United States Department of Agriculture

Climate Change 

Vulnerability Assessment 

of Aquatic and Terrestrial 

Ecosystems in the U.S. 

Forest Service Rocky 

Mountain Region

Janine R. Rice, Linda A. Joyce, Claudia 

Regan, David Winters, Rick Truex

Forest Service General Technical Report

Rocky Mountain Research Station RMRS-GTR-376

June 2018
Cover photos by Robert Rice.

All Rocky Mountain Research Station publications are published by U.S. Forest Service employees and are in the 

public domain and available at no cost. Even though U.S. Forest Service publications are not copyrighted, they 

are formatted according to U.S. Department of Agriculture standards and research findings and formatting cannot 

be altered in reprints. Altering content or formatting, including the cover and title page, is strictly prohibited.

Rice, J.R.; Joyce, L.A.; Regan, C.; Winters, D.; Truex, R. 2018. Climate change vulnerability assessment of aquatic 

and terrestrial ecosystems in the U.S. Forest Service Rocky Mountain Region. Gen. Tech. Rep. RMRS-GTR-376. Fort 

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 216 p.

Abstract

Vulnerability assessments are important tools to assist in climate adaptation planning. Six priority ecosystems were 

identified in the USDA Forest Service, Rocky Mountain Region: alpine turf and dwarf-shrubland; aquatic, riparian, and 

wetland ecosystems in glaciated valleys; subalpine spruce-fir; low-gradient mountain stream reaches; ponderosa pine; and 

Great Plains streams and riparian areas. Vulnerability to nonclimate and climate stressors for these priority ecosystems 

is assessed. Criteria used to assess vulnerability include ecosystem traits related to the sensitivity and adaptive capacity 

of the ecosystem. We engaged scientists through an expert review to vet the vulnerability rankings and confidence in 

the assessment. Aquatic ecosystems were the most vulnerable priority ecosystem, and alpine ecosystems had higher 

vulnerability than lower elevation terrestrial ecosystems. The narrative for each priority ecosystem describes the nature of 

the vulnerability to climate change. 

Keywords: low-gradient mountain stream; subalpine spruce-fir; aquatic, riparian, and wetland ecosystems in glaciated 

valleys; alpine turf and dwarf-shrubland; Great Plains stream reach; ponderosa pine
Acknowledgments

We would like to acknowledge funding from the USFS Climate Change Advisor’s Office. We thank Hector Galbraith for his 

assistance in using his vulnerability assessment method and critiquing our modifications for the Rocky Mountain Region. 

We also acknowledge Lisa-Natalie Anjozian for her editing on the draft narratives. We acknowledge, in particular, the 

technical experts who reviewed the narratives and assessed the vulnerability and confidence for each narrative: Gregor 

Auble, Michael Battaglia, Jill Baron, William Bowman, Shannon Brewer, Jonathan Coop, Kate Dwire, Kelly Elder, Hector 

Galbraith, Carol Howe, Andrew Kratz, Noah Molotch, Robert Musselman, Jason Neff, Laura Perry, David Peterson, Leroy 

Poff, Frank Rahel, William Romme, Wayne Shepperd, Thomas Veblen, Mark Weinhold, and Ellen Wohl. We appreciate 

greatly the three reviewers who reviewed the entire General Technical Report: Greg Aplet, Sarah Sawyer, and Robert Hall. 

We would also like to thank Cynthia Moser for her careful editing of the final draft of the manuscript.

Authors

Janine R. Rice is a Research Ecologist with Rice Consulting LLC in Fort Collins, Colorado.

Linda A. Joyce is a Research Ecologist with the U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station, Human Dimensions Research Program in Fort Collins, Colorado.

Claudia Regan was the Regional Ecologist in the U.S. Department of Agriculture, Forest Service, Rocky Mountain Region, 

Regional Office in Golden, Colorado.

David Winters (retired) was the Regional Fisheries Program Manager and Aquatic Ecologist in the U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Region, Regional Office in Golden, Colorado.

Rick Truex is the Regional Wildlife Program Leader in the U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Region, Regional Office in Golden, Colorado. 
ii

Contents

Chapter 1. Introduction....................................................................................................................1

Janine R. Rice and Linda A. Joyce

Assessing Vulnerability ...........................................................................................................................................................1

Assessing Vulnerability of Priority Ecosystems in the U.S. Forest Service Rocky Mountain Region...........................................2

Regional Focus and Priority Ecosystems of the Rocky Mountain Region ........................................................................2

Vulnerability Assessment Framework, Expert Review, Manager Workshop ....................................................................3

Intended Audience and Potential Uses....................................................................................................................................6

References..............................................................................................................................................................................7

Chapter 2. Climate Overview for the U.S. Forest Service Rocky Mountain Region...................9

Linda A. Joyce and Janine R. Rice

Introduction............................................................................................................................................................ 9

Recent and Historical Climate—State-Level Summaries ........................................................................................ 10

Historical and Recent Climate at the Scale of a National Forest............................................................................. 14

Projecting Climate Over the Next 100 Years.......................................................................................................... 14

Modeling the Climate .................................................................................................................................................14

Understanding Climate Scenarios and Projections ......................................................................................................15

Climate Projections for the Rocky Mountain Region....................................................................................................16

Changing Climate, Changing Risks........................................................................................................................ 17

References............................................................................................................................................................ 21

Chapter 3. Alpine Turf and Dwarf-Shrubland Ecosystems: Vulnerability to Nonclimate 

and Climate Stressors in the U.S. Forest Service Rocky Mountain Region .........................23

Janine R. Rice, Claudia Regan, Dave Winters, Rick Truex, and Linda A. Joyce

Introduction.......................................................................................................................................................... 24

Vulnerability of Alpine Turf and Dwarf-Shrubland Ecosystems to Nonclimate Stressors ......................................... 27

Summary of Key Vulnerabilities to Nonclimate Stressors .............................................................................................27

Current Status and Human Influences................................................................................................................... 28

Invasive and Nonnative Species..................................................................................................................................29

Atmospheric Nitrogen Deposition...............................................................................................................................29

Dust on Snow .............................................................................................................................................................29

Intrinsic Resilience of Ecosystem to Nonclimate Stressors ..................................................................................... 30

Factors that Enhance Resilience to Nonclimate Stressors.............................................................................................30

Factors That Lower Resilience to Nonclimate Stressors................................................................................................30

Future Trends of Nonclimate Stressors................................................................................................................... 31

Vulnerability of Alpine Turf and Dwarf-Shrubland Ecosystems to Climate Stressors................................................ 31

Capacity for Range Shift..............................................................................................................................................31

Elevation Potential for Range Shift...............................................................................................................................32

Ecosystem Connectivity ..............................................................................................................................................32

Vulnerability of Cold-adapted, Foundation, or Keystone Species to Climate Change ............................................. 33

American Pika ............................................................................................................................................................33

Alpine Turf and Dwarf-Shrubland Vegetation ..............................................................................................................34

Pollinators...................................................................................................................................................................35

Sensitivity to Extreme Climatic Events ................................................................................................................... 36

Sensitivity to Drought..................................................................................................................................................36

Sensitivity to Extreme Heat..........................................................................................................................................36

Sensitivity to Freeze Date Variability and Earlier Snowmelt..........................................................................................37

Intrinsic Adaptive Capacity to Climate Change...................................................................................................... 38

Factors That Strengthen Adaptive Capacity to Climate Change ....................................................................................38

Factors That Weaken Adaptive Capacity to Climate Change ........................................................................................39
iii

Dependence on Specific Hydrological Regime ..................................................................................................... 39

Likelihood of Managing Climate Change Effects.................................................................................................... 40

Potential for Climate Change to Exacerbate Effects of Nonclimate Stressors, or Vice Versa..................................... 41

Drought and Dust on Snow, Warmer Temperatures, Atmospheric Nitrogen Deposition ......................................... 41

Climate and Fire ................................................................................................................................................... 42

Summary of the Regional Vulnerability of Alpine Turf and Dwarf-Shrubland Ecosystems....................................... 42

References............................................................................................................................................................ 45

Chapter 4. Aquatic, Riparian, and Wetland Ecosystems in Glaciated Valleys: Vulnerability to 

Nonclimate and Climate Stressors in the U.S. Forest Service Rocky Mountain Region.....52

Janine R. Rice, Dave Winters, Claudia Regan, Rick Truex, and Linda A. Joyce

Introduction.......................................................................................................................................................... 53

Vulnerability of Aquatic, Riparian, and Wetland Ecosystems in Glaciated Valleys to Nonclimate Stressors............ 55

Summary of Key Vulnerabilities to Nonclimate Stressors .............................................................................................55

Current Status and Human Influences................................................................................................................... 56

Invasive and Nonnative Species..................................................................................................................................57

Atmospheric Nitrogen Deposition...............................................................................................................................57

Dust on Snow .............................................................................................................................................................58

Intrinsic Resilience of Ecosystem to Nonclimate Stressors ..................................................................................... 58

Factors That Enhance Resilience to Nonclimate Stressors ............................................................................................58

Factors That Lower Resilience to Nonclimate Stressors ...............................................................................................59

Future Trends of Nonclimate Stressors................................................................................................................... 59

Vulnerability of Aquatic, Riparian, and Wetland Ecosystems in Glaciated Valleys to Climate Stressors .................. 60

Capacity for Network Shift..........................................................................................................................................60

Elevation Potential for Habitat Shift.............................................................................................................................60

Fragmentation ............................................................................................................................................................60

Vulnerability of Cold-Adapted, Foundation, or Keystone Species to Climate Change ............................................................61

Cold-Adapted Amphibians..........................................................................................................................................62

Cold-Adapted Macroinvertebrates...............................................................................................................................63

Cold-Water Native Fish...............................................................................................................................................63

Riparian and Wetland Vegetation ................................................................................................................................64

Pollinators...................................................................................................................................................................64

Sensitivity to Extreme Climatic Events ................................................................................................................... 64

Sensitivity to Drought..................................................................................................................................................64

Sensitivity to Floods....................................................................................................................................................65

Sensitivity to Extreme Heat..........................................................................................................................................66

Intrinsic Adaptive Capacity to Climate Change...................................................................................................... 66

Factors That Strengthen Adaptive Capacity to Climate Change ....................................................................................66

Factors That Weaken Adaptive Capacity to Climate Change ........................................................................................67

Dependence on Specific Hydrological Regime ..................................................................................................... 67

Likelihood of Managing Climate Change Effects.................................................................................................... 68

Potential for Climate Change to Exacerbate Effects of Nonclimate Stressors, or Vice Versa..................................... 69

Drought, Dust on Snow, Warmer Temperatures, Atmospheric Deposition....................................................................69

Warming and Invasive Species....................................................................................................................................71

Summary of the Regional Vulnerability of Aquatic, Riparian, and Wetland Ecosystems in Glaciated Valleys.......... 71

References............................................................................................................................................................ 74

Chapter 5. Subalpine Spruce-Fir Ecosystems: Vulnerability to Nonclimate and 

Climate Stressors in the U.S. Forest Service Rocky Mountain Region.................................81

Janine R. Rice, Claudia Regan, Dave Winters, Rick Truex, and Linda A. Joyce

Introduction.......................................................................................................................................................... 82

Vulnerability of Subalpine Spruce-Fir Ecosystems to Nonclimate Stressors ............................................................ 85

Summary of Key Vulnerabilities to Nonclimate Stressors .............................................................................................85

Current Status and Human Influences.........................................................................................................................85
iv

Invasive and Nonnative Species..................................................................................................................................86

Atmospheric Nitrogen Deposition...............................................................................................................................87

Fire .............................................................................................................................................................................87

Intrinsic Resilience of Ecosystem to Nonclimate Stressors ..................................................................................... 88

Factors That Enhance Resilience to Nonclimate Stressors ............................................................................................88

Factors That Lower Resilience to Nonclimate Disturbances.........................................................................................89

Future Trends of Nonclimate Stressors ........................................................................................................................89

Vulnerability of Subalpine Spruce-Fir Ecosystems to Climate Stressors .................................................................. 90

Capacity for Range Shift..............................................................................................................................................90

Southern Limit Proximity.............................................................................................................................................90

Elevation Potential for Range Shift...............................................................................................................................90

Connectivity ...............................................................................................................................................................91

Vulnerability of Cold-Adapted, Foundation, or Keystone Species to Climate Change............................................. 91

Engelmann Spruce and Subalpine Fir..........................................................................................................................91

Cold-Adapted Understory Vegetation ..........................................................................................................................92

Birds and Mammals....................................................................................................................................................93

Sensitivity to Extreme Climatic Events ................................................................................................................... 94

Sensitivity to Drought..................................................................................................................................................94

Sensitivity to Extreme Heat..........................................................................................................................................95

Sensitivity to Freeze Dates...........................................................................................................................................95

Sensitivity to Wind......................................................................................................................................................96

Intrinsic Adaptive Capacity to Climate Change...................................................................................................... 96

Factors That Strengthen Adaptive Capacity to Climate Change ....................................................................................96

Factors That Weaken Adaptive Capacity to Climate Change ........................................................................................97

Dependence on Specific Hydrological Regime ..................................................................................................... 97

Likelihood of Managing Climate Change Effects.................................................................................................... 98

Potential for Climate Change to Exacerbate the Effects of Nonclimate Stressors, or Vice Versa ............................. 100

Bark Beetle Outbreaks and Warmer Temperatures.....................................................................................................100

Fire, Drought, and Warmer Temperatures..................................................................................................................101

Summary of the Regional Vulnerability of Spruce-Fir Ecosystems ........................................................................ 101

References.......................................................................................................................................................... 104

Chapter 6. Low-Gradient Mountain Stream Reaches: Vulnerability to Nonclimate and Climate 

Stressors in the U.S. Forest Service Rocky Mountain Region.............................................112

Janine R. Rice, Dave Winters, Claudia Regan, Rick Truex, and Linda A. Joyce

Introduction........................................................................................................................................................ 113

Vulnerability of Low-Gradient Mountain Stream Reaches to Nonclimate Stressors.............................................. 115

Summary of Key Vulnerabilities to Nonclimate Stressors ...........................................................................................115

Current Status and Human Influences................................................................................................................. 116

Intrinsic Resilience of Ecosystem to Nonclimate Stressors ................................................................................... 119

Factors That Enhance Resilience to Nonclimate Stressors ..........................................................................................119

Factors That Lower Resilience to Nonclimate Stressors .............................................................................................119

Future Trends of Nonclimate Stressors................................................................................................................. 120

Vulnerability of Low-Gradient Mountain Stream Reaches to Climate Stressors .................................................... 121

Capacity for Network Shift........................................................................................................................................121

Elevation Shift Potential for Stream Network Habitats................................................................................................121

Fragmentation ..........................................................................................................................................................122

Vulnerability of Cold-Adapted, Foundation, or Keystone Species to Climate Change........................................... 123

Beaver.......................................................................................................................................................................124

Cold-Water Native Fish.............................................................................................................................................124

Amphibians ..............................................................................................................................................................126

Benthic Macroinvertebrates ......................................................................................................................................126

Riparian Plants..........................................................................................................................................................127
v

Sensitivity to Extreme Climatic Events ................................................................................................................. 128

Sensitivity to Drought ...............................................................................................................................................129

Sensitivity to Floods..................................................................................................................................................129

Sensitivity to Extreme Heat........................................................................................................................................130

Intrinsic Adaptive Capacity to Climate Change.................................................................................................... 131

Factors That Strengthen Adaptive Capacity to Climate Change .................................................................................131

Factors That Weaken Adaptive Capacity to Climate Change ......................................................................................133

Dependence on Specific Hydrological Regime ................................................................................................... 133

Likelihood of Managing Climate Change Effects.................................................................................................. 134

Potential for Climate Change to Exacerbate Effects of Nonclimate Stressors, or Vice Versa................................... 135

Climate Change and Disturbance..............................................................................................................................135

Climate Change and Human Water Use....................................................................................................................136

Climate Change and Water Quality...........................................................................................................................136

Climate Change and Nonnative or Invasive Species..................................................................................................136

Summary of the Regional Vulnerability of Low-Gradient Mountain Stream Reaches............................................ 137

References.......................................................................................................................................................... 139

Chapter 7. Ponderosa Pine Ecosystems: Vulnerability to Nonclimate and Climate Stressors in 

the U.S. Forest Service Rocky Mountain Region ..................................................................148

Janine R. Rice, Claudia Regan, Dave Winters, Rick Truex, and Linda A. Joyce

Introduction........................................................................................................................................................ 148

Vulnerability of Ponderosa Pine Ecosystems to Nonclimate Stressors................................................................... 152

Summary of Key Vulnerabilities to Nonclimate Stressors ...........................................................................................152

Current Status and Human Influences................................................................................................................. 152

Invasive and Nonnative Species................................................................................................................................154

Intrinsic Resilience of Ecosystem to Nonclimate Stressors ................................................................................... 154

Factors That Enhance Resilience to Nonclimate Stressors ..........................................................................................154

Factors That Lower Resilience to Nonclimate Stressors..............................................................................................155

Future Trends of Nonclimate Stressors................................................................................................................. 156

Vulnerability of Ponderosa Pine Ecosystems to Climate Stressors......................................................................... 156

Capacity for Range Shift............................................................................................................................................156

Southern Limit Proximity...........................................................................................................................................157

Elevational Range Shift Potential ...............................................................................................................................157

 Connectivity ............................................................................................................................................................157

Vulnerability of Cold-adapted, Foundation, or Keystone Species to Climate Change ........................................... 158

Ponderosa Pine .........................................................................................................................................................158

Birds and Mammals..................................................................................................................................................159

Understory Vegetation...............................................................................................................................................160

Sensitivity to Extreme Climatic Events.................................................................................................................................161

Sensitivity to Drought................................................................................................................................................161

Sensitivity to Extreme Heat........................................................................................................................................162

Intrinsic Adaptive Capacity to Climate Change.................................................................................................... 163

Factors That Strengthen Adaptive Capacity to Climate Change ..................................................................................163

Factors That Weaken Adaptive Capacity to Climate Change ......................................................................................164

Dependence on Specific Hydrological Regime ................................................................................................... 164

Likelihood of Managing Climate Change Effects.................................................................................................. 164

Potential for Climate Change to Exacerbate Effects of Nonclimate Stressors, or Vice Versa................................... 166

Climate and Fire .......................................................................................................................................................166

Climate and Bark Beetle Outbreaks...........................................................................................................................167

Climate and Invasive Species....................................................................................................................................168

Summary of the Regional Vulnerability of Ponderosa Pine Ecosystems ................................................................ 168

References.......................................................................................................................................................... 170
vi

Chapter 8. Great Plains Streams and Riparian Areas: Vulnerability to Nonclimate and Climate 

Stressors in the U.S. Forest Service Rocky Mountain Region.............................................179

Janine R. Rice, Dave Winters, Claudia Regan, Rick Truex, and Linda A. Joyce

Introduction........................................................................................................................................................ 180

Vulnerability of Great Plains Streams to Nonclimate Stressors............................................................................. 184

Summary of Key Vulnerabilities to Nonclimate Stressors ...........................................................................................184

Current Status and Human Influences................................................................................................................. 185

Water Use.................................................................................................................................................................186

Invasive and Nonnative Species................................................................................................................................186

Intrinsic Resilience of Ecosystem to Nonclimate Stressors ................................................................................... 187

Factors That Enhance Resilience to Nonclimate Stressors ..........................................................................................187

Factors That Lower Resilience to Nonclimate Stressors .............................................................................................188

Future Trends of Nonclimate Stressors................................................................................................................. 188

Vulnerability of Great Plains Streams to Climate Stressors................................................................................... 189

Ecosystem Capacity for Network Shift.......................................................................................................................189

Elevational and Latitudinal Shift Potential..................................................................................................................189

Fragmentation ..........................................................................................................................................................189

Vulnerability of Foundation or Keystone Species to Climate Change ................................................................... 190

Fish ..........................................................................................................................................................................191

Benthic Macroinvertebrate Species ...........................................................................................................................191

Riparian Plant Communities and Riparian-Dependent Animals.................................................................................191

Sensitivity to Extreme Climatic Events ................................................................................................................. 192

Sensitivity to Drought................................................................................................................................................192

Sensitivity to Floods..................................................................................................................................................193

Sensitivity to Extreme Heat........................................................................................................................................193

Intrinsic Adaptive Capacity to Climate Change.................................................................................................... 195

Factors That Strengthen Adaptive Capacity to Climate Change ..................................................................................195

Factors That Weaken Adaptive Capacity to Climate Change ......................................................................................196

Dependence on Specific Hydrological Regime ................................................................................................... 196

Likelihood of Managing Climate Change Effects.................................................................................................. 197

Potential for Climate Change to Exacerbate Effects of Nonclimate Stressors, or Vice Versa................................... 198

Warmer Temperatures and Invasive Species ..............................................................................................................198

Drought, Warmer Temperatures, and Human Water Use ...........................................................................................199

Summary of the Regional Vulnerability of Great Plains Streams and Riparian Areas ............................................ 199

References.......................................................................................................................................................... 202

Appendix A: Assumptions and Structure of the Rocky Mountain Climate Change Vulnerability Assessment......... 208

Appendix B: Criteria and Rationale for Assessing Ecosystem Vulnerability to Nonclimate and Climate Stressors.. 210

Appendix C: Vulnerability Scoring Categories ..................................................................................................... 216
USDA Forest Service RMRS-GTR-376. 2018 1

Chapter 1. Introduction

Janine R. Rice and Linda A. Joyce

Assessing Vulnerability__________________________________________

One of the first steps in the process of adapting to climate change is assessing the 

vulnerability of natural ecosystems and species to climate change (National Research 

Council 2011; USDA FS 2011). Conducting vulnerability assessments and developing 

adaptation options are a part of the Forest Service, U.S. Department of Agriculture 

(USFS) Roadmap to Climate Change (USDA FS 2011). Vulnerability of a natural 

resource can be defined as a function of the character, magnitude, and rate of climate 

variation to which the natural resource is exposed, and its sensitivity, resilience, and 

adaptive capacity (Glick et al. 2011). Although this definition offers the conceptual 

components, no standard methodological approach to assessing vulnerability currently 

exists (USGCRP 2011).

Assessing vulnerability in a consistent and transparent manner is critical where 

public review of land management planning occurs and where adaptation practices 

may be implemented with partners across ownership boundaries or within a larger 

region (USDA FS 2011). Vulnerability assessments in natural resource management 

have focused on species, habitats, ecosystems, watersheds, or ecosystem processes, or 

a combination thereof (Bagne et al. 2011; Case and Lawler 2011; Furniss et al. 2013; 

Galbraith and Price 2011; McCarthy and Enquist 2011; Rice et al. 2012; Swanston 

et al. 2011, 2016; Young et al. 2012). Various biological and ecological traits and 

processes have been used in these assessment tools to evaluate exposure, sensitivity, 

and adaptive capacity. This variety of approaches can make comparisons across assessments within a geographic area difficult. The legacy of past human activities and 

current underlying ecological stresses, often not considered, can result in a great variation in the status of habitats, influencing adaptive capacity and vulnerability within 

a geographic region. Many of the methods have focused strictly on the physical or 

ecological vulnerabilities and have not integrated social vulnerability into the natural 

resource vulnerability assessment. Although the objective of vulnerability assessments 

is typically stated as assisting managers in developing adaptation options, many of 

the approaches for assessing vulnerability do not help with the difficult conversations about what should be adapted; yet conversations are needed among scientists, 

resource managers, and decisionmakers to identify and implement adaptation actions 

(Joyce and Millar 2014; Yuen et al. 2013). There is a need for a structured vulnerability assessment approach that can bring together the current literature on climate 

change vulnerability, engage experts in vetting the scientific information available to 

assess vulnerability, and initiate the dialogue with managers on what is vulnerable and 

how management may address these vulnerabilities.
2 USDA Forest Service RMRS-GTR-376. 2018

Assessing Vulnerability of Priority Ecosystems in the 

U.S. Forest Service Rocky Mountain Region______________________

Incorporating ecosystem processes as well as species and habitat considerations 

into vulnerability assessments can begin the process of identifying adaptation options to 

maintain and enhance ecosystem services under climate change. In 2008, the Manomet 

Center for Conservation Sciences and the Massachusetts Division of Fisheries and 

Wildlife, with funding from the Wildlife Conservation Society, began a cooperative effort to determine: (1) vulnerability to climate change of Massachusetts fish and wildlife 

habitats and species in greatest need of conservation, and (2) potential adaptations of 

conservation and management practices (Manomet Center for Conservation Sciences 

and Massachusetts Division of Fisheries and Wildlife 2010). This approach was further 

developed when the Manomet Center for Conservation Sciences and the National 

Wildlife Federation collaborated with the Northeastern Association of Fish and Wildlife 

Agencies and Federal agencies. Through this effort, a consistent and transparent 

structure was developed to identify vulnerabilities of key habitats in the northeastern 

United States (Galbraith 2011; Manomet Center for Conservation Sciences and National 

Wildlife Federation 2013). The framework also included current stressors that are a 

focus of resource management. In addition to these two applications in the northeastern 

United States, this assessment framework has been used for assessments of habitat 

and species in the Gunnison Basin in Colorado (Neely et al. 2011) and the Badlands 

National Park in South Dakota (Amberg et al. 2012). 

We build on the work of this northeastern multi-institution collaboration and 

recent applications to extend the framework to consistently assess vulnerability of select 

aquatic and terrestrial ecosystems across a large geographic region. In addition, we 

engaged subject-matter experts to evaluate the vulnerabilities, and managers to initiate consideration of adaptation actions. This collaboration between the USFS Rocky 

Mountain Research Station (hereafter RMRS) and the USFS Rocky Mountain Region, 

as with the northeastern collaboration, has the goal of increasing the capabilities of 

resource agencies to manage natural resources under climate change, and to increase the 

dialogue among resource managers across an ecologically varied region.

Regional Focus and Priority Ecosystems of the Rocky Mountain Region

We assess ecosystem vulnerability at the geographic scale of the USFS Rocky 

Mountain Region (fig. 1.1). Within this Region, 17 national forests and 7 national 

grasslands have been administratively combined into 11 units. These 11 administrative 

units cover 27.8 million ac and include 91 wetland and upland terrestrial ecological 

systems (Comer et al. 2003). These public lands are managed for multiple resources: 

timber, water, range, recreation, and more (USDA FS n.d.). Elevations range from about 

2,500 ft on the central mixed-grass prairie to more than 14,000 ft in the alpine tundra 

of the Rocky Mountains. The mountainous areas receive the most precipitation, and the 

driest areas are on the Great Plains (Nebraska, Kansas) and the Intermountain Basins in 

Wyoming. Mean annual temperatures are coolest in the highest elevations, well below 

freezing for some months, and warmest in the lower elevations of the Great Plains. 
USDA Forest Service RMRS-GTR-376. 2018 3

Figure 1.1—USFS Rocky 

Mountain Region study area.

National Forest System staff in the Rocky Mountain Region identified six ecosystems as priority ecosystems for vulnerability assessment. Three were aquatic: aquatic, 

riparian, and wetland ecosystems in glaciated valleys; low-gradient mountain stream 

reaches; and Great Plains streams and riparian areas. Three of these ecosystems were 

terrestrial: alpine turf and dwarf-shrubland, subalpine spruce-fir, and ponderosa pine. 

These six ecosystems were selected to maximize information gained from such an effort 

to disseminate to the 11 units in the Rocky Mountain Region (table 1.1). 

Vulnerability Assessment Framework, Expert Review, Manager Workshop

The vulnerability framework has two major components, one related to nonclimate 

stressors and one related to climate stressors (Appendix A). Within the nonclimate 
4 USDA Forest Service RMRS-GTR-376. 2018

component, four criteria were used to define factors of vulnerability: current status of 

ecosystem extent, human influences on the ecosystem, intrinsic resilience of the ecosystem to nonclimate stressors, and future trends of nonclimate stressors (table 1.2). The 

rationale for ranking these four criteria is given in Appendix B.

Within the climate change component, seven criteria define factors of vulnerability 

to climate change and the interaction with nonclimate stressors (table 1.3). These seven 

criteria capture the vulnerability of individual species, sensitivity of ecosystem dynamics such as dependence of the ecosystem on the hydrological cycle, adaptive capacity 

of the ecosystem, potential for management to mitigate the effects of climate and nonclimate stressors, and the interaction between climate change and nonclimate stressors. 

The initial ranking of each criterion is based on the synthesis of relevant scientific 

literature and an understanding of the priority ecosystems within the geographic area 

of the Rocky Mountain Region. The literature review considers the factors of exposure, 

sensitivity, and adaptive capacity, and how climate change may affect hydrological 

function, disturbance regimes, and aquatic, riparian, and upland vegetation species. The 

rationale for ranking the seven criteria is given in Appendix B.

The vulnerability of each criterion is ranked as high, moderate, or low based on 

a rationale for scoring each criterion (Appendix B). The initial ranking of vulnerability 

was done by the authors and described in the narrative for that ecosystem within the 

draft assessment. Each narrative underwent a scientific review by experts with field 

research expertise in the Rocky Mountain Region to: (1) critique the narrative content 

and rationale for ranking each criterion, as well as the overall vulnerability ranking 

Table 1.1—Six priority ecosystems of the Rocky Mountain Climate Change Vulnerability Assessment, identified by National 

Forest System Regional Staff.

Priority ecosystem Ecological types Extent within the Rocky Mountain Region

Aquatic

Low-gradient mountain stream 

reaches

Mountain stream reaches with 

slope of less than 2 percent in 

these river basins: Colorado, 

Rio Grande, Arkansas, Lower 

Missouri, Upper Missouri

3,500 miles spanning alpine to foothills in 

national forests

Aquatic, riparian, wetland 

ecosystems in glaciated valleys

Lakes, ponds, and associated 

riparian and wetland ecosystems

A small fraction of area in glaciated valleys of 

Colorado and Wyoming

Great Plains streams and riparian 

areas

Perennial and intermittent streams 

whose headwaters are in the 

Great Plains and their associated 

riparian areas 

100 stream miles across the Great Plains 

landscape within national forests

Terrestrial

Alpine turf and dwarf-shrubland Alpine dry tundra; dwarfshrubland 1 million acres on high mountain peaks in 

Colorado and Wyoming

Subalpine spruce-fir Subalpine spruce-fir forest; dry 

subalpine spruce-fir forest and 

woodlands

4.5 million acres across the subalpine zone in 

Colorado and Wyoming; 119,000 acres of dry 

subalpine spruce-fir forest and woodland type 

primarily in northwestern Wyoming

Ponderosa pine Ponderosa pine woodlands; 

ponderosa pine savannas

Woodlands - 4.8 million ac in the Rocky 

Mountain Region and savannas; 1.6 million ac 

in the Colorado Front Range and in southwestern 

Colorado
USDA Forest Service RMRS-GTR-376. 2018 5

assigned by the author team; and (2) assign a confidence ranking that gauged the quality and amount of information available and whether there was sufficient agreement 

in the literature to assess vulnerability. Four to five experts with academic and natural 

resource backgrounds reviewed each of the six narratives (table 1.1). Two experts on 

terrestrial ecosystems reviewed all three terrestrial ecosystem narratives. Two experts 

on aquatic ecosystems reviewed all three aquatic ecosystem narratives. The remaining experts reviewed only one priority ecosystem narrative each. If experts disagreed 

with the assessment, they could change the rank, using the same system as the authors. 

Vulnerability to nonclimate stressors (four criteria) or to climate change (seven criteria) 

was derived by averaging the expert reviewers’ respective scores. Mean confidence was 

the average of all reviewers’ confidence. The overall regional vulnerability ranking for 

the priority ecosystem was determined by averaging the mean nonclimate and climate 

vulnerability scores (corresponding to a high, moderate, or low rank) into a final ranking 

of very high, high, moderate, low, or very low (appendices B, C).

Table 1.2—Four criteria are used to assess the vulnerability of these priority ecosystems to nonclimate stressors. Each criterion 

is used to rank the vulnerability (high, moderate, low) of the ecosystem to a particular nonclimate stressor, using scientific 

literature and expert elicitation (see appendices B and C for rationale).

Criterion Component

Current status of ecosystem extent Amount and distribution of the ecosystem

Human influences on ecosystem Degree of ecosystem effects of past human activities and stressors 

associated with ongoing human influences.

Intrinsic resilience of ecosystem to nonclimate 

stressors

Degree of resilience to nonclimate stressors such as human influences

Future trends of nonclimate stressors Assessment of likely trends of nonclimate stressors to determine the 

future degree of influence on the ecosystem

Table 1.3—Seven criteria are used to assess the vulnerability of these priority ecosystems to climate stressors. Each criterion 

is used to rank the vulnerability (high, moderate, low) of the ecosystem to a particular climate stress, using scientific 

literature and expert elicitation (see appendices B and C for rationale for scoring).

Criterion Component

Ecosystem shift capacity Current range distribution; elevational ranges; connectivity of the 

ecosystem

Vulnerability of cold-adapted, foundation, or 

keystone species to climate change

Foundation species with their high biomass; keystone species that 

have low biomass yet exert strong effects on the structures of their 

communities; cold-adapted species 

Sensitivity to extreme climatic events Extreme events relevant to the ecosystem, such as drought, flooding, 

heat events, freeze and wind events 

Intrinsic adaptive capacity Degree of resilience to climate stressors; factors such as biotic 

characteristics, physical processes, and recovery times to disturbances 

Dependence on specific hydrological regime Specific and relatively narrow hydrological conditions 

Likelihood of managing or alleviating climate 

change effects

Assessment of the feasibility of management to mitigate potential 

effects of climate change 

Potential for climate change to exacerbate effects 

of nonclimate stressors, or vice versa

Potential for changes in climate to exacerbate effects on current or 

future nonclimate stressors, or for nonclimate stressors to exacerbate 

effects of climate change
6 USDA Forest Service RMRS-GTR-376. 2018

To facilitate a regional dialogue about vulnerability and to begin development of 

adaptation options, a 1-day manager workshop was held. The workshop was assisted 

by additional USFS national forest regional experts. More than 20 Rocky Mountain 

Region resource specialists and planning, public relations, and forest land managers attended the workshop; 9 of the 11 units in the Rocky Mountain Region participated. The 

workshop first familiarized resource managers with the information in the six regional 

vulnerability narratives, scoring, and expert review. Then workshop participants were 

taken through exercises that (1) characterized ecosystem services within their individual 

national forests, (2) used the narratives to characterize vulnerability of ecosystem services, and (3) identified potential management intervention points and opportunities for 

adaptation actions. 

After the workshop and revision of the vulnerability assessments, we asked our 

initial experts to review the chapters again. We incorporated their suggestions for new 

literature and revised the document. As per RMRS publication policy, the entire report 

was reviewed by three technical experts who had not previously seen the vulnerability 

assessment. Their comments and suggestions were included in the revision. 

Intended Audience and Potential Uses_____________________________

We envision that the information in these narratives will inform national forest 

resource managers as to which processes and components of these priority ecosystems 

are most sensitive to potential changes in nonclimate and climate stressors. This vulnerability assessment application used a theme-based approach to identify components of 

vulnerability for select aquatic and terrestrial ecosystems in a spatially nonexplicit manner over a large and spatially varied region. This framework efficiently and consistently 

assessed vulnerability of these priority aquatic and terrestrial ecosystems, including 

processes influenced by climate factors as well as by ecological and social stressors. 

Although these assessments looked at the regional scale, we anticipate that each forest 

may have more site-specific information on past management and current conditions 

that will help complete the picture on a local scale.

The need for the information in this vulnerability assessment is exemplified by 

the assessment process in the new forest planning rule, National Environmental Policy 

Act decisionmaking processes, project-level planning, and coordination of regionwide 

conservation projects. In addition, the information can be used to inform resource management strategies, conservation planning, and monitoring plans. The ecosystem focus 

will provide a means to link vulnerabilities to the provisioning, regulating, supporting, 

and cultural ecosystem services that land managers consider in their planning process. 

The transparent synthesis of the scientific literature also provides a platform for initiating dialogue between land managers with a variety of backgrounds and skill sets, as 

we saw in the manager workshop. It also may help to build support and increase understanding about vulnerability and the capacity to manage natural resources under climate 

change in the ecologically varied Rocky Mountain Region.
USDA Forest Service RMRS-GTR-376. 2018 7

References____________________________________________________

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vulnerability assessment. Natural Resource Report NPS/BADL/NRR—2012/505. Fort 

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Adaptation Strategies for Global Change. 18: 567–590.
USDA Forest Service RMRS-GTR-376. 2018 9

Chapter 2. Climate Overview for the U.S. Forest 

Service Rocky Mountain Region

Linda A. Joyce and Janine R. Rice

Introduction___________________________________________________

Information about weather and climate is used in natural resource planning and 

management. Weather is the information about conditions at a specific time and place, 

such as temperature, precipitation, wind, and humidity. Day-to-day implementation 

of on-the-ground resource management practices, such as the start of prescribed fire 

activities, can be made in response to weather conditions. Climate information is the set 

of characteristics of precipitation, temperature, wind, snowfall, and other measures of 

weather in a particular place over a long period of time. Management practices, such as 

the initiation of grazing on grassland, are based on an understanding of when green-up 

occurs over the long term. The actual start of grazing may be adjusted depending on 

the current weather conditions. Understanding the climate of an area helps managers to 

identify both the general characteristics and the risks associated with climate conditions, 

such as flooding, drought, wildfire, and extreme heat or cold events. Understanding 

these risks assists management decisions, such as determining culvert size to withstand 

expected floods. Thus, longer-term resource management strategies and plans are developed using an understanding of climate, the average conditions over time. With the 

potential for climate change, however, our understanding of climate as static is changing. Given the need for long-term planning and management, an understanding of how 

climate may change in the future is valuable. 

The Forest Service, U.S. Department of Agriculture (USFS) Rocky Mountain 

Region (fig. 1.1) encompasses a wide range of climates, from the semiarid climate of 

the short-grass prairie to the cold and windy climate of the alpine tundra. The climate 

of the Region is influenced by the interactions among moist air masses from the Pacific 

Ocean and the Gulf of Mexico, and cold dry air masses from Canada. These climate 

forces interact with the complex terrain within the Region. The Rocky Mountains 

bisect Wyoming and Colorado, forcing the westerly movement of moist air upslope 

and resulting in higher levels of precipitation on the west side of these mountains. The 

eastern side of the Rockies lies in a rain shadow and annual precipitation along the 

Front Range of Colorado can be less than 14 in. In the Great Plains area of the Region, 

precipitation has a west-east gradient, gradually increasing as the eastern edges of 

Kansas and Nebraska are approached. In the alpine, solar radiation and wind can modify 

surface temperatures and snow cover along gradients in topography over short distances 

(Suding et al. 2015). Snowpack on the north-facing slopes under lodgepole pine (Pinus 

contorta Douglas Ex. Loudon) forests develops over the winter season whereas on the 

south-facing slopes under ponderosa pine (Pinus ponderosa Lawson & C. Lawson) forests, snow accumulation is intermittent. Hinckley et al. (2014) found that north-facing 
10 USDA Forest Service RMRS-GTR-376. 2018

slopes had connected moisture flow to greater soil depths than the drier south-facing 

forested slopes. 

The variety of local climates of the Rocky Mountain Region is reflected in the 

number and variety of ecological systems: 91 aquatic and upland terrestrial ecological 

systems (Comer et al. 2003). Seventeen national forests and seven national grasslands 

are found in the Region. Most of the national forests are located at higher elevations in 

the Rocky Mountains (fig. 1.1). This chapter provides a brief discussion of the historical 

climate of the Region and projected changes in climate under two different futures for 

the Region. 

Recent and Historical Climate—State-Level Summaries_______________

We draw from several sources to review the historical climate at the State level, 

providing a picture of the climate at this larger spatial scale. Typically, the last 30 years 

of climate data is used to determine the normal climate, the likely average conditions 

on an annual, monthly, or daily basis. Where observations are available for a longer 

period of time, we can be more certain about trends in the data. As more observations 

are made, past records, such as the hottest year or coldest year, are broken. For example, 

the annual global surface temperature in 2015 exceeded the annual temperature in 2014, 

which was the previous record for the annual global surface temperature (Blunden and 

Arndt 2016). For the information that follows, the temporal period of the analysis is 

reported for reference. 

Topography and elevation strongly influence temperature in the Rocky Mountain 

Region. With elevation, temperatures are generally cooler (fig. 2.1a). Most of the national forests in this Region are found in cooler areas, whereas the national grasslands 

in these States are found at warmer sites. The southern parts of Kansas are the warmest 

areas in the Region. Precipitation within the Region ranges from less than 4 in annually 

to greater than 50 in. Precipitation is lowest in the canyon, mesa, basin, and grassland 

areas of Colorado and Wyoming. The highest annual precipitation is found in the San 

Juan Mountains and other upper elevation areas, with much of this precipitation coming 

as snow. 

Over the 20th century, all States in the Rocky Mountain Region observed an 

increase of at least 1 °F in mean annual temperature (table 2.1). All States experienced 

the greatest warming in winter with some States also warming in spring or at nighttime. 

Precipitation is highly variable and no trends were reported in annual precipitation. 

Although all States experienced an increase in annual temperatures, the seasonal 

changes differed by State. The 1.4 °F increase in Wyoming mean annual temperature reflects changes in winter and summer temperatures. Wyoming winter temperatures have 

increased 1.9 °F above the historical average since 1995 (fig. 2.2). In addition, the frequency of very hot days (maximum temperature above 95 °F) has increased since 2000. 

Number of days with minimum temperatures above 70 °F has been above the long-term 

average since 2000 in Wyoming. 

South Dakota mean annual temperatures have risen 2 °F over the last 100 years 

(table 2.1); mean temperatures in the early 21st century are exceeded only by temperatures in the Dust Bowl era of the early 1930s. Winter and spring temperatures have 

had the greatest influence on annual warming. The warmest winter temperatures in 
USDA Forest Service RMRS-GTR-376. 2018 11

Figure 2.1—Annual mean temperature (°F) (a) and annual precipitation (inches) (b) over the 1980–2010 period. (Data 

source: Oregon State University 2017. Copyright © 2017.)
12 USDA Forest Service RMRS-GTR-376. 2018

Table 2.1—Observed changes in temperature since the early 20th century for States within the Rocky Mountain Region 

(Frankson et al. 2016a,b,c,d,e).

State

Mean annual change 

since the early 

20th century Seasonal temperature changes

Colorado +2°F Winter warming has been characterized by a much below average occurrence 

of extremely cold days since 1990.

Kansas +2°F Greater warming in the winter and spring than in the summer and fall. The 

number of very cold nights has been much below average since 1990.

Nebraska +1°F Warming has been concentrated during the winter and spring. Winter warming 

has been characterized by a much below average occurrence of very cold nights 

since 1990.

South Dakota +2°F Warming has been concentrated during the winter and spring and nighttime 

minimum temperatures increasing about twice as much as daytime maximums.

Wyoming +1.4°F The annual temperature increase is most evident in winter warming, which 

has been characterized by a below average occurrence of very cold days since 

2000.

Figure 2.2—Observed winter temperature (°F) for 1895–

2014, averaged over 5-year periods for South Dakota (a), 

Wyoming (b), and Nebraska (a). The dark horizontal line 

on each graph is the long-term winter temperature average 

(1895–2014). (Source: Data are from the NCEI climate 

division dataset, version 2. Figures from Frankson et al. 

2016 a,c,e.) 

a) b)

c)
USDA Forest Service RMRS-GTR-376. 2018 13

the historical record were reported from 1995 to 2009 (fig. 2.2). The State has also 

experienced a below-average number of very cold nights (minimum temperature below 

0 °F) since 2000. Summers in South Dakota have not warmed. Nighttime temperatures 

have warmed faster than daytime temperatures in South Dakota. Nebraska mean annual 

temperatures have risen the least of all States in the Rocky Mountain Region (table 2.1). 

The recent warming, since the 2000s, has been observed in winter (fig. 2.2) and spring. 

As with South Dakota, summers have not warmed in Nebraska and the number of days 

above 95 °F has been below the historical average since the 1960s. 

Over the last 100 years, mean annual temperatures have risen 2 °F in both 

Colorado and Kansas (table 2.1). In both States, spring temperatures have warmed 

(fig. 2.3). The hottest year on record in Colorado was 2012, when the average temperature rose to 48.3 °F, reflecting the highest average spring and summer temperatures 

since 2000 (fig. 2.3). The year 2012 was also the hottest year on record in Kansas with 

an average temperature of 58.2 °F; record high temperatures were recorded during the 

summer. Spring temperatures have consistently increased in Kansas over the last three 

decades. No trends were seen in very warm nights (minimum temperature above 75 °F) 

or extremely hot days (maximum temperature above 100 °F).

No significant trends have been reported for annual precipitation. In all of the 

Sstates in the Rocky Mountain Region, annual precipitation is variable. Each State has 

seen multiyear droughts, including the dry years of the Dust Bowl era. Droughts have 

occurred throughout the geographic area of the Rocky Mountain Region in historical 

times (Kunkel et al. 2013a,b) and have been recorded in the paleo record (McWethy 

et al. 2010). Each State has also had multiyear periods of above-average precipitation, 

including significant flooding events. 

Figure 2.3—Observed spring temperature (°F) for 1895–2014, averaged over 5-year periods for Colorado (a) and Kansas (b). The 

dark horizontal line is the long-term average (1895–2014) of 53.3 °F (spring) for Kansas and of 42.9 °F (spring) for Colorado. 

(Source: Data are averages from NCEI›s version 2 climate division dataset. Figures are from Frankson et al. 2016 b,d.)

a) b)
14 USDA Forest Service RMRS-GTR-376. 2018

Historical and Recent Climate at the Scale of a National Forest_________

Climate is influenced by elevation, aspect, and topography. Consequently, temperature and precipitation patterns within a national forest will differ from the State 

patterns. Understanding climate depends on long-term monitoring of weather conditions. Weather stations are sparse in remote areas of the Rocky Mountain Region and 

particularly sparse above 10,000 ft. Most weather stations are established in valleys 

where people live. Weather stations may have short periods of data collection in cases 

where the station was established but then closed. Weather stations may also have 

an intermittent data collection when no data were collected over a period of time. 

Most weather stations will have missing values due to a variety of factors, including 

equipment failure. Station climatologists are the best source of information about the 

robustness of climate data within the State. 

Projecting Climate Over the Next 100 Years_________________________

Modeling the Climate

Scientists continue to explore how climate is influenced by the physical and chemical dynamics of the atmosphere, the interaction between the atmosphere and the surface 

of the Earth, and the ocean currents moving heat around the globe. Climate is affected 

by changes in any one of these processes. Building cities (impervious surfaces) and 

changing land use from forests to agriculture have a local and regional effect on climate. 

Changes in the chemical composition of the atmosphere influence the heat absorbed in 

the atmosphere. Humans contribute trace gases and other components to the atmosphere 

through land management, energy sources, and industrial processes. 

Climate models allow scientists to ask questions about how climate responds to 

changes in the chemical composition of the atmosphere, land use changes, and other disturbances. Global climate models have been an important part of the Intergovernmental 

Panel on Climate Change (IPCC) assessments since 1990. Since 1995, atmospheric 

scientists have been working together to coordinate climate model experiments via the 

Coupled Model Intercomparison Project (CMIP) (Meehl et al. 2007). This coordination 

allows for a rigorous comparison of results from different global climate models around 

the world and improves our understanding of the “range” of possible climate change. 

