or 

density management of planted forests. Thinned stands require less 

water and may be less vulnerable to water stress and insect outbreaks. 

Reduced fuel loads in thinned stands can also reduce wildfire risk. 

Managers can also implement functional changes by favoring or 

planting more drought-adapted species. Management for a diversity 

of species can reduce stand vulnerability to drought, as uncertainty in 

future climate can encourage management for mixtures of droughttolerant species and genotypes. Species diversity can also reduce 

intensity of insect attacks. In some regions of the United States, 

planting or favoring more drought-tolerant species may conflict with 

management objectives that favor rapid accumulation of biomass, as 

fast-growing woody species often use more water and exacerbate 

drought impacts. 

While harvesting increases annual water yield in some forest 

ecosystems, a large reduction of forest cover is needed to have 

an appreciable effect on water yield. Hence, potential increases in 

streamflow through forest cutting are limited by the amount of land 

that managers can harvest. In addition, streamflow responses are 

often short term due to rapid forest regrowth, and the aggrading postcut forest may actually have lower streamflow than the uncut forest. 

In contrast to management actions that are intended to augment 

streamflow, increasing drought stress in some forest ecosystems 

may warrant management strategies that retain water (and hence 

reduce streamflow) on the landscape in order to keep trees alive. 

Land managers may need to plan the timing of some management 

activities to ensure that ecosystems have optimal growing conditions 

and that these activities do not disturb streams during low-flow 

periods. Removal and alteration of riparian vegetation increases stream 

temperatures; therefore, maintaining or increasing shading from solar 

radiation through riparian buffer zone conservation and restoration may 

mitigate any changes in stream temperatures caused by drought.

In summary, this assessment synthesizes information from the 

published literature to better understand the impacts of drought on 

forests and rangelands in the United States. Our expectation is that this 

assessment will provide researchers, land managers, policymakers, and 

other stakeholders a set of realistic inferences of drought effects that 

can be applied to help predict future impacts and evaluate management 

options for adaptation and mitigation. n

Management strategies, such 

as thinning (top) and riparian 

buffer zone conservation 

(bottom), may help to mitigate 

the effects of drought.

Photo credits (from top to bottom):

Scott Roberts, Mississippi State University, Bugwood.org

USDA Natural Resources Conservation Service
Acknowledgments

The editors express their profound thanks to the authors who contributed 

excellent material to this report. We also thank the technical reviewers who 

provided insightful comments and suggestions on earlier drafts. We thank 

Jennifer Plyler and Stephanie Laseter for their assistance with editing and 

formatting. We thank the Southern Research Station for their work to publish this 

report; specifically, we thank Donna Burnett for technical editing and ensuring 

compliance with the U.S. Government Publishing Office, Maureen Stuart for 

technical editing and leading the publication process, and Helen Robinson for 

graphic design. The USDA Forest Service, Southern Research Station (J.M. 

Vose), Rocky Mountain Research Station (C.H. Luce), and National Research 

and Development Washington Office (T. Patel-Weynand and James S. Clark) 

provided financial support for the editors and the publication process.
Photograph: U.S. Geological Survey
Abstract

This assessment provides input to the reauthorized National Integrated Drought 

Information System (NIDIS) and the National Climate Assessment (NCA), and 

it establishes the scientific foundation needed to manage for drought resilience 

and adaptation. Focal areas include drought characterization; drought impacts 

on forest processes and disturbances such as insect outbreaks and wildfire; and 

consequences on forest and rangeland values. Drought can be a severe natural 

disaster with substantial social and economic consequences. Drought becomes 

most obvious when large-scale changes are observed; however, even moderate 

drought can have long-lasting impacts on the structure and function of forests 

and rangelands without these obvious large-scale changes. Large stand-level 

impacts of drought are already underway in the West, but all U.S. forests are 

vulnerable to drought. Drought-associated forest disturbances are expected 

to increase with climatic change. Management actions can either mitigate or 

exacerbate the effects of drought. A first principal for increasing resilience and 

adaptation is to avoid management actions that exacerbate the effects of current 

or future drought. Options to mitigate drought include altering structural or 

functional components of vegetation, minimizing drought-mediated disturbance 

such as wildfire or insect outbreaks, and managing for reliable flow of water.

Keywords: Climate change, drought, forest disturbances, natural disasters, 

water quantity and quality.

Vose, J.M.; Clark, J.S.; Luce, C.H.; Patel-Weynand, T. 2015. Executive 

summary. Effects of drought on forests and rangelands in the United States: 

a comprehensive science synthesis. Gen. Tech. Rep. WO-93a. Washington, 

DC: U.S. Department of Agriculture, Forest Service, Washington Office. 10 p.
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