In these experiments, scientists use scenarios, a similar set of initial conditions and 

changes in the chemistry of the atmosphere and in land cover over time. “The goal of 

working with scenarios is not to predict the future but to better understand uncertainties 

and alternative futures, in order to consider how robust different decisions or options 

may be under a wide range of possible futures” (IPCC 2017). 

The results of the climate experiments have also been used by hydrologists, ecologists, foresters, and other natural resource scientists to explore the potential effects of 

climate on their area of research (USDA Forest Service 2012; Walsh et al. 2014). This 

rich literature has been used to identify the vulnerabilities of the six priority ecosystems 

discussed in this report. As we draw from this research, it is important to keep in mind 

that climate scientists continue to learn about climate and to improve the ability of the 

climate models to capture the climate dynamics. Much of the literature consulted for 

this report uses climate projections from the IPCC Third and Fourth Assessments.
USDA Forest Service RMRS-GTR-376. 2018 15

Understanding Climate Scenarios and Projections

The types of scenarios used in the IPCC Third and Fourth Assessments differed 

slightly. In the Third Assessment, the scenarios involved specific assumptions about 

emissions to the atmosphere, population growth, economic growth, and use of fossil fuel 

sources and alternative energy (Solomon et al. 2007). These scenarios have been called 

the SRES scenarios, as they were based on the Special Report on Emissions Scenarios 

(Nakićenović et al. 2000). Here a suite of models was used to estimate potential future 

population growth, different economic pathways and their associated energy use, and 

different land use pathways—and from these different futures, the changes in emissions. 

Consequently, the projected climates ranged from low emissions (B1) to high emissions (A1FI). For the most recent assessment, the IPCC Fifth Assessment, a new set 

of scenarios was developed (Stocker et al. 2013). For these scenarios, scientists asked, 

“What would happen to the climate if different amounts of heat were added to the atmosphere?” These scenarios are called representative concentration pathways (RCPs) (van 

Vuuren et al. 2011). RCP 2.6 assumes an increase in total radiative forcing (heat) of 2.6 

W m-2 (Watts per square meter) (0.82 BTU (h-1 ft-2) (British thermal unit per hour per 

square foot) by 2100, whereas RCP 8.5 assumes a much larger increase in the radiative 

forcing, 8.5 W m-2 (2.7 BTU h-1 ft-2). Intermediate scenarios include RCP 4.5 and RCP 

6.0. With different amounts of energy coming into the atmosphere, the temperature projections differ by scenario. The global model comparison experiments are archived by 

CMIP. The SRES scenario projections are referred to as “the CMIP3 projections” and 

the RCP scenario projections as “the CMIP5 projections.” 

In reviewing the natural resource literature that used the Third Assessment versus 

the Fourth Assessment projections, the question arises as to how similar these future 

projections are. The global warming under the SRES and RCP scenarios is compared 

in table 2.2. This comparison takes into consideration the understanding of climate sensitivity uncertainty, synthesizes the understanding of climate system and carbon-cycle 

Table 2.2—Probabilistic estimates of temperature increase above pre-industrial 

levels based on representative ECS distribution for six SRES scenarios and 

four RCP scenarios (Rogelj et al. 2012).

Temperature increase above pre-industrial (°F)

2090–2099 period

Scenario Median 66-percent range

SRES B1 4.3 3.6–5.6

SRES A1T 5.2 4.5–6.7

SRES B2 5.2 4.3–6.3

SRES A1B 6.1 5.0–7.6

SRES A2 7.0 5.8–8.6

SRES A1FI 8.5 7.0–10.4

RCP 3-PD (2.6) 2.7 2.3–3.4

RCP 4.5 4.3 3.6–5.2

RCP 6 5.2 4.5–6.5

RCP 8.5 8.3 6.8–10.3
16 USDA Forest Service RMRS-GTR-376. 2018

behavior, and is constrained by the observed historical warming (Rogelj et al. 2012). 

The RCP scenarios were not designed to mimic particular SRES scenarios. Temperature 

increase above pre-industrial temperatures (1850 to 1875 period) ranges from 4.3 to 

7.0 °F in the SRES scenarios, and from 2.7 to 8.3 °F in the RCP scenarios (table 2.2). 

For temperatures projected by individual scenarios, Rogelj et al. (2012) identify similarities between RCP 4.5 and SRES B1, RCP 6 and SRES B2, and RCP 8.5 and SRES 

A1FI. One could conclude that analyses using scenario B1 might produce resource 

effects similar to those of RCP 4.5. Similar comparisons could be made with other 

scenarios (Rogelj et al. 2012). They note that the temporal patterns differ between the 

SRES and RCP scenarios. Broadly, the scenarios can be interpreted as a range of future 

climate, with the hotter scenarios being SRES A2, SRES A1FI, and RCP 8.5; a less 

warm scenario is RCP 3.0.

A comparison has also been made of CMIP3 and CMIP5 projections at the scale 

of the Northern Rocky Mountain Region (Idaho, Montana) and the Intermountain 

Region (Utah, southern Idaho, Nevada) (Joyce et al. 2017, 2018). Annual temperature 

and annual precipitation changes from projections using the A1B scenario developed by 

Littell et al. (2011) were compared with projections from CMIP5 models using RCP 4.5. 

We concluded that projected changes in temperature for the RCP 4.5 and SRES A1B 

scenarios used by Littell et al. (2011) were in the same range. Increase in precipitation, 

however, was slightly greater for the RCP 4.5 scenario than the SRES A1B scenario in 

each region. 

Climate Projections for the Rocky Mountain Region

We draw on climate change projections for each State in the Rocky Mountain 

Region as summaries in Frankson et al. (2016a,b,c,d,e). These figures show the observed historical climate (yellow line) changes compared to the 1901–1960 observed 

historical average (fig. 2.4). The observed historical changes vary over time; the warm 

temperatures of the 1930s and the 1950s are apparent in all States. For most States, the 

changes since 1980 have exceeded the historical average. The observed historical fluctuations in all States are generally within the banded ranges of the historical modeled 

changes (gray band). 

Two projections through 2100 are shown: a higher emissions scenario (red band) 

and a low emissions scenario (blue band). All States are projected to have historically 

unprecedented warming during the 21st century. Differences between the two scenarios 

are not evident until after mid-century, when the upper range of the higher emissions 

scenario rises above that of the low emissions scenario. Typically, the coldest years in 

the low emissions scenario projection are as warm as the hottest years in the historical 

record. For Kansas, the coldest years are about 2 °F warmer than the long-term average 

and the hottest years about 11 °F warmer than the hottest year in the historical record. 

The projections associated with the higher emissions scenario are outside of the historical range by the late 21st century as the range of the projections (red shading) is above 

the historical annual means. 

We explore the changes in winter and spring minimum temperature for the Rocky 

Mountain Region using an ensemble of 20 climate models from the CMIP5 experiments and the RCP 4.5 scenario (fig. 2.5). The comparison here is between the historical 
USDA Forest Service RMRS-GTR-376. 2018 17

Figure 2.4—Observed and projected changes (compared to 

the 1901–1960 average) in near-surface air temperature 

(°F) for the five States in the USFS Rocky Mountain 

Region: Colorado (a); Wyoming (b); Kansas (c); Nebraska 

(d), South Dakota (e). Observed data are for 1900–2014. 

Projected changes for 2006–2100 are from global climate 

models for two possible futures: one in which greenhouse 

gas emissions continue to increase (higher emissions) and 

another in which greenhouse gas emissions increase at a 

slower rate (lower emissions). Shading indicates the range 

of annual temperatures from the set of models. (Sources: 

Colorado, Frankson et al. 2016d; Kansas, Frankson et al. 

2016b; Nebraska, Frankson et al. 2016e; South Dakota, 

Frankson et al. 2016a; Wyoming, Frankson et al. 2016c.)

e)

a) b)

c) d)

period 1971–2000 and the projected period 2040–2069. For winter minimum temperature, areas in the Rocky Mountain Region could see increases in winter temperatures of 

up to 6 °F. These temperature increases could push winter temperatures above a biologically meaningful threshold—above freezing in some areas. For spring temperatures, the 

projected range is 3 to 4 °F above the historical record. 

Changing Climate, Changing Risks________________________________

This information about climate projections can supplement resource managers’

experience-based knowledge. Within the Rocky Mountain Region, climate is highly 

variable, and the averages over time are used to guide expectations about, for example, 
18 USDA Forest Service RMRS-GTR-376. 2018

the average winter temperature, the amount of annual precipitation, or the likely timing 

of spring green-up. This historical information is used to assess potential risks, such 

as drought, flooding, and fire, and to ensure temporal conditions that are appropriate 

for management actions, for example, the presence of frozen soil or the occurrence of 

green-up. What actually happens may differ from the average climate, but over the long 

term, the average typically is a good guide to the future. Recently, however, climatic 

patterns have shifted. The trend is toward warming temperature, suggesting that the 

long-term average to date may be an underestimate of what is likely, even in the near 

term. Hence, relying only on the historical climate may underestimate the potential for 

future risks. 6/6/2018 MACA Statistical Downscaling Method

https://climate.northwestknowledge.net/MACA/tool_summarymaps3.php 1/4

MACA Ensemble Summary Projections

20-Model Mean

Projected Winter (Dec-Jan-Feb) Minimum Temperature

RCP4.5 2040-2069 vs. 1971-2000

Data Source: MACAv2-METDATA, MACA-CMIP5 Ensemble

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Change in Min Temperature(deg F)

bcc-csm1-1

bcc-csm1-1-m

BNU-ESM

a)

Figure 2.5—Change in winter (Dec-Jan-Feb) 

minimum temperature (°F) (a) and spring (MarApr-May) minimum temperature (°F) (b) (next 

page) between historical period (1971–2000) 

and projected period (2040–2069) based on the 

ensemble of 20 models using RCP 4.5 scenario. 

(Source: MACA Ensemble Summary Projections 

2015.)
USDA Forest Service RMRS-GTR-376. 2018 19

Scientists continue to learn about climate at local, regional, and global scales. The 

recent IPCC report reinforced this learning and confidence in the observed changes 

in climate, in the capability of climate models to reproduce climate features, and in 

the likelihood of future changes in climate (table 2.3). Greater certainty is associated 

with the projected temperature changes. These temperature changes will occur within 

the variability of the historical temperatures. In other words, individual months or 

seasons could warm, and yet the annual average could be close to the historical climate 

average. Very cold periods are also likely to be part of the near-term climate patterns. 

6/6/2018 MACA Statistical Downscaling Method

https://climate.northwestknowledge.net/MACA/tool_summarymaps3.php 1/4

MACA Ensemble Summary Projections

20-Model Mean

Projected Spring (Mar-Apr-May) Minimum Temperature

RCP4.5 2040-2069 vs. 1971-2000

Data Source: MACAv2-METDATA, MACA-CMIP5 Ensemble

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Change in Min Temperature(deg F)

bcc-csm1-1

bcc-csm1-1-m

BNU-ESM

b)
20 USDA Forest Service RMRS-GTR-376. 2018

Precipitation projections are less certain, but moisture availability is likely to be lower 

even if precipitation remains the same, given the projected increases in temperature.

What are current biological and physical thresholds? Where will the biological and 

physical thresholds be crossed? The following chapters identify examples of observed 

changes and their observed effects, in addition to identifying potential sensitivities to 

moisture stress, extreme heat, and other climate features. Temperatures moving above 

freezing during winter are likely to have an effect on plants and animals, directly and 

indirectly. Warming winter temperatures have led to earlier snowmelt and an earlier 

onset of vegetative growth of some alpine/subalpine species; earlier flowering was 

then damaged by freezing events during spring (see Chapter 3). For some species, 

new environments may open up; areas of glacial melt have been colonized by midges 

(family Diamesinae) and species that are attached to submerged surfaces (see Chapter 

4). Temperature and moisture availability differentially influence regeneration of tree 

species. An abrupt increase in regional tree establishment at the upper treeline coincided with a shift toward reduced cool-season precipitation (see Chapter 5). During the 

26-month cone development phase of ponderosa pine, below-average temperatures in 

late spring damaged the second-year conelets, whereas high temperatures during the 

first year of cone production coincided with increased cone production (see Chapter 7). 

Late-season moisture stress is another source of potential stress for ecosystems in 

the Region. Although the increased atmospheric carbon dioxide has been reported as 

increasing productivity, its fertilizing effect at high elevations in the Rocky Mountains 

was more than counterbalanced by the increased water stress due to longer growing seasons (see Chapter 5). In low-gradient streams, the response area in the stream network 

is more sensitive to flooding than in higher gradient streams; the large pool volumes 

can become filled with sediment and not be flushed out as rapidly as in higher gradient 

streams, thereby reducing critical habitat area (see Chapter 6). For intermittent streams 

on the Great Plains, local water-table fluctuations and soil moisture conditions can control hydrological responses more than local weather patterns might (see Chapter 8). 

Table 2.3—Key findings from the Fourth IPCC Assessment Working Group I Summary of Policymakers (IPCC 2013).

Conclusion Statement

Observed changes in the 

climate

Warming of the climate system is unequivocal, and since the 1950s, many of the 

observed changes are unprecedented over decades to millennia. The atmosphere and 

ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, 

and the concentrations of greenhouse gases have increased such that changes in the 

atmosphere are leading to changes in the climate.

Evaluation of climate models Climate models have improved since the AR4. Models reproduce observed continentalscale surface temperature patterns and trends over many decades, including the more 

rapid warming since the mid-20th century and the cooling immediately following large 

volcanic eruptions (very high confidence).

Future temperature changes Global surface temperature change for the end of the 21st century is likely to exceed 

1.5 °C (2.7 °F) relative to 1850 to 1900 for all RCP scenarios except RCP 2.6. It is likely 

to exceed 2 °C (3.5 °F) for RCP 6.0 and RCP 8.5, and more likely than not to exceed 2 °C 

for RCP 4.5.
USDA Forest Service RMRS-GTR-376. 2018 21

References____________________________________________________

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19–67.
USDA Forest Service RMRS-GTR-376. 2018 23

Chapter 3. Alpine Turf and Dwarf-Shrubland 

Ecosystems: Vulnerability to Nonclimate and 

Climate Stressors in the U.S. Forest Service 

Rocky Mountain Region

Janine R. Rice, Claudia Regan, Dave Winters, Rick Truex, and Linda A. Joyce

Quick Look: Alpine Turf and Dwarf-Shrubland Ecosystems in the Rocky Mountain 

Region

Topography and climate play large roles in the distribution of plants and animals in alpine 

turf and dwarf-shrubland ecosystems. The alpine turf (dry tundra) ecosystem and the dwarfshrubland ecosystem occur on moderate or gentle slopes, flatter ridges, valleys, and basins 

above treeline on the highest peaks of Colorado and Wyoming. These ecosystems form a mosaic 

along with fell-fields, wet meadows, alpine bedrock and scree, and ice fields across the alpine 

landscape. Temperatures year-round are cold, high winds can desiccate the soil and plants, and 

the growing season is short. At fine spatial scales, temperatures and moisture availability can 

change over short distances from an inhospitable environment to one that is suitable for some 

plants and animals. 

The alpine turf plant community has a rich mixture of species. This diversity includes dense lowgrowing perennial grasses and sedges that produce new plants through rhizomes (horizontal 

underground rootstalks), and mat-forming forbs with thick roots or taproots. The rich diversity 

of the dwarf-shrubland community includes nonwoody (herbaceous) plants, and woody plants, 

such as dwarf shrubs of the heath family, that form a semi-continuous layer. 

Alpine turf and dwarf-shrubland ecosystems provide habitat for a variety of wildlife. Voles, weasels, American pika, yellow-bellied marmots, and pocket gophers live year-round in the alpine. 

Large, hoofed animals, such as elk and bighorn sheep, migrate into the alpine during summer 

for forage. Rosy-finch, white-tailed ptarmigan, and American pipit breed above treeline. Alpine 

plants are pollinated by hummingbirds, and butterflies and other insects.

Quick Look: Vulnerability of Alpine Turf and Dwarf-Shrubland Ecosystems in the 

Rocky Mountain Region 

Vulnerability to nonclimate and climate stressors: High

Confidence: High

Exposure: Warming temperatures year-round, potential early snowpack decline, high variability 

in timing of spring-fall freezing events. 

Current extent: Isolated on high-elevation mountain peaks. 

Sensitivity and adaptive capacity to climate change: Warm temperatures and loss of winter 

snowpack cover cause physiological stress on plants and animals; asynchronous plant and 

pollinator responses could lower successful plant reproduction. Life history traits of wildlife 

and high plant diversity aid persistence. Management practices, such as addressing air 

quality issues and restoring landscapes, may not completely offset the effects of warmer 

temperatures and snowpack loss.
24 USDA Forest Service RMRS-GTR-376. 2018

Nonclimate stressors: Land use activities including domestic livestock grazing, mining, roads, 

and recreation, and air quality issues such as nitrogen deposition and dust on snow. Climate 

change may exacerbate the effects of these nonclimate stressors.

Introduction___________________________________________________

Topography and climate greatly influence the distribution of plants and animals 

in the alpine ecosystem. This high elevation zone begins at treeline, the upper limit of 

upright tree growth, and ends where vascular plant life ceases to occur (Suding et al. 

2015). The growing season is short; year-round temperatures are cold. At fine spatial 

scales, temperatures and moisture availability can change over short distances from 

an inhospitable environment to one that is suitable for specific plants and animals. For 

example, solar radiation and wind modify surface temperatures and snow cover along 

gradients in topography over short distances (Suding et al. 2015). Soil moisture can 

be diminished or increased by small changes in slope and aspect. Understanding how 

alpine plants, animals, and ecosystem processes will respond to climate change requires 

an awareness of how the plants and animals meet their physiological and biological 

needs in the harsh alpine environment (Millar et al. 2016).

Comer et al. (2003) identified six ecological systems that occur in this zone: alpine 

ice field, alpine bedrock and scree, alpine dwarf-shrubland, alpine fell-field, alpine dry 

tundra, and alpine-montane wet meadow. These ecological systems often occur as a 

mosaic in the alpine zone. Areas adjacent to melting snowpack are colonized by alpine 

wet meadows, and often alpine dwarf-shrublands are found next to the wet meadows. 

Comer et al. (2003) describe alpine dry tundra as the matrix of the alpine zone, intermixing with bedrock and scree, ice field, fell-field, alpine dwarf-shrubland, and alpine 

wet meadow. 

These ecological systems are found on the highest peaks of Colorado and 

Wyoming; they extend down to about 11,000 ft in Colorado and about 10,500 ft in 

Wyoming (Comer et al. 2003). Alpine ice field, primarily snow and ice, occurs where 

snowfall exceeds melting. Alpine bedrock and scree is a barren or sparsely vegetated 

area, typically dominated by nonvascular (lichen) communities. Forbs, grasses, and 

low shrubs can also occur (Comer et al. 2003). Alpine fell-field systems occur on 

ridgetops or exposed saddles where wind keeps the area snow-free in winter (Suding et 

al. 2015). Unproductive shallow soils support cushion plants, with a plant cover of 15 

to 50 percent. Alpine wet meadows are often found in small depressions associated with 

late-melting snow patches. Vegetation is often dominated by grasses or grass-like plants. 

Soils may be mineral or organic and show hydric soil features, including high organic 

content. 

The two remaining ecological types in the alpine zone are the focus of this chapter: 

dry tundra (hereafter called alpine turf) and dwarf-shrubland. These two ecosystems 

span the North American mountain ranges, known as cordillera, that run from southern 

Mexico to Alaska (Comer et al. 2003). Within the Forest Service, U.S. Department of 

Agriculture (USFS) Rocky Mountain Region, alpine turf is found throughout the alpine 

zone; dwarf-shrubland predominantly occurs in the alpine zone of Wyoming (fig. 3.1). 
USDA Forest Service RMRS-GTR-376. 2018 25

Figure 3.1—Alpine turf and alpine dwarf-shrubland distribution within USFS Rocky Mountain Region (Comer et al. 2003; 

Data can be found in NatureServe 2014).
26 USDA Forest Service RMRS-GTR-376. 2018

Snow retention, wind desiccation, permafrost, and the short growing season strongly 

influence plant community composition in both of these types. 

Alpine dwarf-shrubland ecosystems occupy level areas within glacial basins, 

where late-lying snow and sub-irrigation from surrounding slopes keep soil moist. 

Ericaceous dwarf shrubs or dwarf willows (Salix spp.) form a semicontinuous layer 

less than 20 in tall in which grasses, grass-like plants, and forbs are found. Dwarf-shrub 

species can include: pink mountain heath (Phyllodoce empetriformis), willows, bilberry 

(Vaccinium spp.), Western Labrador tea (Ledum glandulosum), yellow mountain heath 

(Phyllodoce glanduliflora), and alpine laurel (Kalmia microphylla). Other species 

found in the dwarf-shrubland communities can include: fleabane (Erigeron spp.), 

woolly pussytoes (Antennaria lanata), tundra aster (Oreostemma alpigenum), lousewort 

(Pedicularis spp.), Indian paintbrush (Castilleja spp.), tufted hairgrass (Deschampsia 

caespitosa), Howell’s marsh marigold and white marsh marigold (Caltha leptosepala

DC), fawnlily (Erythronium spp.), Parry’s rush (Juncus parryi), Piper’s woodrush 

(Luzula piperi), showy sedge (Carex spectabilis), black alpine sedge (Carex nigricans), 

and American bistort (Polygonum bistortoides) (Comer et al. 2003).

Alpine turf occurs on gentle to moderate slopes and flat ridges and in valleys, and 

in basins where soil moisture is relatively constant (Comer et al. 2003). This ecosystem has a rich mixture of plant species, which include dense low-growing perennial 

grasses and sedges that produce new plants through rhizomes (a horizontal underground 

rootstalk), and mat-forming forbs with thick roots or taproots. Rhizomatous sedges 

and grasses can include the following: blackroot sedge (Carex elynoides), dryspike 

sedge (Carex siccata), northern singlespike sedge or Western singlespike sedge (Carex 

scirpoidea spp.), spike sedge (Carex nardina), curly sedge (Carex rupestris), alpine 

fescue (Festuca brachyphylla), Idaho fescue (Festuca idahoensis), tufted hairgrass 

(Deschampsia caespitosa), and Bellardi bog sedge (Kobresia myosuroides). Forbs can 

include Ross’ avens (Geum rossii), cushion phlox (Phlox pulvinata), and alpine clover 

(Trifolium dasyphyllum) (Comer et al. 2003). 

Alpine turf and dwarf-shrubland ecosystems provide habitat for a variety of 

wildlife that are permanent residents or that migrate to utilize summer habitat resources. 

Small mammals occupying this ecosystem year-round can include voles (Microtus spp.), 

weasels (Mustela spp.), American pika (Ochotona princeps), yellow-bellied marmots 

(Marmota flaviventris), and pocket gophers (Geomyidae spp). Large, hoofed animals, 

such as elk (Cervus elaphus) and bighorn sheep (Ovis canadensis), migrate into the 

alpine for summer forage (Zeigenfuss et al. 2011). Rosy-finch (Leucosticte australis), 

white-tailed ptarmigan (Lagopus leucura), and American pipit (Anthus rubescens) breed 

above treeline. Alpine plants are pollinated by hummingbirds (Selasphorus spp.), and 

butterflies (Order Lepidoptera) and other insects. Alpine butterflies feed on alpine flower 

nectar and use host plants for reproduction (Matter et al. 2011). 

Alpine turf and dwarf-shrubland ecosystems have been assessed for their vulnerability to climate change. As described in Chapter 1, the vulnerability framework has 

two major components, one related to nonclimate stressors and one related to climate 

stressors. Within the nonclimate component, four criteria were used to define factors of 

vulnerability: current status of ecosystem extent; human influences on the ecosystem, 

intrinsic resilience of the ecosystem to nonclimate stressors, and future trends of nonclimate stressors on the ecosystem. Within the climate change component, seven criteria 
USDA Forest Service RMRS-GTR-376. 2018 27

capture the vulnerability of individual species, sensitivity of ecosystem dynamics such 

as dependence on a specific hydrological cycle, adaptive capacity of the ecosystem, 

potential for management to mitigate the effects of climate and nonclimate stressors, 

and interaction between climate change and nonclimate stressors. More information and 

the rationale for ranking ecosystem vulnerability by using these criteria are given in appendices A, B, and C.

These alpine ecosystems are increasingly being studied on national forest lands 

as well as in other areas throughout the Rocky Mountain Region. We draw from the literature where studies are within the Rocky Mountain Region. Even within this Region, 

these ecosystems vary greatly, particularly at fine spatial scale. Application of these 

results to alpine systems on individual national forests will require an understanding 

of local site characteristics and the site characteristics of the research studies reviewed 

here. 

Vulnerability of Alpine Turf and Dwarf-Shrubland Ecosystems to 

Nonclimate Stressors_________________________________________

Summary of Key Vulnerabilities to Nonclimate Stressors

BOX 3.1

Current status of ecosystem extent

Alpine turf and dwarf-shrubland ecosystems are limited in extent, covering less than 1 percent of the entire 

Rocky Mountain Region. These ecosystems cover about 18 percent of the alpine zone above 10,500 ft, and the 

majority of alpine turf and dwarf-shrubland ecosystems in the Rocky Mountain Region occur within national forest 

boundaries. Consequently, their vulnerability ranking based on extent is very high.

Human influences on ecosystem

The vulnerability of alpine turf and dwarf-shrubland ecosystems is ranked as moderate to the legacy of past 

human influences and to the stressors associated with ongoing human influences. 

Intrinsic resilience of ecosystem to nonclimate stressors

A very high vulnerability ranking is given for the intrinsic resilience of alpine ecosystems to nonclimate stressors.

• Factors that enhance the resilience of alpine turf and dwarf-shrubland ecosystems to nonclimate stressors:

○ Rich plant diversity, life history traits, and variety in community structure enable communities to persist 

by a shift in species composition. 

○ Fire, limited in the alpine, may promote alpine plant establishment; however, postfire recovery can be 

slow.

• Factors that lower the resilience of alpine turf and dwarf-shrubland ecosystems to nonclimate stressors:

○ Alpine plants grow slowly.

○ Alpine plants recover slowly from disturbances, such as road and trail building, fire, trampling, wind, and 

water erosion. Recovery after disturbance can take more than a century.

Future trends of nonclimate stressors

Alpine turf and dwarf-shrubland ecosystems are considered to be moderately vulnerable to future nonclimate 

stressors associated with increased demand for recreation, invasive species, and atmospheric disturbances.
28 USDA Forest Service RMRS-GTR-376. 2018

Current Status and Human Influences______________________________

Alpine turf and dwarf-shrubland ecosystems are limited in extent, covering less 

than 1 percent of the Rocky Mountain Region, or about 1 million ac on high mountain 

peaks in Colorado and Wyoming. These ecosystems cover about 18 percent of the alpine 

zone above about 10,500 ft in the Rocky Mountain Region, the remainder being rock, 

fell-fields, scree, wet meadows, and ice. About 85 percent of the area of alpine turf 

and dwarf-shrubland ecosystems within the boundaries of the Rocky Mountain Region 

occurs on national forest lands. About 55 percent of the area of alpine turf and dwarfshrubland ecosystems is in wilderness areas.

Past human use of these ecosystems has included mining and grazing. Mining in 

the Rocky Mountain Region has harmed sites within some alpine areas, especially in the 

southern Rockies of Colorado (Bowman et al. 2002). Mines and tailings above treeline 

are remnants of mining activity that took place beginning in the mid-1800s. Driven by 

economics, mining is currently less active, but these areas still bear the marks of mining 

disturbance (Bowman et al. 2002). 

Herbivory in alpine areas of the Region includes wild herbivores and domestic 

livestock. Alpine areas are grazed by permanent residents (American pika, pocket 

gophers) and seasonal wild herbivores (bighorn sheep, mountain goat [Oreamnos 

americanus], elk, and grizzly bear [Ursus arctos horribilis]). Domestic livestock grazing continues on national forest lands with high elevation allotments grazed primarily 

by sheep. Overgrazing coupled with drought has left a legacy of grazing impacts in the 

western United States; however, few studies have explored the nature of this legacy in 

alpine ecosystems. Generally, grazing effects, from both domestic and wild animals, 

vary with intensity of grazing, timing, type of grazing animal, grazing behavior of the 

animal, legacy of past grazing onsite, and the current grazing management system including the time available for recovery. 

Recreation is increasing as a nonclimate stressor on these ecosystems. From 2011 

through 2015, the Rocky Mountain Region had more than 27 million annual visits, 

of which 1.37 million visits were in Wilderness Areas (USDA FS 2015b). Nationally, 

visitors to national forests participate in a variety of activities including viewing natural 

features and wildlife, hiking or walking, downhill skiing, fishing, hunting, camping, 

backpacking, and resort use. Recreation can have direct effects on alpine plants and 

animals where humans come into contact with plants and animals through hiking and 

camping, and indirect effects, for example, soil compaction processes that are initiated 

when human use is high. Recreation in Rocky Mountain alpine areas has hindered 

plants due to trampling and erosion (Willard and Marr 1970; Willard et al. 2007). Turftype plants face less potential damage from recreationists than do plants in wetter areas, 

which are more sensitive (Willard and Marr 1970). Turf-forming, matted, or rosette 

alpine plants are more resistant to trampling compared to upright or woody plants 

(Cole and Monz 2002). Lohman (2010) reported that trail and trail margins showed the 

greatest effect of human traffic, but the effects were not confined to the trail, suggesting 

overuse and congestion of the trails associated with the high mountain peak at Quandary 

Park, near Breckenridge, Colorado. Cross-country and downhill skiing are part of the 

recreational experience in alpine settings; Sato et al. (2013) in a meta-analysis describe 
USDA Forest Service RMRS-GTR-376. 2018 29

the ecological effects of downhill skiing in the United States as negative or negligible 

more often than positive.

Invasive and Nonnative Species

The Rocky Mountain Region has a limited distribution of invasive plants in the 

alpine. Although invasive plant species in high elevation areas globally have expanded 

over the last decade (McDougall et al. 2011), the common dandelion (Taraxacum officinale F.H. Wigg.) was the only introduced species found in a survey of Rocky Mountain 

National Park in Colorado (Ashton 2010). Common dandelion displaces native species 

and may alter pollinator behavior when high densities occur (Molina-Montenegro et al. 

2012; Muñoz and Cavieres 2008). The weevil Rhinocyllus conicus Fröelich has been 

found on native thistles on Niwot Ridge in Colorado, but its presence was negatively 

correlated with elevation. Climatic factors apparently limit its current capacity to expand above treeline; warming temperatures could facilitate an upward expansion (Hicks 

et al. 2013). 

Atmospheric Nitrogen Deposition

Human activity is linked to an increase in atmospheric nitrogen deposition in the 

terrestrial and aquatic ecosystems at higher elevations of the Rocky Mountains (Baron 

et al. 2000; Ingersoll et al. 2006; Nanus et al. 2012). Low background rates of nutrient 

cycling and low biomass production of alpine ecosystems reflect the limited availability 

of nitrogen in alpine soils. Increased nitrogen deposition, through natural or experimental changes, has been observed to alter many community and ecosystem processes, 

resulting in increased alpine soil nitrogen cycling, increased soil acidification, greater 

nitrate leaching, shifts in plant and microbial community composition, and increased 

cover of nitrogen-loving grasses, such as tufted hairgrass (Deschampsia cespitosa (L.) P. 

Beauv.) (Bowman 2000; Bowman et al. 2015). 

Dust on Snow

Researchers have linked more intense dust-on-snow events in the Rocky 

Mountains to drought and human activity in areas predominantly to the southwest but 

also to the west of the Rocky Mountains (Painter et al. 2007; Skiles et al. 2015). From 

2010 through 2013, dust events advanced snowmelt 24 to 49 days in the San Juan 

Mountains and 15 to 30 days on Grand Mesa, both sites in Colorado (Skiles et al. 2015). 

As a result of dust-on-snow events in 2006, snow chemistry on 17 sites from southern 

Wyoming to central Colorado increased in pH, calcium content, and acid neutralizing 

capacity; the effects were more pronounced at upper elevations than on densely forested 

lower elevation sites (Rhoades et al. 2010). Steltzer et al. (2009) concluded that dust-onsnow events may tend to reduce the differences in snowmelt timing between north- and 

south-facing slopes and synchronize green-up and timing of flowering for plants across 

the alpine landscape. Earlier snowmelt from dust-on-snow events may temporally 

disconnect the availability of alpine plants for insects and animals seeking forage and 

pollination sources after hibernation or migration. During the growing season, less variety across the landscape in the timing of alpine plant growth and blooming may cause 

increased competition for pollinators (Steltzer et al. 2009). 
30 USDA Forest Service RMRS-GTR-376. 2018

Fire

Fire is limited and infrequent in the alpine; few studies are available to quantify 

the alpine fire regime in the Rocky Mountain Region. Fire return intervals were found 

to be a century or several centuries, based on charcoal records in high elevation lakes 

in Idaho (Anderson et al. 2008; Brunell and Whitlock 2003). Using fire atlases, Rollins 

et al. (2001) suggested that fire suppression during the 1930s through 1970s may 

have reduced the amount of area burned in some alpine areas in the Selway-Bitteroot 

Wilderness complex in Idaho and Montana. More studies are needed to determine human influence on the alpine fire regime in the Rocky Mountain Region.

Intrinsic Resilience of Ecosystem to Nonclimate Stressors____________

Factors that Enhance Resilience to Nonclimate Stressors

Plant species diversity, life history traits, and physical structure of alpine plants 

help to support the persistence of alpine plants exposed to nonclimate stressors. Plant 

diversity and the number of species across the landscape are high, with 150 to 300 species occurring in the Rocky Mountains (Bowman 2000; Komárková and Webber 1978). 

The establishment and survival of alpine plant species can be as high as for perennial 

species in more temperate climates of North America and Europe (Forbis 2003). Plant 

structure and form may provide some protection; for example, turf-forming grasses are 

less impeded by trampling than other plant structures. Plant traits such as being tall or 

prostrate also provide some resistance to grazing. 

The resilience of alpine turf and dwarf-shrubland ecosystems to the limited occurrence of fire in the Rocky Mountain alpine is not well studied (see Current Status and 

Human Influences section). Dwarf-shrubland plants may benefit from fire occurrence; in 

the Washington Cascades, plant diversity eventually increased after recovery (Douglas 

and Ballard 1971). Fire can also promote alpine plant establishment in areas where subalpine trees previously grew (Billings 1969).

Factors That Lower Resilience to Nonclimate Stressors

Alpine turf and dwarf-shrubland ecosystems are not resilient to damage from road 

and trail building, mining, trampling, wind, and soil erosion; recovery can take from 30 

years to more than a century (Conlin and Ebersole 2001; Ebersole 2002; Greller 1974; 

Willard and Marr 1970; Willard et al. 2007). Although recovery is slow, turf-forming 

grasses that grow in clusters are more tolerant of trampling than communities with 

groundcover dominated by primarily low woody shrubs or upright forbs (Cole and 

Monz 2002). 

The alpine plant-herbivore relationship is strongly influenced by plant traits (tall, 

prostrate), environmental conditions (moisture, temperature, nutrients), and grazing 

history and use. The dominance of grasses and grass-like species as in the historically 

grazed alpine plant communities in the central Caucasus Mountains in Eurasia offers 

a contrast to the ungrazed or lightly grazed Rocky Mountain alpine systems; these 

differences may be, in large part, the result of a difference in human history (Bock et 

al. 1995). Warmer summer temperatures on grazed alpine sites in the Medicine Bow 

Mountains in Wyoming led to altered nitrogen cycling and carbon dioxide exchange 
USDA Forest Service RMRS-GTR-376. 2018 31

on the grazed sites. Grazed sites were sources of carbon emissions, in contrast with 

the carbon capture on the ungrazed sites (Welker et al. 2004). Although alpine grazing 

may significantly stimulate nitrogen cycling, Martinsen et al. (2012) concluded from a 

controlled grazing experiment that the effect was not sufficient to shift the nutrient-poor 

alpine system away from nitrogen deficiency. 

Future Trends of Nonclimate Stressors_____________________________

Future land use in the alpine will depend on many factors. Economic factors are 

likely to influence mining activity and domestic livestock grazing. Cattle numbers declined in Colorado and Wyoming between 2000 and 2009 (Reeves and Mitchell 2012); 

however, little information is available to describe changes in livestock grazing numbers 

specific to the alpine zone. Nationally, winter activities projected under future scenarios 

are sensitive to changes in the climate and to economic growth. Downhill skiing is 

projected to go up, depending on income, whereas undeveloped skiing is projected to 

decline, influenced by changes in climate (Bowker et al. 2012). No projections are available for the alpine ecosystem alone. 

Increased atmospheric nitrogen deposition at high elevations is likely to continue 

as human activity is expected to expand in the future. Bowman et al. (2015) suggest that 

dust-on-snow events, with their base cation-laden dust, may have prevented the onset 

of soil and surface water acidification from atmospheric nitrogen deposition; however, 

continued increases in atmospheric nitrogen deposition will shift that balance.

Native alpine plant communities may not be resilient to invasive or nonnative 

species, which disrupt ecosystem function and reduce biodiversity. Dandelion is an 

adaptable species that can successfully replace native species (Molina-Montenegro et al. 

2012) and disrupt the role of pollinators in the ecosystem (Pauchard et al. 2009), especially when dandelions grow in high densities (Muñoz and Cavieres 2008). Although the 

presence of invasive species is currently limited in Rocky Mountain alpine areas, studies 

in Chile suggest that alpine ecosystems are not likely to be resilient to future invasions 

(Pauchard et al. 2009).

Vulnerability of Alpine Turf and Dwarf-Shrubland Ecosystems to Climate 

Stressors __________________________________________________

Capacity for Range Shift

Box 3.2

Key Vulnerabilities

A very high vulnerability ranking is given for the capacity of alpine turf and dwarf-shrubland ecosystems for range 

shift. 

• Plant migration potential is limited. 

• Topographic variation may offer opportunities for plant redistribution within the alpine zone.

• Limited in connectivity, these ecosystems may become more fragmented with future contraction of favorable 

habitat. 
32 USDA Forest Service RMRS-GTR-376. 2018

Elevation Potential for Range Shift

Alpine turf and dwarf-shrubland ecosystems inhabit the area above subalpine 

forests and below any relatively inhospitable area at the highest points of mountains 

in Wyoming and Colorado. The lower boundary of the alpine zone may be vulnerable 

to treeline expansion, which would reduce the existing area of alpine (Williams et al. 

2015b). Upper treeline has expanded into the alpine at rates greater than 200 ft over 

the last 100 years, and at greater rates in some decades in the Big Horn Mountains in 

Wyoming (Elliott 2012b), San Juan National Forest in Colorado (Rondeau et al. 2012), 

and central and southern Rocky Mountains (Elliott 2012a). Even if the area above the 

current alpine zone becomes climatically suitable, soils may not yet support alpine turf 

and dwarf-shrubland plant species (for example, in bedrock and scree, and fell-fields 

areas). Further, the development of suitable soil conditions may lag the rate of climate 

change. 

As topography and climate influence plant distribution within the alpine zone, 

opportunities for expansion may exist within the current area via expansion into areas 

where glaciers have melted, and expansion into areas where moisture and temperature 

become suitable. Studies within the Rocky Mountain Region have quantified areal 

loss of glaciers as well as ice volume lost in recent periods in the Wind River Range in 

Wyoming (DeVisser and Fountain 2015), Arikaree Glacier on the Arapaho-Roosevelt 

National Forest in Colorado (Williams et al. 2015a), and glaciers in Teton National Park 

in Wyoming (Edmunds et al. 2011). Shrinking glaciers can potentially expose area for 

alpine plants to colonize if topography, soils, or microsite conditions allow. 

The alpine zone is a complex terrain; landform features such as steep slopes, 

ridges, northerly versus southerly aspect, and depressions create a diversity of microclimates. Scherrer and Körner (2010) documented how microtopography can result 

in differences in temperature similar to large elevational gradients but over a short 

horizontal distance in the alpine. In such cases, species unable to adapt to a warming 

climate may need only a few yards of horizontal shift to find suitable temperatures. 

Alpine plants that grow in drier areas could migrate into alpine wetland areas that dry 

out from warming, as researchers observed in Glacier National Park in Montana (Lesica 

and McCune 2004). 

Snow distribution and the timing of snowmelt are influenced by wind, slope, and 

aspect as well as air temperature and solar radiation (Suding et al. 2015). Snow distribution and snowmelt influence patterns of alpine plant community composition and plant 

productivity. Changes in wind and temperature could result in different spatial and temporal patterns of snow distribution and snowmelt. At Niwot Ridge, Litaor et al. (2008) 

concluded that decreases in the duration and depth of the snow cover in herbaceous 

alpine tundra ecosystems might result in a shift from dry and wet meadow communities 

toward less productive alpine fell-fields.

Ecosystem Connectivity

Alpine turf and dwarf-shrubland ecosystems are disconnected, existing on isolated 

mountain peaks in the Rocky Mountain Region. Large areas of low elevation land 

separate these ecosystems. Because of this lack of connectivity among alpine plant 

and animal communities, plants and animals in the alpine zone have been identified 
USDA Forest Service RMRS-GTR-376. 2018 33

as possible candidates for assisted migration (Malanson et al. 2015; Wilkening et al. 

2015a). 

Vulnerability of Cold-adapted, Foundation, or Keystone Species to 

Climate Change______________________________________________

BOX 3.3

Key Vulnerabilities

A very high vulnerability ranking is given for cold-adapted, foundation, and keystone species in alpine turf and 

dwarf-shrubland ecosystems. Vulnerability to climate change by species can vary from moderate to very high. 

• American pika have high vulnerability as they are sensitive to warm temperatures and to poor forage quality, 

a consequence of drought. 

• Alpine turf and dwarf-shrubland plant communities are likely to have a very high vulnerability to warming and 

drying, with alpine turf communities the most vulnerable.

• These communities are also vulnerable to tree encroachment where moisture availability and soil conditions 

allow treeline advance.

• Pollinators are typically generalists and are mobile, characteristics that mitigate their vulnerability to climate 

change. However, conditions that decouple flowering phenology and pollinator emergence may contribute to 

moderate vulnerability to climate change.

We selected a subset of species that live in alpine turf and dwarf-shrubland ecosystems for assessment using this criterion. These species capture a range of environmental 

tolerances as well as a range in how their vulnerability may affect ecosystem function. 

We selected American pika, which live year-round in the alpine and are very sensitive 

to warm temperatures. We included alpine turf and dwarf-shrubland plant species as 

foundation species because they have substantial diversity and biomass, and provide 

an important food resource to species that inhabit or migrate to these ecosystems. We 

included pollinators as they have the qualities of keystone species because of their mutually beneficial relationship with blooming alpine plants. 

American Pika

Sensitivity to warm temperatures and drier conditions, along with a low reproductive rate, poor dispersal capability, low population density, and use of scattered 

mountain habitat, suggests a high vulnerability to a changing climate for American 

pika. Recent local extirpations in California and the Great Basin (which spans most 

of Nevada and portions of Califronia, Idaho, Oregon, Utah, and Wyoming) have been 

attributed to drier conditions, poor-quality forage, and human-caused changes, such as 

roads and agricultural grazing (Beever et al. 2003, 2010; Grayson 2005; Stewart et al. 

2015). Warmer temperatures and drier conditions associated with climate change could 

contribute to a decline in pika populations or local extinction in some areas, particularly 

at lower elevations (Calkins et al. 2012). 

Concerns about this species have resulted in an increasing body of scientific 

research. The initial concerns about the resilience of the pika were broadly associated 

with temperature, but Millar et al. (2016) have shown that pika habitat—talus matrices 
34 USDA Forest Service RMRS-GTR-376. 2018

(subsurfaces)—lagged the daily warm-up of air temperature in contrast to the talus 

surfaces. In addition, these matrices remained warmer than air temperature at night. 

The differences in air temperature in these microhabitats afford opportunities for the 

pika to behaviorally adapt to warming air temperatures. Millar et al. (2016) noted that 

warming air temperatures may become limiting for dispersal activities. Another example 

of an important microhabitat was identified by Wilkening et al. (2015b); pika were 

less stressed in matrices where subsurface ice features existed. They noted that pika 

inhabiting areas without subsurface ice may be vulnerable to changing climate. Erb et 

al. (2014) found that climatic factors were not dominant in determining pika relative 

density in Wyoming, Colorado, and New Mexico. Erb et al. (2014) concluded that pika 

populations in sites with poor-quality forage may be at the highest risk of declining. 

Although this new research emphasizes the microclimatic variation in alpine 

habitats, this species uses habitats that are often physically isolated from each other, 

lowering dispersal opportunities under climate change. This limited dispersal capability 

and the critical dependence of pika on high-quality habitat have suggested that the species may be a candidate for assisted migration (Wilkening et al. 2015a). We conclude 

that pika is likely to be highly vulnerable to climate change.

Alpine Turf and Dwarf-Shrubland Vegetation 

Alpine turf and dwarf-shrubland plants are highly vulnerable to warming in association with drying. Solar radiation contributes to the heating of soil surface temperatures 

higher than air temperatures. Although plants on warmer microsite conditions and in 

warmer years show an increase in heat tolerance, increases in air temperature may cause 

heat damage to alpine plants, especially when combined with drought stress (Buchner 

and Neuner 2003). Because moisture is more limited in dry alpine turf communities, 

plants may be more at risk from increased warming and drying. 

Opedal et al. (2015) reported that small-scale topographic variability resulted in 

variations in microclimate (temperature, soil moisture) that benefited plants in alpine 

landscapes; this small-scale microclimatic heterogeneity may benefit plants under future 

climates. For seven community types in the alpine tundra at Niwot Ridge, most of the 

variability in composition over 21 years was nondirectional, both within and between 

community types (Spasojevic et al. 2013). Even with documented directional change in 

climate, nitrogen deposition, and release from grazing, finer-scale variability appears to 

be influencing alpine vegetation responses.

These communities may be at risk from subalpine tree encroachment under a 

warming climate (Williams et al. 2015a). Vegetation models project conifers will 

expand into higher elevations by the mid- and late-21st century (Bachelet et al. 2001; 

Notaro et al. 2012; Rehfeldt et al. 2012). However, projecting treeline expansion 

upslope may be difficult. Microsite conditions, temperature, and soil moisture aid as 

well to suppress tree establishment, for example by warming the alpine environment 

for growth as well as drying out seedlings by wind. These processes over time have led 

to the complex forest cover patterns on the landscape (Elliott 2012a,b; Malanson et al. 

2007; Schrag et al. 2008; Weisberg and Baker 1995). Experimental treatments warming 

the soil with and without additional moisture have provided insights into the potential 
USDA Forest Service RMRS-GTR-376. 2018 35

for treeline advance (Moyes et al. 2013, 2015; Winkler et al. 2016). Warming may 

enhance climate suitability for trees; however, summer moisture stress, along with high 

soil surface temperatures, was found to limit seedling success for limber pine (Pinus 

flexilis James) (Moyes et al. 2013). Encroachment of subalpine trees into alpine areas 

may occur with climate change, and is likely to be highly variable.

Pollinators

Pollinators in the high elevations of the Rocky Mountain Region are dominated 

by bumblebees (Bombus spp.; Bingham 1998), other bees (Apis spp.; Bingham 1998; 

Korb 2012), butterflies (e.g., Rocky Mountain Apollo butterfly [Parnassius smintheus

Doubleday; Matter et al. 2011], beeflies [family Bombyliidae; Korb 2012], hoverflies 

[family Syrphidae; Korb 2012], and other flies [order Diptera; Korb 2012]). Pollinators 

in the alpine are likely to persist under climate change, but may be hindered by the 

direct effect of extreme climatic events (drought, extreme cold), and indirectly through 

the effects of climate change on plants in the alpine (Forrest 2014). In addition, factors 

other than climate may indirectly influence changes in pollinator communities. Franzén 

and Öckinger (2012) reported that over the last 60 years, alterations in pollinator communities were greatest at highest elevation in Sweden. This change was influenced by 

increased area from glacier decline, increased area of birch (Betula L. spp.) forest, and 

colonization by high elevation species previously absent from these sites.

Snowpack loss may play a role in hampering pollinators. A study of Rocky 

Mountain Apollo butterfly showed reduced snowfall hinders butterfly productivity, as snowpack provides insulation from freezing temperatures (Matter et al. 2011). 

Pollinators may be more vulnerable to bloom scarcity during dry years, or when early 

spring frost damages and reduces flower abundance, as researchers observed in montane 

meadows and subalpine forests (Aldridge et al. 2011; Inouye 2008). Warming may 

cause the connection between life histories of plants and pollinators to break. As plants 

may be more responsive to warming than pollinators and flower before pollinators 

emerge (Forrest and Thompson 2011; Thompson 2010), reductions in pollinator food 

sources could occur. Further, some plants that undergo shorter chilling periods may 

delay flowering beyond the time that pollinators emerge (Willmer 2012). However, 

researchers think the potential break in connected life histories is not a major threat 

(Willmer 2012). In a global study, pollinators flew distances (several hundred feet 

to miles depending on body size) to find other food sources (Greenleaf et al. 2007). 

Pollinators are typically generalists, not tied to a specific life history or plant species 

(Forrest and Thompson 2011; Willmer 2012). 

Although there are research studies that identify potential vulnerabilities to pollinators under climate change, few of these studies have focused on identifying the 

major factors influencing pollinators under climate change in the alpine zone in the 

Rocky Mountain Region. Based on the available information, pollinators are moderately 

vulnerable to climate change. 
36 USDA Forest Service RMRS-GTR-376. 2018

Sensitivity to Extreme Climatic Events______________________________

BOX 3.4

Key Vulnerabilities

A very high vulnerability ranking is given for the sensitivity of alpine plants and animals to extreme climatic events. 

• Seeding success and productivity are lower under drought stress and these effects can cascade through the 

trophic system. Drought also lowers the heat tolerance of alpine plants.

• Extreme heat can damage, stress, and kill plants and animals in alpine turf and dwarf-shrubland ecosystems. 

Although rosette, cushion, and dwarf shrub plants tolerate higher temperatures more than do nonwoody 

herbaceous species, they may not tolerate increases in warming and drying. Heat stress hinders the ability of 

animals to absorb nutrients, thus affecting their survival. 

• Earlier springtime melting of the protective snowpack coupled with earlier warm air temperatures may 

promote emergence of plants which increases exposure of plants to springtime freezing and thus hampers 

plant reproduction and growth. 

Sensitivity to Drought

Alpine turf and dwarf-shrubland plant and animal species experience stress, 

reduced reproduction, and death during drought. Seedling success was lower in drier 

areas, such as alpine dry tundra (Forbis 2003). Drought stress lowered the heat tolerance 

of alpine plants for all but the xerophytic, or highly drought tolerant, species (Buchner 

and Neuner 2003). Photosynthesis and productivity also declined during drought 

(Billings and Mooney 1968). Seedlings may be particularly sensitive to drought until 

their roots have reached a reliable water source (Billings and Mooney 1968). These 

drought effects to plants can cascade through the trophic system, limiting food for insects and animals (Aldridge et al. 2011; Calkins et al. 2012). These ecosystems are very 

sensitive to drought. 

Sensitivity to Extreme Heat

Plants and animals in the alpine system potentially can be sensitive to extreme 

heat or even warming, given their life histories in the alpine environment. The paucity 

of long-term monitoring of alpine animal species has hampered research exploring the 

effects of warming temperatures or extreme heat on alpine plants and animals. Recent 

studies have experimentally manipulated the heat regime in plots, expanding our understanding of plants and increased heat, described next. Where available, long-term 

monitoring records can be analyzed in tandem with long-term records of climate to 

investigate the potential effects of warming and in some cases extreme heat. Recent 

work by Wann et al. (2014, 2016) is such an example. White-tailed ptarmigan populations have been monitored since 1968 at two locations in central Colorado: Rocky 

Mountain National Park and Mount Evans Wilderness. Temperatures in biologically 

relevant seasons (pre-nesting, nesting, and brood-rearing) warmed significantly at both 

sites. Median hatch dates advanced 3.7 and 1.9 days per decade for the Rocky Mountain 

National Park site and the Mount Evans Wilderness site, respectively; however, there 

were no trends in survival over the study period. Using a shorter time period, Wang 

et al. (2002) suggested future population declines with higher mean winter monthly 
USDA Forest Service RMRS-GTR-376. 2018 37

minimum and maximum temperatures; however, Wann et al. (2016) found reproductive 

success only weakly linked to climate and concluded that the populations were invariant 

to fluctuations in seasonal weather. Reproductive success is likely to be influenced by 

many interrelated factors, and indirect consequences of warming may result from competition with other species for resources or increased predation rates under warming. 

Species with a narrow range of temperature tolerance would be hindered or suffer 

population reductions when heat exceeds their tolerance limits; however, recent research 

has suggested that some alpine species exploit the complexity of the alpine terrain 

to escape extreme heat (e.g., pika; see Vulnerability of Cold-adapted, Foundation, or 

Keystone Species section). Wildlife (deer [Odocoileus spp.], elk) can regulate their temperatures behaviorally by migrating to cooler areas, such as the alpine zone. Parker and 

Robbins (1983) found that deer appeared to be more sensitive to warming temperatures 

than elk. Heat stress hinders the ability of animals to absorb nutrients, thus affecting 

their survival; examples are moose (Alces alces; Lenarz et al. 2009) and mountain goat 

(White et al. 2011).

Alpine plants can withstand high temperatures. Researchers found that rosette, 

cushion, and dwarf-shrub plants tolerated higher temperatures better than did nonwoody 

herbaceous species (Buchner and Neuner 2003). In the Swiss Alps, researchers found 

cushion plants died when leaf temperatures reached 135 to 136 °F, and glaucous sedge 

(Carex firma) shoots and rhizomes withstood temperatures up to 140 °F (Larcher et al. 

2010; Neuner et al. 1999). Studies show that the heat tolerance of alpine plants develops 

over time as plants are exposed to high heat that remains below their critical thresholds 

(Buchner and Neuner 2003). Variety of microsite conditions over the landscape may 

create nearby refugia for species to move into and escape from warm temperatures 

(Scherrer and Körner 2010). However, these plant species may not be able to tolerate 

greater heat, especially when combined with drought stress, as researchers saw in heating experiments (Buchner and Neuner 2003). Because alpine turf and dwarf-shrubland 

plants and wildlife are damaged, stressed, or killed by extreme heat, these ecosystems 

are expected to be very sensitive to extreme heat.

Sensitivity to Freeze Date Variability and Earlier Snowmelt

Alpine turf and dwarf-shrubland species survive freezing temperatures, but can be 

sensitive to earlier snowmelt and more variability in freeze dates. Subalpine nonwoody 

plant growth is controlled by snowpack in the Rocky Mountains (Inouye 2008). Late 

flowering species may benefit from a longer growing season with earlier snowmelt. 

However, earlier flowering species could have an earlier onset of vegetative growth, 

with increased risk of frost damage, particularly with variable springtime freeze events 

(Aldridge et al. 2011; Inouye 2008). Munson and Sher (2015) found that the flowering 

date of rare species in the alpine has accelerated every decade since the late 1800s, 

with high spring temperatures explaining the advanced phenology. The reproduction of 

perennial forbs in montane and subalpine areas is hampered by frost damage that occurs when the protective snowpack cover is lost with earlier snowmelt (Miller-Rushing 

and Inouye 2009). Frost damage may be especially problematic for taller alpine plants, 

as was reported for alpine dwarf shrubs in the Swiss Alps (Rixen et al. 2012). Earlier 

snowmelt was found to increase the risk of lethal spring freezing exposure for less 
38 USDA Forest Service RMRS-GTR-376. 2018

freeze-resistant species (Wheeler et al. 2014). In the Swiss Alps, freezing damage was 

increased for alpine dwarf shrubs grown under experimentally elevated atmospheric 

carbon dioxide levels compared to ambient conditions. Sensitivity to freezing damage 

was greater with advanced plant phenology in the spring after soil warming caused 

earlier snowpack melt (Rixen et al. 2012). Mondoni et al. (2012) demonstrated that 

fall temperature warming could result in seed germination in fall instead of the typical 

springtime germination, thus exposing seedlings to harsh winter environmental conditions. Alpine turf and dwarf-shrubland ecosystems are expected to be sensitive to freeze 

date variability and earlier snowmelt. 

Intrinsic Adaptive Capacity to Climate Change_______________________

BOX 3.5

Key Vulnerabilities

A moderate vulnerability ranking is given for the intrinsic adaptive capacity of alpine ecosystems to climate 

change.

• Factors that strengthen adaptive capacity:

○ Species are highly tolerant to large variations in weather.

○ Microtopographic variation over alpine landscapes and rich plant species diversity support persistence 

by enabling transitions in community composition in response to environmental changes.

○ Varied life history strategies have aided plant and animal growth in the harsh alpine climate.

• Factors that weaken adaptive capacity:

○ Plants, especially in dry environments, have very slow growth and a long recovery time after 

disturbance.

○ Many alpine plants are not tolerant of early freezing events that may occur with earlier snowmelt.

○ Bird species, such as white-tailed ptarmigan, and mammal species, such as snowshoe hare, that 

undergo seasonal color molt may have limited capacity to respond to widely varying temporal changes 

in snowpack accumulation and snowmelt. 

Factors That Strengthen Adaptive Capacity to Climate Change

Alpine turf and dwarf-shrubland plants and animals have acclimated and adapted 

to the widely varying and harsh conditions of cold, heat, and wind (Billings and 

Mooney 1968). Animal species have adapted by migrating to warmer environments 

(birds, ungulates), hibernating (yellow-bellied marmot), or staying active all year 

through development of life history traits to conserve energy and survive the cold alpine 

winters (American pika) (Beever et al. 2003). Some alpine plant species grow close to 

the ground to reduce wind impacts, grasses and rosette plants keep a large portion of 

their biomass belowground to escape cold, and plants reproduce during a short growing 

season (Forbis 2003). The chemical characteristics of alpine plants make them more 

tolerant to heat and cold (Körner 2003). 

Alpine turf communities have high species diversity. Life history strategies vary; 

for example, Forbis and Doak (2004) contrasted cushion plants where one strategy was 

successful reproduction whereas the other strategy was long life spans and persistence 

through environmental conditions. Plant species resistance to environmental stress 
USDA Forest Service RMRS-GTR-376. 2018 39

varies. For example, enhanced snowpack hindered Bellardi bog sedge, whereas Parry’s 

clover (Trifolium parryi) increased cover (Bowman 2000). Given this diversity, some 

species under climate change will benefit and possibly expand, and some will decline, 

which will change the composition of the plant community. 

Factors That Weaken Adaptive Capacity to Climate Change

The adaptive capacity of alpine turf and dwarf-shrubland ecosystems is limited 

by slow growth rates. Recovery after disturbance can take from 30 years to more than 

a century, with communities at drier sites, such as alpine turf, recovering more slowly 

than communities at moister sites, such as wet meadows (Ebersole 2002; Willard et al. 

2007). 

Species that live in alpine turf and dwarf-shrubland habitats have a low ability 

to adapt to changes in temperature, reduced moisture, or changes in freeze dates that 

could reduce food sources (see Vulnerability of Cold-adapted, Foundation, or Keystone 

Species section, and Sensitivity to Extreme Climatic Events section). Changes in snowpack accumulation or melt are problematic for species that undergo seasonal molts, 

where their coats change color to match the presence or absence of snow. Zimova et 

al. (2014) found that snowshoe hares (Lepus americanus) had limited plasticity in coat 

color molt phenology in the spring, and showed no capacity to change their fall molt 

phenology. Predation is the primary cause of mortality for this species. When snowshoe 

hares were mismatched against their background, Zimova et al. (2016) reported a 

decline in weekly survival. Based on this information and climate change projections, 

snowshoe hare annual survival would decrease by 23 percent in Montana at the end 

of this century. There is great variation at the individual level in molt phenology for 

snowshoe hares, but without an evolutionary response, population growth rates and 

population numbers of these species could decline under climate change. See also 

Vulnerability of Species section in Chapter 5.

Dependence on Specific Hydrological Regime_______________________

BOX 3.6

Key Vulnerabilities

A very low vulnerability ranking is given for alpine turf and dwarf-shrubland ecosystem dependence on a specific 

hydrological regime. These ecosystems have adapted to a wide range of variability in moisture.

Alpine turf and dwarf-shrubland ecosystems are not dependent on a specific and 

narrow hydrological regime. These ecosystems are not limited by forms of moisture that 

are largely received as snow but can also be rain. Yet wind, slope, and aspect as well as 

air temperature and solar radiation influence snow distribution, snowmelt, and moisture availability on the alpine landscape (Litaor et al. 2008; Suding et al. 2015). Plant 

phenology and growth are greatly controlled by snowpack distribution and snowmelt. 

Less snow or drier conditions may hinder some species, for example, alpine turf plants 

(Forbis 2003; Oberbauer and Billings 1981) and pika (Erb et al. 2011). But the high 
40 USDA Forest Service RMRS-GTR-376. 2018

elevation of alpine ecosystems may buffer the near-term climate effects, such as less 

snowpack, on these systems compared to lower elevations (Lukas et al. 2014).

The availability of moisture in the alpine is influenced also by the advance and retreat of glaciers. Although there is a consensus that glaciers in the western United States 

will continue to retreat over the next 100 years, the local effect of climate change on 

glaciers differs according to glacier elevation, size, shape, slope or aspect position, and 

local monthly temperature and precipitation (Hall and Fagre 2003). Changes in glaciers 

can be incremental, though glacier outburst floods have the potential to alter the rate and 

magnitude of changes in valley bottoms. Researchers observed this effect in a flood that 

originated from meltwater ponded by glacial ice from the Grasshopper Glacier in the 

Wind River Range (Oswald and Wohl 2008). 

Likelihood of Managing Climate Change Effects______________________

BOX 3.7

Key Vulnerabilities

Management options for mitigating climate change effects in localized areas of alpine ecosystems are available, 

but there are few management options to offset effects from heat, earlier snowmelt, snowpack loss, variability in 

freezing dates, and drought stress projected with climate change. Consequently, the vulnerability ranking is very 

high. 

Management strategies for mitigating the effects of climate change on alpine 

turf and dwarf-shrubland ecosystems include reducing current stressors, enhancing 

the resilience of the system, and, as climate continues to change, helping plants and 

animals adapt without substantial loss of soil, soil nutrients, and plant cover (Millar et 

al. 2007; Peterson et al. 2011). Alpine turf and dwarf-shrubland ecosystems have high 

vulnerability to nonclimate stressors; reducing these current stressors would enhance 

alpine resilience, potentially reducing near-term effects of climate change. Management 

actions include controlling or limiting invasive species, rehabilitating disturbed sites, 

reseeding and transplanting turf, restoring endemic communities, restoring connectivity, managing populations of key native animals, and reducing soil erosion (Bay 

and Ebersole 2006; Conlin and Ebersole 2001; McDowell et al. 2014; Morrison and 

Pickering 2013). Characteristics that enhance the intrinsic adaptive capacity may be 

important to conserve, such as ensuring that migration and dispersal patterns are intact 

to allow mobile species to move to refugia (see Intrinsic Adaptive Capacity to Climate 

Change section). 

Adaptation strategies to address the potential for increased natural hazards under 

climate change have been the focus of discussion. In Europe, risk reduction strategies 

have been identified for increased floods, debris flow, glacial hazards, mass movement 

(rockfalls), and avalanches. Glacial hazards include the loss of stability in shelf glaciers, 

increase in glacial lakes, and risk of outburst flooding such as occurred in Wyoming 

(Oswald and Wohl 2008). PLANALP (2012) identified 10 recommendations for integrated climate-proof risk management in the alpine space: 

1. Prepare for emergency intervention; 
USDA Forest Service RMRS-GTR-376. 2018 41

2. Review the climate change fitness of existing structural protection measures;

3. Set up and optimize long-term monitoring and warning; 

4. Anticipate and deal with new risks; 

5. Adapt hazard and risk mapping to a changing climate; 

6. Enhance coordination between spatial planning and risk management; 

7. Establish a risk culture and initiate risk dialogue; 

8. Strengthen individual preparedness and precaution; 

9. Improve the knowledge base and transfer to practice; and 

10. Maintain and improve the functionality of protection forests (forests that protect 

people, buildings and infrastructure from natural hazards). 

Climate change management options, however, may not completely offset the 

harm in alpine areas from heat, severe drought stress, earlier snowmelt, snowpack loss, 

and shifts in freezing dates. In addition, ecosystem services from the alpine ecosystems 

to support water supply and the tourism industry may be affected by climate change, 

motivating local adaptations. Morrison and Pickering (2013) reported a range of adaptation strategies in the Australian Alps by conservation managers, the tourism industry, 

and local communities. Some adaptation strategies complemented each other, such as 

efforts to control invasive species, but other adaptation strategies resulted in concern 

and conflicts among the stakeholders, such as year-round recreational use in the alpine 

and increased use of water for snowmaking. 

Potential for Climate Change to Exacerbate Effects of Nonclimate 

Stressors, or Vice Versa_______________________________________

BOX 3.8

Key Vulnerabilities

Climate change may exacerbate the magnitude, intensity, and effect of nonclimate stressors. Consequently, the 

vulnerability ranking for alpine turf and dwarf-shrubland ecosystems is very high.

• Future increases in drought occurrence and intensity, combined with human activities to the west of the 

Rocky Mountain Region, may exacerbate dust-on-snow events that reduce duration of snow cover. 

• Increased atmospheric nitrogen deposition attributed to human sources is projected to exceed critical levels, 

resulting in ecosystem damage. 

• Warmer temperatures and drought may increase fire occurrence in Rocky Mountain forested areas, but it is 

not clear how the fire regime will change in the alpine under a changing climate.

Drought and Dust on Snow, Warmer Temperatures, Atmospheric 

Nitrogen Deposition__________________________________________

Climate change could potentially exacerbate the effects of dust on snow in alpine 

areas of the Rocky Mountain Region. Future dust-on-snow events coupled with regional 

warming could shift peak snowmelt by 2050 an additional 45 to 50 days earlier than 

current conditions, depending on the intensity of the dust events (Deems et al. 2013). 
42 USDA Forest Service RMRS-GTR-376. 2018

Climate change coupled with projected nitrogen emissions could result in critical 

load exceedances in the western United States. A critical load is defined as an annual 

deposition flux in nitrogen (kg ha-1 year-1), below which significant ecosystem damage does not occur. Nitrogen gases include both nitrogen oxide and ammonia. Future 

emissions of nitrogen oxide are projected to decline by 52 to 73 percent, but ammonia 

emissions are projected to increase by 19 to 50 percent by 2050 (Ellis et al. 2013). 

Using these assumptions about future nitrogen emissions, Ellis et al. (2013) quantified 

future ecosystem-dependent critical loadings in national parks across the United States. 

Under these future scenarios, atmospheric deposition may exceed critical loads by 

mid-century for many western national parks, including Rocky Mountain National Park 

(Ellis et al. 2013). They conclude that returning all parks to nitrogen deposition below 

their ecosystem-dependent critical load by 2050 would require at least a 50-percent 

decrease in anthropogenic ammonia emissions relative to the projected 2050 level. See 

also Atmospheric Nitrogen Deposition section in Chapter 4.

Climate and Fire________________________________________________

Fire history records collected at treeline between subalpine forest and alpine show 

that fire occurrence is closely tied to drought (Buechling and Baker 2004; Sherriff et al. 

2001; Sibold and Veblen 2006). Increased drought is likely in the future. Little information on fire in the alpine in the Rocky Mountain Region is available; in other alpine 

areas, wildfire can have significant effects, but recovery of some plant species can be 

fast (Takahashi 2012; Williams et al. 2008). How the alpine fire regime in the Rocky 

Mountain Region may change in the future is not certain.

Summary of the Regional Vulnerability of Alpine Turf and DwarfShrubland Ecosystems_______________________________________

The overall regional vulnerability ranking for the ecosystem is a combination of 

the vulnerability to nonclimate stressors and the vulnerability to climate stressors. For 

alpine turf and dwarf-shrubland ecosystems, the mean overall vulnerability ranking was 

high with mean expert reviewer rankings in the high or very high category (fig. 3.2, 

table 3.1). Mean vulnerability rankings were high for both the nonclimate stressor 

vulnerability and the climate vulnerability (fig. 3.3). Expert reviewer rankings were in 

the high or very high category for the nonclimate stressor vulnerability and in the high 

category for the climate vulnerability.

The expert reviewers had high overall confidence in the ranking of the overall 

regional vulnerability (fig. 3.2). The five experts’ individual rankings for overall confidence ranged from the moderate to the very high category (table 3.2). Mean confidence 

was high for the nonclimate stressor vulnerability ranking, but individual confidence 

rankings ranged from the low to the very high category (fig. 3.3a). Mean confidence was 

also high for the climate vulnerability ranking with individual rankings in the high or 

very high category (fig. 3.3b). 
USDA Forest Service RMRS-GTR-376. 2018 43

Figure 3.2—Overall regional vulnerability ranking 

and confidence ranking for alpine turf and dwarfshrubland ecosystems in the Rocky Mountain Region. 

Large arrow points to the mean score for vulnerability 

and for confidence. Bar represents the range of scores.

Table 3.1—Original and reviewer rankings for the regional vulnerability of alpine turf and dwarf-shrubland ecosystems in the Rocky 

Mountain Region. Underline indicates a reviewer score change from the original assigned score. 

Criterion 

Original 

Score

(Reviewer 1)

Score

(Reviewer 2)

Score

(Reviewer 3)

Score

(Reviewer 4)

Score

(Reviewer 5)

Score

Reviewer 

criterion 

rank mean

Nonclimate 

1. Extent 5 5 5 5 5 5 Very High

2. Human influences 3 3 3 3 3 3 Moderate

3. Resilience 5 5 5 5 5 5 Very High

4. Future trends 3 3 3 3 5 3 Moderate

Total 16

High

16

High

16

High

16

High

18

Very High

16

High

16

High

Climate

1. Ecosystem shift 5 5 5 5 5 5 Very High

2. Species groups 5 5 5 5 5 5 Very High

3. Climatic events 5 5 5 5 5 5 Very High

4. Adaptive capacity 3 3 3 3 3 3 Moderate

5. Hydrology 1 1 1 1 1 1 Very Low

6. Management 5 5 5 5 5 5 Very High

7. Interactions 5 5 5 5 5 5 Very High

Total 29

High

29

High 

29

High

29

High

29

High

29

High

29

High

Overall Vulnerability 

Rank

23

High 

23

High

23

High

23

High

24

Very High

23

High

23

High
44 USDA Forest Service RMRS-GTR-376. 2018

Table 3.2—Reviewer rankings for confidence in the vulnerability assessment of alpine turf and dwarf-shrubland ecosystems in 

the Rocky Mountain Region 

Criterion 

(Reviewer 1)

Score

(Reviewer 2)

Score

(Reviewer 3)

Score

(Reviewer 4)

Score

(Reviewer 5)

Score

Criterion rank 

confidence 

mean

Nonclimate 

1. Extent 3 5 5 5 5 Very High

2. Human influences 1 3 3 5 5 Moderate

3. Resilience 3 3 5 5 5 High

4. Future trends 1 3 3 5 5 Moderate

Total 8

Low

14

High

16

High

20

Very high

20

Very high

16

High

Climate

1. Ecosystem shift 3 4 5 5 5 High

2. Species groups 1 4 5 5 5 High

3. Climatic events 3 5 3 5 5 High

4. Adaptive capacity 5 3 3 5 3 High

5. Hydrology 5 3 1 1 5 Moderate

6. Management 5 5 5 5 5 Very high

7. Interactions 3 5 5 5 5 Very high

Total 25

High

29

High

27

High

31

Very high

33

Very high

29

High

Overall confidence rank 17

Moderate 

22

High 

22

High 

26

Very high 

27

Very high

23

High

Figure 3.3—Regional vulnerability of alpine turf and dwarf-shrubland ecosystems to nonclimate (a) and climate (b) stressors 

in the Rocky Mountain Region. Large arrow points to the mean score for vulnerability and for confidence. Bar represents 

the range of scores.

a) b)
USDA Forest Service RMRS-GTR-376. 2018 45

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52 USDA Forest Service RMRS-GTR-376. 2018

Chapter 4. Aquatic, Riparian, and Wetland 

Ecosystems in Glaciated Valleys: Vulnerability 

to Nonclimate and Climate Stressors in the U.S. 

Forest Service Rocky Mountain Region

Janine R. Rice, Dave Winters, Claudia Regan, Rick Truex, and Linda A. Joyce

Quick Look: Aquatic, Riparian, and Wetland Ecosystems in Glaciated Valleys in the 

Rocky Mountain Region

Glaciated valleys in the Rocky Mountain Region have unique geological and topographic features 

that result in a heterogeneous landscape. Streams, kettle basins, cirques, tarns, and wetlands 

provide habitats for a diversity of plants, invertebrates, and animals. Valleys typically have a “U” 

shape, with steep side walls and a wide, low-gradient valley floor. Glaciers or permanent snowfields can be found in these valleys. The tarns, cirques, and moraines formed by glaciers retain 

and redirect water. Groundwater strongly influences the moisture availability in these valleys. 

Valley floors have riparian areas and well-developed soils in the low-gradient stretches, where 

gravel, sand, and silt eroded from the hillsides are deposited. 

Climate is harsh: a short growing season, high snowfall, cold temperatures, and high winds that 

can cause drying. Aquatic, riparian, and wetland ecosystems provide important and unique habitats for cold-adapted plants and animals that live there year-round or that migrate into these 

areas seasonally. Cold-adapted species include benthic macroinvertebrates, cold-water fish species, and amphibians such as the boreal toad. Benthic macroinvertebrates found in these valleys 

provide essential ecosystem services, such as sediment mixing and the acceleration of detrital 

decomposition, nutrient cycling, and energy flow through food webs. Wetland communities are 

strongly influenced by the hydrological regime. Riparian areas adjacent to streams are sensitive 

to flow dynamics in streams; fens are present where water tables are high and are sensitive to 

groundwater availability, and wet meadows develop where water tables are not perennially 

high. This complex of habitats results in highly diverse plant taxa distinct from lower elevations. 

Quick Look: Vulnerability of Aquatic, Riparian, and Wetland Ecosystems in Glaciated 

Valleys in the Rocky Mountain Region

Vulnerability to nonclimate and climate stressors: Very high

Confidence: Very high

Exposure: Warming temperatures, particularly in winter and spring. Retreat of glaciers, potential 

loss of annual snowpack, and potential early melt of snowpack. Higher elevations may be 

less affected. 

Current extent: Aquatic, riparian, and wetland ecosystems in glaciated valleys account for a 

small fraction of area in the high elevation mountain valleys of Colorado and Wyoming.

Sensitivity and adaptive capacity to climate change: These ecosystems are highly dependent on 

a snow-dominated hydrological regime. They are sensitive to drought, declines in snowpack, 

and longer growing seasons with potential late-season drying. Insect and plant diversity is 

relatively high, which helps adaptive capacity to environmental changes. However, warm 
USDA Forest Service RMRS-GTR-376. 2018 53

and dry conditions may exceed plant and animal tolerance levels under climate change. Early 

plant flowering in response to warm early spring temperatures could affect pollinators. Low 

streamflows, drying ponds, and lower water tables will stress aquatic and wetland plants 

and animals. At the highest elevations, plants and animals have little potential to migrate upstream or upslope. Additionally, climate change may exacerbate nonclimate stressors, such 

as dust on snow. 

Nonclimate stressors: Mining, livestock grazing, and development of roads, trails, and infrastructure affect water quality as well as water flow patterns in these broad valleys. Atmospheric 

nitrogen deposition and dust on snow also affect snowpack and water chemistry. These stressors—particularly those that occur because of increased human influences, such as declines in 

air quality and increases in recreation—will continue to stress these ecosystems. 

Introduction___________________________________________________

Understanding the process of glaciation is critical for understanding where and in 

what form wetland and riparian ecosystems are found (Gage and Cooper 2013). No national forest or national grassland in the Forest Service, U.S. Department of Agriculture, 

Rocky Mountain Region was covered by the continental glaciers in the Pleistocene, 

but colder climates contributed to the development of mountain glaciers in the San 

Juan Mountains in Colorado, the Colorado Rocky Mountains, and the Medicine Bow 

Mountains, the Big Horn Mountains, and the Yellowstone-Teton-Highlands in Wyoming 

(see figure 8 in Cage and Cooper 2013; Richmond 1986; Wohl et al. 2007). Glaciation 

was a major factor in creating the wide variety of wetland habitats in the Rocky 

Mountains (McKinstry et al. 2010). 

Localized ice caps and valley glaciers from the Pleistocene and the Holocene 

have influenced physiographic features in alpine and subalpine valleys of the Rocky 

Mountains (Gage and Cooper 2013; Pierce 2003; Veblen and Donnelly 2005). The most 

commonly understood impact of glaciers is the formation of a deep U-shaped valley, 

with a wide flat valley floor. Glacial retreat can result in the formation of kettle basins, 

small depressions resulting from the melting of buried ice masses (Molnia 2013). 

Cirques, formed through glacial erosion, are the half-open steep-sided amphitheatershaped hollows at the head of a valley or on a mountainside. Tarns, which are lakes that 

form in the basin of a cirque, and mountain lakes are also a result of glaciation. Melt 

from existing glaciers or permanent snowfields is captured in the depressions of cirques, 

tarns, and mountain lakes. Water flow is influenced by glacial moraines. 

Geomorphic and hydrological processes vary between these glacially formed valleys and the fluvially formed valleys (those formed by river dynamics). Streams within 

glaciated valleys have a snowmelt-dominated hydrograph; seasonal flooding is primarily 

from snowmelt. In contrast, lower elevation streams in fluvial valleys are subjected to 

rainfall and flash floods as well as snowmelt (Livers and Wohl 2015). Channel geometry 

characteristics are more variable in glaciated valleys than in reaches in fluvial valleys. 

Streams in glaciated valleys inherit the topographic conditions from glacial processes 

in the Holocene and Pleistocene and are not necessarily adjusted to current fluvial 

sediment, water, and disturbance regimes. In unglaciated valleys, the stream channel 

formation is likely to have co-occurred with the historical and current sediment, water, 

and disturbance regimes (Livers and Wohl 2015). 
54 USDA Forest Service RMRS-GTR-376. 2018

Glaciated valleys are characterized by a harsh climate, with short growing seasons, 

high snowfall and snowpack, and cold temperatures—all of which are associated with 

alpine and spruce-fir ecosystems. Streams, lakes, and wetland areas in glaciated valleys 

create a heterogeneous landscape that provides a diversity of habitats for plants, invertebrates, and animals. Invertebrates include benthic macroinvertebrates, small animals that 

live among the sediment and stones at the bottom of streams, ponds, and lakes (i.e., the 

benthos). These animals can be insects, including mayflies (order Ephemeroptera), stoneflies (order Plecoptera), caddisflies (order Trichoptera), beetles (order Coleoptera), midges 

(family Diamesinae), crane flies (Tipula spp.), and dragonflies (suborder Anisoptera). 

Other members of the benthic macroinvertebrate community are snails (class Gastropoda), 

clams (class Pelecypoda), aquatic worms (class Oligochaeta), and crayfish (Cambarus

spp.). These benthic macroinvertebrates provide essential ecosystem services, such as sediment mixing and the acceleration of detrital decomposition, nutrient cycling, and energy 

flow through food webs (Covich et al. 1999). Amphibians are food sources for predators 

such as birds, mammals, and fish, and can include species such as boreal toad (Anaxyrus 

boreas), boreal chorus frog (Pseudacris maculata), barred tiger salamander (Ambystoma 

mavortium), and Columbia spotted frog (Rana luteiventris) (Corn et al. 2011). These high 

elevation areas are often too cold to support native trout populations.

A diversity of wetland habitats can be found in glaciated valleys. The valley floors 

may have well-developed soil in the lowest gradient reaches, where soils and detritus 

from erosional processes in the upper hillsides are deposited. The riparian areas are 

strongly influenced by the unidirectional flow of water in the streams (Cage and Cooper 

2013). Groundwater-influenced fens of the Rocky Mountain Region are most abundant 

in glaciated valleys (Gage and Cooper 2013). Wet meadows are also found in glaciated 

valleys; in contrast to fens, they typically do not have perennially high water tables. 

This diversity of wetland habitats results in highly diverse plant taxa distinct from 

lower elevations (Cooper and Andrus 1994; Vance et al. 2012). On the Grand Mesa in 

northwestern Colorado, Austin and Cooper (2016) identified 16 different wetland habitat 

communities including fen, modified fen, marsh, and wet meadows. Dominant life 

forms across these communities ranged from semiaquatic species, small sedges, large 

sedges, and floating mats and bryophytes, to shrubs and trees. Peatland expanses in the 

Wind River Range in Wyoming were dominated by mountain sedge (Carex scopulorum), silvery sedge (Carex canescens), fewflower spikerush (Eleocharis quinqueflora), 

Boott Northwest Territory sedge (Carex utriculata), and water sedge (Carex aquatilis) 

(Cooper and Andrus 1994). Trees such as Engelmann spruce (Picea engelmannii Parry 

ex Engelm.), subalpine fir (Abies lasiocarpa (Hook.) Nutt.), and lodgepole pine (Pinus 

contorta Douglas Ex. Loudon) can be present in glaciated valleys.

Aquatic, riparian, and wetland ecosystems in glaciated valleys have been assessed 

for their vulnerability to climate change. As discussed in Chapter 1, the vulnerability 

framework has two major components, one related to nonclimate stressors and one 

related to climate stressors. Within the nonclimate component, four criteria were used to 

define factors of vulnerability: current status of ecosystem extent; intrinsic resilience of 

the ecosystem to nonclimate stressors, human influences on the ecosystem, and future 

trends of nonclimate stressors on the ecosystem. Within the climate change component, 

seven criteria capture the vulnerability of individual species, sensitivity of ecosystem 

dynamics such as dependence on a specific hydrological cycle, adaptive capacity of the 
USDA Forest Service RMRS-GTR-376. 2018 55

ecosystem, potential for management to mitigate the effects of climate and nonclimate 

stressors, and interaction between climate change and nonclimate stressors. More information and the rationale for ranking ecosystem vulnerability by using these criteria are 

given in appendices A, B, and C.

Glaciated valleys are increasingly being studied on national forest lands as well 

as in other areas throughout the Rocky Mountain Region. We draw from this literature 

where studies are within the Rocky Mountain Region. Even within this Region, these 

glaciated valleys have great variation. Application of these results to glaciated valleys 

on individual national forests will require an understanding of local site characteristics 

and the site characteristics of the research studies reviewed here. 

Vulnerability of Aquatic, Riparian, and Wetland Ecosystems in Glaciated 

Valleys to Nonclimate Stressors________________________________

Summary of Key Vulnerabilities to Nonclimate Stressors

BOX 4.1

Current status of ecosystem extent

With limited extent in the Rocky Mountain Region, the aquatic, riparian, and wetland ecosystems in glaciated 

valleys have very high vulnerability.

Human influences on ecosystem

Glaciated valleys are considered to have moderate vulnerability to the legacy of past human influences and to the 

stressors associated with ongoing human influences. 

• Ecosystems have not completely recovered from early 19th-century mining and livestock grazing. 

• Roads, grazing, and recreation have hindered and sometimes continue to hinder hydrological and wetland 

function. 

• Invasive terrestrial and aquatic species appear to be limited. Pathogens such as chytrid fungus are affecting 

amphibian populations.

• Atmospheric nitrogen deposition has altered lakes and has potential to alter terrestrial and stream 

ecosystems. 

• Dust-on-snow events alter snow chemistry and result in earlier snowmelt, notably in southwestern Colorado. 

Intrinsic resilience of ecosystem to nonclimate stressors

A very high vulnerability ranking is given for the intrinsic resilience of aquatic, riparian, and wetland ecosystems in 

glaciated valleys to nonclimate stressors.

• Factors that enhance the resilience of aquatic, riparian, and wetland ecosystems in glaciated valleys to 

nonclimate stressors: 

○ Life history strategies facilitate adaptation to fluctuating environments.

○ Functioning riparian and wetland habitats can maintain resilience to land use effects that cause erosion. 

• Factors that lower the resilience of aquatic, riparian, and wetland ecosystems to nonclimate stressors:

○ Increases in atmospheric nitrogen deposition, alterations in snowmelt dynamics, and presence of 

aquatic and terrestrial invasive species compromise these ecosystems.

○ This ecosystem is susceptible to altered function from disturbances such as nitrogen deposition and 

other human-caused disturbances. 

Future trends of nonclimate stressors

Glaciated valley ecosystems have very high vulnerability to future nonclimate stressors: recreation use, invasive 

species, and atmospheric nitrogen deposition.
56 USDA Forest Service RMRS-GTR-376. 2018

Current Status and Human Influences______________________________

Aquatic, riparian, and wetland ecosystems in glaciated valleys of the Rocky 

Mountain Region cover a small fraction of area in Colorado and Wyoming highelevation mountain valleys. Historical land uses in glaciated valleys included mining, 

livestock grazing, beaver (Castor canadensis) trapping, timber harvest, and log floating. 

Placer mining for gold and other precious metals was a common practice beginning 

in the mid-1800s (Wohl 2006). High elevation mining disturbed stream channels, and 

caused the release of toxic materials (Wohl 2006, 2011). Aquatic species and channel 

function have somewhat recovered from these early Euro-American activities (Wohl 

2006), but researchers are still seeing elevated metal concentrations downstream of 

mines (Schmidt et al. 2012). Land uses in glaciated valleys share a similar history with 

low-gradient streams in high elevation areas (see Chapter 6). 

Glaciated valleys in the Rocky Mountain Region are currently grazed by many of 

the same herbivores found in the montane, subalpine, and alpine (see chapters 3 and 5). 

Overgrazing by domestic livestock in the 19th and early 20th century has had negative 

effects in most Rocky Mountain Region riparian areas (Wohl 2006) by compacting 

soils; reducing riparian vegetation cover, leading to erosion; reducing aquatic invertebrate production; causing siltation of stream bed channels; destabilizing channels; 

and introducing noxious weeds (Cannon and Knopf 1984; Saunders and Fausch 2012; 

Winters et al. 2004a). Heavy grazing during the early 20th-century summer seasons 

reduced shrub density and narrowed riparian areas in Colorado glaciated valleys, and 

these effects lasted for decades after grazing intensity had been reduced (Cannon and 

Knopf 1984). Livestock grazing mostly by sheep, but also by cows and horses, continues on some high-elevation national forest allotments (USDA FS 2016). Generally, 

grazing effects—from both domestic and wild animals—vary with the intensity and 

timing of grazing activities as well as with species, land use, grazing legacy, and current 

grazing management system. 

Roads connecting populated areas to national forest lands are widespread in the 

Rocky Mountain Region. Roads provide important access for a suite of economic and 

social uses, including recreation, hunting, camping, and skiing. However, the effects 

of roads include animal mortality from road development, mortality from collision 

with vehicles, modification of animal behavior, alteration of the physical environment, 

alteration of the chemical environment, spread of invasive plant species, and increased 

use of areas by humans (Hauer et al. 2016; Mortensen et al. 2009; Pollnac et al. 2012; 

Trombulak and Frissell 2000). Roads can alter drainage patterns in watersheds and 

increase surface runoff from soil compaction. Roads can intercept overland water 

flow that is important for healthy function and maintenance of wetland habitats. Road 

culverts can block aquatic passage, and thus fragment aquatic species habitats. Native 

fish species in the Upper Colorado Basin have declined because of more intense land 

use and greater road density (Dauwalter et al. 2011). Many roads are currently designed, 

managed, and maintained to reduce erosion, soil compaction, and contaminants; managers are also decommissioning roads to benefit stream and riparian areas (Winters et al. 

2004a). 
USDA Forest Service RMRS-GTR-376. 2018 57

Recreation demands have increased over the past decade (USDA FS 2012). 

Recreation in high elevation ecosystems of the Rocky Mountain Region can hinder the 

functioning of aquatic, riparian, and wetland ecosystems. On the Big Horn National 

Forest, recreational activities, especially dispersed camping, were closely related to 

water in streams, valley bottoms, and lakes in wilderness areas (Winters et al. 2004b). 

Seventy percent of the mapped dispersed campsites were within 300 ft of a road on 

the Big Horn National Forest (Winters et al. 2004a). Visitor use can result in trampled 

alpine plants, cause erosion, and set back restoration efforts for decades (Willard et al. 

2007). Ski areas that make artificial snow alter local hydrological function by removing 

water from streams, thus creating low winter streamflows for fish (Baron 2002). 

Invasive and Nonnative Species

The presence of invasive species appears to be limited in aquatic, riparian, and 

wetland ecosystems in glaciated valleys, but more scientific documentation is needed. 

Although invasive plant species in high elevation areas have expanded globally over the 

last decade (McDougall et al. 2011), the common dandelion (Taraxacum officinale F.H. 

Wigg) was the only introduced species found in a survey of Rocky Mountain National 

Park (Ashton 2010). The weevil Rhinocyllus conicus Fröelich has been found on native 

thistles (Cirsium spp.) at Niwot Ridge in Colorado, but its presence was negatively correlated with elevation. Climatic factors apparently limited its current capacity to expand 

above treeline; warming temperatures could facilitate an upward expansion (Hicks et 

al. 2013). Chytrid fungus (Batrachochytrium dendrobatidis) is widespread in the Rocky 

Mountains and amphibians are not resistant to this disease (Muths et al. 2008). Disease 

detection rates were lower in amphibians inhabiting higher elevation sites (42 percent of 

clusters at higher elevation sites in Colorado and Wyoming vs. 84 percent of clusters at 

sites in lower elevations in Montana, Idaho, and Wyoming). Muths et al. (2011) suggested that when the disease is present, survival and recruitment may be compensatory in 

boreal toad populations. Where the pathogen was emerging, boreal toad populations had 

high recruitment rates, possibly compensating for low survival, and this combination 

resulted in a relatively slow rate of decline. In contrast, populations with no evidence of 

disease had high survival probability but lower recruitment rates. 

Perennial invasive aquatic species are mostly absent in high-elevation glaciated 

valleys, and it is unclear if these snow-dominated areas would support many invasive 

species. Although high elevation lakes are often seasonally stocked with nonnative 

trout (Salvelinus spp.; Colorado Parks and Wildlife 2007), the environment tends to be 

nonsustaining due to very cold temperatures. Where native (Onchorhynchus spp.) and 

nonnative trout share the stream habitat, nonnative trout compete with, and in some 

cases, outcompete native fish, especially where stream conditions are degraded, angler 

harvest is high, temperatures are warmer, or a combination thereof (Dunham et al. 2004; 

Gresswell 2011).

Atmospheric Nitrogen Deposition

Researchers have linked human activity to an increase in atmospheric nitrogen 

deposition in high-elevation terrestrial and aquatic ecosystems (Baron et al. 2000; 

Benedict et al. 2013; Bowman et al. 2014; Nanus et al. 2012). From 1993 through 2012, 
58 USDA Forest Service RMRS-GTR-376. 2018

significant trends in snowpack and wetfall chemistry mirrored changes in emissions of 

ammonia, nitrogen oxides, and sulfur dioxide (Ingersoll et al. 2016). Downward trends 

were seen in emissions of nitrogen oxides and in the regional trends in snowpack and 

wetfall concentrations of nitrate. However, ammonia emissions and ammonium concentrations and deposition in snowpack and wetfall all showed upward trends (Ingersoll et 

al. 2016). 

Slow rates of nutrient cycling and low biomass production of alpine ecosystems 

reflect limited availability of nitrogen. Given adaptation to low nitrogen levels, these 

ecosystems have a low capacity to accommodate increases in nitrogen. Increased 

nitrogen deposition, through atmospheric deposition or by experimental changes, has 

been observed to increase soil nitrogen cycling, increase soil acidification, increase 

nitrate leaching, shift plant and microbial community composition, and increase cover 

of nitrogen-loving grasses in the terrestrial systems (Bowman et al. 2014). Changes 

in aquatic ecosystems from greater nitrogen deposition have resulted in higher nitrate 

concentrations in lakes (Baron et al. 2000), changes in diatom community assemblages 

(Wolfe et al. 2003), episodic acidification (Baron et al. 2011), and shifts in the nutrient 

status and productivity of lakes (Baron et al. 2011). Mast et al. (2014) examined three 

decades of stream nitrate export and atmospheric nitrogen deposition data from Loch 

Vale, a high elevation watershed in the Colorado Front Range. They documented the 

close relationship between the patterns of atmospheric nitrogen deposition and the 

temporal dynamics in stream chemistry. Similar patterns had been observed by Mast et 

al. (2011) in their study of sulfur deposition and stream chemistry. Effects of nitrogen 

deposition on aquatic systems are greatest on the eastern side of the Continental Divide 

in Colorado, but effects are also seen to the west (Wolfe et al. 2003). 

Dust on Snow

Agricultural activities and drought to the west and southwest of the Rocky 

Mountains have caused increased intensity of dust-on-snow events in the Colorado 

Rocky Mountains (Painter et al. 2007; Toepfer et al. 2006). Dust-on-snow events have 

reduced duration of snow cover (Painter et al. 2007), increased rate of snowmelt associated with more extreme dust deposition, and produced earlier peak streamflow rates on 

the order of 1 to 3 weeks (Livneh et al. 2015; Painter et al. 2012; Steltzer et al. 2009). 

As a result of these dust-on-snow events, snow chemistry increased in pH, calcium 

content, and acid neutralizing capacity and the effects were more pronounced at upper 

elevations when compared to densely forested lower elevation sites (Rhoades et al. 

2010) (see also Dust on Snow section in Chapter 3). 

Intrinsic Resilience of Ecosystem to Nonclimate Stressors____________

Factors That Enhance Resilience to Nonclimate Stressors

Variation in life history strategies has helped aquatic and riparian species to persist 

in these high elevation habitats. Amphibians and invertebrates are adapted to fluctuating environments, through either the ability to rapidly colonize a site or the capacity to 

withstand very cold temperatures (Ryan et al. 2014). Functioning riparian areas serve 

to enhance resilience to nonclimate stressors. These moister areas are generally less at 
USDA Forest Service RMRS-GTR-376. 2018 59

risk from fire (Dwire and Kaufmann 2003). Resilience is strengthened when functioning 

riparian areas are present to stabilize banks, and filter or modify water flows and matter 

moving through the landscape (Naiman et al. 2005). 

Factors That Lower Resilience to Nonclimate Stressors 

Rocky Mountain high elevation areas are not resilient to the effects of atmospheric 

nitrogen deposition, aerosol deposition from dust-on-snow events, introduction of 

nonnative fish, and human activities such as ditch construction or off-road vehicle use. 

Aquatic, riparian, and wetland ecosystems in glaciated valleys are sensitive to nutrient 

additions from natural and human-caused sources. These additions can alter ecosystem 

function. Bowman and Steltzer (1998) found that plants in moist meadows can use the 

increased nitrogen inputs from the atmosphere for growth, but this addition resulted in a 

change in plant dominance. Increased nitrogen deposition from human-caused sources 

promotes a trend toward oxygen depletion in some alpine lakes of the Colorado Front 

Range (Baron et al. 2000), increases acidity, and shifts species composition (Wolfe et 

al. 2003). The introduction of nonnative fish also weakens resilience of native species in 

the aquatic systems. Disruptions to the hydrological cycle affect these systems. Austin 

and Cooper (2016) reported that 79 percent of the fens surveyed on the Grand Mesa in 

western Colorado had been impacted by human activities such as ditch construction, 

drainage, flooding, or vehicular rutting. These disturbances, and factors such as water 

rights or the cost of restoration, led to the conclusion that there is little restoration potential for many fens.

Future Trends of Nonclimate Stressors_____________________________

Increasing human populations in the Rocky Mountain Region will expand the 

urban-wildland interface, will increase the demand for ecosystem services from wildland areas, and are likely to increase several current stressors. Protected lands are altered 

by the human communities that surround them and by visiting recreationists’ activities 

(Hansen et al. 2002; Wood et al. 2014). Recreational skiing may increase the demand 

for water to make snow, potentially reducing water in some glaciated valleys near these 

areas. Where roads exist in or near glaciated valleys, they may continue to alter plant 

and animal communities and the hydrology of these ecosystems. Although invasive 

species are currently rare in high-elevation glaciated valleys, some invasive species can 

adapt to high elevation environments (Pauchard et al. 2009). Other species introductions 

may occur, and travel corridors, changes in land use, and grazing improve their chances 

of spreading (Bradley et al. 2010; Pauchard et al. 2003). Atmospheric nitrogen deposition may reach critical levels by mid-century (Ellis et al. 2013), causing further shifts 

in plant community composition and changes in the nitrogen cycle. Increases in dust-onsnow events are likely to occur as agricultural activities continue and drought becomes 

more frequent in the southwestern United States. 
60 USDA Forest Service RMRS-GTR-376. 2018

Vulnerability of Aquatic, Riparian, and Wetland Ecosystems in Glaciated 

Valleys to Climate Stressors___________________________________

Capacity for Network Shift

BOX 4.2

Key Vulnerabilities

A very high vulnerability is given for the capacity for network shift for glaciated valley ecosystems. 

• Species associated with aquatic, riparian, and wetland ecosystems in glaciated valleys are unlikely to find 

aquatic and wetland habitats farther upslope or upstream of these high elevation ecosystems.

• Wetlands, ponds, and lakes in glaciated valleys can be connected to streams by both surface water and 

groundwater; alterations in these connections through changes in annual or seasonal precipitation or late 

season drying will affect the life history dynamics of plants and animals, especially hindering biota that have 

limited migration potential. 

• Fragmentation of aquatic habitat could occur through changes in stream temperatures. 

Elevation Potential for Habitat Shift

Glaciated valleys largely occur in the highest elevations of the Rocky Mountains 

in Colorado and Wyoming. Species associated with aquatic, riparian, and wetland ecosystems in these valleys are unlikely to find suitable habitats upslope or upstream; there 

is little potential for migration. Further, these high elevation valleys are isolated from 

each other by lower elevation landscapes, essentially preventing migration among the 

glaciated valleys.

Glacial mass is shrinking (Bell 2009; Cheesbrough et al. 2009) and retreating glaciers may provide area for colonization by plants and animals. Glaciers currently cover 

about 1.8 mi2 in Colorado and about 28 mi2 in Wyoming (Portland State University 

2009). At the snout of a retreating glacier, newly formed streams develop. Insects 

(midges) and species that are attached to submerged surfaces (algae, cyanobacteria, 

microbes) rapidly grew and colonized these newly formed streams in the Wind River 

Range in Wyoming (Thompson et al. 2013). The local effect of climate change on 

glaciers varies according to elevation, size, shape, slope or aspect position, and local 

monthly temperature and precipitation. With climate change, researchers project that 

glaciers will melt entirely by 2030 in Glacier National Park in Montana (Hall and Fagre 

2003). 

Fragmentation 

Glaciated valleys are part of a larger river network. Natural and human barriers 

can fragment or geographically isolate glaciated valleys. Isolation is a benefit when it 

serves as a barrier to downstream invasive species. It can be a detriment when stream 

habitat warms and organisms are unable to reach new habitat. 

Wetlands, ponds, and lakes in glaciated valleys can be connected to streams by 

both surface water and shallow groundwater. These connections help aquatic organisms 

move between wetlands, ponds, and lakes in glaciated valleys. Changes in precipitation 
USDA Forest Service RMRS-GTR-376. 2018 61

patterns or streamflows could disrupt these connections. Benthic macroinvertebrates can 

drift downstream and potentially out of glaciated valleys; winged adults move freely 

upstream as well as downstream (Graham et al. 2017). Amphibians can move between 

wetlands, ponds, and lakes of the ecosystem, as well as in and out of the glaciated 

valley. Seeds and regenerative stem segments can drift downstream. Plants have little 

ability to move upstream other than by wind or attached to the outside of birds and 

mammals or in their intestinal tracts. 

Within the cold-water aquatic systems, habitat can become fragmented in these 

glaciated valleys. At the downstream end of valleys, terminal moraines can act as dams 

or create steep cascading stream segments (Gage and Cooper 2013) that block aquatic 

organisms from migrating upstream. Similarly, water diversion structures that occur 

downstream may fragment or isolate these upstream glaciated valley ecosystems. Fish 

movement can also be impeded during seasons in which streams have low flow, for 

example during fall and winter, or when ice blocks passage during late fall, winter, and 

early spring. This type of fragmentation is also found in low-gradient stream habitats 

(see Chapter 6).

Vulnerability of Cold-Adapted, Foundation, or Keystone Species to 

Climate Change______________________________________________

BOX 4.3

Key Vulnerabilities

A very high vulnerability ranking is given for cold-adapted, foundation, and keystone species in glaciated valleys, 

although vulnerability ranking for individual species can vary from moderate to very high.

• Given projections for increased warming and increased variability in precipitation and snowpack, amphibians 

are likely to have a high or very high vulnerability.

• Given increased warming and potential changes in streamflow with drier conditions, benthic 

macroinvertebrates are likely to have a high or very high vulnerability.

• Cold-water native fish have moderate to high vulnerability. New habitat with suitable temperature upstream 

may become available under a warmer climate. However, the smaller high-elevation tributaries, with more 

limited and disconnected habitats in glaciated valleys, may require assistance for populations to reach and 

survive in these areas. 

• Riparian and wetland plants have moderate vulnerability. Plant communities may suffer from warming and 

drying. 

• Pollinators have moderate vulnerability to climate change; however, under a changing climate, plant flowering 

may occur earlier than pollinator emergence.

We selected a subset of species that live in aquatic, riparian, and wetland communities of glaciated valleys that are foundation species (those whose high population 

presence substantially influences community structure), or keystone species (those 

exerting strong effects on the structure of their community, despite a low population 

presence). Amphibians, macroinvertebrates, and riparian plants were chosen as foundation species. Amphibians are important food sources for other species and are very 

sensitive to drying conditions. Macroinvertebrates were selected for their importance to 

aquatic food webs and provision of ecosystem services such as breaking down organic 
62 USDA Forest Service RMRS-GTR-376. 2018

material. Riparian plants are a group of foundation species that substantially influence 

ecosystem function through biogeochemical cycling, regulate and filter runoff, and provide habitat for breeding, feeding, and sheltering of animals. 

Pollinators and cold-water native fish were selected as keystone species. 

Pollinators have a symbiotic relationship with plants, but are a relatively small fraction 

of all life forms in these areas. Cold-water native fish, although rare in these ecosystems, 

are sensitive to warmer temperatures. Beaver, a keystone species that has been found in 

glaciated valleys, is not assessed here; for more information, see Chapter 6.

Cold-Adapted Amphibians

Recent declines in amphibian populations in the western United States are attributed to many factors including nonnative or invasive species, land use, overexploitation, 

climate change, ultraviolet radiation, contaminants, and emerging infectious diseases 

such as chytrid fungus (Hussain and Pandit 2012). Adams et al. (2013) analyzed the 

rate of change in the probability that amphibians occupy ponds and other comparable 

habitat features; they concluded, as have others, that amphibians are currently declining. Their analysis included species not typically considered of conservation concern, 

and suggested that these species may also be declining. Lack of sufficient data limits a 

robust conclusion on which factors, such as climate change, habitat loss, introduction of 

predatory nonnative fish, disease, and pollution, have directly or indirectly contributed 

to declining amphibian populations (Hossack et al. 2015; Hussain and Pandit 2012; Ray 

et al. 2016). 

Amphibians are sensitive to temperature and moisture. Many species of amphibians rely on shallow or intermittently flooded wetland habitats for breeding; snowpack 

and runoff strongly influence the suitability of these wetland habitats (Amburgey et al. 

2014; Corn 2005; Ray et al. 2016). Using climate and inventory data from Yellowstone 

and Grand Teton National Parks, Ray et al. (2016) concluded that models of amphibian 

breeding dynamics using climate variables outperformed models that were exclusively 

habitat based. Extinction rates were most sensitive to climate variation. Amburgey et 

al. (2014) reported that tadpoles of boreal chorus frogs were found more often in ponds 

with intermediate-length hydroperiods (length of time water remains in the pond), in 

contrast to ephemeral or permanent wetlands. Boreal chorus frog presence was negatively related to fish presence. 

Future climate may pose a challenge for amphibians, as higher temperatures, 

reduced snowpack, lower soil moisture, and drought potentially decrease the occurrence 

of intermediate-length hydroperiods in wetland habitats (Amburgey et al. 2014; Ryan et 

al. 2014). Loss of these habitats may concentrate amphibians in permanent wetlands. If 

boreal chorus frogs shift to breeding in permanent wetlands, predation risk to tadpoles 

increases as fish are more likely to be found in these habitats. Ray et al. (2016) emphasized that the strength and direction of relationships between occupancy parameters and 

climate drivers differed across the three species they studied: boreal frog, western tiger 

salamander, and Columbia spotted frog. In addition, community relationships in these 

wetland habitats are complex. The type of habitat, type of predator, life stages of each 

species, and availability of other habitats will affect options for amphibians under a 

changing climate. Given the projections for increased warming and increased variability 
USDA Forest Service RMRS-GTR-376. 2018 63

in precipitation and snowpack, amphibians are likely to have a very high vulnerability to 

climate change.

Cold-Adapted Macroinvertebrates

Different benthic macroinvertebrate species can be prey or predators. As prey, 

they are food for aquatic and terrestrial vertebrate consumers (fishes, birds). As predators, they may control the numbers, locations, and sizes of their prey (Covich et al. 

1999). Benthic macroinvertebrates contribute to nutrient cycling and energy flow in 

these ecosystems. Herbaceous litter is an important source of energy to small streams. 

High-elevation benthic macroinvertebrates are shredders that break down organic matter. Other species also serve as collectors, further processing organic matter and thereby 

releasing nutrients bound up in this material (Covich et al. 1999; Vannote et al. 1980). 

Although such specialization might suggest the loss of these ecosystem processes if 

shredders or collectors are lost under climate change, few studies have explored the 

interaction among macroinvertebrates species in these high elevation valleys and how 

community structure affects these ecosystem services. Finn and Poff (2005) found that 

reach-scale environmental variables in four high-elevation Colorado streams were most 

important in defining communities; taxonomic variance and functional variance provided no greater understanding of benthic communities than did environmental variables. 

Finn and Poff (2005) also found that benthic communities differed more in the alpine 

than in the spruce-fir or lodgepole pine ecological zones and suggested this difference 

may be the result of increased isolation of streams in the alpine zone. Tronstad et al. 

(2016) also reported greater differences in stream invertebrate assemblages at higher 

elevations than at lower elevations in Grand Teton National Park. 

Milner et al. (2016) explored statistical approaches to determine years significantly 

outside of reference conditions for benthic macroinvertebrate communities in three 

streams in the Wind River Wilderness Area in Wyoming. For most sites, diversity and 

community persistence trends that were not correlated with precipitation, appeared to be 

on trajectories away from a reference condition, suggesting further research is necessary 

to determine if a nonclimate stressor was acting on the stream. Trait-based responses 

(related to life history, including trophic level; morphology; mobility; ecology) to environmental conditions are complex and are more likely to be multivariate responses (Poff 

et al. 2006). 

These species are sensitive to changes in temperature and changes in streamflow 

as well as to environmental disturbances (Pyne and Poff 2016). Macroinvertebrates in 

the Upper Colorado River Basin in southwestern Wyoming and western Colorado and 

the Great Basin are most sensitive to increases in July temperatures and reductions in 

snowfall, conditions projected under climate change (Poff et al. 2010). Because of the 

warmer temperatures, changes in streamflow, and drier conditions expected from climate change, macroinvertebrates are considered to have high vulnerability. 

Cold-Water Native Fish

Many streams, lakes, and ponds of high elevation valleys do not support fish 

because of extremely low temperatures, barriers to migration such as terminal moraines, 

or high gradients that isolate them from downstream fish populations. Aquatic habitats 
64 USDA Forest Service RMRS-GTR-376. 2018

in glaciated valleys may become more hospitable for native cold-water fish and artificially stocked fish species as temperatures warm (Cooney et al. 2005; Isaak et al. 2015; 

Roberts et al. 2013). At higher elevations, however, current populations are often more 

isolated (due to topography), tend to be smaller, and have lower genetic diversity and 

higher extinction risk (Hilderbrand and Kershner 2000). Such populations would most 

likely need human intervention to sustain their size and diversity (Cook et al. 2010; 

Hilderbrand and Kershner 2000). Consequently, cold-water native fish have moderate to 

high vulnerability to climate change (for more information, see Cold-Water Native Fish 

in Chapter 6). 

Riparian and Wetland Vegetation

Plants associated with riparian and wetland habitats in glaciated valleys may 

face warming and drying that could inhibit their growth and survival in some areas. 

However, the high elevation of these valleys may buffer drying and warming, which 

may reduce plant sensitivity to climate change. Conditions are wetter in glaciated valleys, snowpack loss is buffered, and species are highly likely to survive and regenerate; 

therefore, the vulnerability of riparian and wetland plants to climate change is very 

likely moderate (for more information, see Chapter 6).

Pollinators

Pollinators are likely to persist under climate change in these glaciated valleys, 

but may be hindered by the direct effect of extreme climatic events (drought, extreme 

cold), and indirectly through the effects of climate change on plants in these valleys 

(Forrest 2014). Warming temperatures and the early melt of snowpack could result in 

plants flowering earlier than pollinators emerge, limiting food sources (Inouye 2008). 

However, pollinators are mobile and typically generalists; these capabilities may 

strengthen the adaptive capacity of pollinators to climate change. Researchers do not expect that pollinators, in general, will be severely hindered by shifts to earlier blooming 

of plants, or changes in the species composition of plant communities (Willmer 2012). 

Although there are research studies that identify potential vulnerabilities to pollinators 

under a changing climate, few of these studies have focused on identifying the major 

factors influencing pollinators under climate change at high elevations (for more information about pollinators and climate change, see Chapter 3). Based on the available 

information, pollinators have moderate vulnerability to climate change. 

Sensitivity to Extreme Climatic Events_____________________________

Sensitivity to Drought

Extreme events, such as drought, are expected to increase in frequency and magnitude under climate change (Field et al. 2012). At the level of a national forest, Sun et 

al. (2015) documented the impacts of the five worst droughts between 1962 and 2012. 

Across the Rocky Mountain Region, the most intense drought occurred in 75 percent of 

the national forests in 2012. Across all national forests, the top five droughts were associated with a 22-percent reduction in precipitation and a 9-percent reduction in gross 

primary productivity (Sun et al. 2015). 
USDA Forest Service RMRS-GTR-376. 2018 65

Glaciated valley ecosystems are sensitive to drought and are likely to be sensitive 

under projected climate change. Droughts in the glaciated valleys can arise through 

lower snowpack, earlier melt of snowpack, or less precipitation (snow and rain). Less 

precipitation (snow or rain) results in declining soil moisture and runoff, and consequently lower flows in streams and lower potential to recharge groundwater. Using 

future climate projections, Dhungel et al. (2016) explored the potential for changes to 

occur in streamflow, for example, from perennial to intermittent or from snow-fed to 

rain-fed. Small and large perennial snow-fed streams in Colorado and Wyoming were 

included in their study. No changes in streamflow regimes were projected for these 

streams, in contrast to other parts of the United States. They acknowledged that further 

work exploring the subtler aspects of snow-fed flow regimes may be needed. Little 

quantitative information on projected drought is available for glaciated valleys in the 

Rocky Mountain Region.

Riparian vegetation and aquatic organisms can be stressed by less moisture. Life 

history traits, biotic interactions, and availability of water in ponds will influence the 

future success of aquatic insects. Wissinger et al. (2008) report that pond-drying can 

increase the risk of desiccation-induced, pre-reproductive mortality of caddisflies, 

as well as decrease adult body size at maturity. Life history traits, such as caddisfly 

species laying eggs on the water, may limit these species to permanent ponds under 

climate change (Wissinger et al. 2003). Species with traits such as rapid larval growth, 

ovarian diapause, and terrestrial oviposition of desiccation-tolerant eggs, may be able 

to use both permanent and temporary wetlands. Detrivorous insects, such as the larvae 

of caddisflies, supplement their diets with animal material, and Wissinger et al. (2004) 

concluded that supplementation can have strong effects on fitness, especially in temporary habitats that constrain larval development. These interactions could become more 

important under limiting moisture availability. 

Sensitivity to Floods

If extreme precipitation and flooding increase in the Rocky Mountain Region, water retention capacity of most glaciated valleys may magnify flooding effects; however, 

the relatively small drainage area of these headwater basins would limit the amount of 

water contributing to flooding (Luce et al. 2012). Where glaciers are present, glacial 

BOX 4.4

Key Vulnerabilities

A moderate ranking is given for the sensitivity of glaciated valley ecosystems to extreme climatic events:

• Aquatic, riparian, and wetland ecosystems in glaciated valleys are sensitive to drought and highly dependent 

on a snow-dominant hydrology; projections indicate snowpack will decline. However, snowpack loss at the 

highest elevations may be minimal, allowing glaciated valleys to continue to retain water throughout the 

year. These factors may buffer plants and organisms in glaciated valleys from drought. Additional research is 

needed. 

• These ecosystems are only moderately sensitive to flooding given their small drainage areas in headwater 

areas; however, glacial outburst floods have potential to alter this ecosystem. 

• High elevation ecosystems are buffered somewhat from extreme heat, but are very sensitive to warming that 

may exceed species tolerance limits.
66 USDA Forest Service RMRS-GTR-376. 2018

outburst floods can occur and have the potential to alter valley bottoms. Such was the 

situation researchers observed in the Wind River Range: Meltwater, ponded by glacial 

ice from the Grasshopper Glacier, was suddenly released when the ice burst; the ensuing 

flood altered the valley bottom (Oswald and Wohl 2008). Glaciated valley aquatic and 

riparian habitats could have more floods with climate change but are likely to be only 

moderately sensitive to flooding.

Sensitivity to Extreme Heat

Extreme heat may have the most impact on stream habitats where species live at 

the edge of their heat tolerance levels, for example, benthic communities (Poff et al. 

2010), salmonids (Isaak et al. 2015; Wenger et al. 2011), and Colorado River cutthroat 

trout (Oncorhynchus clarkii pleuriticus) (Roberts et al. 2013). Aquatic habitats in glaciated valleys are somewhat buffered from extreme heat because of their elevation, but 

species are likely to be sensitive to extreme heat that may happen with climate change.

Intrinsic Adaptive Capacity to Climate Change_______________________

BOX 4.5

Key Vulnerabilities

A high vulnerability ranking is given for the intrinsic adaptive capacity of glaciated valley ecosystems to climate 

change.

• Factors that strengthen adaptive capacity:

○ Physical features and functioning riparian areas enhance moisture availability and adaptive capacity to 

drought. 

○ Biota and hydrological function recover relatively quickly following disturbance. 

• Factors that weaken adaptive capacity: 

○ Connectivity among the wetlands, ponds, and lakes is helped by both surface water and groundwater; 

drier and warmer conditions may diminish these connections with implications for the diversity of 

habitats as well as population and community dynamics in glaciated valleys. 

Factors That Strengthen Adaptive Capacity to Climate Change

Functioning riparian areas demonstrate the adaptive capacity of aquatic, riparian, 

and wetland areas in glaciated valleys. Riparian and wetland communities are diverse 

(Cooper and Andrus 1994; Gage and Cooper 2013; Vance et al. 2012), facilitating 

shifts in community composition in response to environmental changes (Bowman 

2000). Low areas of these valleys tend to retain water in wetlands, lakes, and ponds 

following flooding and high flows (Naiman et al. 2005; Winters et al. 2004b), which 

strengthens adaptive capacity to drought. Riparian plants stabilize banks and provide 

food and habitat for aquatic and terrestrial species (Naiman et al. 2005). Biota and hydrological conditions recover relatively quickly after disturbances that disrupt function, 

such as riparian recovery after fire in the western United States (Dwire and Kauffmann 

2003).
USDA Forest Service RMRS-GTR-376. 2018 67

Factors That Weaken Adaptive Capacity to Climate Change

Drought and drying of aquatic, wetland, and riparian ecosystems could hinder 

plants and animals (Poff et al. 2010). Wetlands, ponds, lakes, and riparian areas in 

glaciated valleys tend to have lower productivity and population densities of plants and 

animals, which weaken the capacity to recover from disturbances that reduce populations. Amphibians, such as boreal toads, are sensitive to drying of ponds and lakes. In 

areas where riparian vegetation has been removed or damaged, adaptive capacity to 

floods, drought, and heat is weakened. Large woody and leaf litter inputs are necessary 

for insect and fish productivity. Riparian vegetation also benefits the ecosystem by shading streams and stabilizing streambanks (Naiman et al. 2005).

Dependence on Specific Hydrological Regime_______________________

Aquatic, wetland, and riparian ecosystems in glaciated valleys are highly dependent on a snow-dominated hydrological regime. Regonda et al. (2005) found that 

snowpack at elevations above 8,200 ft stayed relatively unchanged from 1950 through 

1999; in contrast, warmer temperatures at lower elevations caused snowpack loss. 

Klos et al. (2014) projected high-elevation precipitation regimes throughout the Rocky 

Mountains to remain relatively snow-dominated with lower elevations becoming more 

rain-dominated. Although snowpack loss is buffered in the high elevations of glaciated 

valleys, drying of wetlands and riparian areas could occur and hinder plants and animals. Loss of snowpack could contribute to drier conditions late in the growing season 

that stress plants and aquatic organisms in glaciated valleys.

Stream temperatures are projected to warm under a changing climate. Isaak and 

Rieman (2013) developed equations to calculate isotherm shift rates in streams that 

can be used to look at future warming scenarios and the associated shifts in stream 

temperatures. For example, for streams with an existing lapse rate of 1.4 °F per 328 ft 

and a projected warming of 0.2 to 0.4 °F per decade, the isotherms are projected to shift 

upstream at a rate of 0.08 to 0.8 mi per decade in steep streams (2–10 percent slope) and 

0.80 to 15.5 mi per decade in flat streams (0.1–1 percent slope). 

Glaciers contribute to the water balance of glaciated valleys. Edmunds et al. (2011) 

reported a volume loss of three glaciers in the Teton Range in Wyoming to be 3.20 ± 

0.46 million m-3 (113.0 ± 16.8 million ft-3) between 1967 and 2002. They identified 

summer (June, July, and August) temperature as the primary climatic driver in glacial 

melt by comparing historical temperatures from 1911 to 1967 and from 1968 to 2006. 

Over the period of study, no significant differences in snowpack were observed. The 

Wind River Range contains 269 glaciers and snowfields, including the largest glacier in 

BOX 4.6

Key Vulnerabilities

A high vulnerability ranking is given for dependence on the hydrological regime in glaciated valley ecosystems. 

Aquatic, wetland, and riparian ecosystems are dependent on a snow-dominated hydrological regime.
68 USDA Forest Service RMRS-GTR-376. 2018

the conterminous United States, the Gannet Glacier. Numerous studies have explored 

the dynamics of these glaciers and snowfields (Cheesbrough et al. 2009; DeVisser and 

Fountain 2015). DeVisser and Fountain (2015) reported that between 1900 and 2006 the 

ice-covered area of the 269 glaciers shrank by 47 percent. As with other studies, they 

identified air temperature as the primary climatic driver influencing this loss. Glaciers at 

lower elevations had a faster rate of shrinkage than those at higher elevations. The contribution of this glacial melt to late summer streamflow ranges from 0.4 to 10.9 percent 

across four different watersheds. 

Most researchers agree that glaciers in the western United States will continue 

to retreat in the future, though the local effect of climate change on glaciers varies according to glacier elevation, size, shape, slope or aspect position, and local monthly 

temperature and precipitation (Hall and Fagre 2003). Changes in glaciers can be incremental, but glacier outburst floods can occur and potentially alter the rate and magnitude 

of change in valley bottoms (Oswald and Wohl 2008).

Likelihood of Managing Climate Change Effects______________________

BOX 4.7

Key Vulnerabilities

Management options exist to promote resilience in aquatic, riparian, and wetland ecosystems, but these options 

are not always feasible or applicable in wilderness or remote areas, and may not succeed in mitigating effects 

from extreme heat and drought. Consequently, vulnerability is ranked as high.

Management options for mitigating the effects of climate change on aquatic, 

riparian, and wetland ecosystems include reducing current stressors, strengthening 

ecosystem resilience, and, as climate continues to change, helping plants and animals to 

adapt (Millar et al. 2007; Rieman and Isaak 2010; Shoo et al. 2011; Winters et al. 2004a; 

Wohl et al. 2005). Quantifying the water needs of riparian and wetland vegetation may 

be helpful in setting the basis for management options (Cooper and Merritt 2012). 

Management options that have been identified include: 

• maintaining or restoring the natural hydrological regime; 

• maintaining and restoring upslope plant cover to minimize flooding impacts; 

• maintaining or restoring riparian areas, floodplains, and wetlands, and their 

connection to streams; 

• reintroducing beaver; 

• protecting or restoring critical or unique habitats that buffer survival during stressful 

conditions; 

• disconnecting roads from the drainage network; 

• limiting or stopping the introduction of nonnative species;

• eliminating or controlling pollutants or contaminants; 

• removing or modifying barriers to fish movement; 
USDA Forest Service RMRS-GTR-376. 2018 69

• maintaining or reconnecting large networks of habitat; 

• conserving or restoring diverse habitats across basins;

• with respect to amphibians, installing irrigation sprayers to manipulate water 

potentials at breeding sites and retaining or supplementing natural and artificial 

shelters (e.g., logs, cover boards) to reduce desiccation and thermal stress;

• manipulating canopy cover over ponds to reduce water temperature; 

• creating hydrologically diverse wetland habitats capable of supporting larval 

development under variable rainfall regimes;

• targeted fish removals in wetland habitats to aid amphibians (Ryan et al. 2014);

• aiding species migration, such as transporting individuals to inaccessible habitat or 

places of refuge to maintain genetic diversity;

• allowing new species to colonize when the resident species can no longer survive, so 

that ecosystem processes continue and loss of nutrients and soil is minimized;

• reducing the impacts from livestock grazing; 

• increasing cover of riparian vegetation to provide shading and maintain stream 

temperature;

• allowing the natural variability of stream movement across the landscapes; and

• minimizing soil compaction by limiting roads, trails, and campgrounds. 

Management in these high-elevation glaciated valleys may be challenging; the remoteness of these glaciated valleys and the feasibility of reversing the effects of climate 

change may limit or prevent management actions. Many glaciated valleys lie within 

designated wilderness or identified roadless areas (Colorado Roadless Areas in Colorado 

and Inventories Roadless Areas in Wyoming), where interventions may not be allowed. 

Hotaling et al. (2017) suggested that a worthwhile goal for alpine streams is to identify 

and prioritize the protection of robust local populations of cold stenothermic species associated with meltwater habitat, as well as meltwater-associated habitats that are likely 

to be most resistant to climate change. 

Potential for Climate Change to Exacerbate Effects of Nonclimate 

Stressors, or Vice Versa_______________________________________

BOX 4.8

Key Vulnerabilities

Climate change may exacerbate the magnitude, intensity, and effect of nonclimate stressors, such as atmospheric 

nitrogen deposition, dust-on-snow events, and introduction of invasive or undesirable native species, in glaciated 

valley ecosystems. Consequently, the vulnerability ranking is very high.

Drought, Dust on Snow, Warmer Temperatures, Atmospheric Deposition

Currently, drought, dust on snow, warmer temperatures, and atmospheric deposition affect glaciated valleys in different ways. Drought affecting a region far from these 

glaciated valleys, with the right weather conditions, can result in a dust-on-snow event 
70 USDA Forest Service RMRS-GTR-376. 2018

in a glaciated valley. Atmospheric deposition from the eastern Plains of Colorado brings 

nitrogen and the risk of acidification to these historically nitrogen-limited ecosystems 

(Benedict et al. 2013). Warming temperatures across glaciated valleys may be speeding 

up the weathering processes and altering watershed chemistry (Heath and Baron 2014). 

Climate change (increased drought, warmer temperatures) could potentially exacerbate the effects of dust on snow in high elevation ecosystems of the Rocky Mountain 

Region. Drought- affected areas of the southwestern United States have been the source 

of intensified dust-on-snow events in the Colorado Rockies. Temperature and drought 

are projected to increase, particularly in the southwestern United States. Human activities that disturb soil in dust-source regions are also likely to continue. Dust-on-snow 

events in the Upper Colorado River and surrounding basins of the Rocky Mountains 

have reduced duration of snow cover, increased the rate of snowmelt, produced earlier 

peak streamflow rates on the order of 1 to 3 weeks, and altered the chemistry of snow 

(see Current Status and Human Influences section). Deems et al. (2013) explored a future under climate change that included dust-on-snow events. While projected warming 

temperatures have a large impact on snowpack accumulation and melt, dust-on-snow 

events combined with future climate moved peak runoff earlier by 2 to 3 weeks, and 

peak flows at Lee’s Ferry on the Colorado River in Utah decreased by 14 to 18 percent 

relative to the historical scenario. These results have important implications for water 

management, planning, and policy (Painter et al. 2012).

Climate change could exacerbate the effect of atmospheric nitrogen deposition in 

high-elevation aquatic systems. It has been documented that these aquatic systems are 

sensitive to nutrient additions from natural and human-caused sources as these additions 

shift aquatic community composition, increase the risk of aquatic oxygen depletion in 

lakes, increase alpine lake acidity, and alter water chemistry (see Current Status and 

Human Influences section). Flanagan et al. (2009) identified links between atmospheric 

deposition and the alpine lake response to drought. Response of the phytoplankton 

community was most strongly correlated with water quality changes associated with 

the drought, rather than temperature and hydraulic residence time, and the dominant 

post-drought phytoplankton species were found to be associated with nitrate; both 

findings suggest the influence of atmospheric deposition in the aquatic response to 

drought (Flanagan et al. 2009). Mast et al. (2014) documented that stream chemistry 

has responded rapidly to changes in atmospheric nitrogen deposition over the last three 

decades at Loch Vale in Rocky Mountain National Park. Bowman et al. (2014) suggested that these dust-on-snow events, with introduction of base cation-laden dust, may 

have delayed the onset of soil and surface water acidification from atmospheric nitrogen 

deposition in the Rocky Mountains. Continued increases in atmospheric nitrogen deposition could eventually overwhelm this delayed response and result in soil and surface 

water acidification.

Warming temperatures are apparently having an increasing effect on weathering 

dynamics at the basin scale. Heath and Baron (2014) explored historical data collected 

at Loch Vale watershed. They concluded that increasingly warm summer temperatures 

are melting what was once permanent ice; elements entrained in the ice are released, 

resulting in a rapid change in the flux of weathering products and inorganic nitrogen 

from the basin. 
USDA Forest Service RMRS-GTR-376. 2018 71

Warming and Invasive Species

Warming may improve habitat conditions for some invasive species that have 

expanded into alpine areas (Ashton 2010). Invasive plants can displace native species, alter nutrient cycling, and disrupt food webs (Pauchard et al. 2003, 2009) (see 

also Current Status and Human Influences and Future Trends of Nonclimate Stressors 

sections).

Summary of the Regional Vulnerability of Aquatic, Riparian, and Wetland 

Ecosystems in Glaciated Valleys_______________________________

The overall regional vulnerability ranking for the ecosystem is a combination of 

the vulnerability to nonclimate stressors and the vulnerability to climate stressors. For 

aquatic, riparian, and wetland ecosystems in glaciated valleys, the mean overall vulnerability ranking was very high, and mean expert reviewer rankings were in the high or 

very high category (fig. 4.1, table 4.1). Mean vulnerability rankings were very high for 

both the nonclimate stressor vulnerability and the climate vulnerability (fig. 4.1), with 

expert reviewer rankings in the high or very high category.

The expert reviewers had very high overall confidence in the ranking of the overall 

regional vulnerability (fig. 4.2). The four experts’ individual rankings for overall confidence ranged from the high to the very high category (table 4.2). Mean confidence was 

very high for the nonclimate stressor vulnerability ranking and high for the climate vulnerability ranking. Individual experts’ confidence rankings ranged from the moderate to 

the very high category for both the nonclimate stressor and climate vulnerability rankings. Calculations of mean vulnerability and confidence included only the four complete 

reviewer rankings. 
72 USDA Forest Service RMRS-GTR-376. 2018

Figure 4.1—Overall regional vulnerability ranking and 

confidence ranking for aquatic, riparian and wetland 

ecosystems in glaciated valleys in the Rocky Mountain 

Region. Large arrow points to the mean score for 

vulnerability and for confidence. Bar represents the range 

of scores. Only four reviewer scores are included in mean 

calculations here and in tables 4.1 and 4.2.

Table 4.1—Original and reviewer rankings for the regional vulnerability assessment of aquatic, riparian, and wetland ecosystems 

in glaciated valleys in the Rocky Mountain Region. Underline indicates a reviewer score change from the original assigned 

score. 

Criterion 

Original 

score

(Reviewer 1)

Score

(Reviewer 2)

Score

(Reviewer 3)

Score*

(Reviewer 4)

Score

(Reviewer 5)

Score

Reviewer 

criterion 

rank mean

Nonclimate 

1. Extent 5 5 5 3 5 5 Very high

2. Human influences 3 3 3 3 3 3 Moderate

3. Resilience 5 5 5 5 5 5 Very high

4. Future trends 5 5 5 3 5 3 Very high

Total 18

Very high

18

Very high

18

Very high

14

High

18

Very high

16

High

18

Very high

Climate

1. Ecosystem shift 5 5 5 5 5 5 Very High

2. Species groups 5 5 5 3 5 5 Very High

3. Climatic events 3 3 3 3 4 3 Moderate

4. Adaptive capacity 3 5 3 * 4 3 High

5. Hydrology 5 5 5 5 5 1 High

6. Management 3 5 3 5 4 3 High

7. Interactions 5 5 5 5 5 5 Very High

Total 29

High

33

Very high 

29

High

* 32

Very high

25

High

30

Very high

Overall vulnerability 

rank

24

Very high 

26

Very high

24 

Very high

* 25

Very high

21

high

24

Very high

* Calculation of mean vulnerability and confidence (table 4.2) included only the four complete reviewer rankings.
USDA Forest Service RMRS-GTR-376. 2018 73

Table 4.2—Reviewer rankings for confidence in the vulnerability assessment of aquatic, riparian, and wetland ecosystems in 

glaciated valleys in the Rocky Mountain Region.

Criterion 

(Reviewer 1)

Score

(Reviewer 2)

Score

(Reviewer 3)

Score *

(Reviewer 4)

Score

(Reviewer 5)

Score

Criterion rank 

confidence 

mean

Nonclimate

1. Extent 5 5 3 5 5 Very high

2. Human influences 5 5 3 4 5 High

3. Resilience 5 5 3 4 3 Very high

4. Future trends 5 5 3 4 5 Very high

Total 20

Very high 

20

Very high

12

Moderate 

17

High

18

Very high

19 

Very high

Climate

1. Ecosystem shift 3 5 3 5 5 Very high

2. Species groups 5 5 3 4 3 High

3. Climatic events 3 5 3 4 3 High

4. Adaptive capacity 3 5 * 4 3 High

5. Hydrology 5 3 5 5 1 High

6. Management 5 3 3 4 3 High

7. Interactions 3 5 3 4 3 High

Total 27

High

31

Very high

* 30

Very high

 21 

Moderate

27 

High

Overall confidence 

rank

24

Very high 

26

Very high

* 24

Very high

20

High

24

Very high

* Calculation of mean vulnerability (table 4.1) and confidence include only the four complete reviewer rankings.

Figure 4.2—Regional vulnerability of aquatic, riparian, and wetland ecosystems in glaciated valleys to nonclimate (a) and 

climate (b) stressors in the Rocky Mountain region. Large arrow points to the mean ranking for vulnerability and for 

confidence. Bar represents the range of scores. Only four reviewer scores are included in mean calculations here and in 

tables 4.1 and 4.2.

a) b)
74 USDA Forest Service RMRS-GTR-376. 2018

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Chapter 5. Subalpine Spruce-Fir Ecosystems: 

Vulnerability to Nonclimate and Climate Stressors 

in the U.S. Forest Service Rocky Mountain Region

Janine R. Rice, Claudia Regan, Dave Winters, Rick Truex, and Linda A. Joyce

Quick Look: Subalpine Spruce-Fir Ecosystems in the Rocky Mountain Region

Subalpine spruce-fir ecosystems are moderately widespread in the subalpine zone of the Rocky 

Mountain Region. Climate is cold year-round with frost possible at any time. Precipitation is 

mainly in the form of snow. Winter snowpack and the associated slow melt-out in the spring 

serve as a reservoir for water in these ecosystems. The fire regime is characterized by long return intervals, and fires usually are stand-replacing. Vegetation is largely composed of mixed- or 

single-species stands of Engelmann spruce and subalpine fir. Lodgepole pine and quaking aspen 

also commonly grow in these ecosystems, especially in western Colorado. Moderately moist 

meadows intermix with spruce-fir ecosystems. On drier sites, spruce-fir ecosystems can grow 

with lodgepole pine, limber pine, Rocky Mountain bristlecone pine in Colorado, and whitebark 

pine in Wyoming. Moisture availability influences the shrub and herbaceous plant species associated with spruce-fir forests.

Wildlife can be year-round residents, and may include red squirrel, chipmunks, snowshoe hare, 

American marten, and, in Colorado, reintroduced Canada lynx. Elk, mule deer, bighorn sheep, 

mountain goat, black bear, and moose find habitat here in the summer. Numerous bird species 

use spruce-fir habitats, including the year-round resident, white-tailed ptarmigan, and the tropical migrants, flycatchers and hummingbirds.

Quick Look: Vulnerability of Subalpine Spruce-Fir Ecosystems in the Rocky Mountain 

Region 

Vulnerability to nonclimate and climate stressors: Moderate

Confidence: High

Exposure: Warming temperatures will occur year-round. In winter and spring, warming temperatures will contribute to early snowpack melt and a longer growing season, both of which 

could result in late growing season moisture stress. High variability in timing of spring freezing events could also occur. Spruce-fir forest on drier sites will have greater exposure under 

climate change. 

Current extent: Subalpine dry-mesic spruce-fir forests and woodlands occupy more than 4.5 

million ac in the Rocky Mountain Region. The less common subalpine mesic-wet spruce-fir 

forest and woodland occupies more than 119,000 ac, mostly in northwestern Wyoming. 

Sensitivity and adaptive capacity to climate change: Spruce-fir ecosystems are sensitive to 

warmer temperatures and drought, and to disturbances, such as insect outbreaks and fire, 

that could be exacerbated by warming and drying. The ability of Engelmann spruce and 

subalpine fir to successfully recolonize after these disturbances could be limited due to 

warming temperatures and moisture availability. Landscape changes in mature forest habitat 

under increased insect outbreaks could reduce available habitat for some wildlife. Warming 

can also stress wildlife species when their temperature limits are reached. Wildlife species 
82 USDA Forest Service RMRS-GTR-376. 2018

with seasonal color coat molts will be increasingly vulnerable to predators as duration of 

snowpack changes. Where moisture availability and site conditions allow, spruce and fir trees 

potentially could move into higher elevations, but less area is available. Spruce-fir ecosystems may fragment and contract at the lower edge of their elevational range. Management 

options currently exist to increase the resilience of spruce-fir ecosystems to nonclimate 

stressors, such as insects and wildfire; however, large areas of the subalpine are inaccessible 

or occur in wilderness, limiting implementation of some management treatments. 

Nonclimate stressors: Human activities of harvesting and road development have affected 

forest structure and composition, and landscape pattern. Mining impacts are localized. 

Future trends in recreation, atmospheric nitrogen deposition, and possibly dust on snow 

may impact ecosystem processes and species composition, but will not eliminate subalpine 

spruce-fir ecosystems in the Region. Climate change is likely to alter the dynamics and the 

effects of nonclimate stressors such as fire and insect outbreaks.

Introduction___________________________________________________

Spruce-fir forests are the matrix forests of the subalpine zone, intermixing with 

mesic meadows and other forest types, in the Forest Service, U.S. Department of 

Agriculture (USFS) Rocky Mountain Region. Snow is the dominant form of precipitation and the snowpack can remain onsite for 6 to 8 months of the year. These forests 

are long-lived with major disturbances such as infrequent but stand-replacing wildfire, 

insect outbreaks, windstorms and windthrow, and avalanches. Land use has added other 

stressors: logging, grazing, road development, and recreation (Romme et al. 2009a). 

Of the two spruce-fir ecological systems found in the Rocky Mountain Region, the 

subalpine dry-mesic spruce-fir forest and woodland type occupies by far the greater area 

in the Region: more than 4.5 million ac (Comer et al. 2003). The less common subalpine 

mesic-wet spruce-fir forest and woodland type covers more than 119,000 ac, mostly in 

northwestern Wyoming (fig. 5.1). The commonly used term “spruce-fir forest” typically 

focuses on the dry-mesic subalpine spruce-fir forest and woodland type that is prevalent in the Region. We henceforth group the two ecological systems and use the term 

“spruce-fir ecosystems” interchangeably with “spruce-fir forests.” 

These forests are largely composed of mixed- or single-species stands of 

Engelmann spruce (Picea engelmannii Parry ex Engelm.) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) (Comer et al. 2003). Co-occurring lodgepole pine (Pinus contorta

Douglas ex Louden) and quaking aspen (Populus tremuloides Michx.) are common, especially in western Colorado. On more xeric subalpine sites, lodgepole pine, limber pine 

(Pinus flexilis James), and Rocky Mountain bristlecone pine (Pinus aristata Engelm.) 

can occur within subalpine spruce-fir stands in Colorado. In Wyoming, whitebark pine 

(Pinus albicaulis Engelm.) can occur within subalpine spruce-fir stands. At the alpinesubalpine transition zone, spruce-fir forest can intermix with alpine bedrock and scree, 

ice field, fell-field, alpine turf, alpine dwarf-shrublands, and alpine meadows. 

Common shrubs in subalpine spruce-fir ecosystems can include: blueberry, huckleberry, or whortleberry (Vaccinium spp.); serviceberry (Amelanchier alnifolia Nutt.); 

thimbleberry (Rubus parviflorus Nutt.); and willow (Salix spp.). The shrub Cascade azalea (Rhododendron albiflorum Hook.) is found in the subalpine spruce-fir in Colorado. 

Shrubs found only in Wyoming include western Labrador tea (Ledum glandulosum

Nutt.) and pink mountainheath (Phyllodoce empetriformis (Sm.) D. Don). Herbaceous 
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Figure 5.1—Subalpine dry-mesic (in blue) and mesic-wet (in red) spruce-fir forest and woodland distribution (Comer et 

al. 2003). Mesic-wet spruce-fir forests and woodlands only occur in Wyoming (northern Medicine Bow Mountains and 

Shoshone National Forest). (Data for Comer et al. 2003 can be found in NatureServe 2014.)
84 USDA Forest Service RMRS-GTR-376. 2018

species include: red baneberry (Actaea rubra (Aiton) Willd.), starry false lily of the valley (Maianthemum stellatum (L.) Link), bunchberry dogwood (Cornus canadensis L.), 

sprucefir fleabane (Erigeron eximius Green), yellowdot saxifrage (Saxifraga bronchialis

L.), and mountain bluebells (Mertensia ciliata). A herbaceous species found only in 

Wyoming is the Hitchcock’s smooth woodrush (Luzula glabrata (Hoppe ex Rostk.) 

Desv. var. hitchcockii (Hämet-Ahti) Dorn). A common grass is bluejoint (Calamagrostis 

Canadensi (Michx.) P. Beauv.) (Comer et al. 2003). Xeric understory species that can 

occur in spruce-fir ecosystems include: common juniper (Juniperus communis L.), twinflower (Linnaea borealis L.), creeping barberry (Mahonia repens (Lindl.) G. Don.), and 

grouse whortleberry (Vaccinium scoparium Leiberg ex. Coville) (Comer et al. 2003). 

Spruce-fir ecosystems provide food and shelter for permanent wildlife residents 

such as red squirrel (Tamiasciurus hudsonicus), chipmunks (Tamias spp.), snowshoe 

hare (Lepus americanus), and American marten (Martes americana), and Canada lynx 

(Lynx canadensis), which was reintroduced in Colorado. Summer habitat resources are 

used by large wildlife such as elk (Cervus elaphus), mule deer (Odocoileus hemionus), 

bighorn sheep (Ovis canadensis), mountain goat (Oreamnos americanus), black bear 

(Ursus americanus), and moose (Alces americanus). Numerous birds utilize spruce-fir 

habitats such as Clark’s nutcracker (Nucifraga columbiana), ruby-crowned kinglet 

(Regulus calendula), nuthatches (Sitta spp.), juncos (Junco spp.), thrushes (Catharus

spp.), mountain chickadee (Poecile gambeli), red crossbill (Loxia curvirostra), pine 

siskin (Spinus pinus), blue grouse (Dendragapus obscurus), white-tailed ptarmigan 

(Lagopus leucura), owls such as the boreal owl (Aegolius funereus), cavity nesting 

birds such as the three-toed woodpecker (Picoides dorsalis) or other woodpeckers, and 

migrating birds such as flycatchers (Empidonax spp.) and hummingbirds (Selasphorus

spp.) (Uchytil 1991a,b).

Subalpine spruce-fir ecosystems have been assessed for their vulnerability to 

climate change. As described in Chapter 1, the vulnerability framework has two major 

components, one related to nonclimate stressors and one related to climate stressors. 

Within the nonclimate component, four criteria were used to define factors of vulnerability: current status of ecosystem extent, intrinsic resilience of the ecosystem to 

nonclimate stressors, human influences on the ecosystem, and future trends of nonclimate stressors on the ecosystem. Within the climate change component, seven criteria 

capture the vulnerability of individual species, sensitivity of ecosystem dynamics such 

as dependence of the ecosystem on the hydrological cycle, adaptive capacity of the 

ecosystem, potential for management to mitigate the effects of climate and nonclimate 

stressors, and interaction between climate change and nonclimate stressors. More information and the rationale for ranking ecosystem vulnerability by using these criteria are 

given in appendices A, B, and C.

Subalpine spruce-fir ecosystems are increasingly being studied on national forest 

lands as well as in other areas throughout the Rocky Mountain Region. We draw from 

this literature where studies are within the Region to assess the vulnerability of sprucefir ecosystems to climate change at the scale of the Rocky Mountain Region. Even 

within this Region, these ecosystems have great variation. Application of these results to 

spruce-fir ecosystems on individual national forests will require an understanding of local site characteristics and the site characteristics of the research studies reviewed here. 
USDA Forest Service RMRS-GTR-376. 2018 85

Vulnerability of Subalpine Spruce-Fir Ecosystems to Nonclimate 

Stressors___________________________________________________

Summary of Key Vulnerabilities to Nonclimate Stressors

BOX 5.1

Current extent

Subalpine spruce-fir ecosystems, though limited in area, are spread throughout the subalpine zone of the Rocky 

Mountain Region; their vulnerability ranking is moderate.

Human influences on ecosystem

Subalpine spruce-fir ecosystems are moderately vulnerable to the legacy of past human influences and to the 

stressors associated with ongoing human influences.

• Management since the mid-20th century has influenced forest structure and composition, and landscape 

pattern. Silvicultural activity, recreation, and fire control activities have increased road density.

• Mining activity has left a legacy of localized effects including compacted, displaced, and chemically 

contaminated soils.

• Currently, nonnative and invasive species occur minimally at high elevations. 

Intrinsic resilience of ecosystem to nonclimate stressors

A moderate vulnerability ranking is given for the intrinsic resilience of subalpine spruce-fir ecosystems to 

nonclimate stressors.

• Factors that enhance subalpine spruce-fir ecosystem resilience to nonclimate stressors: 

○ Plants and animals, such as understory vegetation, snowshoe hare, and red squirrel, are adapted to 

subalpine climate and the use of post-disturbance habitats.

• Factors that lower subalpine spruce-fir ecosystem resilience to nonclimate stressors:

○ Spruce-fir are susceptible to disturbances such as severe fires, insect outbreaks, wind events, and 

avalanches.

○ Tree regeneration and forest stand recovery are very slow following disturbance.

Future trends of nonclimate stressors

Subalpine spruce-fir ecosystems are considered to be moderately vulnerable to future nonclimate stressors, such 

as road development, increased demand for recreation, nonnative and invasive species, atmospheric deposition, 

and possibly dust-on-snow events.

Current Status and Human Influences

Stand-replacing fire, insect outbreaks, and windthrow events have been and continue to be the primary natural disturbances in the subalpine zone. From 1500 to 1850, 

the forested landscape across the northern Colorado Front Range subalpine zone would 

have consisted of very large patches of even-aged forests varying in composition from 

pure lodgepole pine or aspen to spruce-fir (Veblen and Donnegan 2005). Disturbances 

that promoted this pattern would have been infrequent, high-severity fires followed 

by successional replacement of species or recovery to the same dominant tree species 

according to site conditions and availability of seeds and sprouts. For south-central 

and southwestern Colorado, Romme et al. (2009a) described the subalpine landscape 

pattern as a coarse-grained mosaic of intermingling spruce-fir, aspen, cold-wet and 

cool-moist mixed conifer forests, bristlecone pine woodlands, meadows, grasslands, 

willow carrs, and sparsely vegetated rock fields. This landscape pattern was the result of 
86 USDA Forest Service RMRS-GTR-376. 2018

stand-replacing fires at the large scale, periodic insect outbreaks at a finer scale, and, at 

the finest scale, windthrow events. 

The past effects of Euro-American settlement in these high elevation forests 

have been less intense than effects at lower elevations in the Rocky Mountain Region 

(Romme et al. 2009a; Veblen and Donnegan 2005). Mining, logging, and road development were seen as the most significant impacts in the southern part of Colorado 

(Romme et al. 2009a,b) and in the Colorado Front Range (Veblen and Donnegan 2005). 

Although extensive grazing by domestic livestock occurred in subalpine meadows from 

the mid-to-late 1800s to the early 1900s, little information is available on impacts within 

subalpine forests. Some evidence suggests that spruce-fir tree species expanded into 

subalpine meadows during the mid-20th century during a period of heavy livestock grazing in the Wind River Mountains in Wyoming as lack of tree establishment coincided 

with cessation of grazing (Dunwiddie 1977). 

Mining for gold, silver, copper, lead, molybdenum, zinc, and tungsten started in 

the mid-1800s. Mining activities include removal of subsurface materials and associated 

vegetation, and creation of tailings piles, all of which can alter hillslope stability (Wohl 

2006). These effects remain as localized remnants of mines and tailings in subalpine 

areas of the Rocky Mountain Region. On the White River National Forest, previous 

mining activity compacted, displaced, and chemically contaminated soils and impaired 

water quality in many streams; soils in some areas are still not healthy or productive 

(USDA FS 2002). The long-term effects are primarily on the aquatic systems in the 

form of increased sediment yield to streams and pollution from heavy metals and other 

contaminants from mining (Schmidt et al. 2012; Wohl 2006). 

In contrast to lower elevations, logging of spruce-fir forests was minimal before 

the 1950s. Logging was heaviest along the Front Range and southern Colorado during 

the 1950s to 1970s (Romme et al. 2009a; Veblen and Donnegan 2005). After 1950, 

foresters used clearcutting to reduce losses from windthrow and beetle kill and to 

lower costs (Alexander 1987). As clearcutting resulted in regeneration failure, foresters 

turned to partial cutting and small-patch clearcutting after the 1970s (Alexander 1987). 

After the 1950s, fire control and recreation interests increased development. Between 

1950 and 1993, McGarigal et al. (2001) reported that the greatest impact on landscape 

structure on the San Juan National Forest in Colorado was a three-fold increase in road 

density to support timber harvest, fire control, and recreation. The harvest of trees along 

with road construction has continued to influence subalpine forest structure and composition (Romme et al. 2009b; Veblen and Donnegan 2005).

Invasive and Nonnative Species

Invasive and nonnative plants can influence dynamics of native plants, insects, and 

animals (Bradley et al. 2010). For example, fewer well-developed seeds were produced 

from flowers of two native species when hand-pollinated with pollen mixtures containing dandelion (Taraxacum officinale), suggesting an allelopathy effect (Loughnan et 

al. 2014). High elevations, including subalpine areas in the Rocky Mountain Region, 

have lower levels of nonnative plant species invasions than other areas (Averett et al. 

2016; Banks and Baker 2011; Pauchard et al. 2009). Researchers have found common 

dandelion, Kentucky bluegrass (Poa pratensis), and smooth brome (Bromus inermis) in 
USDA Forest Service RMRS-GTR-376. 2018 87

Colorado Front Range subalpine areas (Coop et al. 2010). Low abundances of common 

dandelion, dark goosefoot (Chenopodium atrovirens), and many-flowered woodrush 

(Luzula multiflora) have been reported in the subalpine Medicine Bow Mountains in 

Wyoming (Musselman 1994). The invasive winter annual cheatgrass (Bromus tectorum L., also called downy brome) was found to be increasing in Rocky Mountain 

National Park in Colorado (Bromberg et al. 2011). Thirty-four nonnative species were 

recorded by Pollnac et al. (2012) in the Greater Yellowstone Ecosystem in northwestern 

Wyoming, spanning an elevational gradient from sagebrush to alpine ecosystems. As 

with other studies, they found nonnative plant richness decreased with increased elevation and increased distance from the road. 

Atmospheric Nitrogen Deposition

Researchers have observed increased nitrogen deposition in high elevation areas of 

the Rocky Mountain Region. McDonnell et al. (2014) concluded that increased nitrogen 

deposition from 1900 to present has probably changed treeline vegetation. Simulating 

historical changes in atmospheric chemistry, they found a 25-percent increase in cover 

of subalpine fir saplings, mixed response in grasses, generally decreased abundance in 

forbs, and, specifically for the forb Ross’ avens (Geum rossii), a reduction of 50 percent. 

Subalpine forests are still nitrogen limited even with these additions from human-caused 

sources (Baron et al. 2000; Burns 2004), but continued increases could result in less 

nitrogen limitation, possibly short-term increased productivity, and likely alterations in 

soil microbial communities leading to less soil carbon (Boot et al. 2016; Burns 2004). 

At low levels of nitrogen fertilization, measurable changes in the biogeochemistry of 

Engelmann spruce forests east of the Continental Divide were observed (Rueth and 

Baron 2002). If deposition continues to increase, plant species composition will continue to change (McDonnell et al. 2014), and the short-term increase in productivity 

may shift to a decline in productivity, as reported for other forested regions. Reaching 

nitrogen saturation will depend on atmospheric nitrogen deposition and concurrent 

changes in temperatures as well as on existing soil nitrogen levels, which vary across 

the subalpine zone (Boot et al. 2016; McDonnell et al. 2014; Rueth et al. 2003).

Fire

Fire occurrence in the subalpine is characterized by infrequent, high-severity 

crown fires that are primarily climate driven, with forest structure and topography affecting fire characteristics (Higuera et al. 2014; Romme and Knight 1981; Sibold et 

al. 2006). Fire suppression effects can include increases in fuel loadings and canopy 

cover at the stand level. However, fire return intervals in spruce-fir ecosystems are 100 

to several hundred years long; hence it is difficult to detect whether fire suppression 

for the last 80 years has influenced the natural fire regime in the subalpine. Keane et al. 

(2002) concluded that fire exclusion has yet to significantly alter stand conditions and 

forest health in Rocky Mountain subalpine ecosystems. Buechling and Baker (2004) 

found longer fire intervals in Rocky Mountain National Park over the 20th century, and 

attributed this change to less severe drought impacts over this time compared to before 

the 20th century. Veblen and Donnegan (2005) found a modern pattern of less-frequentthan-expected fire along the Colorado Front Range. Conversely, a central and northern 
88 USDA Forest Service RMRS-GTR-376. 2018

Colorado Front Range study conducted near alpine treelines by Sherriff et al. (2001) 

found a slightly higher occurrence of fire, and attributed it to drought associated with 

La Niña events. It is possible that modern fire suppression efforts may have altered the 

fire return intervals at the landscape level in some subalpine forests (Kipfmueller and 

Baker 2000; Romme et al. 2009a; Veblen and Donnegan 2005). The naturally long fire 

return interval, variability of fire over landscapes, and interactions with drought make 

it challenging to determine if fire suppression efforts have significantly affected the fire 

regime. At this point, the fire regime is apparently within the range of natural variability.

Intrinsic Resilience of Ecosystem to Nonclimate Stressors____________

Factors That Enhance Resilience to Nonclimate Stressors

Resilience of spruce-fir ecosystems to disturbances, such as fire or beetle outbreaks, can be enhanced by variety in size and age of spruce and fir trees across the 

subalpine landscape. Disturbance processes, such as insect outbreaks, windthrow, 

and fire, affect the age and structural variety of stands (Alexander 1987; Veblen and 

Donnegan 2005). Variety across landscapes is also created in areas disturbed by snow 

avalanches, which can create fire breaks (Veblen et al. 1994). Wildfires may reduce 

susceptibility to spruce beetle (Dendoctronus rufipennis) outbreaks by reducing large 

Engelmann spruce trees (Bakaj et al. 2016). 

These ecosystems are resilient to natural disturbance in the long term, and recover 

over time through changes in species that occur in plant communities. For example, 

pioneer species, such as lodgepole pine, or at higher elevations five-needle pines, are 

shade-intolerant and recover on the site quickly. Spruce and fir are shade-tolerant and 

seed into the understory of lodgepole trees (Minckley et al. 2012; Veblen 1986). The 

infrequent, large, and stand-replacing fires in subalpine Rocky Mountain forests have 

also been observed to increase plant understory diversity (Coop et al. 2010; Sibold et al. 

2006).

Some wildlife and plant species are adapted to fire disturbance. Snowshoe hares 

apparently escape fire and suffer minimal mortality rates. After fire, they consume 

charred tree bark, and visit burned areas as plants regrow (Sullivan 1995a). Red squirrels and snowshoe hares and their predator, Canada lynx, use woody debris for dens 

(Ulev 2007). Red squirrels also use snags (Sullivan 1995b). Several understory plant 

species can resprout readily after low- or moderate-intensity fire that has killed the 

aboveground portion. Postfire sprouting is common for bunchberry dogwood as long 

as fires fail to raise soil temperatures appreciably or produce long-term soil heating 

(Gucker 2012a). In addition to sprouting from rhizomes, thimbleberry can emerge from 

seed in the soil on burned sites (Gucker 2012b). Serviceberry can sprout from the upper 

portion of the root crown, or if that has been killed, from rhizomes farther below the 

surface (Fryer 1997). Additional information about plant species and fire effects can be 

found at the Fire Effects Information System (USDA FS 2017).

Continued increases in atmospheric nitrogen deposition could lessen the current 

nitrogen limitation for subalpine plants, possibly increasing productivity (Burns 2004); 

however, long-term implications for subalpine forests are unclear. Dust on snow has 

been documented to affect the pH and raise acid neutralizing capacity of snow samples, 
USDA Forest Service RMRS-GTR-376. 2018 89

but additional research is needed to assess the impact of atmospheric deposition as well 

as wind-blown deposition on snowpack chemistry, snowpack duration, plant communities, nutrient and water relations, and quality of the water exported from high elevation 

ecosystems (Rhoades et al. 2010).

Factors That Lower Resilience to Nonclimate Disturbances

Spruce-fir forests have historically burned in infrequent, stand-replacing fires 

that return about every 100 to several hundred years (Keane et al. 2002; Veblen 2000). 

Engelmann spruce and subalpine fir have shallow roots, and thin, less fire-resistant bark 

that makes them susceptible to fire (Alexander and Shepperd 1990; Alexander et al. 

1990; Schoennagel et al. 2004). Recovery periods for trees are long following mortality from disturbances. Old-growth spruce-fir stands may take more than 300 years to 

develop after disturbance (Aplet et al. 1988; Veblen et al. 1989). Although recovery is 

slow, local reproduction is common and may occur by seeding, or by layering (when 

low branches touch the soil and take root) where forest cover is developing (Alexander 

and Shepperd 1990; Alexander et al. 1990). Disturbances such as severe fires, insect 

outbreaks, avalanches, and windthrow can reduce plant and animal habitat as well as 

cause mortality.

Mature spruce-fir forests have a high likelihood of insect outbreaks with associated mortality. Spruce beetle outbreaks have affected more than 1 million ac of spruce 

forest in Colorado and Wyoming since 1996 (Harris et al. 2013). Subalpine fir decline, a 

complex that includes two species of root disease fungi and western balsam bark beetle 

(Dryocoetes confusus) (McMillin et al. 2003), has affected more than 180,000 ac of 

this ecosystem in Colorado (Colorado State Forest Service 2012). Engelmann spruce 

and subalpine fir are also susceptible to other insects and pathogens, such as western 

spruce budworm (Choristoneura freemani), balsam fir bark beetle (Dryocoetes confusus

Swaine), and broom rust (caused by the fungus Chrysomyxa arctostaphyli) (Alexander 

and Shepperd 1990; Alexander et al. 1990). 

Future Trends of Nonclimate Stressors 

Subalpine spruce-fir ecosystems are considered to be moderately vulnerable to 

future nonclimate stressors associated with human population growth and related infrastructure and road development, increased demand for recreation, atmospheric nitrogen 

deposition, and possibly dust-on-snow events. Although nonnative and invasive species 

occur minimally at high elevations in the Rocky Mountains, their spread threatens to 

disrupt ecosystem function, reduce biodiversity, displace native species, and disrupt pollination processes (Bradley et al. 2010; Jones 2004; Pauchard et al. 2009).
90 USDA Forest Service RMRS-GTR-376. 2018

Vulnerability of Subalpine Spruce-Fir Ecosystems to Climate Stressors__

Capacity for Range Shift

Southern Limit Proximity

Spruce-fir ecosystems are moderately widespread throughout the subalpine areas 

of the Rocky Mountain Region (fig. 5.1), covering about 20 percent of the area above 

an elevation of 8,200 ft. The range of the spruce-fir types considered here encompasses 

the Cascades and Rocky Mountains, extending from British Columbia and Alberta in 

the north, to northern New Mexico in the south (Comer et al. 2003). Trees in southern 

Colorado spruce-fir forests are close to the southern limit of their current distribution 

and would have greater vulnerability to a changing climate.

Elevation Potential for Range Shift

Subalpine spruce-fir ecosystems cover high-elevation mountain areas of Wyoming 

and Colorado starting at about 9,000 ft and extending up to treeline at about 11,500 ft 

(Comer et al. 2003). Establishment of Engelmann spruce or subalpine fir at the interface 

between upright trees and alpine depends on seed dispersal by local sources or birds 

and wind, and sufficient moisture for seed germination in a microsite where climate 

and soil support establishment (Malanson et al. 2007). Treeline expansion has been 

observed to be limited by moisture availability during dry years (Villalba et al. 1994), 

topography, alpine microsite conditions, and geomorphic and soil factors (MaciasFauria and Johnson 2013; Weisberg and Baker 1995). Elliott (2012) documented that 

regional climate variability (moisture and temperature) can initiate abrupt changes in 

tree establishment at broad regional scales (New Mexico to Montana); an abrupt increase in regional tree establishment coincided with a shift toward reduced cool-season 

precipitation. Under a changing climate, warmer temperatures may enhance subalpine 

tree expansion, but without adequate moisture availability and suitable site conditions, 

tree establishment will be inhibited. Although this ecosystem has the potential for upward elevational shifts, it may contract at the lower edge of its elevational range (Notaro 

et al. 2012; Rehfeldt et al. 2006, 2012). Because less area is available at higher elevations where this ecosystem may expand, subalpine spruce-fir ecosystems may contract 

overall.

BOX 5.2

Key Vulnerabilities

A moderate vulnerability ranking is given for the capacity of spruce-fir ecosystems for range shift.

• Trees in southern Colorado spruce-fir forests are close to the southern limit of their current distribution and 

would have greater vulnerability to a changing climate. 

• Overall, spruce-fir ecosystems may contract; however, expansion of tree species into higher elevation areas 

under a warming climate is possible if sufficient moisture is available and where microsite, geomorphic, and 

soil conditions allow. 

• Spruce-fir forest species may expand into associated subalpine ecosystems, such as subalpine meadows. 
USDA Forest Service RMRS-GTR-376. 2018 91

Connectivity

Within the subalpine zone of the Rocky Mountain Region, spruce-fir forests are 

relatively connected (fig 5.1). Therefore, range shifts within the subalpine zone are not 

likely to be constrained, provided suitable site conditions exist. Spruce-fir forests at the 

southern range limit of these trees have smaller, more fragmented patches not typical of 

similar forests at higher latitudes, and range shifts may be the most constrained in these 

areas. Along with site conditions and the degree of fragmentation, climate may be a factor supporting spruce-fir expansion into subalpine meadows (Hessl and Baker 1997). 

Vulnerability of Cold-Adapted, Foundation, or Keystone Species to 

Climate Change______________________________________________

BOX 5.3

Key Vulnerabilities

A moderate vulnerability ranking is given for cold-adapted, foundation, and keystone species in subalpine sprucefir ecosystems.

• The foundation species Engelmann spruce and subalpine fir are likely to shift ranges but persist under 

climate change. 

• Herbaceous understory plants may be hindered by earlier warming and higher spring temperatures coupled 

with variability in freeze dates, which could limit reproduction and productivity. However, these species are 

expected to persist on the landscape.

• Habitat suitability may increase for some bird and mammal species and not for others. Cavity nesting birds 

may find increased, albeit transient, availability of snags and food from more fire and insect outbreaks. 

Some species may have more reproductive success with warmer spring temperatures and earlier snowmelt. 

Population declines or stress on species, such as the boreal owl and snowshoe hare, is likely with warmer 

temperatures.

We selected a subset of species that occupy spruce-fir ecosystems to capture a 

range of environmental tolerances and to evaluate how their vulnerability may affect 

ecosystem function. Engelmann spruce and subalpine fir are selected as they are both 

cold-adapted and foundation species that have substantial biomass and influence within 

spruce-fir ecosystems. Understory shrub, forb, and grass species are also included as 

they are cold-adapted species, important food resources, and habitat for wildlife living 

in or migrating to these ecosystems. Bird and mammal species are selected as many 

are cold adapted. We identify groups of species, rather than specific individual species, 

because plant and animal species differ across spruce-fir forests from southern Colorado 

to northern Wyoming. 

Engelmann Spruce and Subalpine Fir

Engelmann spruce and subalpine fir, alone or mixed, dominate the tree canopy of 

subalpine spruce-fir ecosystems. A variety of climate and species models suggest that 

spruce-fir ecosystems will contract overall in their extent, but continue to have a presence in the Rocky Mountain Region (Mathys et al. 2016; Notaro et al. 2012; Rehfeldt et 

al. 2006, 2012; Schrag et al. 2008). Differences in results are tied to how site and growth 

factors are modeled. When the modeling framework includes elevated carbon dioxide, 
92 USDA Forest Service RMRS-GTR-376. 2018

researchers found reduced stresses associated with climate change and expansion of 

high elevation forests in the Colorado Rockies (Notaro et al. 2012). However, field 

research by Hu et al. (2010) showed that the fertilizing effect of increased atmospheric 

carbon dioxide was more than counterbalanced by the increased water stress due to 

longer growing seasons, suggesting that the mitigating factor of elevated carbon dioxide 

may not persist in the future. Using individual bioclimate models based on forest inventory plot data, Gray and Hamann (2012) reported that current populations of Engelmann 

spruce lag behind their optimal climate niche in the Rocky Mountains. Under climate 

change, they project that these lags will be even greater, delaying new tree establishment. Thus, under climate change, these tree species will continue to persist, but 

increasingly will occupy areas outside of their optimal climate. Given this information, 

Engelmann spruce and subalpine fir are moderately vulnerable to climate change. 

Cold-Adapted Understory Vegetation

Little information is available on climate effects and understory plant dynamics 

in subalpine spruce-fir forests. We draw from literature exploring climate effects and 

subalpine meadow plants. Warming of early spring temperatures and early snowmelt 

have been documented to initiate plant growth and flowering in subalpine meadow 

plants (Iler et al. 2013; Inouye 2008). In an experimental treatment, subalpine meadow 

plants flowered 10 days earlier in plots where snow was removed (Gezon et al. 2016). 

Plants that survived freeze events also had more pollinator visitation and reproduction 

than controls, but early blooming forbs were vulnerable to variability in spring freeze 

dates (see Sensitivity to Extreme Climatic Events section). Iler et al. (2013) reported 

that early flowering species were more responsive to a shift in flowering onset, peak 

flowering, and flowering end days. Although many forb and shrub species can reproduce 

by sprouting from belowground structures, periods of sexual reproduction are necessary 

to produce the genetic variation on which selection can work. Further, flower blooming 

in subalpine meadows has been observed to shift away from the mid-growing season to 

an earlier or late period, negatively affecting pollinators (Aldridge et al. 2011). Loss of 

flowers and seeds associated with unsuccessful sexual reproduction has an impact on 

food sources for pollinators, herbivores, and seed predators. 

Nine spruce-fir associated understory plant species were assessed using the 

NatureServe climate change vulnerability index (Neely et al. 2011). Six species are 

moderately vulnerable: reflected moonwort (Botrychium echo), forkleaved moonwort 

(B. furcatum), Mingan moonwort (B. manganese), pale moonwort (B. pallidum), northern moonwort (B. pinnatum), and Black Canyon gillia (Gillia penstemonoides). One 

species was ranked highly vulnerable, rockcress draba (Draba globose); and one species 

extremely vulnerable, peculiar moonwort (Botrychium paradoxum). 

Growing conditions for understory subalpine spruce-fir plants may improve with 

the loss of tree canopy from insect outbreaks and the associated tree mortality. After 

bark beetle outbreaks, forbs, grasses, and shrubs increased in amount, height, and coverage (Jorgenson and Jenkins 2011). Understory vegetation of spruce-fir ecosystems is 

likely to persist under climate change along with their overstory components, according 

to studies, but reproduction could be hindered by a combination of climate change 
USDA Forest Service RMRS-GTR-376. 2018 93

factors. Therefore, cold-adapted understory plants are likely to be moderately vulnerable 

to climate change.

Birds and Mammals

Warming can affect wildlife indirectly through changes in snowpack and directly 

through increases in air temperature. Temporal changes in snowpack accumulation or 

snowpack melt are problematic for species that undergo seasonal molts where their 

pelage or plumage changes color to match the presence or absence of snow (Mills et 

al. 2013; see Snowshoe Hare section in Chapter 3). Research observed that American 

marten population distributions are highest in areas of deep snow (>9 in) and may 

be hindered where snowpacks shrink (Stone 2010). Warming can stress wildlife (See 

Sensitivity to Extreme Heat section). White-tailed ptarmigan and its European counterpart, alpine rock ptarmigan (Lagopus muta), were projected to decline with warmer 

winter temperatures and delayed fall snowpacks in the Rocky Mountains and the Alps, 

respectively (Imperio et al. 2013; Wang et al. 2002). However, warmer spring temperatures may provide ptarmigan with earlier access to forage with earlier snow-free ground, 

and earlier breeding increases their reproductive success (Imperio et al. 2013). A suite 

of factors that included rising temperatures, less snow or shorter duration of snowpacks, 

insect infestations, periodic drought, and shorter fire return intervals contributed to a 

ranking of most vulnerable for boreal owl in southwestern Colorado (Rhea et al. 2013). 

Similar factors contributed to the highly vulnerable ranking for the owl in the Gunnison 

Basin in Colorado (Neely et al. 2011). 

Greater loss of mature forest structure on the landscape as a result of increased 

disturbance, such as fire or bark beetle outbreaks, may harm some species. Declines 

in American marten populations have been linked to prey availability, weather conditions, and loss of habitat, particularly their preferred mature forest habitat structure 

(Stone 2010). In a study in northern Idaho, western Montana, and parts of the Greater 

Yellowstone Ecosystem, Wasserman et al. (2013) reported that projected climate 

warming reduces the extent of American marten dispersal habitat and increases the 

fragmentation of marten populations in the western and northwestern parts of the study 

area. In the Greater Yellowstone Ecosystem, however, climate change is not predicted to 

fragment dispersal habitat of marten. In southwestern Colorado, American marten was 

assessed as vulnerable to climate change based on these factors: long life span and small 

litters, decreasing abundance of prey (red-backed vole [Myodes gapperi]), and competition for prey by other predators (Rhea et al. 2013). 

Birds that use forests after disturbance may benefit from disturbances that are likely to increase with warming (Norris et al. 2013; Saab et al. 2014). For example, cavity 

nesting birds and species nesting in shrubs respond positively to bark beetle outbreaks 

(Saab et al. 2014). The olive-sided flycatcher (Contopus cooperi) uses forests that have 

been disturbed in southwestern Colorado; however, this bird is a long-distance migrant, 

and may also be subjected to climate changes occurring in wintering areas outside the 

Rocky Mountain Region (Kotliar 2007; Rhea et al. 2013). The American three-toed 

woodpecker was identified as resilient to climate change in southwestern Colorado 

(Rhea et al. 2013). Neverthless, a high uncertainty was given for this ranking as little 

information is available on how climate change will affect competitive interactions, 
94 USDA Forest Service RMRS-GTR-376. 2018

predators, diseases, and parasites, all of which influence woodpecker population dynamics. In addition, these opportunities to use forests after disturbance depend on the return 

of this forested type following the disturbance. Detailed species-specific information 

about vulnerability is currently limited. Given the available information, bird and mammal species of spruce-fir ecosystems will have differing degrees of vulnerability to 

climate change (Pavlacky and Sparks 2016). Vulnerability will be high for some species 

and low for others.

Sensitivity to Extreme Climatic Events______________________________

BOX 5.4

Key Vulnerabilities

A moderate vulnerability ranking is given for the sensitivity of spruce-fir ecosystems to extreme climatic events.

• Plants and animals in the subalpine spruce-fir ecosystems may be susceptible to increased mortality under 

extreme drought.

• Plants and animals may undergo heat stress, which can reduce population numbers. 

• Earlier warm periods and earlier snowmelt may promote earlier plant emergence and flowering and increase 

plant vulnerability to greater variability in freeze dates.

• Engelmann spruce and subalpine fir are vulnerable to extreme wind events, especially on ridgetops and 

saddles and where wind exposure is highest.

Sensitivity to Drought

Although drought is a rare event in the moderately wet subalpine ecosystems 

(Schoennagel et al. 2004), drought has caused stress, population declines, and increased 

mortality rates for animal and plant species of spruce-fir ecosystems. Researchers reported decreased avian abundances at the ecoregional spatial scale in the western United 

States when extreme weather events occurred in breeding and post-fledging periods; declines were greatest when drought coincided with heat waves (Albright et al. 2010). van 

Mantgem et al. (2009) concluded that noncatastrophic tree mortality rates in the western 

United States have increased over time and are correlated with increasing temperatures 

and water deficits. Specifically, for the Colorado Front Range, monitoring of more than 

5,000 marked trees in permanent plots since 1982 showed an accelerating rate of mortality for all the common conifers, which was attributed primarily to increased moisture 

stress (Smith et al. 2015). In the subalpine forest at Niwot Ridge in Colorado, Smith et 

al. (2015) reported an increase in the frequency of maximum daily temperatures exceeding 75 °F for the 1986–2012 period compared to the 1953–1985 period. It is noteworthy 

that this increase in background tree mortality rates has been occurring even in stands 

not affected by bark beetles or other lethal insects or pathogens. 

Drought strongly affects mortality of both Engelmann spruce and subalpine fir 

(Bigler et al. 2007), with subalpine fir having higher drought sensitivity than Engelmann 

spruce (Villalba et al. 1994), and more pronounced mortality from early season drought 

(Bigler et al. 2007). Engelmann spruce mortality can continue for up to 5 years after a 

drought, and subalpine fir can be affected 11 years after a drought (Bigler et al. 2007). 
USDA Forest Service RMRS-GTR-376. 2018 95

Although topographic differences strongly influenced the growth of Engelmann spruce, 

subalpine fir, and lodgepole pine, warm falls without early snow favored lodgepole pine 

over the other two species (Villalba et al. 1994). Such conditions may increase in the 

future, favoring lodgepole pine in the subalpine zone. Other infrequent disturbances 

such as insect epidemics and wildfire can follow drought (Bigler et al. 2005; DeRose 

and Long 2012; Hart et al. 2014; Sibold and Veblen 2006). Due to drought-associated 

population declines and mortality in spruce-fir ecosystems, we expect these ecosystems 

will be moderately sensitive to drought.

Sensitivity to Extreme Heat

Wildlife and plant species associated with spruce-fir ecosystems are sensitive to 

extreme heat. Researchers have observed that numbers of birds of resident species decline during extended heat waves in the Rocky Mountains (Albright et al. 2011). Studies 

project that ptarmigan population numbers will decline with warmer temperatures associated with climate change (Wang et al. 2002). Heat may also stress other wildlife that 

use subalpine habitat in summer; thresholds are as follows: 77 °F for American marten 

(Stone 2010), 59 °F for moose (Lowe et al. 2010), and 77 °F for elk and deer (Parker 

and Robbins 1983). Studies link warmer temperatures to reduced wildlife populations 

of large mammals, as heat inhibits their ability to assimilate nutritional resources; examples are moose (Lenarz et al. 2009) and mountain goat (White et al. 2011). 

Engelmann spruce and subalpine fir grow where summer temperatures are relatively cool. Extreme heat events may stress mature trees and negatively affect seedlings 

through drying (Alexander 1987). Buechling et al. (2016) reported increases in seed 

output of Engelmann spruce in the southern Rocky Mountains over the last 40 years 

but found that seed output declined when summer temperatures in the maturation year 

were more than 20 percent above the long-term mean temperature. These results suggest 

future declines in seed output with warming temperatures. 

Sensitivity to Freeze Dates

Variability in freezing dates combined with warming temperatures and earlier 

snowmelt could increase plant and pollinator exposure to frost damage in spring. Heavy 

frosts may especially damage or kill Engelmann spruce and subalpine fir seedlings 

(Alexander and Shepperd 1990). Herbaceous vegetation growth in subalpine meadows 

is controlled by snowpack, and earlier melt of snowpack has resulted in earlier blooming, earlier emergence of seedlings, earlier onset of flowering, frost damage to flowers, 

and thus loss of seeds (Iler et al. 2013; Inouye 2008). Caradonna and Bain (2016) found 

subalpine meadow plant species in the Colorado Rocky Mountains had greater frost 

sensitivity of reproductive structures compared to vegetative structures and argue that 

this pattern may be widespread for long-lived perennial plants. Flower blooming in 

subalpine meadows has been observed to shift away from the mid-growing season to 

an earlier flower blooming state, potentially increasing frost exposure and negatively 

affecting pollinators, birds, and animals dependent on these plants for food (Aldridge 

et al. 2011). Plants and pollinators of spruce-fir ecosystems are likely to be sensitive to 

variability in freeze dates.
96 USDA Forest Service RMRS-GTR-376. 2018

Sensitivity to Wind

Mature Engelmann spruce and subalpine fir tree mortality caused by windthrow 

and windstorms is historically common, and attributed to shallow root systems and 

shallow, poorly drained soils (Alexander 1987). The impact of wind events on subalpine 

forests was reported for a 1934 wind event (Kulakowski and Veblen 2003), a large 

blowdown event in northern Colorado (Kulakowski and Veblen 2002), and a small 

wind event in Rocky Mountain National Park (Veblen et al. 1989). Topography strongly 

influences the impact of the wind events; trees on ridgetops, ridge saddles, or moderate to steep south- and west-facing slopes (Alexander 1987), or at higher elevations 

and on east-facing slopes (Kulakowski and Veblen 2002) may be most sensitive to 

wind. Younger stands are less sensitive to wind events (Kulakowski and Veblen 2002). 

Advance regeneration in the understory of a lodgepole pine forest impacted by wind 

shifted the forest dominants to spruce and fir (Veblen et al. 1989). Although strong and 

frequent winds are common in the Rocky Mountain subalpine (Alexander 1987), no 

change, or a reduction in extreme wind compared to the late 20th century, is projected 

for the western United States (Pryor et al. 2012). Given this information, a moderate 

vulnerability ranking is assigned.

Intrinsic Adaptive Capacity to Climate Change_______________________

BOX 5.5

Key Vulnerabilities

A moderate vulnerability ranking is given for the intrinsic adaptive capacity of subalpine spruce-fir ecosystems to 

climate change. 

• Factors that strengthen adaptive capacity:

○ Topographic positions in moister valley bottoms, and a higher percentage on north-facing slopes, 

enhance the ability of spruce-fir ecosystems to adapt to a warmer and drier climate.

○ Different life history strategies of Engelmann spruce and subalpine fir create flexibility for transitions in 

community composition under future climate variation. 

○ Wildlife species have adapted to the cold- and snow-dominated subalpine climate.

• Factors that weaken adaptive capacity:

○ Dominant tree species need a long time to recover from disturbances; warming temperatures and drier 

conditions, and increases in wildfire, could affect successful regeneration. 

○ The potential for invasive species may stress native understory plants. 

Factors That Strengthen Adaptive Capacity to Climate Change

Because the subalpine zone is one of great topographic and consequently environmental variation, these forests are highly variable in composition and structure (Romme 

et al. 2009a). In the Rocky Mountain Region, spruce-fir forests occur in the following 

proportions: 34 percent on north-facing slopes, 25 percent on west-facing slopes, 22 

percent on east-facing slopes, and 19 percent on south-facing slopes (derived from 

Comer et al. [2003] GIS data). North-facing slopes may be more buffered from warming 

and drying (Hinckley et al. 2014). Subalpine spruce-fir forests can occur in sheltered ravines and valley bottoms where moist conditions exist and fire is less frequent (Romme 
USDA Forest Service RMRS-GTR-376. 2018 97

and Knight 1981; Veblen et al. 1994). In some cases, these wetter subalpine areas may 

mitigate negative effects from a warmer and drier climate. 

Life history traits of the dominant tree species facilitate adaptive responses. 

Coexistence of Engelmann spruce and subalpine fir is a function of their different 

life history strategies: Subalpine fir has high fecundity and shorter longevity whereas 

Engelmann spruce has longer survival (adult survivorship) (Smith et al. 2015; Veblen 

1986). Shade tolerance of spruce and fir may be advantageous in some settings. 

Researchers found mortality events in mixed-species stands can accelerate a shift from 

lodgepole pine to spruce-fir forests, as the understory in these mixed-species stands 

often includes a component of spruce and fir regeneration (Veblen et al. 1989). 

Animals of spruce-fir ecosystems have adaptive characteristics that enhance their 

ability to exist in the snow-dominated subalpine ecosystem. These ecosystems are likely 

to remain cold and snow dominated under climate change although with some snowpack losses (Christensen and Lettenmaier 2006; Regonda et al. 2005). Squirrels cache 

food to sustain themselves through the winter (Sullivan 1995b). American martens, 

well adapted to snow, use it as a protection against cold (Stone 2010). Snowshoe hares 

develop white winter coats to reduce risk of predation (Sullivan 1995a). 

Factors That Weaken Adaptive Capacity to Climate Change

A factor contributing to lower adaptive capacity of spruce-fir ecosystems is the long 

recovery times of spruce and fir trees after disturbance; it can take centuries to develop 

mature stands (Aplet et al. 1988; Veblen et al. 1989). Reestablishment is slow as a thick 

understory layer often develops before trees can establish, requiring centuries before sufficient tree regeneration forms a closed-canopy forest (Peet 1981; Turner et al. 1998). Cone 

production begins after trees are 20 to 40 years old, and seeds are distributed primarily by 

wind and gravity (Alexander and Shepperd 1990; Alexander et al. 1990). 

Coop et al. (2010) also found decreased regeneration of spruce and fir in burned 

areas as a function of elevation and distance from unburned edges. Increases in fire 

extent or fire at high elevations will result in delayed recovery. Although information is 

available on genetic adaptation of spruce and fir to a changing climate, it is unlikely that 

tree adaptation will match the rate of warming in the future. Thus, trees will have a low 

capacity to survive in higher temperatures or moisture-limited conditions. 

Although high elevation areas have minimal invasive or nonnative species, climate 

change coupled with human influences could provide new habitat and dispersal mechanisms (Pauchard et al. 2009). West et al. (2015) modeled future habitat for cheatgrass under 

an ensemble of six future climate scenarios. Habitable area for this species in the Rocky 

Mountain National Park is projected to expand to four times the current area by 2050.

Dependence on Specific Hydrological Regime_______________________

BOX 5.6

Key Vulnerabilities

A very low vulnerability ranking is given for dependence of subalpine spruce-fir ecosystems on a specific 

hydrological regime. These systems are dependent on snowpack, but their high elevation location may mitigate 

snowpack changes. 
98 USDA Forest Service RMRS-GTR-376. 2018

Subalpine spruce-fir ecosystems are not dependent on a specific and narrow hydrological regime that has little variability from year to year. Precipitation is variable, and 

primarily comes as snow. Snowpack in the subalpine zone acts as a reservoir for water 

within that zone as well as for areas downstream. Studies project that snowpack loss as a 

result of warmer temperatures will be less in the high-elevation Rocky Mountain subalpine compared to lower elevations (Christensen and Lettenmaier 2006; Lukas et al. 2014).

Even though the vulnerability ranking for this criterion is given as very low, 

subalpine spruce-fir ecosystems and the dominant trees, in particular, are dependent on 

snowpack. A 9-year study in Colorado found that years with longer growing seasons 

were correlated with shallower snowpacks and an earlier start of spring (Hu et al. 2010). 

For these 9 years, the longer growing season was associated with an earlier start to 

spring, not a lengthening of the fall growing period. The dominant subalpine tree species, lodgepole pine, Engelmann spruce, and subalpine fir, were very dependent on the 

snowmelt water in the soil, even late into the growing season. Most importantly, summer rains failed to compensate for years with earlier springs and shallower snowpacks. 

The researchers suggested that the lower reliance on summer rains is a function of the 

tree roots going deeper in the soil for a more consistent source of water and that summer 

rain events are often too small to penetrate deep into the soils (Hu et al. 2010). Projected 

changes in snowpack will impact spruce-fir ecosystems.

Likelihood of Managing Climate Change Effects______________________

BOX 5.7

Key Vulnerabilities

A very high vulnerability ranking is given for mitigation of climate change effects on spruce-fir ecoystems. 

• Spruce beetle management strategies exist for prevention, suppression, and restoration of beetle-impacted 

spruce-fir forests. The effectiveness of these treatments varies; most studies emphasize the importance of 

managing for regeneration as part of the treatment strategy.

• No techniques have been shown to offset negative effects from heat, snowpack variability, shifts in freezing 

dates, and drought stress. 

• Large areas of spruce-fir forests are inaccessible, occur in complex terrain, or are in wilderness, limiting 

potential management options. 

Management options for mitigating the effects of climate change in spruce-fir 

forests include reducing current stressors, enhancing ecosystem resilience and, as climate continues to change, helping plants and animals adapt without substantial loss of 

soil, soil nutrients, and plant cover (Millar et al. 2007). In response to the recent widespread outbreaks of spruce beetle, several research studies have evaluated silvicultural 

techniques in the Rocky Mountain Region as well as across the western United States. 

Spruce beetle management strategies include prevention to enhance stand diversity 

and resilience before an outbreak, suppression during an outbreak to reduce population 

levels and the rate of insect spread, and restoration activities to reestablish vegetation 

and promote the long-term resilience of forests to insects and diseases (Alexander 1987; 

Jacobs 2012; Jenkins et al. 2014). 
USDA Forest Service RMRS-GTR-376. 2018 99

In a retrospective assessment, Hansen et al. (2010) evaluated the effectiveness 

of partial cutting to reduce spruce beetle-caused mortality across the southern Rocky 

Mountains. They conclude that partial cutting apparently results in reduced losses to 

spruce beetle but caution that spruce beetle populations are likely to infest residual 

mature trees regardless of previous treatment. Windmuller-Campione and Long (2015) 

evaluated silvicultural treatments in a spruce-fir forest type in northern Utah, using a 

resistance index and a metric for resilience (minimum amount of Engelmann spruce 

regeneration necessary to maintain a spruce component after a spruce beetle epidemic). 

They argued that resilience should be considered in active management for beetles. 

Based on these indices, they determined that shelterwood with reserves increased 

short-term resistance and long-term resilience to spruce beetle outbreaks. Temperli et 

al. (2014) reported limited effectiveness of density reduction treatments for mitigating 

stand-level beetle infestation; rather than attempt to increase resistance to the spruce 

beetle, they recommended increasing spruce advance regeneration in the understory, 

thereby enhancing ecosystem resilience. The degree of protection to residual spruce 

following treatment is influenced by insect population pressure (Hansen et al. 2010). 

Large and older trees can die under such conditions whether stands were thinned or not 

(Black et al. 2013; Romme et al. 2006). DeRose and Long (2014) offered a framework 

for evaluating silvicultural options for resistance and resilience to spruce beetle (table 

5.1). The importance of managing for regeneration was emphasized in most studies. We 

Table 5.1—Conceptual framework of stand and landscape resistance and resilience to disturbance (DeRose and Long 2014).

Resistance Resiliencea

Stand Influence of structure and composition on disturbance 

severity 

Wildfire: influence of structure and composition on 

the severity of fire behavior 

Spruce beetle: influence of structure and composition 

on the severity of spruce mortality due to high beetle 

population levels arising from within the stand 

Influence of disturbance on subsequent structure 

and composition 

Wildfireb : Influence of fire on subsequent 

structure and composition 

Spruce beetlec : Influence of spruce beetle 

infestation on subsequent structure and 

composition 

Landscape Influence of structure and composition on the spread 

of disturbance 

Wildfire: influence of multistand structure and 

composition on the spread of fire 

Spruce beetle: influence of multistand structure and 

composition on the severity of spruce mortality due 

to the transition from endemic to epidemic beetle 

populations 

Influence of disturbance on subsequent forest 

structure and composition

Wildfire: Influence of fire on subsequent 

proportion of landscape age classes and species 

dominance 

Spruce beetle: Influence of spruce beetle 

outbreak on proportion of landscape age classes 

and spruce-dominated stands

a Structural and compositional indicators of stand and landscape resilience are a function of management goals relating to desired 

conditions in a specified period after a disturbance, for example, immediately after the disturbance or longer term. Desired conditions 

need not be limited to live trees and may include important ecosystem attributes such as snags, coarse woody debris, or decadent 

crowns. 

b Indicators of stand and landscape resilience to wildfire typically reflect specified reference conditions, for example, large, widely spaced 

trees of fire-tolerant species (stand) and diversity of successional stages (landscape).

c Indicators of stand and landscape resilience to spruce beetle might include, e.g., surviving large trees (stand) and the potential for future 

spruce dominance (landscape).
100 USDA Forest Service RMRS-GTR-376. 2018

know of no feasible management techniques to completely mitigate the negative effects 

of drought; heat stress; earlier snowpack melt, which intensifies late season drought; and 

shifts in freezing dates, which hamper seedling and herbaceous vegetation productivity. 

Jenkins et al. (2014) concluded that the spruce beetle management recommendations of the 1970s were not sufficiently applied at the landscape scale to mitigate the 

recent outbreaks. They note that many factors influence effective management, such as 

lack of timber harvesting infrastructure in some areas of the western United States and 

cost versus benefit realities. In the Rocky Mountain Region, spruce-fir forests encompass nearly 5 million ac. In addition, large areas of these subalpine spruce-fir forests 

are inaccessible, occur in complex terrain, or are in wilderness areas. These factors also 

limit the potential to implement silvicultural treatments.

Potential for Climate Change to Exacerbate the Effects of Nonclimate 

Stressors, or Vice Versa_______________________________________

BOX 5.8

Key Vulnerabilities

Several factors exacerbate climate and nonclimate stressors in spruce-fir ecosystems. Consequently, a very high 

vulnerability ranking is given.

• Warming and drought contribute to increased insect outbreaks and fire activity. 

Bark Beetle Outbreaks and Warmer Temperatures

Engelmann spruce is the primary host for the spruce beetle. The insect typically 

completes a generation in 2 years, but high summer temperatures are correlated with 

a rising proportion of beetles that complete a generation in 1 year, thus increasing population growth and levels of spruce beetle-caused tree mortality (Bentz et al. 

2010). Tree-ring reconstructions document widespread spruce beetle outbreaks over 

the past several centuries, long before forest management practices were implemented 

in the subalpine zone (Baker and Veblen 1990; Eisenhart and Veblen 2000; Jenkins et 

al. 2014); studies document regional spruce beetle outbreaks associated with warmer 

and drier climate (Hart et al. 2014). In Colorado, for example, large areas of relatively 

even-aged spruce-fir forests resulted from widespread burning in the second half of 

the 19th century so that by the late 1900s much of the subalpine forest had reached an 

age known to be susceptible to bark beetle outbreaks (Romme et al. 2006; Sibold et al. 

2006). Outbreaks have affected spruce forests on more than 1 million ac in Colorado 

and Wyoming (Harris et al. 2013). Spruce beetle outbreaks have greatly affected stand 

structure and composition, causing mortality of Engelmann spruce, shifting dominance 

to subalpine fir, and accelerating growth of remaining trees (Veblen et al. 1991). 

Researchers project that, with greater probability of occurrence, future outbreaks will 

cause tree mortality at higher elevations, but with a large amount of variability over the 

landscape and over time (Bentz et al. 2010).
USDA Forest Service RMRS-GTR-376. 2018 101

Fire, Drought, and Warmer Temperatures

Many studies have linked warm temperatures, drought conditions, and fire in 

subalpine ecosystems. Recent temperature warming and longer fire seasons have 

contributed to an upswing in forest fire activity in the western United States since the 

mid-1980s (Westerling et al. 2014). Drought and warmer temperatures are two factors associated with increased fire occurrence in paleo records (Pierce et al. 2004). 

Drought is well documented as a primary factor controlling historical fire occurrence 

in Rocky Mountain subalpine forests (Buechling and Baker 2004; DeRose and Long 

2012; Sherriff et al. 2001; Sibold et al. 2006). Throughout the Rocky Mountain Region, 

multiple centuries of tree-ring records from subalpine forests clearly show fire occurring 

with warmer and drier conditions linked to cooler Pacific Ocean and warmer Atlantic 

Ocean sea surface temperatures. This indicates synchronized warm-drying trends on 

a subcontinental scale, and increased wildfire in these relatively cool, moist subalpine 

forests (Schoennagel et al. 2005, 2007; Sibold and Veblen 2006). 

Warmer and drier climates in the future may contribute to increased fire occurrence 

in subalpine forests of the Greater Yellowstone Ecosystem (Westerling et al. 2011). 

High elevation spruce-fir stands that have not burned in the past 400 years may be more 

susceptible to burning (Sibold et al. 2006). Researchers found stands that burned during 

the 1800s had a lower likelihood of burning during recent fires (Bigler et al. 2005). In 

some cases, compound disturbances, such as fire and wind, may favor aspen regrowth 

over spruce-fir; spruce-fir is less resilient to fire than aspen, as observed in northwestern 

Colorado (Kulakowski et al. 2012). In contrast, Higuera et al. (2014) reported that 

pollen and macroscopic charcoal from high-resolution lake-sediment records in Rocky 

Mountain National Park suggested that fire severity may be more responsive than fire 

frequency to climate change in Rocky Mountain subalpine forests. 

Summary of the Regional Vulnerability of Spruce-Fir Ecosystems_______

The overall vulnerability ranking for the ecosystem is a combination of the vulnerability to nonclimate stressors and the vulnerability to climate stressors. For spruce-fir 

ecosystems, the mean overall vulnerability ranking was moderate with mean expert 

reviewer rankings ranging from the moderate to the high category (fig. 5.2, table 5.2). 

Mean vulnerability rankings were moderate for both the nonclimate stressor vulnerability and the climate vulnerability (fig. 5.2). Expert reviewer rankings were in the 

moderate category for nonclimate vulnerability and the moderate or high category for 

climate vulnerability. 

The expert reviewers had high overall confidence in the ranking of the overall regional vulnerability (fig. 5.3). The five experts’ individual rankings of overall confidence 

ranged from the moderate to the very high category (table 5.3). Mean confidence was 

high for the nonclimate vulnerability ranking, but individual confidence rankings ranged 

from the moderate to the very high category (fig. 5.3a). Mean confidence was also high 

for the climate vulnerability ranking with individual rankings ranging from the moderate 

to the very high category (fig. 5.3b). 
102 USDA Forest Service RMRS-GTR-376. 2018

Figure 5.2—Overall regional vulnerability ranking and 

confidence ranking for spruce-fir ecosystems in the 

Rocky Mountain Region. The large arrow points to the 

mean score for vulnerability and for confidence. Bar 

represents the range of scores.

Table 5.2—Original and reviewer rankings for the regional vulnerability assessment of subalpine spruce-fir ecosystems in the 

Rocky Mountain Region. Underline indicates a reviewer score change from the original assigned score.

Criterion 

Original 

score

(Reviewer 1)

Score

(Reviewer 2)

Score

(Reviewer 3)

Score

(Reviewer 4)

Score

(Reviewer 5)

Score

Reviewer 

criterion

rank mean

Nonclimate 

1. Extent 3 3 3 3 3 3 Moderate

2. Human influences 3 3 3 3 3 3 Moderate

3. Resilience 3 3 3 3 3 3 Moderate

4. Future trends 3 3 3 3 3 3 Moderate

Total 12

Moderate

12

Moderate 

12

Moderate 

12

Moderate

12

Moderate

12

Moderate

12

Moderate

Climate

1. Ecosystem shift 3 3 3 3 3 3 Moderate

2. Species groups 3 4 3 3 3 3 Moderate

3. Climatic events 3 3 4 3 3 3 Moderate

4. Adaptive capacity 3 3 2 3 3 3 Moderate

5. Hydrology 1 1 1 3 1 1 Very low

6. Management 5 5 5 5 5 5 Very high

7. Interactions 3 5 5 5 5 3 Very high

Total 21

Moderate

24

High 

23

Moderate

25 

High

23

Moderate 

21

Moderate

23

Moderate

Overall vulnerability 

rank

17

Moderate 

18

Moderate 

18

Moderate 

19 

High

18

Moderate

17

 Moderate 

18

Moderate
USDA Forest Service RMRS-GTR-376. 2018 103

Figure 5.3—Regional vulnerability of spruce-fir ecosystems to nonclimate (a) and climate (b) stressors in the Rocky Mountain 

Region. The large arrow points to the mean score for vulnerability and for confidence. Bar represents the range of scores.

a) b)

Table 5.3—Reviewer rankings for confidence in the vulnerability assessment of subalpine spruce-fir ecosystems in the 

Rocky Mountain Region. 

Criterion 

(Reviewer 1)

Score

(Reviewer 2)

Score

(Reviewer 3)

Score

(Reviewer 4)

Score

(Reviewer 5)

Score

Criterion rank 

confidence 

mean

Nonclimate

1. Extent 5 3 5 3 5 High

2. Human influences 5 5 5 3 5 Very high

3. Resilience 5 5 5 3 5 Very high

4. Future trends 3 3 5 1 5 Moderate

Total 18

Very high 

16

High

20

Very high

10

Moderate 

20

Very high

17

High

Climate

1. Ecosystem shift 5 3 5 3 5 High

2. Species groups 1 3 5 3 5 Moderate

3. Climatic events 3 3 5 3 5 High

4. Adaptive capacity 5 2 5 1 5 High

5. Hydrology 5 5 1 3 5 High

6. Management 5 5 5 5 5 Very high

7. Interactions 1 4 1 5 5 Moderate

Total 25

High

25

High

27

High 

23

Moderate

35 

Very high

27

High

Overall confidence 

rank

22

High 

21

High 

24

High 

17

Moderate

28

Very high

22

High
104 USDA Forest Service RMRS-GTR-376. 2018

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Chapter 6. Low-Gradient Mountain Stream 

Reaches: Vulnerability to Nonclimate and Climate 

Stressors in the U.S. Forest Service Rocky 

Mountain Region

Janine R. Rice, Dave Winters, Claudia Regan, Rick Truex, and Linda A. Joyce

Quick Look: Low-Gradient Mountain Stream Reaches in the Rocky Mountain Region

Low-gradient mountain stream reaches can be found in headwater alpine areas and downstream through subalpine, montane, and foothill zones in five river basins in the Rocky 

Mountain Region: Colorado, Rio Grande, Arkansas, Lower Missouri, and Upper Missouri. For 

this regional assessment, we include the stream reach as well as aquatic and riparian plants and 

animals that live there. Low-gradient mountain stream reaches typically have relatively broad 

valley bottoms and increased residence time of water, sediment, and organic matter as a result 

of the sinuous or multi-thread channels and extensive riparian zones and floodplains. Lowgradient stream reaches are also generally dominated by complex and diverse habitats, given 

the relative abundance and range of water and soil conditions. Cattle and wildlife prefer these 

areas for grazing, people enjoy recreational activities in and near them, and development tends 

to be focused adjacent to them. 

Quick Look: Vulnerability of Low-Gradient Mountain Stream Reaches in the Rocky 

Mountain Region

Vulnerability to nonclimate and climate stressors: Very high

Confidence: High

Exposure: Warming stream and soil temperatures; potentially earlier local snowmelt. Greater 

frequency of disturbances (fire, flooding) could increase debris flow. 

Current extent: Limited; 11 percent of all stream lengths on national forest lands in the Rocky 

Mountain Region. 

Sensitivity and adaptive capacity to climate change: Low-elevation low-gradient streams may 

warm more than high-elevation low-gradient streams. The warming of low elevation streams 

may reduce the diversity of associated aquatic species. Low-gradient reaches in higher 

elevation streams may become more productive as stream temperatures increase. However, 

these high-elevation stream networks are smaller and habitats are typically more limited. 

Riparian plant communities may shift in elevation as temperature and streamflow conditions 

change, but species in high-elevation riparian areas have limited options for upstream migration. Beaver, a keystone species, can tolerate wide ranges of water and air temperature and 

therefore is likely to persist. However, cold-water native fish, amphibians, benthic macroinvertebrates, and riparian plants are highly vulnerable to climate change. 

Nonclimate stressors: Many stream reaches have not recovered from the legacy of human activities during the mid-1800s to early 1900s. Current stressors include recreation, wildlife and 

agricultural grazing, roads, and other land use development. These stressors can constrict 

streamflows, reduce channel size, contribute to pollution, and introduce invasive species. 
USDA Forest Service RMRS-GTR-376. 2018 113

Introduction___________________________________________________

We focus this assessment on low-gradient mountain stream reaches that flow 

through national forests in the Forest Service, U.S. Department of Agriculture (USFS) 

Rocky Mountain Region. We define low-gradient mountain stream reaches as those 

streams with slopes of less than 2 percent (Montgomery and Buffington 1997; Wohl 

and Merritt 2005). These stream reaches occur in the mountainous areas of the following drainage basins in the Rocky Mountain Region: Colorado, Rio Grande, Arkansas, 

Lower Missouri, and Upper Missouri (fig. 6.1). This assessment examines the vulnerability of the physical and hydrological characteristics of the stream reach as well as 

the vulnerability of the aquatic and riparian flora and fauna associated with the stream 

reach. Hereafter, we refer to this ecosystem as “low-gradient mountain stream reaches.”

Stream gradient influences the energy available to transport sediment and modify 

channel and floodplain dynamics (Buffington and Montgomery 2013; Wohl and Merritt 

2005). Steep-gradient stream channels are typically armored with large rocks and 

boulders and can rapidly transport water and sediment downstream with relatively small 

changes in their channels (Wohl and Merritt 2005). Segments of streams that are transport-limited sediment storage sites are referred to as “source segments.” These reaches 

undergo intermittent debris-flow scour (i.e., colluvial channels) (Montgomery and 

Buffington 1997). Low-gradient stream channels, referred to as “response segments,” 

can have small to large changes in channel morphology depending on the increase in 

sediment supply. Consequently, low-gradient reaches in the mountainous watersheds 

are more likely to exhibit changes in floodplain and channel geometry in response to 

upstream and within-reach activities that alter water and sediment inputs (Wohl et al. 

2017). 

In the Rocky Mountain Region, low-gradient stream reaches are found in alpine 

areas at the highest elevations (>13,000 ft) and down through the subalpine, montane, 

and foothills at the lowest elevations (3,000 ft). Small alluvium from upstream typically composes the streambanks. Pool-riffle and plane-bed channels occur in these 

low-gradient stream reaches, whereas step-pool and cascade channels occur on steeper 

slopes (Montgomery and Buffington 1997). Riparian vegetation affects bank cohesion, channel width, and channel pattern in these low-gradient streams (Buffington and 

Montgomery 2013). Root strength contributes to channel stability; when root strength 

is diminished, channel width can widen considerably. Habitat can be diverse with high 

productivity, leading to abundant terrestrial and aquatic fauna. In addition, beaver 

(Castor canadensis) activity can increase the complexity of habitat in these low-gradient 

stream reaches. Activities upstream can result in changes in the movement of water, 

sediment, and contaminants, all of which affect channel dynamics. Activities within the 

low-gradient stream reach also can alter channel geometry, the dynamics of water and 

sediment movement, contaminants in the stream, or aquatic and riparian communities 

(Wohl 2006), affecting the resilience of these ecosystems. 

Low-gradient mountain stream reaches have been assessed for their vulnerability 

to climate change. The vulnerability framework has two major components, one related 

to nonclimate stressors and one related to climate stressors. Within the nonclimate 

component, four criteria were used to define factors of vulnerability: current status 

of ecosystem extent, human influences on the ecosystem, intrinsic resilience of the 
114 USDA Forest Service RMRS-GTR-376. 2018

Figure 6.1—Low-gradient mountain stream reaches on national forest lands in the Rocky Mountain Region represent only 

about 11 percent of all stream reaches on national forests in this region. Low gradient is defined as mountain stream 

reaches with slope of less than or equal to 2 percent. (Data source: National Hydrography Dataset Plus (Horizon Systems 

Corporation n.d.); lake edges, diversions and irrigation ditches removed.) 
USDA Forest Service RMRS-GTR-376. 2018 115

BOX 6.1

Current status of ecosystem extent

A very high vulnerability ranking is given as low-gradient stream reaches have a limited extent in the Rocky 

Mountain Region.

Human influences on ecosystem

Low-gradient streams and associated habitats have a very high vulnerability to the legacy of past human activities 

and to the stressors associated with ongoing human activities.

• The arrival of European settlers had an almost immediate influence on low-gradient stream channels and 

their ecology from beaver trapping, placer mining, logging, road and railroad development, domestic livestock 

grazing, and water development and diversion. In many areas, reaches have not recovered from activities of 

the mid-1800s to early 1900s.

• Current stressors include recreation, wildlife and agricultural grazing, roads, and land use development. 

These stressors can lower streamflows, reduce channel size, contribute to pollution, and introduce and 

spread invasive species. 

Intrinsic resilience of ecosystem to nonclimate stressors

A high vulnerability ranking is given for the intrinsic resilience of low-gradient mountain streams to nonclimate 

stressors.

• Factors that enhance low-gradient stream resilience:

○ These ecosystems have stream structure complexity, high productivity, and diversity of species, 

particularly at lower elevations. 

○ Beavers can add habitat complexity. 

○ Barriers can protect native fish from competition and hybridization with nonnative species.

ecosystem to nonclimate stressors, and future trends of nonclimate stressors on the ecosystem. Within the climate change component, seven criteria capture the vulnerability 

of individual species, sensitivity of ecosystem dynamics such as dependence of the 

ecosystem on the hydrological cycle, adaptive capacity of the ecosystem, potential for 

management to mitigate the effects of climate and nonclimate stressors, and interaction 

between climate change and nonclimate stressors. More information and the rationale 

for ranking ecosystem vulnerability by using these criteria are given in appendices A, B, 

and C.

These low-gradient mountain streams are increasingly being studied throughout 

the Rocky Mountain Region, as well as in other mountainous regions globally. We draw 

from this literature where studies were conducted within the Rocky Mountain Region, 

and then in similar settings when local information is unavailable. Even within this 

Region, these streams vary greatly in geology, valley formation, and land use. Assessing 

the vulnerability of specific low-gradient mountain stream reaches in individual national 

forests will require an understanding of local site characteristics and the site characteristics of the research studies reviewed here. 

Vulnerability of Low-Gradient Mountain Stream Reaches to Nonclimate 

Stressors___________________________________________________

Summary of Key Vulnerabilities to Nonclimate Stressors
116 USDA Forest Service RMRS-GTR-376. 2018

Current Status and Human Influences______________________________

Low-gradient stream reaches are limited in extent, consisting of only 11 percent 

of all stream lengths on national forest lands in the Rocky Mountain Region (table 6.1). 

Across the Rocky Mountain Region, the Black Hills National Forest in South Dakota 

has the largest share of low-gradient mountain stream reaches (24 percent), and the Big 

Horn National Forest in Wyoming has the smallest share (2 percent). At the scale of an 

individual national forest, the Black Hills National Forest has the highest percentage of 

low-gradient stream miles as a function of all stream miles. In contrast, low-gradient 

stream miles in the Big Horn National Forest were only 5 percent of total stream miles. 

Winters et al. (2016) found that low-gradient stream systems were prevalent in only 8 

• Factors that lower low-gradient stream resilience:

○ Physical and biological function of low-gradient stream reaches is hindered by a variety of historical and 

current activities including water management and changes in runoff timing from dust-on-snow events 

that reduce late season flow.

○ Low-gradient streams are also the response areas of the stream network. As such, they can be greatly 

affected by increased disturbances upstream, such as fire, which increases water flow or sediment 

deposition, or both.

○ Constructed barriers can fragment streams, preventing aquatic species movement.

○ Nonnative fish and pathogens can hinder native species.

Future trends of nonclimate stressors

Low-gradient streams are considered to be very highly vulnerable to future stressors associated with human 

population growth and related effects on land use, water quality, air quality, increased demand for recreation, 

increased fire, and the spread of invasive species. 

Table 6.1—Low-gradient mountain stream reach and total stream miles by national forest in the Rocky Mountain Region, 

given as miles within each national forest, and as a percent of all stream miles within the national forest, and as 

percent of all low-gradient stream miles in the Rocky Mountain Region. Data Source: NHDPlus data set (lake edges, 

diversions and irrigation ditches removed).

National Forest

Low-gradient 

stream miles

Total stream 

miles

Low-gradient stream 

miles as percentage 

of all stream miles in 

National Forest

Percentage of Rocky 

Mountain Region lowgradient stream miles

Black Hills 851 2,826 30 24

Medicine Bow-Routt 536 4,150 13 15

San Juan 454 2,998 15 13

Shoshone 341 4,222 8 9

Grand Mesa, Uncompahgre 

and Gunnison

327 4,332 8 9

Pike-San Isabel 286 3,504 8 8

Rio Grande 259 3,037 9 7

Arapaho-Roosevelt 236 2,792 8 7

White River 226 3,734 6 6

Big Horn 66 1,379 5 2

Rocky Mountain Region 3,582 32,974 n.a. 11
USDA Forest Service RMRS-GTR-376. 2018 117

of 162 subwatersheds on the Rio Grande National Forest in Colorado, a reflection of 

the dominance of steep topography in the southern Rocky Mountains landscape. At the 

regional scale, low-gradient streams, approximately 3,582 mi, span the elevational gradient from the alpine to the foothills zone. 

The arrival of European settlers in the Rocky Mountains had an almost immediate 

influence on low-gradient stream channels and their ecology. Beaver trapping removed 

most beaver from Rocky Mountain streams by the mid-1800s (Wohl 2006), reducing 

the influence of beavers on low-gradient stream habitats (Smith and Tyers 2012). Placer 

mining for gold and other precious metals was a common practice starting in the mid1800s. Mining increased stream sediment movement, altered stream and riparian health, 

and released toxic materials into stream channels (Wohl 2006). As human population 

increased in the late 1800s and early 1900s, logging, tie-driving, and road and railroad 

building altered channel shape and the supply of water and sediment, and destabilized 

slopes (Wohl 2006). Year-round logging practices removed trees from upland as well 

as riparian areas, primarily for railroad ties. Riparian trees trap sediment during floods, 

provide habitat for a variety of wildlife and fish species, and generally contribute to 

heterogeneous riparian habitats. Harvested wood was stored on floodplains and within 

stream channels so that high spring flows could transport wood loads downstream, a 

practice called tie-driving (Ruffing et al. 2015). Road and other transportation infrastructure development straightened the meanders of streams and promoted continued 

development in valleys, thus reducing floodplain extent. The introduction of domestic 

livestock grazing in these ecosystems altered the riparian communities (Wohl 2006). 

Human activities from the mid-1800s to the mid-1900s have left a legacy such 

that aquatic species and channel functions in low-gradient streams are still recovering 

from their effects. In a recent study on the Medicine Bow National Forest in Wyoming, 

tie-driven streams had a more simplified channel morphology; narrower, shallower low 

cross-sectional roughness; and higher width-to-depth ratios than streams that were not 

tie-driven (Ruffing et al. 2015). Wood loading in these streams is still in the early stage 

of recovery. Fornwalt et al. (2009) concluded that settlement-era logging and grazing 

had a greater long-term influence on riparian understories of ponderosa pine (Pinus ponderosa Lawson & C. Lawson) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) 

forests than on the understories of upland ponderosa pine and Douglas-fir forests. 

Grazing can reduce riparian vegetation, compact soils, alter species communities, destabilize channels, and introduce noxious weeds (Kauffmann and Krueger 1984). Cannon 

and Knopf (1984) found that heavy grazing during summer seasons in the early 20th

century reduced shrub density and narrowed the width of riparian areas in higher elevations of Colorado; such effects can last decades after grazing intensity has been reduced.

Wohl (2006) ranks the impact of human activities to all mountain streams in 

the Rocky Mountains (from Montana through New Mexico) based on several threats. 

The impact on all mountain streams was ranked as severe from activities that regulate 

water flow, such as construction of dams and water diversion. Stream and trans-basin 

diversions for agricultural, municipal, and recreational uses alter physical processes 

and can harm aquatic communities. Diversions reduce sediment movement, increase 

downstream erosion, and alter riparian plant composition (Caskey et al. 2015). Unless 

modified for fish passage, diversions can create migration barriers that fragment aquatic 

habitats but also may restrict movement upstream of nonnative species. 
118 USDA Forest Service RMRS-GTR-376. 2018

The Rocky Mountain Region has 910 dams with 146 occurring on national 

forests and grasslands, 127 of which are in Colorado (U.S. Geological Survey 2005). 

Reservoirs and dams regulate flow and flooding, are used for power generation, and 

make water available for municipal and agricultural uses throughout the year. Flow 

regulation by dams can harm downstream hydrology by: 

• reducing meanders; 

• reducing sediment transport; 

• modifying flows; 

• increasing erosion and downcutting of stream beds; 

• altering composition and abundance of bottom-dwelling communities; 

• narrowing channels; 

• modifying algae presence, and populations of macroinvertebrates and fish; and 

• increasing invasive species (Baker et al. 2011; Merritt and Wohl 2006; Ward and 

Stanford 1982; Wohl and Cenderelli 2000).

Impacts on the biotic integrity of all mountain streams in the Rocky Mountains 

were ranked by Wohl (2006) as moderate to limited based on the current presence of 

nonnative species, percentage of at-risk native species, status of endemic species, and 

comparisons with measures of original versus current biodiversity. Aquatic nonnative 

and invasive species have been intentionally or unintentionally introduced to many 

aquatic ecosystems in the Rocky Mountain Region. These species have caused reductions in native populations, altered streamflows and nutrient cycling, and disrupted 

food webs (Rahel et al. 2008; Stromberg et al. 2007). Cutthroat trout (Oncorhynchus 

clarkii) populations have declined, as rainbow trout (O. mykiss) breed with native species (Gresswell 2011). Nonnative brook (Salvelinus fontinalis), brown (Salmo trutta), 

and rainbow trout can hinder and outcompete native cutthroats (Van Kirk et al. 2009). 

Leafy spurge (Euphorbia esula L.), purple loosestrife (Lythrum salicaria), cheatgrass 

(Bromus tectorum L.), and Canada thistle (Cirsium arvense) are particularly problematic 

in riparian areas, where they displace native species (Al-Chokhachy et al. 2013b; Poff et 

al. 2011).

Though a native species, the freshwater diatom Didymosphenia geminata can be 

a nuisance during blooms in low productivity streams (Elwell et al. 2014; Spaulding 

and Elwell 2007). This diatom has been reported in various locations in the western 

United States (Spaulding and Elwell 2007) and in the Rocky Mountain Region, 

notably in Rapid Creek, South Dakota (James et al. 2010). Research in Rapid Creek 

documented the role of this diatom in shifting the invertebrate species composition 

(James et al. 2010) and supplementing the diet of brown trout (James and Chipps 2016). 

Management recommendations have focused on preventing its spread (Elwell et al. 

2014).

Impacts associated with water pollution were seen as moderate to limited based 

on the presence of organic (human and animal) wastes, nutrients (fertilizers), organochlorine compounds (herbicides, pesticides, industrial chemicals), and heavy metals 

(Wohl 2006). Increased atmospheric nitrogen deposition from human activities has been 

observed in the snowpack and in high elevation lakes of the Rocky Mountain Region 

(Baron et al. 2000b; Ingersoll et al. 2008; Nanus et al. 2012; Wolfe et al. 2001). 
USDA Forest Service RMRS-GTR-376. 2018 119

Impacts associated with channel alteration to mountain streams were ranked as 

moderate to limited depending on the presence of in-channel structures, channelization, levees, and beavers, and the occurrence of log drives and placer and aggregate 

mining (Wohl 2006). Land use impacts on mountain streams have been locally intensive 

through minimal or extensive deforestation and grazing. 

Roads are sources of sediment and contaminants; they can alter drainage patterns 

in watersheds, increase surface runoff from soil compaction, remove riparian vegetation, 

and facilitate the spread of invasive plant species (Mortensen et al. 2009). They can 

also block aquatic passage with structures such as culverts, thus fragmenting habitats 

(Forman et al. 2003). Populations of native fish species, some of which prefer low-gradient reaches (e.g., roundtail chub [Gila robusta] or flannelmouth sucker [Catostomus 

latipinnis]), have declined as a result of more-intense land use and higher road density 

in the Upper Colorado Basin (Dauwalter et al. 2011). Recreational demands have 

increased in the Rocky Mountains and such demands can affect low-gradient streams 

through water withdrawals for snowmaking, soil compaction and increased surface 

runoff from parking lots, and concentrated use of riparian areas, all of which result in 

reduced bank stability and degradation of riparian plant communities (Winters et al. 

2004; Wohl 2006).

Intrinsic Resilience of Ecosystem to Nonclimate Stressors_____________

Factors That Enhance Resilience to Nonclimate Stressors

Resilience of low-gradient stream reaches to nonclimate stressors is enhanced 

by stream structure complexity, high productivity, and diversity of species, especially 

at low elevations. Natural disturbances such as fire and landslides are infrequent and 

low-gradient streams can be resilient, in the long term, to these disturbances. Riparian 

areas along low-gradient reaches generally burn less severely and less frequently than 

upland areas (Dwire and Kaufman 2003). Harm from fire on these stream reaches is 

temporary, and, under natural conditions, most riparian areas recover quickly (Dwire 

and Kaufmann 2003), although few studies have documented postfire riparian recovery. 

Degraded water quality from erosion after fire eventually diminishes (see figure 1 

in Goode et al. 2012). Additionally, these stream reaches can have extensive willow 

(Salix spp.) communities, and may support beavers, which add habitat complexity by 

building dams that keep water and sediment in these areas (Polvi and Wohl 2012). 

Rocky Mountain streams can have barriers that protect native fish living upstream from 

competition or hybridization as nonnative species are blocked from migrating upstream 

(Gresswell 2011). 

Factors That Lower Resilience to Nonclimate Stressors 

Low-gradient stream processes and aquatic and riparian species are not resilient to 

long-term stressors that alter hydrology of low-gradient systems. Diversions for human 

uses have lowered water tables and streamflow and have altered physical processes 

and riparian vegetation (Caskey et al. 2015; Wohl 2006). Ground-disturbing activities 

and drought to the west of the Rocky Mountain Region have contributed to increased 
120 USDA Forest Service RMRS-GTR-376. 2018

incidence of dust-on-snow events; these occurrences increase snowmelt, and have 

shifted the timing of runoff up to 3 weeks earlier (Painter et al. 2010).

Depending on geological and watershed conditions, water quality may be harmed 

by greater hillslope erosion and sediment transport associated with increased fire 

(Goode et al. 2012; Ryan et al. 2011; Smith et al. 2011; Wondzell and King 2003). 

Low-gradient stream reaches in the Rocky Mountain Region that occur in areas with 

noncalcareous geology may be sensitive to added nutrients and disturbances that change 

sediment supply (Wohl et al. 2007). 

Fish and other aquatic species are not resilient to many other nonclimate stressors. 

Natural and constructed barriers that are distributed throughout Rocky Mountain Region 

streams limit migration of aquatic species (Gresswell 2011). Native fish are not resilient 

to nonnative fish that outcompete or hybridize with them; lack of resilience contributes 

to the decline of native populations in Rocky Mountain Region streams (Dunham et al. 

2004; Gresswell 2011). Aquatic species are at risk from pathogens and disease that kill 

them. Chytrid fungus (Batrachochytrium dendrobatidis), for example, kills boreal toads 

(Anaxyrus boreas) (Muths et al. 2008). Whirling disease (Myxobolus cerebralis), introduced by humans from Europe, affects fish species, causing skeletal and neurological 

damage (Bartholomew and Reno 2002). 

Future Trends of Nonclimate Stressors_____________________________

Future human population growth will increase the demand for drinking water as 

well as for water associated with land uses such as agriculture and energy. Using projections of population and climate from the 2010 Resources Planning Act Assessment, Foti 

et al. (2012) reported that increased population growth and potential changes in climate 

increase the vulnerability of water supply in several Rocky Mountain river basins. These 

analyses did not consider management to specifically address water use efficiency or 

any climate change mitigation actions. Expanding human populations in the Rocky 

Mountain Region are likely to increase the wildland-urban interface. Given the right 

fire weather conditions and an ignition, Haas et al. (2015) found wildfires could have 

high potential to impact human populations along the Front Range of Colorado. In this 

area, they also reported that overall, private ignitions have the potential to impact more 

people than Federal ignitions. Population growth, land use development and energy 

development, and the associated air and water quality challenges are stressors that will 

continue to affect low-gradient mountain streams.

Some populations of invasive plant species, such as cheatgrass, spotted knapweed 

(Centaurea biebersteinii DC), and Canada thistle, may expand (Al-Chokhachy et al. 

2013b; Bradley et al. 2009). Additional introductions may occur, and the spread of 

invasive species may be exacerbated by roads, changes in land use, fire, grazing, and 

agriculture (Al-Chokhachy et al. 2013b; Bradley et al. 2010), as well as flow changes 

in streams (Merritt and Wohl 2006). The population of native fish species may continue 

to decline, and local extinction may happen due to competition and hybridization with 

nonnative populations (Wenger et al. 2011).
USDA Forest Service RMRS-GTR-376. 2018 121

BOX 6.2

Key Vulnerabilities

A very high vulnerability ranking is given for the capacity for network shift as low-gradient stream reaches and 

their associated floodplains have a constrained capacity for shifting network ranges. 

• Range shift potential for species associated with low-gradient streams will be influenced by the rate and 

degree to which these streams warm and develop unsuitable habitat conditions.

• At higher elevations, stream reaches may become more hospitable with warming temperatures. However, 

these stream networks are smaller and more geomorphically restricted, providing limited habitat as climate 

warms. 

• Warming temperatures, particularly in the mid- and late growing seasons, and reduced flows may fragment 

individual stream reaches, restricting movement for fish in the stream network.

Vulnerability of Low-Gradient Mountain Stream Reaches to Climate 

Stressors___________________________________________________

Capacity for Network Shift

Elevation Shift Potential for Stream Network Habitats

Low-gradient mountain stream reaches occur as part of a larger network within 

each major river system. They span elevations from 3,000 ft to more than 13,000 ft 

within the Rocky Mountain Region. The highest concentration of low-gradient stream 

miles is found between elevations of about 7,500 and 9,500 ft (fig. 6.2). 

Range shift potential for species associated with low-gradient streams will be 

influenced by the rate at which streams develop unsuitable conditions for species, 

particularly at lower elevations (cold-water fish, Rahel et al. 1996; benthic macroinvertebrates, Poff et al. 2010), as well as by the rate at which high-elevation stream reaches 

become more hospitable. The potential for elevational or latitudinal shifts for species 

ranges in the low-gradient mountain stream reaches is limited. Researchers project habitat ranges will contract for native cold-water fish as well as nonnative rainbow, brook, 

and brown trout by the 2040s (Luce et al. 2012; Wenger et al. 2011). Higher elevation 

streams that are currently too cold to support fish species may warm and provide new 

habitats as temperatures rise (Cooney et al. 2005; Isaak et al. 2015; Roberts et al. 2013). 

But tributaries of these high elevation streams are less likely to support cold-water trout 

populations; these streams offer less stream habitat, which limits the genetic and species 

diversity of the fish (Cook et al. 2010; Hilderbrand 2003). 

Some low-gradient streamside plants may be able to migrate to more climatically suitable upstream or upslope areas and more northern latitudes by wind or animal 

dispersal (Perry et al. 2012). However, aquatic organisms have limited mobility for 

latitudinal shifts and may be constrained in upstream migration by natural and built 

barriers. Range expansion by plant species in highest elevations may be constrained 

as available upslope area is limited (Perry et al. 2012). Downstream drift, which is the 

primary way in which seeds are dispersed over long distances (Merritt and Wohl 2002), 

would not assist upstream migration of riparian plants.
122 USDA Forest Service RMRS-GTR-376. 2018

Fragmentation 

Increased fragmentation of low-gradient stream reaches associated with changes 

in climate will limit range shifts in response to climate change. Fragmentation can be 

considered at the basin-wide or the reach scale. At the basin-wide scale, fragmentation 

can result from natural or constructed barriers, such as waterfalls, steep slopes, roads, 

and water diversion structures that segment the relatively flat valley bottoms and further 

fragment low-gradient stream habitats. At the reach scale, fragmentation can occur 

through disturbances (hillside erosion), but also through changes in temperature and 

streamflow.

The presence of nonnative species can fragment or restrict the current range of 

native fish species. Large, connected stream corridors allow aquatic and semiaquatic 

species to move and track environmental changes as well as allow for genetic exchange 

(Rahel et al. 2008). Fragmentation can occur within a reach through disturbances such 

as wildfire and flooding or high flows that transport sediment (Dunham et al. 2003). As 

population abundance of native trout is sensitive to stream fragment length (Roberts et 

al. 2013), the creation of shorter fragments reduces area for cold-water fisheries. 

With climate change, low-gradient streams can become fragmented for cold-water 

aquatic species by warmer temperatures (Isaak et al. 2010) or changes in the timing and 

magnitude of streamflow (Jones et al. 2013; Lake 2000; Roberts et al. 2013). Earlier 

snowmelt, which lowers late season streamflow, and drought can also fragment and 

decrease habitat (Rahel et al. 1996; Wenger et al. 2011). Increased fragmentation may 

result in loss of connections between riparian and aquatic species and among their populations, reduce aquatic habitat, and restrict migration and survival of aquatic species 

(Fischer and Kummer 2000; Lake 2000; Leppi et al. 2012; Rahel et al. 1996; Rieman 

et al. 2007; Perry et al. 2012). Although higher elevation streams may become thermal 

refugia for some species (Isaak et al. 2015), higher summer temperatures may thermally 

fragment and isolate these smaller tributaries (Isaak et al. 2015; Jones et al. 2013; Rahel 

et al. 1996). 

Figure 6.2—Low-gradient 

mountain stream reach 

miles by elevation 

category on Rocky 

Mountain Region National 

Forest lands. Low gradient 

is defined as mountain 

stream reaches with slopes 

of less than or equal to 

2 percent. (Data source: 

National Hydrography 

Dataset Plus (Horizon 

Systems Corporation n.d.); 

lake edges, diversions 

and irrigation ditches 

removed.)
USDA Forest Service RMRS-GTR-376. 2018 123

BOX 6.3

Key Vulnerabilities

A very high vulnerability ranking is given for cold-adapted, keystone, or foundation species in low-gradient stream 

reaches. Most species are highly to very highly vulnerable to climate change. One exception is beaver, which has 

moderate vulnerability.

• Beaver, a keystone species, is likely to persist. It may grow more vulnerable with drier conditions and 

drought, changes in riparian plant communities, and increased flooding or fire.

• Cold-water native fish are highly vulnerable to warming and modified streamflows, both of which can 

eliminate thermally suitable habitat, especially at lower elevations. 

• Cold-adapted amphibians are very highly vulnerable to the warming and drying that are likely to occur with 

climate change, especially at their lower elevational ranges. Reduced precipitation and stream discharge are 

likely to degrade habitat conditions considerably. 

• Cold-adapted benthic macroinvertebrates are very highly vulnerable to warming temperatures, and altered 

stream dynamics may exceed their heat tolerance as well as interfere with the success of life history 

strategies, with implications for food chains in these ecosystems. 

• Riparian plants, especially at low elevations and mid-elevations are likely to be highly vulnerable to climate 

change. Riparian plant species at all elevations are dependent on the moist soil conditions and periodic 

flooding that low-gradient reaches provide. Woody vegetation (for example, willows and cottonwoods) in 

particular, as well as other plants that depend on saturated and relatively shallow water tables, may be most 

vulnerable to warming and drying with climate change.

Disturbances (fire or flooding) are current stressors that are likely to interact with 

climate change effects on individual stream reaches. High-elevation stream reaches are 

projected to warm, but temperatures are likely to remain suitable for cold-water fish 

habitat. Reaches that are currently too cold for cold-water fisheries may become suitable 

(Isaak et al. 2015). Native trout populations in short stream fragments and at lowest 

elevations are projected to have the highest risk of extirpation; however, this risk is a 

result of disturbances such as wildfire that are likely to be a greater threat than warming 

(Roberts et al. 2013). Although the high elevation streams appear to be buffered from 

warming, the researchers emphasize that the near-term threat is from stochastic disturbances such as wildfire and that future conservation efforts for this species should focus 

on preventing further loss of habitat from conservation segments. 

Vulnerability of Cold-Adapted, Foundation, or Keystone Species to 

Climate Change______________________________________________

For this criterion, we selected species groups that demonstrate a range of 

environmental tolerances and have a major influence on ecosystem function. These lowgradient ecosystems provide habitat to cold-adapted species of fish, amphibians, and 

macroinvertebrates. Beaver is considered a keystone species in these ecosystems as it 

has a large effect: Dam building by beavers promotes water and sediment retention and 

landscape heterogeneity in low-gradient stream reaches. We included riparian plants as a 

group of foundation species that substantially influence ecosystem functions: They cycle 

organic matter and nutrients, reduce the effects of environmental disturbances, regulate 

and filter runoff, and offer breeding, feeding, and shelter habitat for animals.
124 USDA Forest Service RMRS-GTR-376. 2018

Beaver

Beavers typically inhabit low-gradient stream reaches (Baker and Hill 2003). 

Their dams can increase storage of precipitation, decrease stream velocity, increase 

water depth, elevate the water table locally, distribute water and sediment across valley 

bottoms, create habitat and increase habitat diversity, decrease water turbidity, improve 

water quality, increase nutrient availability, and enhance resistance to disturbance 

(drought, fire) (Gibson and Olden 2014; McCaffery and Eby 2016; Polvi and Wohl 

2012; Rosell et al. 2005; Westbrook et al. 2011). Although beaver dams can restrict fish 

passage in some places (Schlosser 1995; but see Baker and Hall 2003; Lokteff et al. 

2013) and reduce dissolved oxygen levels (Call 1966), beaver activity creates habitat 

complexity and enhances ecosystem capacity to withstand streamflow variations that 

may increase with climate change (Gibson and Olden 2014). Beaver activity may help 

buffer the effects of reduced snowpack and earlier runoff due to climate change within 

these low-gradient ecosystems (Wohl 2000). 

We draw from historical studies in North America as no studies have explored the 

effect of projected climate change on beavers in the Rocky Mountain Region. In eastern 

Canada, Jarema et al. (2009) concluded that beavers were sensitive to projected climate 

change, with range expansions to the north. However, population density increased 

more in the core of the current range than at the leading edge. Using historical climate 

and beaver dam deposits as proxy records, Perisco and Meyer (2012) documented the 

increase of beaver dams during cool and wet periods and when willow is abundant, 

and declines during drought in the Greater Yellowstone Ecosystem of northwestern 

Wyoming. Greater streamflow variation and flooding due to less snow and more rain 

could hamper beaver populations in the Greater Yellowstone Ecosystem (Perisco and 

Meyer 2012). In the dryland stream environments of North America, beaver distribution is influenced by the availability of perennial water (flowing streams; small pools 

in intermittent streams, lakes, and wetlands), and the availability of riparian vegetation 

(Gibson and Olden 2014). Though beaver population numbers and distributions fluctuate in response to environmental variations, beavers have continued to live in Rocky 

Mountain landscapes despite severe droughts (Perisco and Meyer 2012). Beavers are 

also found currently in dryland stream environments (Gibson and Olden 2014). We conclude that beavers are likely to persist under climate change as long as plant sources are 

available, drought does not cut off water supplies, or flooding magnitude and frequency 

do not hinder them. 

Cold-Water Native Fish

Life history traits and populations of cold-water native fish are closely linked to 

the dynamics of Rocky Mountain climate and hydrological systems (Gresswell 2011). 

Warming stream and lake temperatures will lower habitat suitability for cold-water fish 

species, and unless they can move to suitable habitat in a connected stream network, 

these changing conditions will increase stress and metabolic rates (Al-Chokhachy et al. 

2013a; Isaak et al. 2012). High summer temperatures can also prevent native fish from 

colonizing small headwater streams (Mullner and Hubert 2005). Changes in winter precipitation, from snow to rain, may result in altered flow regimes and increased scouring; 
USDA Forest Service RMRS-GTR-376. 2018 125

this scouring may impact fall-spawning species, such as brook trout and brown trout 

(Goode et al. 2013). 

Wildfires and landslides have positively (via gravel bars) and negatively (via 

sedimentation) affected fish habitat. Low-gradient stream reaches are response areas in 

the stream network. Greater frequency of these watershed disturbances would increase 

habitat loss (Williams et al. 2015) and challenge the ability of fish to recover from them. 

Stream and river temperatures have increased in many locations across the western 

United States (Isaak et al. 2012, 2013). Bioclimatic models have been used to explore 

the potential future effect of climate change on suitable habitat. In a large regional 

analysis of climate change effect on native and nonnative trout, Wenger et al. (2011) 

reported a 47-percent decline in suitable habitat by the end of the 21st century for all 

trout analyzed. Native cutthroat trout is projected to lose an additional 58 percent of its 

currently restricted habitat; habitat for nonnative brook trout and brown trout declines 

by 77 and 48 percent, respectively. Warming temperatures negatively affect nonnative 

rainbow trout habitat, but favorable changes in flow regime result in benefits for rainbow trout (Wenger et al. 2011). 

Thermal risks from climate change were analyzed in tandem with fragmentation risks for Colorado River cutthroat trout (Onchorhynchus clarkii pleuriticus) in the 

Upper Colorado River Basin. The model study by Roberts et al. (2013) concluded that 

the likelihood of disturbances such as wildfire and debris flow impacting the cutthroat 

habitat was greater than the acute mortality associated with future higher temperatures. 

Past habitat loss has restricted most of the Colorado River cutthroat populations to 

high-elevation stream fragments that would be buffered by elevation from the potential 

consequence of future warming. Isaak et al. (2015) projected high probabilities of 

cold-water trout occupancy at new habitats that become thermally suitable at higher 

elevations. Cooney et al. (2005) determined that 10 streams where greenback cutthroat 

trout (O. c. stomias) had been translocated would improve as suitable habitat under 

several degrees’ warming. As in the Colorado River cutthroat trout study, greenback cutthroat trout have been restricted to suboptimal temperature because of nonnative species 

expansion or introduction. 

Environmental changes associated with climate change will vary across the complex terrain of the Rocky Mountain Region, as will the biotic response to these changes. 

As the climate changes, site condition changes will differ depending on local climate or 

microclimate, sensitivity of streams to climate change, availability of climate refugia in 

the stream network, and soil and geological factors. In addition, individual cold-water 

fish species will have differing responses to these environmental factors. Maximum 

growth temperature of Rio Grande cutthroat trout (Onchorhynchus clarkii virginalis) fry 

was similar or higher than Colorado River cutthroat trout, Yellowstone cutthroat trout 

(O.c. bouvieri), Snake River cutthroat trout (O.c. behnkei), and Westslope cutthroat trout 

(O.c. lewisi) (Ziegler et al. 2015). The Rio Grande cutthroat trout could be expected to 

be relatively more warm-adapted because it has the southernmost distribution of these 

cutthroat trout subspecies. However, Ziegler et al. (2015) cautioned that sub-lethal 

effects of temperature in wild fish populations should be explored to understand how 

temperatures affect other ecosystem components, such as disease, with a changing 

climate. Further, native populations have already suffered large range contractions 

that researchers attribute to introduction of nonnative trout and habitat degradation 
126 USDA Forest Service RMRS-GTR-376. 2018

(Gresswell 2011). Lepori et al. (2015) concluded that ecosystem function in streams 

with nonnative species differs in several ways from when these species are not present; 

for example, they reported that populations of brook trout at natural density consumed 

seven times more benthic invertebrates than did cutthroat trout. 

Understanding the potential effects of climate change on cold-water fish is a rapidly expanding area of research, and results of this research repeatedly identify warming 

temperatures as a significant threat to these species. Warming of higher elevation 

streams may open up new habitat for fish, but this gain may be offset by losses from 

more fragmentation, less accessibility, and smaller tributary size. We conclude that coldwater native fish will become increasingly vulnerable to changes in climate.

Amphibians

Researchers continue to document amphibian declines globally. Adams et al. 

(2013) analyzed the rate of change in the probability that amphibians occupy ponds 

and other comparable habitat features; they concluded that amphibians, including 

species not typically considered of conservation concern, are declining across the 

conterminous United States. While reporting more declines than increases in national 

parks in the Rocky Mountains, Hossack et al. (2015) also observed higher colonization 

rates and occupancy of boreal toads and Columbia spotted frogs (Rana luteiventris) in 

beaver-influenced wetlands than in wetlands without beaver influence. Recent declines 

in amphibian populations in the western United States are attributed to many factors 

including nonnative or invasive species, land use, overexploitation, climate change, ultraviolet radiation, contaminants, and emerging infectious diseases (Hussain and Pandit 

2012). Researchers have seen connections between warmer temperatures and the pathogen chytrid fungus (Batrachochytrium dendrobatidis), with declines in some amphibian 

populations (Muths et al. 2008). Lack of sufficient data limits a robust conclusion on 

which factors (e.g., climate change, habitat loss, introduction of predatory nonnative 

fish, disease, and pollution) have directly or indirectly contributed to dying amphibian 

populations (Hossack et al. 2015; Hussain and Pandit 2012).

Amphibians are very sensitive to temperature and moisture. Future climate may 

pose a larger challenge for amphibians, as higher temperatures, reduced snowpack, 

reduced soil moisture, and drought potentially decrease habitat availability (Corn 2005; 

Ryan et al. 2014). Future conditions in alpine areas could be more tolerable to amphibian populations as compared to lower montane elevations, where snowpack losses in 

the future could be substantial. Snowpack loss causes earlier breeding, and results in a 

potentially higher risk of freezing (Corn 2005). Other factors, along with changes in climate, prey species, and availability of wetland habitat, could also influence amphibians. 

Given the climate sensitivity and potential for population loss, amphibians have very 

high vulnerability to climate change.

Benthic Macroinvertebrates 

Benthic macroinvertebrates include all aquatic animals exhibiting an exoskeleton 

(no internal backbone) and existing in or on the substrate of standing or flowing water 

(e.g., streams and lakes). Benthic macroinvertebrates in the low-gradient streams may 

include grazers, which consume plant material and serve as food for organisms such as 
USDA Forest Service RMRS-GTR-376. 2018 127

fish. Disturbances such as seasonal flooding in high elevation catchments affect the composition of primary consumers such as these benthic macroinvertebrates; for example, 

Peckarsky et al. (2015) reported that as disturbances increased, the community composition of grazers switched from disturbance-intolerant taxa (caddisflies; order Trichoptera) 

to disturbance-tolerant taxa (mayflies; order Ephemeroptera). Disturbances also alter 

dispersal behavior, grazing performance, and, consequently, the benthic algae biomass.

Many of these organisms (including some insects, crustaceans, and mollusks) 

tolerate fairly narrow ranges of ecological conditions. At higher elevations, changes in 

temperature over time can influence life history traits such as emergence and mating. 

In a field-manipulated experiment, Harper and Peckarsky (2006) found that warming 

stream temperatures resulted in earlier emergence of mayflies, but reducing the streamflow did not accelerate the onset of mayfly emergence. These warming temperatures 

serve as a cue that enables mayflies to time their emergence during the descending limb 

of the hydrograph, when protruding rocks are available as sites for laying eggs. Thus, 

shifts in temperature and declining snowpack and streamflow could potentially lower 

mayfly reproductive success. At lower elevations, many stream and lake species are 

more tolerant to the relatively wider range of thermal conditions. 

Many benthic macroinvertebrates are mobile. They drift freely in stream currents 

and settle onto the substrate at fairly short distances downstream. When stream insects 

become adults, however, they can travel upstream to deposit eggs (Graham et al. 2015). 

In this manner, they are not depopulated in the upper reaches of streams. However, it 

can be postulated that if significant warming in water temperature or greater discharge 

occurs, these insects would not be able to adjust their life history strategies, with consequences to population size.

Non-insect groups of benthic macroinvertebrates, such as mollusks, have little 

ability to move upstream or downstream to find preferable conditions. Indeed, many of 

these benthic macroinvertebrates are found in spring environments, where water temperature is relatively constant. Changes in the temperature regimes can result in local 

elimination of certain taxa.

Climate warming and subsequent drying may contribute to large shifts in the 

composition of benthic macroinvertebrate communities (Poff et al. 2010). These communities, with higher concentrations of cold-adapted species that survive in a narrow 

temperature range, are the most sensitive to projected warmer July temperatures and to 

less snowfall (Poff et al. 2010). Temperature-sensitive, cold-adapted species that live in 

higher elevations have a very high vulnerability to climate change; less temperaturesensitive, bottom-dwelling aquatic stream species that live in lower elevations will also 

have more stress from a warmer and drier climate (Poff et al. 2010). See also ColdAdapted Macroinvertebrates in Chapter 4. 

Riparian Plants

Dominant plant species in riparian areas of low-gradient stream reaches vary 

across elevations in the Rocky Mountain Region. Woody species occurring at lower 

elevations include cottonwoods (Populus spp.); at mid-elevations, species include alders 

(Alnus spp.) and birches (Betula spp.). Willows (Salix spp.) grow across a range of 

elevations from alpine to montane where low-gradient streams occur. Riparian plants 
128 USDA Forest Service RMRS-GTR-376. 2018

associated with headwaters and smaller streams are primarily herbaceous species, such 

as blue bells (Mertensia spp.), mannagrass (Glyceria spp.), groundsel (Senecio spp.), 

and monkey flower (Mimulus spp.) (Cooper and Merritt 2012). At higher elevations, 

these plant communities can have distinctly different compositions that include sedges 

(Carex spp.), quaking aspen (Populus tremuloides), and Engelmann spruce (Picea engelmannii Parry ex Engelm.).

Riparian ecosystems are influenced by the periodic or perennial influence of flowing water, water table depths associated with stream dynamics, and flood events related 

to the hydrological regime (Cooper and Merritt 2012). Riparian plants are likely to 

respond to the projected changes in streams: earlier snowmelt, drier conditions in late 

summer with lowering of water tables, and fluctuations in timing and magnitude of peak 

flows. Earlier snowmelt that reduces late season flows and shifts the timing of runoff 

may cause drier conditions that stress riparian plants and inhibit them from establishing seedlings (Perry et al. 2012). Willow species, in particular, may be most vulnerable 

to drying as a result of more frequent and intense drought and heat stress (Perry et al. 

2012). Alder population dynamics in the southern Rocky Mountains have been tied to 

cool climate phases, when populations expand, and to warm phases when they shrink 

in response to epidemics of Cytospora canker (Worrall et al. 2010). Decline of alder 

has been noted in the southern Rocky Mountains, with canker expansion and killing of 

branches and stems occurring during the warmest parts of the summer. Warmer phases 

and fewer cool phases, a distinct possibility under climate change, could result in further 

decline of alder in the southern Rocky Mountains.

Neely et al. (2011) rated low-elevation riparian areas as highly vulnerable, midelevation riparian areas as moderately vulnerable, and high-elevation riparian areas 

as of low to moderate vulnerability in the Gunnison Basin in Colorado. Many factors 

contribute to making low- and mid-elevation riparian areas more vulnerable: changes in 

the timing of snowmelt, flooding, and drought; decreased base flows; impacts from land 

use, grazing, and browsing; and increased invasive species (Neely et al. 2011). Because 

most low-gradient stream reaches in the Rocky Mountain Region are at mid- and low elevations (fig. 6.2), riparian plants along low-gradient mountain stream reaches are likely 

to be highly vulnerable to climate change. 

Sensitivity to Extreme Climatic Events______________________________

BOX 6.4

Key Vulnerabilities

A high ranking is given for the overall vulnerability of low-gradient stream reaches to extreme climatic events; they 

are sensitive to climate extremes of drought and heat, and are moderately sensitive to flooding.

• These reaches are very sensitive to droughts, which can reduce stream habitat, limit water supply, shift and 

hinder aquatic organisms, reduce the flow of sediment, and increase erosion potential when streamflow 

returns to higher levels. 

• Although flooding offers long-term benefits for stream and habitat maintenance, the increased energy 

associated with projected changes in annual hydrographs could destabilize streambanks and increase 

sedimentation, hindering aquatic habitat in the short term. 

• Warming of air temperature and surface waters will result in some aquatic habitat becoming thermally 

unsuitable for aquatic species.
USDA Forest Service RMRS-GTR-376. 2018 129

Sensitivity to Drought 

Low-gradient stream ecosystems are sensitive to drought. Droughts can arise 

through lower snowpack, earlier melt of snowpack, or less seasonal or annual precipitation (snow and rain). Less precipitation (snow or rain) results in declining soil moisture 

and runoff and consequently lower flows in the stream. Groundwater may also decline. 

Riparian vegetation and aquatic organisms can be affected. Canton et al. (1984) studied 

the effects of a 1-year flow on a low-gradient stream in Colorado; total macroinvertebrate density decreased by 50 percent in the low- flow year with taxon response widely 

variable. Trophic group responses also varied, with shredder and predator species increasing in relative abundance during low flow, and collector-gatherer species abundant 

only in the normal flow year. Fish returned to the stream reaches during the normal year. 

The processes of bank erosion and sediment transport in streams are altered when 

climate shifts rapidly (Johnson et al. 2011). Reduced flows deliver less sediment to lowgradient stream reaches, and the reduction in transported sediment increased streambank 

vulnerability. During subsequent higher flows and flooding, streambanks may erode 

more intensely (as less sediment is present to buffer the scouring effect) (Wohl et al. 

2007). During the Holocene, valley filling occurred during moister periods, whereas 

stream incision occurred during a warmer, drier period that reduced plants and increased 

fire (Johnson et al. 2011). 

Drought of the magnitude that is possible with climate change could also reduce 

instream habitat connectivity, stress riparian plants, deteriorate water quality, alter food 

resources and interactions between species, and reduce water supply (Lake 2003; Perry 

et al. 2012). Because of the potential for the increased harm that drought can bring to 

low-gradient mountain stream reaches, these ecosystems are likely to be very sensitive 

to drought.

Sensitivity to Floods

Low-gradient streams can undergo spring floods from snowmelt, and flooding 

from storm precipitation. Flooding can restructure channels, riparian areas, and habitats, 

and increase sediment delivery, but can also introduce long-term benefits to lowgradient streams. Benefits include maintaining and creating channels, and increasing 

the habitat complexity and diversity of streams with additions of wood and sediment 

(Benda et al. 2003, 2004; Miller et al. 2003). Low-gradient streams are more sensitive 

to flooding than higher gradient streams; the large pool volumes can become filled 

with sediment and not flushed out as rapidly as in higher gradient streams, causing loss 

of critical habitat (Wohl 2000; Wohl et al. 2007). Fire-induced flooding can increase 

sediment loads in the short term, as well as degrade water quality (Agnew et al. 1997; 

Benavides-Solorio and MacDonald 2001; Goode et al. 2012). 

Flooding can result in streambed scouring and in fish embryo mortality; the risk 

of critical scour to fish is a function of the timing of floods relative to embryo incubation periods, location of spawning within the stream network, and egg burial depths 

(Goode et al. 2013). Risk of critical scour will be species-specific; for example, smaller 

fish bury eggs in shallow depths (Goode et al. 2013). But a modeling study by McKean 

and Tonina (2013) found that bank-full flows had minimal movement of sediment and 

gravel that would displace embryos, and overbank flows did not increase scouring or 
130 USDA Forest Service RMRS-GTR-376. 2018

movement of sediment in the unconfined low-gradient stream they studied, a conclusion 

similar to that of Goode et al. (2013). 

Flooding can remove riparian plants (Perry et al. 2012), and wash out beaver 

dams. The loss of beaver dams can reduce fish numbers, decrease water retention within 

the riparian area, shift riparian plant communities, and increase erosion (Westbrook et 

al. 2011). Although flooding may hinder species in some cases, plants and animals are 

likely to recover and survive in the future as the ecosystem responds. Because of this 

response over the long run and the long-term benefits of flooding, low-gradient stream 

reaches are likely to be moderately sensitive to flooding.

Sensitivity to Extreme Heat

The temperature of low-gradient streams has a strong seasonal signal, with cooler 

temperatures in spring, fall, and winter, and the warmest temperatures typically in 

summer. Stream temperature is influenced by solar radiation, heat transfer between air 

and water, conduction from the stream bed, and precipitation and groundwater input 

(Isaak et al. 2012). Climate change will affect stream temperatures by the warming of 

air temperature and surface waters, and potential shifts in canopy cover along riparian 

streams. A warming climate has already been observed to increase stream temperatures 

and stress fish in the Rocky Mountains (Isaak et al. 2012, 2013).

Extreme heat may most significantly impact stream habitats where species are 

at the edge of their tolerance to heat (Perry et al. 2012; Poff et al. 2010; Roberts et al. 

2013; Wenger et al. 2011). Physiological processes for cold-water fish, such as trout, 

are controlled by the temperature of the ambient environment. Within the Greater 

Yellowstone Ecosystem in northwestern Wyoming, future climate changes include 

increased peak stream temperatures and protracted periods of warming from May to 

September. Al-Chokhachy et al. (2013a) found that these climate changes result in 

pronounced growth during early and late summer for Yellowstone cutthroat trout. In 

contrast, projected mid-21st century temperatures result in periods of increased thermal 

stress from July to mid-August, and low elevation streams become less suitable habitat. 

At high elevation, warming of cold-water temperatures was projected to extend the 

growing season with positive responses by fish.

Stream temperatures could lag changes in air temperature if the streams are connected to groundwater sources. Springs and seeps may reduce the sensitivity of streams 

to warmer conditions from climate change. Floodplains associated with low-gradient 

reaches historically acted as natural “sponges” for water in the drainage; the floodplain 

would release water slowly, helping to maintain cooler temperatures (Wohl 2000). In 

addition, the lack of healthy riparian vegetation along the stream may result in higher 

energy gains (increased temperatures) to the stream because of reduced canopy cover 

(Garner et al. 2014).

Isaak and Rieman (2013) modeled how fast stream temperature has shifted or will 

shift over future warming scenarios. Based on their analysis, stream isotherms in the last 

century shifted 1.5 to 43.0 mi in many streams as air temperatures increased by 1.1 °F. 

In the first half of the 21st century, they would shift another 3.1 to 88.8 mi if mid-range 

projections of an increase in air temperature of 3.6 °F occur. Slope influences the rate 

of change. For central Idaho, a projected 0.2 to 0.4 °F temperature increase per decade 
USDA Forest Service RMRS-GTR-376. 2018 131

BOX 6.5

Key Vulnerabilities

The intrinsic adaptive capacity of low-gradient streams is ranked as moderately vulnerable to climate change. The 

capacity of low-gradient stream reaches to withstand precipitation variability and warming can be strengthened 

as well as weakened by geomorphic and geological conditions, some of which can fluctuate with environmental 

forces. 

• Factors that strengthen adaptive capacity:

○ Wider valley bottoms generally slow water flows and sediment transport; low-gradient slopes contribute 

to increased residence time of water in this ecosystem.

○ Functioning riparian areas stabilize streambanks and provide shade, which helps maintain cool stream 

temperatures.

○ Large wood, typically deposited on floodplains and in stream channels, helps sort sediment and increase 

habitat complexity. 

○ Low-gradient stream reaches underlain by calcareous bedrock tend to have greater contributions from 

groundwater flow and may be better able to withstand drier and warmer conditions.

• Factors that weaken adaptive capacity:

○ Large variations in high flows and flood-related alterations of sedimentation from upstream disturbances, 

particularly over extended periods, can affect recovery of these systems.

○ Streams that do not have functioning riparian areas are less adapted to flooding, drought, and 

temperature increases. 

would result in isotherms shifting in steep streams (2- to 10-percent slope) at a rate of 

0.08 to 0.80 mi per decade, while isotherms in flat streams (slope of <1 percent) shift 

at 0.80 to 15.5 mi per decade. Models are available that provide stream-specific probabilistic projections about the occurrence of juvenile bull trout (Salvelinus confluentus) 

and cutthroat trout in association with different scenarios for climate change and brook 

trout invasions (USDA FS n.d.). Low-gradient streams are likely to be more sensitive to 

temperature warming than streams on steep slopes. 

Because modified temperature conditions can hinder species or cause mortality, 

low-gradient stream reaches have a very high sensitivity to the potential effects of extreme heat events in the Rocky Mountains.

Intrinsic Adaptive Capacity to Climate Change_______________________

Factors That Strengthen Adaptive Capacity to Climate Change 

The capacity of low-gradient stream reaches to adapt to precipitation variability 

and warming can be strengthened as well as weakened by geomorphic and geological conditions, some of which can fluctuate with environmental forces. Low-gradient 

stream reaches generally occur in wider valley bottoms, which allow water to flow 

and move across wider areas compared to stream segments in narrower valleys and 

steeper slopes. Water flows and sediment transport occur less rapidly in these wider 

valley bottoms, thus helping to develop more complex habitat that features meandering streams, runs, and pools (Wohl et al. 1993) (fig. 6.3). The low-gradient slope and 

loose gravel and silt contribute to the likelihood that subsurface water will be retained 

in these streams. Watersheds underlain by calcareous bedrock also tend to have greater 

contributions from groundwater flow (Wohl et al. 2007). Springs, seeps, and other 
132 USDA Forest Service RMRS-GTR-376. 2018

groundwater sources may also exist in many areas of the Rocky Mountain Region, 

potentially mitigating drier conditions from climate change (Jones and Petreman 2012). 

Glacial meltwater can also mitigate drier conditions. In the Wind River Range in 

Wyoming, glacial melting caused by warmer temperatures has contributed 4 to 10 percent of July-to-October streamflow since 1985 (Cheesbrough et al. 2009). Streamflow 

from glacial meltwater is temporary, however, as projections show it will disappear 

before mid-century (for example, Glacier National Park in Montana; Hall and Fagre 

2003). Low-gradient reaches retain sediment and develop landform complexity with 

their meandering, thereby helping deciduous riparian communities establish, which can 

contribute high-quality plant matter to the channel (Gregory et al. 1991). This plant matter—leaves, bark, and branches—provides habitat and food for bottom-dwelling stream 

insects and macroinvertebrates. Low-gradient stream reaches often support beavers; 

beavers increase hydrological and ecological complexity by building dams, which help 

to retain water in the riparian area. 

In places where functioning riparian areas border low-gradient stream reaches, 

adaptive capacity to flooding, drought, and heat is strengthened. Low-gradient stream 

reaches are commonly dominated by extensive willow communities and other plants 

that recover relatively quickly after a disturbance (Dwire and Kauffmann 2003). 

Riparian plants provide large wood to the stream, bank stability, food for aquatic and 

riparian species, and shade (Naiman et al. 2005). 

Adaptive capacity to disturbances is strengthened by high biodiversity; high 

productivity and high biodiversity contribute to adaptive capacity at lower elevations 

and larger stream sizes (Ward 1998). The typically wider floodplains have finer grained 

soils, and support diverse plant and macroinvertebrate communities (Ward 1986). Kozel 

et al. (1989) found that these channel types are generally dominated by riffles and pools, 

habitat that benefits the life history of larger fish. 

Figure 6.3—Comparison between stream (blue) and 

riparian area (green-gray) of a meandering (left) vs. a 

straight (right) stream structure. A meandering stream 

supports a greater amount of riparian area, which in 

turn promotes more water retention on the landscape. 
USDA Forest Service RMRS-GTR-376. 2018 133

BOX 6.6

Key Vulnerabilities

A very high vulnerability ranking is given for dependence of low-gradient stream reaches on a specific 

hydrological regime, as the functioning of these ecosystems and of the species living within them, is highly 

dependent on a snow-driven hydrological system. 

Factors That Weaken Adaptive Capacity to Climate Change

Low-gradient stream reaches are response areas; their large wide valleys receive 

high flows and disturbances from upstream. However, these systems are not adapted to 

large shifts in the hydrological system from drought and flooding over extended periods 

(see Sensitivity to Drought and Sensitivity to Floods sections). In areas where riparian 

plants have been removed or damaged along low-gradient streams, adaptive capacity to 

drought and floods is weakened. Additions of large wood and leaf litter to the stream, 

which are necessary for insects and fish (Naiman et al. 2005), are reduced or eliminated 

under low flows. Benefits that riparian plants provide—stabilizing streambanks and 

providing shade, which maintains cool water temperatures (Naiman et al. 2005)—can be 

compromised where plants are removed. 

Dependence on Specific Hydrological Regime_______________________

Low-gradient mountain stream reaches are dependent on a snow-dominated hydrological regime. In mountainous areas of the Rocky Mountain Region, hydrological 

regimes range from snow-driven above an elevation of about 8,200 ft, through a mixed 

snow- and rain-dominated system in the montane below about 8,200 ft, to a more raindominated hydrological system in the foothills (Regonda et al. 2004). Spring flooding 

from snowmelt depends on an upstream drainage area that is snow dominated. Peak 

snowmelt benefits streams by delivering a bank-full flooding pulse that maintains channel size (Poff et al. 1997). Willow decline in Rocky Mountain National Park was tied 

to a decrease in peak streamflows and an increase in moose (Alces alces) populations 

(Kaczynski et al. 2014). 

The western United States has had greater reductions in snowpack at elevations 

below 8,200 ft that appear to be caused by earlier snowmelt, and from precipitation 

shifting to more rain instead of snow (Knowles et al. 2006; Mote et al. 2005; Regonda 

et al. 2004; Stewart et al. 2005). Projections for the Colorado River Basin also show 

relatively greater reductions in snowpack over the next century at lower elevations, and 

moderate losses at higher elevations (Christensen and Lettenmaier 2007; Lukas et al. 

2014). A warmer climate that shifts the current snow-driven hydrological system to one 

with more snow mixed with rain, or rain dominated, would cause greater flow variation 

in winter, shift peak flows earlier during spring, and lower summer late-season flows 

(Baron et al. 2000b; Stewart et al. 2004; Stonefelt et al. 2000). Earlier peak flows and 

reductions in summer low flows are likely to occur as temperatures warm (Stonefelt 

et al. 2000), resulting in more variation in seasonal water availability for wildlife. 

Hydrological changes caused by climate, such as less snowpack, could reduce channel 
134 USDA Forest Service RMRS-GTR-376. 2018

maintenance benefits, degrade stream habitat, and increase riparian plant stress, especially during summer low flows.

Likelihood of Managing Climate Change Effects______________________

BOX 6.7

Key Vulnerabilities

A moderate vulnerability ranking is given, as there is a likelihood of managing and mitigating climate change 

impacts on low-gradient stream reaches, where feasible.

• Many management options are available to mitigate the negative effects from flooding or reduced water 

supply, and to offset the effects of heat and drought. 

• Numerous aquatic and riparian management options are known to promote resilience of aquatic habitats; 

however, some management actions may not always be economically, politically, socially, or ecologically 

feasible. 

Management options for lessening the effects of climate change on aquatic habitats 

include reducing current stressors, strengthening ecosystem resilience and, as climate 

continues to change, helping the ecosystem adapt without substantial loss of soil, 

soil nutrients, and plant cover (Millar et al. 2007). Managers have successfully used 

methods to reduce current stressors to aquatic communities and enhance resilience to 

climate change (Luce et al. 2012; Rieman and Isaak 2010). These management practices 

(Rieman and Isaak 2010; Wohl et al. 2005) include:

• maintaining or restoring the natural hydrological regime; 

• maintaining and restoring forest and streamside plants to minimize flooding impacts; 

• maintaining or restoring riparian areas, floodplains, and wetland areas and their 

connection to streams; 

• reintroducing beaver; 

• protecting or restoring critical or unique habitats that buffer survival of the species 

when conditions are unfavorable; 

• disconnecting roads from the drainage network; 

• limiting or stopping the introduction of nonnative species; 

• eliminating or controlling pollutants or contaminants; 

• removing or modifying barriers to the movement of fish; 

• maintaining or reconnecting large networks of habitat; 

• conserving or restoring diverse habitats across basins;

• helping the ecosystem make a transition to a new state, which may require 

transporting fish to inaccessible habitat or places of refuge to maintain genetic variety 

and to allow new species to colonize when native fish species can no longer survive;

• reducing the impacts of livestock grazing; 

• increasing riparian plant cover to provide shading and maintain cool stream 

temperatures; 
USDA Forest Service RMRS-GTR-376. 2018 135

BOX 6.8

Key Vulnerabilities

A very high vulnerability ranking is given, as several factors exacerbate climate and nonclimate stressors in lowgradient mountain stream reaches.

• Climate change is likely to continue to increase fire frequency, causing erosion and subsequent harm to lowgradient stream function.

• A warmer climate is projected to reduce streamflows; human water use could considerably exacerbate the 

effects of drying from climate change.

• Some nonnative and invasive species may expand in range with climate change, potentially altering 

ecosystem function.

• allowing the natural change of stream channels; and 

• minimizing soil compaction by limiting roads, trails, campgrounds, and grazing. 

Even though many management options exist to reduce stressors, they may not 

completely alleviate the effects of severe flooding, warming, and drought. Management 

actions can reduce stressors if the right technique is used at the right scale (Furniss et 

al. 2010). Riparian plants maintain stream temperatures by providing shade, but this 

maintenance of water temperature may or may not be enough to maintain or create 

thermally suitable habitats for aquatic species. Water diversions reduce low-gradient 

stream resilience; however, human water demands from agriculture and cities are expected to increase in the future. Development of management strategies also requires 

incorporation of new information. For example, if some stream habitats where species 

are targeted for restoration develop unsuitable temperatures, management strategy may 

have to change to address species viability (Auerbach et al. 2012). 

Potential for Climate Change to Exacerbate Effects of Nonclimate 

Stressors, or Vice Versa_______________________________________

Climate Change and Disturbance

Climate change has exacerbated the effects of some nonclimate disturbance 

stressors on low-gradient stream reaches. Recent increases in fire frequency have been 

linked to drought, warmer temperatures, and reduced snowpack, trends that are likely to 

continue in the future (Westerling et al. 2011, 2014). Increased fire activity could cause 

more sediment to be added to streams; alter stream chemistry; create short-term, harmful consequences for local aquatic populations; and increase frequency of disturbances, 

such as debris flows that degrade water quality (Luce et al. 2012; Rhoades et al. 2011; 

Rieman and Isaak 2010). Researchers have also seen a link between warmer temperatures and increased bark beetle (Dendroctonus spp.) outbreaks (Bentz 2009), but there is 

no scientific consensus on the effects of these outbreaks on the quantity of streamflow or 

stream temperatures (Lukas and Gordon 2010). 

In high elevation areas of Colorado and Wyoming where glaciers exist, glacial outburst floods may occur as temperatures warm. Many glaciers lie within national forests 
136 USDA Forest Service RMRS-GTR-376. 2018

in the Rocky Mountain Region (Gage and Cooper 2013; Oswald and Wohl 2008; Veblen 

and Donnelly 2005; see Dependence on Hydrological Regime in Chapter 4). 

Climate Change and Human Water Use

Climate warming and drying in combination with water withdrawals for human 

use have ecological consequences for low-gradient streams. Water is taken by diversions, sometimes across basins, and can be stored in reservoirs for use by agriculture 

and municipalities, or can be used for snowmaking by the ski industry. The demand for 

water is likely to increase as human populations grow (Foti et al. 2012), with further 

consequences to hydrological and ecological function. Crop water use is projected 

to grow in the Colorado River Basin (Colorado Water Conservation Board 2012). 

Rajagopalan et al. (2009) projected that increased human water use in combination with 

a reduced water supply from a warmer and drier climate threatens the sustainability of 

the water supply in the Colorado River Basin. Less available water can change the hydrological and ecological function in low-gradient streams. These changes could mean 

the following: a shift in the riparian plant community composition to fewer aquatic plant 

species, lower plant biodiversity, reduced channel sizes and sediment movement, and 

changes in stream chemistry and nutrient transport (Caskey et al. 2014; Wohl 2006).

Climate Change and Water Quality

Expected changes in climate could alter stream chemistry and increase channel 

disturbance and sediment dynamics from a possibly greater number of floods (Clow 

2010; Hamlet and Lettenmaier 2007). In areas where glaciers, and especially glaciers 

embedded with rocks, are melting from warming temperatures, meltwater can flush 

nitrogen from exposed sediments. Researchers suggest that in the Colorado Rocky 

Mountains, water quality limits were exceeded by the addition of nitrogen from glacial 

meltwater (Baron et al. 2009). Historical and recent land use development has increased 

nutrient levels in many streams. In addition, historical mining activity has contributed 

pollutants as well as increased sediment to areas downstream of mine operations (Wohl 

2015). A reduction in streamflows would concentrate pollutants, which potentially could 

reach lethal levels for aquatic and riparian taxa. Low-gradient streams may be particularly sensitive to the addition of nutrients (and the potential for nuisance algae growth) 

because the residence time of water is longer in those streams (Wohl et al. 2007).

Climate Change and Nonnative or Invasive Species

Some invasive plant species that occur in riparian areas, such as leafy spurge, 

may decrease in cover as a result of climate change, but others may expand their range, 

displacing native species and disrupting ecological function (cheatgrass and tamarisk 

[Tamarix spp.], Bradley et al. 2009, 2010; Canada thistle, Al-Chokhachy et al. 2013b). 

Nonnative fish that are more tolerant of warmer temperatures and earlier runoff flows, 

such as rainbow trout, may interbreed with native species, or displace native populations 

(Gresswell 2011; Wenger et al. 2011). 
USDA Forest Service RMRS-GTR-376. 2018 137

Figure 6.4—Overall regional vulnerability ranking 

and confidence ranking for low-gradient 

mountain stream reaches in the Rocky Mountain 

Region. The large arrow points to the mean 

score for vulnerability and for confidence. Bar 

represents the range of scores. Only four reviewer 

scores are included in mean calculations here 

and in tables 6.2 and 6.3.

Summary of the Regional Vulnerability of Low-Gradient Mountain Stream 

Reaches___________________________________________________

The overall regional vulnerability ranking for the ecosystem is a combination 

of the vulnerability to nonclimate stressors and the vulnerability to climate stressors. 

For low-gradient mountain stream reaches, the mean overall vulnerability ranking was 

very high with mean expert reviewer rankings in the high or very high category (fig. 

6.4, table 6.2). Mean vulnerability rankings were very high for the nonclimate stressor 

vulnerability and high for the climate vulnerability (fig. 6.4). Expert reviewer rankings 

were in the high or very high category for both vulnerabilities. 

The expert reviewers had high overall confidence in the ranking of the overall 

regional vulnerability (fig. 6.5). The four experts’ individual rankings of overall confidence ranged from the high to the very high category (table 6.3). Mean confidence 

was very high for the nonclimate stressor vulnerability ranking, and individual experts’

confidence rankings were in the high or very high category (fig. 6.5a). Mean confidence 

was high for the climate vulnerability ranking, with individual rankings ranging from 

the moderate to the very high category (fig. 6.5b). Calculations of mean vulnerability 

and confidence were based on the four complete reviewer rankings.
138 USDA Forest Service RMRS-GTR-376. 2018

Figure 6.5—Regional vulnerability of low-gradient mountain stream reaches to nonclimate (a) and to climate (b) stressors in 

the Rocky Mountain Region. The large arrow points to the mean score for vulnerability and for confidence. Bar represents 

the range of scores. Only four reviewer scores are included in mean calculations here and in tables 6.2 and 6.3.

a)

b)

Table 6.2—Original and reviewer rankings for the regional vulnerability of low-gradient mountain stream reaches of the Rocky 

Mountain Region. Underline indicates a reviewer score change from the original assigned score. 

Criterion 

Original 

score

(Reviewer 1) 

Score

(Reviewer 2) 

Score

(Reviewer 3) 

Score

(Reviewer 4) 

Score

(Reviewer 5) 

Score *

Reviewer 

criterion 

rank mean

Nonclimate

1. Extent 5 5 5 5 5 5 Very high

2. Human influences 5 5 5 5 5 5 Very high

3. Resilience 3 3 5 3 3 3 High

4. Future trends 5 5 5 3 5 5 Very high

Total 18

Very high

18

Very high

20

Very high

16

High

18

Very high

18

Very high

18 

Very high

Climate

1. Ecosystem shift 5 3 5 5 5 5 Very high

2. Species groups 5 5 5 5 5 5 Very high

3. Climatic events 5 4 5 3 5 5 High

4. Adaptive capacity 3 3 3 3 3 3 Moderate

5. Hydrology 5 4 5 5 5 5 Very high

6. Management 1 3 3 3 1 1 Moderate

7. Interactions 5 5 5 5 5 5 Very high

Total 29

High

27

High 

31

Very high

29 

High

29

High 

29

High

29 

High

Overall vulnerability 

rank

24

High 

23

High 

26

Very high 

23

High

24

Very high

24

Very high 

24

Very high

* Calculation of mean vulnerability and confidence (table 6.3) included only the four complete reviewer rankings.
USDA Forest Service RMRS-GTR-376. 2018 139

Table 6.3—Reviewer rankings for confidence in the vulnerability assessment of low-gradient stream reaches in the 

Rocky Mountain Region. 

Criterion 

(Reviewer 1) 

Score

(Reviewer 2) 

Score

(Reviewer 3) 

Score

(Reviewer 4) 

Score

(Reviewer 5) 

Score *

Criterion rank 

confidence 

mean

Nonclimate

1. Extent 5 5 5 5 5 Very high

2. Human influences 5 5 5 5 5 Very high

3. Resilience 5 3 3 5 3 High

4. Future trends 3 3 3 5 3 High

Total 18

Very high 

16

High

16

High 

20

Very high 

16

High

18 

Very high

Climate

1. Ecosystem shift 3 3 5 5 5 High

2. Species groups 5 5 3 5 5 Very high

3. Climatic events 3 3 3 5 3 High

4. Adaptive capacity 5 3 3 5 1 High

5. Hydrology 3 3 3 5 5 High

6. Management 3 3 5 5 * High

7. Interactions 5 3 5 5 5 Very high

Total 27

High

23

Moderate 

27

High 

35

Very high

 * 28

High

Overall confidence 

rank

23

High 

20

High 

22

High 

28

Very high 

* 23

High

* Calculation of mean vulnerability (table 6.2) and confidence included on the four complete reviewer rankings.

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American Fisheries Society. 142:5, 1395–1405, DOI: 10.1080/00028487.2013.811104
148 USDA Forest Service RMRS-GTR-376. 2018

Chapter 7. Ponderosa Pine Ecosystems: 

Vulnerability to Nonclimate and Climate Stressors 

in the U.S. Forest Service Rocky Mountain Region

Janine R. Rice, Claudia Regan, Dave Winters, Rick Truex, and Linda A. Joyce

Quick Look: Ponderosa Pine Ecosystems in the Rocky Mountain Region 

Two ecological systems of ponderosa pine forests are found in the Rocky Mountain Region. 

Ponderosa pine woodlands have dense stands; ponderosa pine savannas have widely spaced 

trees and open, park-like grassy areas. Although this narrative focuses only on these two 

ponderosa pine-dominated forests, ponderosa pine trees grow in other types, such as mixedconifer. Ponderosa pine ecosystems provide habitat for a wide variety of wildlife including mule 

deer, wild turkey, cavity nesting birds, and avian predators. 

Quick Look: Vulnerability of Ponderosa Pine Ecosystems in the Rocky Mountain Region

Vulnerability to nonclimate and climate stressors: Moderate

Confidence: High

Exposure: Variability in annual and seasonal precipitation, warming temperatures, more frequent and intense drought, longer growing season. 

Current extent: Ponderosa pine savannas occur on approximately 1.6 million ac on the Colorado 

Front Range and in southwestern Colorado. Ponderosa pine woodlands dominate on an additional 4.8 million ac in the Rocky Mountain Region.

Sensitivity and adaptive capacity to climate change: The natural range of ponderosa pine extends well beyond the Rocky Mountain Region, to the south as well as to the north. Upslope 

expansion is possible in areas of Colorado and Wyoming. Ponderosa pine ecosystems in 

Nebraska, eastern Wyoming, and lower elevations of the Black Hills of South Dakota, the 

Front Range, and southwestern Colorado may be most at risk of being replaced by grassland, 

shrub, or other woodland types. Warming and highly variable precipitation may hinder seedling establishment and seed and cone production of ponderosa pine as well as regeneration 

of understory plants. Insect outbreaks and wildfires increase habitat for some wildlife, such 

as cavity nesting birds that depend on snags; however, these disturbances reduce habitat for 

species such as northern goshawks and squirrels that depend on mature forest structure. In 

addition, fire size may be larger and severity higher with climate change. 

Nonclimate stressors: Natural and human-caused disturbances (beetle outbreaks, fire, logging, 

grazing) have altered the structure, composition, and function of these ecosystems. Their impacts are likely to continue, with climate change potentially exacerbating their effects. 

Introduction___________________________________________________

Ponderosa pine (Pinus ponderosa Lawson & C. Lawson) ecosystems, also called 

forests in this report, include the two ecological systems of ponderosa pine woodlands 

and savannas defined by Comer et al. (2003), and cover mountainous areas of the Forest 
USDA Forest Service RMRS-GTR-376. 2018 149

Service, U.S. Department of Agriculture (USFS) Rocky Mountain Region and beyond 

in western North America. Ponderosa pine woodlands have denser stands, whereas the 

less common ponderosa pine savannas have widely spaced trees and open, park-like 

grassy areas. Ponderosa pine woodland canopy cover can range from 26 to 100 percent, 

whereas savannas have sparse canopy cover. Within the Rocky Mountain Region, ponderosa pine ecosystems are widespread, dominating the lower montane of Colorado and 

Wyoming, the Black Hills in South Dakota, and high areas in Nebraska (fig. 7.1). 

Ponderosa pine ecosystems can mix with higher elevation vegetation types, and 

ponderosa pine trees are scattered in cliff and canyon plant communities across the 

western Great Plains area in the Region. Ponderosa pine woodlands and savannas can 

mix with Rocky Mountain montane grasslands, montane dry-mesic and mesic mixedconifer forests and woodlands, subalpine montane limber-bristlecone pine woodlands, 

and aspen forests and woodlands. Tree species that can mix with ponderosa pine in the 

upper elevational ranges of ponderosa pine ecosystems include: white spruce (Picea 

glauca (Moench) Voss) found in the Black Hills, lodgepole pine (Pinus contorta 

Douglas ex Louden), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), bristlecone 

pine (Pinus aristata Engelm.), and limber pine (Pinus flexilis James), and in more mesic 

sites, Engelmann spruce (Picea engelmannii Parry ex Engelm.) and quaking aspen 

(Populus tremuloides Michx.). At lower elevational ranges, ponderosa pine savannas 

and woodlands can mix with the central mixed grass prairie and foothills grasslands, 

juniper woodlands, Gambel oak shrublands, lower montane woodlands and shrublands, 

western Great Plains canyons, and cliff and canyons semi-desert shrub steppe. Cooccurring species at lower elevational ranges include pinyon pine (Pinus edulis) found 

in Colorado, Gambel oak (Quercus gambelii Nutt.), and juniper (Juniperus spp.), as 

well as numerous grass species (Comer et al. 2003).

Ponderosa pine ecosystems provide habitat for a wide variety of wildlife including mule deer (Odocoileus hemionus) and wild turkey (Meleagris gallopavo); cavity 

nesting birds such as woodpeckers (family Picidae), northern flicker (Colaptes auratus), 

and nuthatches (Sitta spp.); and predators such as northern goshawk (Accipiter gentilis) 

and great horned owl (Bubo virginianus). Typical understory plants of ponderosa pine 

ecosystems include the following shrubs: antelope bitterbrush (Purshia tridentata) 

and mountain snowberry (Symphoricarpos oreophilus), both of which occur in much 

of the Rocky Mountain Region but are absent in the Black Hills; kinnickinnick 

(Arctostaphylos uva-ursi); black sagebrush (Artemisia nova); big sagebrush (Artemisia 

tridentata); greenleaf manzanita (Arctostaphylos patula) only in Colorado; mountain 

mahogany (Cercocarpus spp.); Stansbury cliffrose (Purshia stansburiana) in Colorado; 

Gambel oak, absent in the Black Hills; and chokecherry (Prunus virginiana), Saskatoon 

serviceberry (Amelanchier alnifolia), and rose (Rosa spp.). Common grasses in ponderosa pine woodlands can include bluebunch wheatgrass (Pseudoroegneria spicata), 

and species of needle and thread (Hesperostipa spp.), needlegrass (Achnatherum spp.), 

fescue (Festuca spp.), muhly (Muhlenbergia spp.), and grama (Bouteloua spp.). Grasses 

common in ponderosa pine savannas include: Arizona fescue (Festuca arizonica) in 

Colorado; bluebunch wheatgrass, big bluestem (Andropogon gerardii), little bluestem 

(Schizachyrium scoparium), fescue species (Festuca spp.), and blue grama (Bouteloua 

gracilis) (Comer et al. 2003). 
150 USDA Forest Service RMRS-GTR-376. 2018

Figure 7.1—Ponderosa pine savanna and woodlands (Comer et al. 2003). (Data can be found in NatureServe 2014.) 
USDA Forest Service RMRS-GTR-376. 2018 151

We spatially differentiate and describe vulnerability of the ponderosa pine ecosystems in the Rocky Mountain Region by three areas based on distinct disturbance 

regimes and climate: southwestern Colorado, the Black Hills, and the Front Range. 

Southwestern Colorado ponderosa pine forests have had a historical fire regime 

more similar to southwestern ponderosa pine found in Arizona and New Mexico. 

Southwestern ponderosa pine ecosystems had a pre-Euro-American settlement fire 

regime of more frequent and less severe fires (Roos and Swetnam 2011), most of which 

occurred before the summer monsoon season (Hunter et al. 2007). The Black Hills and 

Colorado Front Range have historically had a low- or mixed-severity fire regime, burning less often and creating greater diversity over the landscape than in southwestern 

ponderosa pine forests (Ehle and Baker 2002; Sherriff et al. 2014). Some ponderosa 

pine forests, typically more dense woodlands and forests in the upper montane of the 

Colorado Front Range, have been shaped by a fire regime characterized by moderate to 

high severity and less frequent fire (Schoennagel et al. 2011; Sherriff et al. 2014). Lower 

elevation areas of the Front Range had more frequent and less severe fires (Sherriff et al. 

2014; Veblen et al. 2000). 

Climate plays a role in differentiating southwestern, Black Hills, and Front Range 

ponderosa pine ecosystem characteristics and fire regimes. The Southwest, which 

includes southwestern Colorado, has the driest periods during late spring and early 

summer; it receives the most moisture as snow during winter, and rain from the late 

summer monsoon. The Black Hills receive precipitation year-round, with the highest 

precipitation amounts falling over the summer growing season; this supports growth 

and prolific regeneration. The Front Range has more erratic establishment (League and 

Veblen 2006) and less prolific growth as this area generally receives less moisture than 

the Southwest and the Black Hills; the most precipitation arrives in late spring or early 

summer. 

Ponderosa pine ecosystems have been assessed for their vulnerability to climate 

change. The vulnerability framework has two major components, one related to nonclimate stressors and one related to climate stressors. Within the nonclimate component, 

four criteria were used to define factors of vulnerability: current status of ecosystem 

extent, intrinsic resilience of the ecosystem to nonclimate stressors, human influences 

on the ecosystem, and future trends of nonclimate stressors on the ecosystem. Within the 

climate change component, seven criteria capture the vulnerability of individual species, 

sensitivity of ecosystem dynamics such as dependence of the ecosystem on the hydrological cycle, adaptive capacity of the ecosystem, potential for management to mitigate 

the effects of climate and nonclimate stressors, and interaction between climate change 

and nonclimate stressors. More information and the rationale for ranking ecosystem 

vulnerability by using these criteria are given in appendices A, B, and C.

Ponderosa pine ecosystems are increasingly being studied on national forest lands 

as well as in other areas throughout the Rocky Mountain Region. We draw from this literature where studies are within the Rocky Mountain Region. Even within this Region, 

these ponderosa pine ecosystems vary greatly. Application of results to ponderosa pine 

ecosystems on individual national forests will require an understanding of local site 

characteristics and the site characteristics of the research studies reviewed here.
152 USDA Forest Service RMRS-GTR-376. 2018

Vulnerability of Ponderosa Pine Ecosystems to Nonclimate Stressors___

Summary of Key Vulnerabilities to Nonclimate Stressors

BOX 7.1

Current status of ecosystem extent

Ponderosa pine ecosystems are widespread in the lower montane zone of the Rocky Mountains, and occur in 

isolated and higher areas of the Great Plains within the Rocky Mountain Region. Consequently, their vulnerability 

based on current extent is low. 

Human influences on ecosystem

Regionally, ponderosa pine ecosystems are considered to be moderately vulnerable to the legacy of past human 

influences and to the stressors associated with ongoing human influences.

• Intensive livestock grazing and 20th-century fire suppression have altered the structure and function of 

ponderosa pine ecosystems. 

• Fire exclusion has created denser forest structures and has altered the fire regime.

• Forest management, urbanization, road building, and recreational uses have fragmented ponderosa pine 

forests in some areas.

• Invasive species have been introduced and have altered the function of ponderosa pine ecosystems. 

Intrinsic resilience of ecosystem to nonclimate stressors

A moderate vulnerability ranking is given for the intrinsic adaptive capacity of ponderosa pine ecosystems to 

nonclimate stressors.

• Factors that enhance the resilience of ponderosa pine ecosystems to nonclimate stressors:

○ These ecosystems occur over a wide range of soil and site conditions.

○ Plant species are generally fire-tolerant or fire adapted with understory species recovering quickly. 

• Factors that lower the resilience of ponderosa pine ecosystems to nonclimate stressors:

○ Regeneration is sensitive to moisture availability and seed dispersal ability. Periodic masting is limited, 

delaying recovery in response to disturbance.

○ Ponderosa pine trees are susceptible to bark beetle outbreaks, which have caused widespread mortality 

in recent years.

Future trends of nonclimate stressors

Ponderosa pine ecosystems are considered to be very highly vulnerable to future nonclimate stressors. Current 

stresses associated with demand for natural resources, recreation use, and the expansion of the wildland-urban 

interface are likely to continue.

Current Status and Human Influences______________________________

Euro-American settlement of the late 1800s brought extensive timber harvest and 

large numbers of livestock to montane areas of the Rocky Mountain Region (Baron 

2002; Veblen and Donnegan 2005). Research shows that domestic livestock grazing reduces tree seedling competition and can promote denser tree establishment (Milchunas 

2006). Grazing also reduces understory biomass, disturbs and compacts soils, increases 

soil erosion, and reduces the fast-drying dead or live grass, leaves, and needles that 

formerly carried fires. The heavy grazing associated with 19th-century settlement very 

likely altered understory plant composition and productivity; however, lack of historical 

grazing information makes assessing the effects of grazing on specific areas challenging 

(Veblen and Donnegan 2005). Grazing has influenced riparian areas more than upland 
USDA Forest Service RMRS-GTR-376. 2018 153

areas (Fornwalt et al. 2009). Grazing after the arrival of Euro-American settlers probably contributed to reduced fire occurrence (Veblen et al. 2000).

Ponderosa pine forests have had a long history of timber harvest in the Rocky 

Mountain Region. Logging of ponderosa pine along the Front Range of Colorado was 

widespread from the mid-1800s to the early 1900s (Veblen and Lorenz 1991). Before 

the establishment of the USFS in 1897, forests in the Black Hills were extensively 

clearcut (Boldt and VanDeusen 1974; Graham et al. 2015). Forests in southwestern 

Colorado were also harvested and were affected by domestic grazing in the early 19th

century (Fulé et al. 2009). Harvest of trees continues today (USDA 2017). The most 

timber comes from the highly productive ponderosa pine stands in the Black Hills 

National Forest in South Dakota. Timber harvest and road building have fragmented 

landscapes, which can harm wildlife (Anderson and Crompton 2002; Montgomery et al. 

2013; Reed et al. 1996). Of the 20 commonly seen bird species in the Black Hills, shelterwood cutting resulted in no effect on 6 species, greater abundance in 11 species, and 

less abundance in 3 bird species (Anderson and Crompton 2002). However, red-breasted 

nuthatch (Sitta candensis), brown creeper (Certhia americana), and ovenbird (Seirus 

aurocapilla), species associated with dense forests, were more abundant in untreated 

stands, suggesting a balance of treated and untreated stands is necessary to maintain 

interior forest bird species on the landscape (Anderson and Crompton 2002). Thinning 

can reduce the risk of crown fires (Hunter et al. 2007). Thinning, in some cases, has also 

reduced the risk from bark beetle (Dendroctonus spp.) outbreaks (Fettig et al. 2014). 

Researchers found, however, that not all silvicultural treatments are effective when high 

bark beetle populations exist (Schmid and Mata 2005; Six et al. 2014; see additional 

discussion in Likelihood of Managing Climate Change Effects). 

Fire suppression has resulted in ponderosa pine forests that are more similarly 

aged, less patchy, and denser in Arizona (Fulé et al. 1997), the Colorado Front Range 

(Kaufmann et al. 2000; Veblen et al. 2000), and Montana and Idaho (Naficy et al. 2010). 

Of importance is understanding which habitats, prior to fire exclusion, were formed by 

a low- or high- or mixed-severity fire regime; this information is critical for determining 

where and whether fuel conditions are outside historical variability. In low-elevation 

Front Range forests, Sherriff et al. (2014) determined that forests historically had a 

low-severity fire regime. Mixed-severity fires (areas of high severity and low severity) 

are typical of higher elevation ponderosa pine forests in the Colorado Front Range 

(Schoennagel et al. 2011; Sherriff et al. 2014). Using historical observations and fire 

modeling, Sherriff et al. (2014) determined that less than 20 percent of the Colorado 

Front Range has seen a shift from frequent surface fires to less frequent, higher severity crown fires, and this shift is predominantly at elevations lower than 7,200 ft. In 

southwestern Colorado, fire regimes in the dry mixed-conifer forests before European 

settlement have been described as low severity (Brown and Wu 2005; Fulé et al. 2009). 

The mesic gradient of these dry mixed-conifer forests has a mixed-severity fire regime, 

similar to the mesic mixed-conifer forests (Tepley and Veblen 2015). 

Expansion of the wildland-urban interface has occurred across lower montane 

areas (Theobald and Romme 2007). Colorado and Wyoming were among the 16 States 

with the greatest proportional expansion of wildland-urban interface from 1970 to 2000. 

Based on data for 2000, Colorado and Wyoming (all vegetation types) were found to 

have about 50 percent and 78 percent, respectively, of their wildland-urban interface 
154 USDA Forest Service RMRS-GTR-376. 2018

within high-severity fire classes. Expansion of human communities has fragmented 

and reduced habitats, increased wildfires, contributed to water and air pollution, and 

spread invasive and nonnative species (Hansen et al. 2005). Theobald and Romme 

(2007) projected that Colorado wildland-urban interface would increase 21 percent by 

2030. Additionally, high numbers of recreationists use Rocky Mountain Region national 

forests—about 28.3 million visits per year between 2011 and 2015 (USDA FS 2015b). 

Construction of recreation travel corridors (roads and trails) fragments ecosystems. 

Heavy recreational use in some areas has degraded these landscapes (Hansen et al. 

2002). Along recreational trails, researchers found increased destruction of bird eggs 

and nestlings by predators (Miller et al. 1998). Human activity along trails can affect interactions between wildlife predators and prey, such as elk (Cervus elaphus) and wolves 

(Canis lupus) in Canada, in complex ways (Rogala et al. 2011). 

Invasive and Nonnative Species

Many species of invasive plants have been introduced to ponderosa pine ecosystems (Brooks et al. 2016; Fornwalt et al. 2010; Symstad et al. 2014). The most 

frequently mentioned include cheatgrass (Bromus tectorum L.), leafy spurge (Euphorbia 

esula L.), Canada thistle (Cirsium arvense), and spotted knapweed (Centaurea biebersteinii DC). These and other invasive species have altered ecosystem function and 

nutrient cycling, displaced native plants and animals, increased fire probability and erosion, reduced biodiversity and forage, and displaced native species (Brooks et al. 2016; 

Kohl et al. 2012; Seig et al. 2003). One mega-wildfire introduced new invasive species 

to the understory of a Front Range ponderosa pine forest; however, native species accounted for more than 89 percent of cover in burn-affected communities at 10 years 

postfire (Abella and Fornwalt 2015; Fornwalt et al. 2010). Seventeen exotic species 

increased their presence postfire, including the following noxious weeds in Colorado: 

cheatgrass, musk thistle (Carduus nutans), Canada thistle, butter and eggs (Linaria vulgaris), and common mullein (Verbascum thapsus) (Abella and Fornwalt 2015).

Intrinsic Resilience of Ecosystem to Nonclimate Stressors____________

Factors That Enhance Resilience to Nonclimate Stressors

Plants are more resilient to nonclimate stressors when they have adapted to a wide 

range of environmental conditions. Ponderosa pine trees can grow in a wide variety of 

soil conditions, including low nutrient status and a wide range of soil pH (Oliver and 

Ryker 1990). 

Ponderosa pine and associated understory species are generally classified as fireand drought-tolerant. Studies show ponderosa pine of 2-in diameter at breast height 

survives most surface fires because of its insulating thick bark (Battaglia et al. 2009; 

Schoennagel et al. 2011). Grasses, forbs, and shrubs found in ponderosa pine ecosystems can regenerate or resprout quickly. In the Black Hills, Lentile et al. (2005) found 

regeneration densities varied by burn severity: about 1,500 and 1,100 seedlings ac-1 in 

low and moderate burn severity patches, respectively, with no regeneration in the interior of high burn severity patches. Fornwalt and Kaufmann (2014) documented no impact 

or a stimulatory impact to the understory vegetation in a ponderosa pine/Douglas-fir 
USDA Forest Service RMRS-GTR-376. 2018 155

forest 5 years after the Hayman fire in Colorado. At 10 years postfire and with a decade 

of below-average precipitation, Abella and Fornwalt (2015) concluded that burning 

resulted in diverse and productive native understories in the ponderosa pine/Douglas-fir 

forest in the Colorado Front Range. 

Among forest types, ponderosa pine forests have high biodiversity of birds and 

mammals (e.g., compared to lodgepole pine, Douglas-fir, pinyon-juniper, spruce-fir) 

(Finch and Ruggiero 1993). Taken together, the environmental tolerances of bird and 

mammal species contribute to strengthening the adaptive capacity of the ponderosa pine 

ecosystem to changes from disturbances such as fire and bark beetle outbreaks (i.e., 

nonclimate stressors). Some wildlife species are dependent on different successional 

stages of the ponderosa forest structure, such as mature ponderosa pine forests. Other 

species in ponderosa pine ecosystems benefit from bark beetle outbreaks and associated 

tree deaths. By increasing the amount of snags on a landscape, high-severity fires have 

benefited the black-backed woodpecker (Picoides arcticus), which uses postfire habitats 

in ponderosa pine forests.

Factors That Lower Resilience to Nonclimate Stressors

Ponderosa pine establishment is localized; the heavy cones with small winged 

seeds do not disperse far from seed-producing trees (Oliver and Ryker 1990). This 

limited ability to spread in large areas of fire-caused tree mortality can potentially slow 

recovery. Ponderosa pine establishment in the Southwest and Front Range is erratic, and 

dependent on wetter than average conditions (League and Veblen 2006; Savage et al. 

1996, 2013). In the Black Hills, ponderosa pine tends to be more prolific (Shepperd and 

Battaglia 2002), as most moisture falls during the growing season. Seed crop periodicity 

varies across the Rocky Mountain Region: Colorado Front Range, 4 to 6 years or more; 

Southwest, 3 to 4 years; Black Hills, 3 years (Oliver and Ryker 1990). Mooney et al. 

(2011) found high interannual variability and synchrony in the production of both pollen 

and seed cones within populations along the Front Range. Synchrony among populations was evident at local scales, but was less so across elevations and at distances up 

to 17 mi. The authors suggested this lack of synchrony is the result of highly varied 

topographic conditions and variations in local climate. 

Ponderosa pine forests may not be resilient to high-severity fires, particularly 

when far from a surviving forest. Chambers et al. (2016) studied five 11- to 18-year-old 

Colorado Front Range wildfires. They concluded that regeneration has occurred in high 

burn severity areas. However, regeneration is at densities lower than in unburned and in 

low-to-moderate severity burn areas. Regeneration declined with distance from surviving forest and as elevation decreased. 

Ponderosa pines are at risk of mortality from outbreaks of mountain pine beetle 

(Dendroctonus ponderosae Hopkins), western pine beetle (Dendroctonus brevicomis

LeConte), and pine engraver (Ips spp.) (Negrón and Fettig 2014; West et al. 2012). 

Bark beetle species are present at endemic levels in ponderosa pine ecosystems. With 

favorable conditions, outbreaks can occur, resulting in reduced habitat for some wildlife 

and changes in forest composition. Epidemic mountain pine beetle populations have affected approximately 1.483 million ac of ponderosa pine forests in Colorado, Wyoming, 

Nebraska, and South Dakota (Harris 2014). Warmer temperatures and limited moisture 
156 USDA Forest Service RMRS-GTR-376. 2018

conditions that stress trees have exacerbated bark beetle outbreaks (Negrón and Fettig 

2014; Negrón et al. 2009). Negrón and Popp (2004) reported, as have others, that infestation by mountain pine beetle increased with higher host tree stocking. In northern 

Colorado, they found that plots infested with mountain pine beetle exhibited higher 

basal area and stand density index for ponderosa pine and for all tree species combined. 

In addition, there were higher numbers of ponderosa pine trees per unit area. The likelihood of infestation of a stand was greater than 0.7 when the stand level basal area of 

ponderosa pine exceeded 75 ft2 ac-1. Likelihood of infestation of dominant or codominant individual trees was 0.77 when diameter at breast height was greater than 7.2 in. 

Many other insects, pathogens, and diseases occur in ponderosa pine forests. 

For example, dwarf mistletoe (Arceuthobium M. Bieb. spp.; USDA FS 2015a) and 

Armillaria root disease (Klutsch et al. 2012) can damage the pine. The spatially discontinuous stand structure of ponderosa pine forests can serve to reduce mortality from bark 

beetle outbreaks (Chapman et al. 2012). Outbreaks can also have social and economic 

consequences for people: degrading aesthetic values of forests, reducing timber production, leaving hazard trees on landscapes that pose falling-tree risks for recreationists, 

and declining property values (Gebert et al. 2014; Price et al. 2010).

Future Trends of Nonclimate Stressors_____________________________

Increased population growth, expansion of the urban-wildland interface, and 

increased demands for natural resources and recreational opportunities will continue 

to affect the ponderosa pine ecosystem (Hansen et al. 2002; Theobald 2000). Increased 

fragmentation of forested landscapes, reduced wildlife habitats, and increased spread of 

invasive plant species may result. Effects from a legacy of fire exclusion will continue 

to act as a nonclimate stressor. Combined with increasing human populations and urban 

expansion, these activities may lead to an increase in future fire occurrence. Invasive 

plants are expected to continue to negatively affect ponderosa pine ecosystems. 

Vulnerability of Ponderosa Pine Ecosystems to Climate Stressors______

Capacity for Range Shift

BOX 7.2

Key Vulnerabilities

A low vulnerability ranking is given for the capacity of ponderosa pine ecosystems for range shift.

• Ponderosa pine ecosystems in the Rocky Mountain Region are well north of their southern range limit in 

Mexico. 

• Upslope area is available for range shifts in the mountains of Colorado and Wyoming, but eastern Wyoming, 

Nebraska, and the Black Hills have limited upslope area. 

• Ponderosa pine ecosystems are widely distributed across the Rocky Mountain Region, but are isolated by 

lower elevation grasslands in the Great Plains. This may limit opportunities to migrate into new areas within 

the Region.
USDA Forest Service RMRS-GTR-376. 2018 157

Southern Limit Proximity

Portions of ecosystems that are close to the southern limits of their geographical 

ranges may be more vulnerable to a warming climate than counterparts farther north 

of these edge zones. To assess the southern range limit, we use the native range of the 

Rocky Mountain ponderosa pine variety (Pinus ponderosa Lawson & C. Lawson var. 

scopulorum Engelm.; Oliver and Ryker 1990). It is one of three varieties of ponderosa 

pine in western North America. The Rocky Mountain ponderosa pine variety grows 

in areas east of the Continental Divide in north-central Montana to South Dakota, in 

Wyoming, and in north-central Nebraska. This variety also grows on both sides of the 

Continental Divide in Colorado and New Mexico, extending into Arizona and the eastern edge of the Great Basin in Nevada. Rocky Mountain ponderosa pine is well north 

of the southern limit of its species range: about 300 to 900 mi north of the southernmost 

extent of the Rocky Mountain ponderosa pine variety in northern New Mexico, and 

about 550 to 1,200 mi north of the southernmost extent of the Arizona ponderosa pine 

species varieties (P. ponderosa Lawson. & C. Lawson var. arizonica (Engelm.) and P. 

ponderosa Lawson. & C. Lawson ssp. arizonica (Engelm.) A.E. Murray) in Mexico 

(Oliver and Ryker 1990). As ponderosa pine are well north of their southern range limit, 

a low vulnerability is assigned.

Elevational Range Shift Potential

Potential upslope expansion areas for ponderosa pine forests exist in Colorado 

and Wyoming (Rehfeldt et al. 2006), although range contraction at lower elevations 

could occur (Shafer et al. 2001). Colorado ponderosa pine ecosystems currently occur at 

elevations ranging from about 6,000 to 9,500 ft in the Rocky Mountain Region (derived 

from Comer et al. 2003; NatureServe 2014). More than 9.7 million ac of land area in the 

Colorado Rocky Mountains are above an elevation of 9,500 ft (Cartesian area calculated 

from digital elevation model). Although ponderosa pine forests do not grow above an 

elevation of 9,000 ft on the Bighorn and Medicine Bow Mountain Ranges in Wyoming, 

there is potential for upslope expansion above this elevation. In South Dakota, eastern 

Wyoming, and Nebraska, ponderosa pine ecosystems generally occur at the highest 

elevations available, from about 3,200 to 7,200 ft, with limited potential area to expand 

upslope. 

Areas upslope of where ponderosa pines currently grow could become climatically suitable to support the species (Rehfeldt et al. 2006). Migration into upslope areas 

would depend on a variety of conditions: future fires (frequency, severity, and size) 

that would not restrict ponderosa pine from growing, suitable soils, microclimate, and 

the effects of climate change on existing vegetation. Given the potential for upslope 

expansion of ponderosa pine forests in the Rocky Mountain Region, the potential for an 

elevational range shift is assigned a low vulnerability. 

Connectivity

Ponderosa pine forests are spread throughout the Rocky Mountain Region. In addition to the lower montane in Colorado and Wyoming, they almost entirely cover the 

Black Hills, and higher areas of the Great Plains in eastern Wyoming and Nebraska. 

These forests are absent in vast expanses of low elevation shrublands and grasslands 
158 USDA Forest Service RMRS-GTR-376. 2018

in Wyoming, Colorado, Kansas, Nebraska, and South Dakota. Ponderosa pine trees 

are rare in the northwestern Wyoming area of the Greater Yellowstone Ecosystem (fig. 

7.1). Ponderosa pine trees are widely scattered across the western Great Plains region, 

and grow in isolated areas such as canyons and cliffs. However, large expanses of low 

elevation shrublands and grasslands do not support ponderosa pine ecosystems, which 

would be an inhibiting factor for ponderosa pine migration across the Great Plains part 

of the Rocky Mountain Region. Kaye et al. (2010) observed that ponderosa pine expanded over the past two centuries into the west-central Great Plains, but suggested the 

expansion would be negatively affected by drought under future climate. Given the wide 

distribution of ponderosa pine throughout the Rocky Mountain Region, and the large 

expanses of other vegetation types that could inhibit regional migration, connectivity is 

assigned a moderate vulnerability ranking.

Vulnerability of Cold-adapted, Foundation, or Keystone Species to 

Climate Change______________________________________________

BOX 7.3

Key Vulnerabilities

A moderate vulnerability ranking is given for cold-adapted, foundation, and keystone species of ponderosa pine 

ecosystems, although vulnerability to climate change by species can vary from low to high.

• At lower elevations, the range of ponderosa pine may contract or disappear, particularly in eastern Wyoming, 

Nebraska, and some areas of the Black Hills. At higher elevations, the ponderosa pine range may expand. 

• Some bird and mammal species of ponderosa pine forests (such as cavity nesting birds that use snags 

and feed on bark beetles) may benefit from an increase in disturbance associated with climate change that 

provides more habitat resources. Other species dependent on mature forest structures such as squirrels or 

northern goshawk may decline. Responses of birds to changes in forest structure associated with changes in 

fire regimes will vary by species. 

• Understory grasses and shrubs in ponderosa pine ecosystems are tolerant of drought and heat, and thus are 

expected to have low vulnerability to climate change. They may benefit from more disturbances that result in 

lower tree density and open tree canopies.

We selected a subset of species that live in ponderosa pine forests to capture a 

range of environmental tolerances and roles in ecosystem function. We selected ponderosa pine as it is a foundation species with high biomass, and an important habitat 

resource for wildlife. Mammals and birds are included as keystone species that greatly 

affect the function of ponderosa pine ecosystems; for example, squirrels distribute 

seeds, and cavity nesting birds feed on insects. We also assessed understory plants, as 

they are important food resources for wildlife.

Ponderosa Pine

Considering a variety of climate scenarios and modeling methods, researchers 

projected ponderosa pine extent will expand at higher elevations while declining at 

lower elevations (Bachelet et al. 2000; King et al. 2013; Notaro et al. 2012; Rehfeldt 

et al. 2006; Shafer et al. 2001). Dynamic vegetation models used by Bachelet et al. 

(2000) and King et al. (2013) more fully represent ecosystem processes such as fire, 

carbon dioxide enrichment, and plant interactions; climate envelope models consider the 
USDA Forest Service RMRS-GTR-376. 2018 159

relationships between climate and current species distribution. Using a dynamic vegetation model, King et al. (2013) projected that by the end of the 21st century ponderosa 

pine trees will persist at the ponderosa pine and grassland ecotone in the Black Hills, 

but will decline in productivity, especially during drought. This finding was contrary to 

climate envelope models, which projected ponderosa pine to contract or to disappear 

from Nebraska and eastern Wyoming and some lower elevation areas of the Black Hills 

by mid-century, but continue to expand in the higher elevations of the Black Hills, 

Colorado, and Wyoming (Rehfeldt et al. 2006; Shafer et al. 2001). 

Climate envelope models projected greater pine extinctions and range movement 

expansions compared to dynamic vegetation models (Morin and Thuiller 2009), but 

both model approaches projected some hindrance to ponderosa pine under climate 

change. Hansen and Phillips (2015) reviewed modeling studies focused on the Great 

Northern Landscape Conservation Cooperative in the Northern Rockies and ecosystems surrounding Glacier National Park in Montana and Yellowstone and Grand Teton 

National Parks in Wyoming. They found that the models suggested increased climate 

suitability for ponderosa pine, including in areas east of the Continental Divide where 

historically this pine had low occurrence. They concluded that for this area in Wyoming 

and in the Northern Rockies, ponderosa pine has moderate vulnerability to climate 

change. 

Ponderosa pine in areas that remain climatically suitable under climate change are 

likely to persist if future intensity of drought or fire does not greatly hinder or limit this 

species. Ponderosa pine trees are, therefore, given a moderate ranking for vulnerability 

to climate change.

Birds and Mammals

Bird and mammal populations of ponderosa pine ecosystems can be closely tied 

with forest structure, which is influenced by climate and disturbance. Future changes 

in forest structure caused by warming, drought, bark beetle outbreaks, and fire may 

hinder some wildlife species, while benefiting others. Bird and mammal species tied to 

mature forest structures would be hindered by more fire and bark beetle disturbance, 

which reduces the amount of mature forest on the landscape. Tassel-eared squirrels 

(Sciurius aberti), for example, prefer a dense forest structure, and use mature ponderosa 

pine trees in northern Arizona (Kalies et al. 2012), habitat and food resources that are 

likely to become less abundant under climate change. Northern goshawks live in mature 

conifer forests in Colorado, Wyoming, and the Black Hills. These birds of prey prefer 

the largest trees for nesting areas (Squires and Reynolds 1997), and a loss of mature forest structure from climate change-induced fire and bark beetle disturbance could hinder 

reproduction. Drought and warming could also play a major role in reducing resource 

availability and stressing animals, such as deer (Odocoileus spp.) (Parker and Robbins 

1983) and northern goshawk (Reynolds et al., 2017). Predator birds such as the northern 

goshawk are dependent on the availability of prey, and their populations are projected to 

decline when prey animal numbers decline during drought (Reynolds et al., 2017).

Cavity nesting birds may benefit from changes in forest structure associated with 

warming temperatures, and disturbances that create more snags. Snags in ponderosa 

pine forests provide them with nesting areas and shelter (Howard 2003). Bonnot et 
160 USDA Forest Service RMRS-GTR-376. 2018

al. (2008) found that bark beetle outbreaks in the Black Hills benefited black-backed 

woodpeckers, offering nesting sites comparable in numbers to a postfire landscape. In 

other areas of the Rocky Mountain Region, black-backed woodpeckers did not have as 

positive a response to beetle outbreaks as bark-drilling specialists such as the three-toed, 

downy (Picoides pubescens), and hairy (P. villosus) woodpeckers, which rely on beetle 

larvae (Saab et al. 2014). In ponderosa pine forest across the western United States, 

American robin (Turdus migratorius), mountain bluebird (Sialia currucoides), chipping 

sparrow (Spizella passerina), Cassin’s finch (Haemorhous cassinii), Swainson’s thrush 

(Catharus ustulatus), and warbling vireo (Vireo gilvus) exhibited a positive relationship 

with beetle outbreaks, a larger degree of positive avian response than in lodgepole pine 

forests (Saab et al. 2014). Kotliar et al. (2008) found that three-toed woodpecker response to burn severity was strongly scale-dependent. Burn severity at the largest scale 

(home range) accounted for woodpecker use, but at the smallest scale (individual trees) 

tree size and beetle occurrence were important. Avian relationships with wildfire were 

found to be different across sites with low-severity and mixed-severity fires; researchers suggested that intensive fuels management may be ecologically less appropriate for 

promoting biodiversity in areas where mixed-severity wildfires and dense forest stands 

were historically more common (Latif et al. 2016). 

Fire can also benefit wildlife by enhancing the diversity of native grass species 

(Griffis et al. 2001); mule deer or other grazing wildlife species that use open-canopy 

pine-grasslands may benefit from increased grass after fire. Generalists and mobile 

species, such as deer or birds, are able to move to areas where food resources exist 

(Amberg et al. 2012), a factor that can strengthen their ability to persist under climate 

change with its associated changes in disturbance.

Detailed species-specific information about vulnerability is currently limited; 

further study is needed. Given the current information, however, bird and mammal species of ponderosa pine ecosystems will have varying degrees of vulnerability to climate 

change. Vulnerability is high for some species, such as those dependent on mature forest 

structure, and low for others; still others may opportunistically be able to benefit from 

climate change impacts on the forest structure and composition. 

Understory Vegetation

Amberg et al. (2012) ranked the vulnerability of grasslands in the Badlands 

National Park in South Dakota as “least vulnerable.” Although this assessment was not 

in a forested area, many of the grass species that they assessed also occur in the understory of ponderosa pine ecosystems of the Rocky Mountain Region (e.g., blue grama, 

needle and thread, western wheatgrass [Pascopyrum smithii (rydb.) Á. Löve], and little 

bluestem). Amberg et al. (2012) gave a ranking of moderate vulnerability for the shrubs 

that they assessed, such as sagebrush and chokecherry, which also grow in ponderosa 

pine ecosystem understories.

Shrubs and grasses may become more productive and denser in areas where 

increased fires or bark beetle outbreaks open up dense tree canopies. The 10-year study 

by Abella and Fornwalt (2015) recorded increases in ponderosa pine understory graminoids, and short- and long-lived forbs after the 2002 Hayman fire in the Colorado Front 

Range. Researchers found grass density increased when ponderosa pine trees in the 
USDA Forest Service RMRS-GTR-376. 2018 161

BOX 7.4

Key Vulnerabilities

A high vulnerability ranking is given for the sensitivity of ponderosa pine ecosystems to extreme climatic events. 

• Ponderosa pine ecosystem plants are generally drought-tolerant, but mortality of understory plants and trees 

has been observed under historical drought. Future drought conditions are expected to exceed historical 

levels. 

• Ponderosa pines are very sensitive to the availability of moisture during seed germination, seedling 

establishment, and, in mature trees, seed and cone development and production. Success of ponderosa 

seedlings may also be sensitive to warming temperatures. Increased temperatures and changes in the 

seasonal availability of moisture will increase the vulnerability of ponderosa pine regeneration success.

• Extreme heat events could exceed the environmental tolerances of some plant species, especially during 

drought conditions. 

overstory were removed. Shrubs that can tolerate more shade than grasses have highest 

productivity when the density of ponderosa pine trees is intermediate, as in the Black 

Hills (Uresk and Severson 1989). Given that grasses and most shrubs are highly tolerant of warmer and drier climates and fire, and may benefit from increased disturbance 

associated with climate change, understory plants in ponderosa pine ecosystems are 

expected to have low vulnerability to climate change.

Sensitivity to Extreme Climatic Events_____________________________

Sensitivity to Drought

Drought is the accumulated imbalance between the supply of water and the 

demand for water by plants, animals, the atmosphere, the soil column, and humans 

(Kunkel et al. 2013a,b). Ponderosa pine trees and shrubby or grassy understories are, 

for the most part, tolerant to historical drought conditions (Oliver and Ryker 1990), but 

have suffered mortality or reduced reproduction during drought events. Researchers 

observed that mature ponderosa pine tree deaths occurred in the Southwest during the 

drought periods of the early 2000s and 1950s, when annual moisture was as low as 4 in 

(Allen and Breshears 1998; Ganey and Vojta 2011). Droughts of the 1930s, 1950s, and 

early 2000s in Nebraska greatly reduced ponderosa pine establishment; however, the 

pine did not undergo the diebacks that occurred in the southwestern United States (Kaye 

et al. 2010). Drought in combination with other stressors, such as heat, insect outbreaks 

(Negrón et al. 2009), and topographic position (e.g., south-facing slopes; Hinckley et 

al. 2014), increases the likelihood of mortality in ponderosa pine. Future drought is expected to far exceed historical conditions (Cook et al. 2015) and to have greater effects 

on ponderosa pine than have been observed (Allen and Breshears 1998).

Regeneration of ponderosa pine is strongly linked with moisture availability 

(Mooney et al. 2011; Petrie et al. 2016). Krannitz and Duralia (2004) noted that during 

the 26-month cone development phase, below-average temperatures in late spring can 

kill the second-year conelets. High temperatures during the first year of cone production 

were associated with high cone production. Mooney et al. (2011) observed high interannual variation of tree pollen and cone production associated with climate, with similar 

production patterns at small scales, but wide variation across populations separated 
162 USDA Forest Service RMRS-GTR-376. 2018

by 17-mi and 4,400 ft in elevation. In the Southwest, Feddema et al. (2013) projected 

future reductions in ponderosa pine regeneration, especially where seed producing trees 

were growing in the dry end of climate conditions (presumably around 8 inches a year 

of moisture). The erratic establishment in the Front Range is associated with annual 

variation in spring and fall moisture (League and Veblen 2006; Shepperd et al. 2006). 

Based on an extensive literature survey, Petrie et al. (2016) reported that ponderosa 

pine seedling success is highest at temperatures of 68 to 77 °F with higher precipitation 

and higher moisture availability. A warming climate with decreasing precipitation may 

reduce ponderosa pine seedling success. They cautioned that few studies have explored 

the effects of climate and environmental variables directly in an experimental framework. Greater understanding is needed here to project the potential effects of climate 

change on seedling emergence and establishment for ponderosa pine. 

Cregg (1994) found that ponderosa pine seedlings from South Dakota and 

Nebraska generally survived drought conditions longer than seedlings from central 

Wyoming, Colorado, and especially New Mexico and Montana. Shorter needles, 

less needle surface area, and fewer stomata per needles in the more drought-tolerant 

seedlings could play a role (Cregg 1994). Another explanation may be that stomate 

sensitivity limits water loss under drought conditions, and maximizes growth during 

wet conditions (Zhang et al. 1997). However, warming temperatures with no change in 

precipitation could increase stress on seedlings and affect establishment success in the 

future. Despite the drought tolerance of these trees, future drought in combination with 

higher temperatures would very likely reduce ponderosa pine regeneration and raise the 

potential for a change to a grassland ecosystem (Kaye et al. 2010), especially in drier, 

low elevation areas where ponderosa pine may not recover after a disturbance. 

Although plants within the ponderosa pine ecosystem may tolerate drought of 

historical magnitude and duration, ponderosa pine trees can die from extreme drought 

events. Seedling establishment and survival are sensitive to both temperature and lack 

of moisture. Given that drought intensity and duration are projected to exceed historical 

conditions, ponderosa pine ecosystems are likely to be very sensitive to drought under a 

warming climate.

Sensitivity to Extreme Heat

Ponderosa pine and associated plants are heat-tolerant. Understory plants common 

to ponderosa pine ecosystems can endure high temperatures (e.g., sagebrush: 115 °F) 

(Loik and Harte 1996). Mature ponderosa pine can survive extreme temperatures to 

110 °F, and the pine is more successful in resisting high soil surface temperature with 

increasing age (Oliver and Ryker 1990). Kolb and Robberecht (1996) determined lethal 

temperature limits for ponderosa pine seedlings to be 145 °F at exposure for 1 minute. 

Though ponderosa pine seedlings are not particularly sensitive to extreme heat, they die 

when extreme heat is combined with drying (Oliver and Ryker 1990). Future extreme 

heat events could exceed the heat tolerance of ponderosa pine, grasses, and shrubs in 

some areas. Ponderosa pine ecosystems will be sensitive to extreme heat, especially 

when seedlings are also exposed to drought.
USDA Forest Service RMRS-GTR-376. 2018 163

BOX 7.5

Key Vulnerabilities

A moderate vulnerability ranking is given for the intrinsic adaptive capacity of ponderosa pine ecosystems to 

climate change.

• Factors that strengthen adaptive capacity:

○ Ponderosa pine ecosystems occur across a wide range of climate conditions.

○ Understory species can recover from disturbances quickly. 

○ Ponderosa pine trees have some genetic variability and have life history traits that help them adapt to 

heat and drought.

• Factors that weaken adaptive capacity:

○ Ponderosa pine trees require a long time for reproduction, potentially delaying regeneration and 

persistence under climate change. 

Intrinsic Adaptive Capacity to Climate Change_______________________

Factors That Strengthen Adaptive Capacity to Climate Change

Ponderosa pine ecosystems occur across a wide range of climate conditions, in 

association with a diversity of plants and animals. Patton et al. (2014) found 753 bird, 

plant, and animal species associated with ponderosa pine forests in Arizona. Finch and 

Ruggiero (1993) identified up to 128 bird species and 57 mammals in ponderosa pine 

forests in Colorado, the Rocky Mountains, and the northern Great Plains. Ponderosa 

pine plant communities typically have 3 to 7 dominant understory types; several plant 

community types exist across the region: 12 in the Big Horn Mountains in Wyoming 

(Hoffman and Alexander 1976), 6 in the Black Hills (Thilenius 1971), and 5 in the 

Southwest (Hanks et al. 1983). Taken together, the high level of biodiversity widens the 

range of climates where these species have been found, and contributes to raising the 

adaptive capacity of species in ponderosa pine ecosystems.

Differences in the genetic composition of ponderosa pine trees can strengthen 

adaptive capacity to climate change. For example, studies show that genetic variation 

among ponderosa pine populations is abundant in the Southwest and Great Plains 

(Rehfeldt 1999; Van Haverbeke 1986). Genetic variability among ponderosa pine populations has been observed to differentiate regeneration responses to climate that varies 

geographically across the landscape (Mooney et al. 2010). A range of genetic composition among ponderosa pine widens the degree of potential responses and adaptation to 

variations in climate.

The ability of species to regenerate quickly also strengthens adaptive capacity to 

climate change. Grasses and shrubs found in ponderosa pine ecosystems can regenerate 

or resprout quickly (Griffis et al. 2001), enhancing the ability of plants to persist under 

climate change. 

Ponderosa pine trees have physical characteristics that strengthen adaptive capacity to climate change. The long taproot system of mature ponderosa pine provides 

stability, enabling the trees to persist during floods better than shallow-rooted plants 

(Lawrence 1939). Studies show that ponderosa pine prefers coarse-textured soils of 

sandstone origin, which allow the trees to draw the limited moisture more easily than 
164 USDA Forest Service RMRS-GTR-376. 2018

fine-textured soils of limestone origin permit (Oliver and Ryker 1990), thus increasing 

their capacity to adapt to drought. 

Factors That Weaken Adaptive Capacity to Climate Change

Regeneration of ponderosa pine trees is slower than for understory plants, and can 

be variable. Ponderosa pine trees can take 20 or more years to start producing seeds, 

and viability of seeds from trees between 60 and 160 years is greater than trees outside 

of this age range, younger or older (Oliver and Ryker 1990). Seed crops can be smaller 

than for other conifers, and many birds and mammals can eat large quantities (Shepperd 

et al. 2006). Ponderosa pine establishment is localized; the heavy cones with small 

winged seeds do not disperse far from seed-producing trees (Oliver and Ryker 1990). 

These hindrances to ponderosa pine tree reproduction serve to delay recovery or persistence of trees undergoing climate change, thus weakening adaptive capacity.

Dependence on Specific Hydrological Regime_______________________

BOX 7.6

Key Vulnerabilities

A very low vulnerability ranking is given for ponderosa pine ecosystem dependence on a specific hydrological 

regime. These ecosystems have adapted to a wide range of variability in moisture.

BOX 7.7

Key Vulnerabilities

Management options for mitigating some climate change effects (fire, insect outbreaks, drought) on ponderosa 

pine ecosystems are available. Management approaches to alleviate the direct effects of extreme temperatures 

and temperature variability may be limited. Vulnerability is ranked as low.

Ponderosa pine ecosystems occur in areas that receive moisture as rain or snow, 

and survive in areas that have a wide range of annual precipitation. Regeneration of 

ponderosa pine forests is sensitive to moisture availability, which may be a critical 

factor in a potentially changing climate. Plants and animals in these ecosystems are 

particularly adapted to semiarid conditions and not dependent on a specific hydrological 

regime.

Likelihood of Managing Climate Change Effects______________________

Management options for mitigating the effects of climate change include reducing 

current stressors, enhancing ecosystem resilience, and, as climate continues to change, 

helping plants and animals adapt without substantial loss of soil, soil nutrients, and 

plant cover (Millar et al. 2007; USDA FS 2011). Management actions are available to 

mitigate climate change effects from wildfire, insect attack, and drought. However, successful implementation depends on many factors. One of those factors is the capacity to 
USDA Forest Service RMRS-GTR-376. 2018 165

implement the treatments at the appropriate scale, which for ponderosa pine ecosystems 

may be very large. 

Specific management options exist to reduce the effects of drought. For example, 

where projections suggest a site will be suitable for ponderosa pine, mechanical removal of Douglas-fir trees in a mixed-conifer stand can favor the future survival and 

establishment of ponderosa pine (Underhill et al. 2014). Thinning or prescribed fire can 

reduce competition for resources and increase growth in young trees (Kolb et al. 2007). 

However, management treatments need to consider the possibility that drought may 

have negated silvicultural treatments aimed at growth stimulation, particularly for older 

trees (Kolb et al. 2007). Older trees may also have negative growth response to fire, as 

seen in north-central Idaho (Keeling and Sala 2012). 

Managers have used many practices to reduce wildfire risk, and a careful examination of the treatments is critical in evaluating success or failure. Factors such as the size 

of the treatment, the degree of density reduction, completion of the treatment, historical 

fire regime in the area, and presence of invasive species all contribute to attaining the 

management objective of reducing wildfire risk. Restoring a more open forest structure 

and reducing surface fuels decrease the potential for stand-replacing fires (Fitzgerald 

2005; Reynolds et al. 2013; Strom and Fulé 2007). Lezberg et al. (2008) compared 

prefire and postfire conditions in an experimental area that was burned in the Hayman 

fire in the Colorado Front Range. They reported that the treatments of soil scarification and overstory harvest influenced the severity of the fire. Other studies found that 

reducing fuels and burning prescriptively to reduce wildfire hazard were minimally 

effective because of extreme weather conditions in some instances, such as the Hayman 

fire (Graham 2003) and the Fourmile Canyon fire in Colorado (Graham et al. 2012). 

The scale and capacity of fuels treatments may not be broad enough to have an effect 

in some cases. Additional research to unravel treatment effects and wildfire is needed 

(Lezberg et al. 2008; Rocca et al. 2014), particularly as extreme fire weather is projected 

to be more common in the future (Tang et al. 2015) with the potential to overwhelm the 

effects of fuels treatments.

Researchers found that repeated moderate- and low-severity prescribed burns 

lowered seedling densities and surface fuel loads, and reduced crown fire potential 

and death in ponderosa pine forests in the Black Hills (Battaglia et al. 2008) and New 

Mexico (Hunter et al. 2011). Use of prescribed fire to maintain low densities is influenced by the size of seedlings and saplings and frequency of burns. The effectiveness 

is lost within 10 to 20 years if regeneration densities are not controlled (Battaglia et al. 

2008). 

Fire has been implicated in the spread of invasive and nonnative plant species. 

Symstad et al. (2014) suggested that postfire invasive outbreaks associated with prescribed fires may be reduced if the invasive species are noted and controlled on the 

landscape prior to fire. They acknowledged that finding these invasive species on large 

landscapes before the prescribed fire may be difficult, and suggested that moderating fire 

intensity or targeting areas of high severity for postfire invasive control may be more 

strategic in the Black Hills area. In southwestern Colorado, Korb et al. (2012) found 

that burn-and-thin treatments were more effective in restoring the historical reference 

conditions for the warm and dry mixed-conifer forests; burn-only treatments were not as 

effective, but they were less expensive. 
166 USDA Forest Service RMRS-GTR-376. 2018

Management options have successfully mitigated tree death from bark beetle infestation under particular beetle pressures and spatial scales. Direct methods to address 

current infestations use one or a combination of methods, such as fire, tree removal, 

insecticides, or semiochemicals (chemicals that mediate responses between organisms) 

(Fettig et al. 2014). Some studies show that indirect methods, such as thinning, increase 

tree resilience and change forest stand structure to make forests less favorable for beetles; several studies reported reduced death from beetle outbreaks in thinned ponderosa 

pine stands (Egan et al. 2010; McCambridge and Stevens 1982; Sartwell and Dolph 

1976; Sartwell and Stevens 1975). Studies show higher beetle populations occurred in 

denser ponderosa pine forests, and lower beetle populations occurred in forests where 

managers used thinning and prescribed burning (Fettig et al. 2014; Zausen et al. 2005). 

The Schmid and Mata (2005) study in the Black Hills found growing stock levels (GSL) 

of 60 to 70 ft2 ac-1 had 9 percent of trees attacked, whereas GSLs of 80 to 90 and 100 to 

110 ft2 ac-1 had 53 and 48 percent trees attacked, respectively. They questioned the effectiveness of thinning treatments when treatment areas were surrounded by unmanaged 

stands infested with outbreaks. Although more information is needed on the efficacy of 

thinning treatments, as results are not always consistent, researchers found most of the 

thinning treatments reduced mortality from bark beetle outbreaks (Fettig et al. 2007, 

2014; Six et al. 2014). Additionally, the effects on birds and mammals may not be as expected with thinning, and guidelines for restoration are likely to differ by region (Hutto 

et al. 2014; Latif et al. 2016). 

Despite the availability of several management options for reducing nonclimate 

stressors, these options are limited in their effectiveness in mitigating or completely 

offsetting the direct climate effects, and results may not always be consistent. 

Potential for Climate Change to Exacerbate Effects of Nonclimate 

Stressors, or Vice Versa_______________________________________

BOX 7.8

Key Vulnerabilities

Climate change may exacerbate the magnitude, intensity, and effect of nonclimate stressors. Consequently, the 

vulnerability ranking for ponderosa pine ecosystems is very high.

• Drought exacerbates fire occurrence.

• High-severity fire could increase damage from flooding and the associated increased erosion following fire.

• Warmer temperatures and drought contribute to bark beetle population success and the spatial extent of the 

outbreaks. 

• Increased risk from invasive plant species may also occur with climate change.

Climate and Fire

Climate change may alter fire dynamics. Recent drought has contributed to an 

increase in the frequency of fires larger than 988 ac in fire size, and lengthened the 

fire season since the mid-1980s in the western United States (Dennison et al. 2014; 

Westerling et al. 2014). Researchers expect fire to increase in frequency and to affect 

more area of the Rocky Mountains in the future (Rocca et al. 2014; Westerling et al. 
USDA Forest Service RMRS-GTR-376. 2018 167

2011). The paleo record shows a link between fire occurrence and drought, combined 

with warmer temperatures (Kitzberger et al. 2007; Pierce et al. 2004). Researchers 

project that drought and extreme fire weather will intensify under climate change (Cook 

et al. 2015; Tang et al. 2015). Drought in combination with high-severity fire was found 

to inhibit southwestern ponderosa pine regeneration in New Mexico more than drought 

alone (Savage et al. 2013). Tree death caused by drought may increase as temperatures 

warm, especially at lower latitudes and elevations. The potential increase in fire and 

drought could lower timber production and carbon storage capacity, contribute to 

less productive soils, reduce aesthetic values of landscapes, and create fewer, or less 

dependable, recreation opportunities. How these potential changes in fire will influence 

ponderosa pine ecosystems in the Black Hills, Front Range, and southwestern Colorado 

is complicated by the human-influenced changes in the current fire regimes. 

Events associated with fire or the consequences of fire, such as postfire flooding, 

affect ponderosa pine ecosystems. Ponderosa pine forests are most susceptible to negative effects from flooding during a short-term increased erosion period following fire 

(e.g., 3–6 years for the Colorado Front Range; MacDonald and Larsen 2009). Paleo 

records show that during periods when ecosystems have an increase in fire frequency 

and stand-replacing fires, postfire erosion contributes greatly to long-term erosion (e.g., 

Idaho; Pierce et al. 2004). Areas with rugged topography and narrow canyons, which are 

typical of many ponderosa pine forested areas in the Front Range, are inherently at risk 

from postfire flash floods, and debris flows from summer thunderstorms (e.g., Waldo 

Canyon Fire; Stanley et al. 2015). 

Climate and Bark Beetle Outbreaks

Recent warming has resulted in more frequent and severe bark beetle outbreaks 

(Bentz 2009; Negrón and Fettig 2014). Drought reduces the ability of southwestern ponderosa pine to resist bark beetle attacks (Negrón et al. 2009). In mixed ponderosa pine 

and lodgepole pine forests, West et al. (2014) found similar susceptibility to mountain 

pine beetle infestations for these two tree species. A high potential exists for other bark 

beetle species that are at their northern range limit in the southwestern United States and 

Mexico to expand their range northward (Bentz et al. 2010) as temperatures increase 

and isotherms shift northward. Evangelista et al. (2011) projected the western pine 

beetle population distribution will move farther north in Colorado and into Wyoming. 

The mountain pine beetle and pine engraver beetle will expand into areas of northwestern Colorado and the Greater Yellowstone Ecosystem. 

Future beetle outbreaks will be sensitive to both climate and changes in vegetation. 

Using a mechanistic, phenology-based demographic model, Bentz et al. (2016) explored 

how climate change will affect thermal suitability for mountain pine beetle population 

growth in the Northern Rockies. This area is outside of the Rocky Mountain Region, 

yet their conclusions about host and beetle interactions may be important for future 

outbreaks in the Region. They reported that the best thermal habitats for mountain pine 

beetle will be at the lowest and the highest elevations. Changes in hosts were not simulated. Drawing on the modeling studies for ponderosa pine, which for this area suggest 

that ponderosa pine will move upslope, they suggested that this migration may be beneficial. Ponderosa pine would be moving out of the lower elevations where bivoltinism 
168 USDA Forest Service RMRS-GTR-376. 2018

(two insect generations per year) is projected to be the greatest. Similar detailed analysis 

would be beneficial for the Rocky Mountain Region. 

Climate and Invasive Species

Researchers expect that some invasive plants will expand in the Rocky Mountain 

Region as a result of climate change, while others may contract (Bradley et al. 2009). 

Leafy spurge will contract, according to projections (Bradley et al. 2009). Cheatgrass 

and spotted knapweed are likely to shift ranges, potentially expanding into mountain 

and grassland regions of Colorado and Wyoming (Bradley et al. 2010). This expansion 

of invasive and nonnative species could disrupt ponderosa pine ecosystem function by 

reducing soil moisture, increasing fire probability, reducing biodiversity and forage, 

displacing native species, and increasing erosion (Bradley et al. 2010). 

Summary of the Regional Vulnerability of Ponderosa Pine Ecosystems__

The overall regional vulnerability ranking for the ecosystem is a combination 

of the vulnerability to nonclimate stressors and the vulnerability to climate stressors. 

For ponderosa pine ecosystems, the mean overall vulnerability ranking was moderate 

(fig. 7.2, table 7.1), as was the ranking by each expert reviewer. Mean vulnerability 

rankings were moderate for both the nonclimate stressor vulnerability and the climate 

vulnerability (fig. 7.2). Expert reviewer rankings ranged from the low to high category 

for the nonclimate stressor vulnerability and were in the moderate category for the climate vulnerability.

The expert reviewers had high overall confidence in the ranking of the overall 

regional vulnerability (fig. 7.3). The five experts’ individual rankings were in the high 

or very high category (table 7.2). Mean confidence was high for the nonclimate stressor 

vulnerability ranking, but individual experts’ confidence rankings ranged from the 

moderate to the very high category (fig. 7.3a). Mean confidence was also high for the 

climate vulnerability ranking, with consistently high confidence across all reviewers 

(fig. 7.3b). 

Figure 7.2—Overall regional vulnerability ranking 

and confidence ranking for the ponderosa pine 

ecosystem in the Rocky Mountain Region. 

The large arrow points to the mean score for 

vulnerability and for confidence. Bar represents 

the range of scores.
USDA Forest Service RMRS-GTR-376. 2018 169

Table 7.1—Original and reviewer rankings for the regional vulnerability assessment of ponderosa pine ecosystems in the Rocky 

Mountain Region. Underline indicates a reviewer score change from the original assigned score. 

Criterion 

Original 

score

(Reviewer 1) 

Score

(Reviewer 2) 

Score

(Reviewer 3) 

Score

(Reviewer 4) 

Score

(Reviewer 5) 

Score

Reviewer 

criterion 

rank mean

Nonclimate

1. Extent 3 3 3 1 3 1 Low

2. Human influences 3 3 3 3 3 3 Moderate

3. Resilience 3 3 3 1 3 3 Moderate

4. Future trends 5 5 5 5 5 5 Very high

Total 14

High

14

High

14

High

10

Low

14

High

12

Moderate

13

Moderate 

Climate

1. Ecosystem shift 3 3 1 1 3 3 Low

2. Species groups 3 3 3 1 3 3 Moderate

3. Climatic events 3 3 5 5 3 3 High

4. Adaptive capacity 3 3 3 3 3 3 Moderate

5. Hydrology 1 1 1 1 1 1 Very Low

6. Management 1 1 1 5 1 1 Low

7. Interactions 5 5 5 5 5 5 Very high

Total 19

Moderate

19

Moderate 

19

Moderate

21

Moderate

19

Moderate

19

Moderate

19 

Moderate

Overall vulnerability 

rank

17

Moderate 

17

Moderate

17

Moderate

16

Moderate

17

Moderate

16

Moderate

17

Moderate

Figure 7.3—Regional vulnerability of ponderosa pine ecosystems to nonclimate (a) and to climate (b) stressors in the Rocky 

Mountain Region. The large arrow points to the mean score for vulnerability and for confidence. Bar represents the range 

of scores.

a) b)
170 USDA Forest Service RMRS-GTR-376. 2018

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Table 7.2—Reviewer rankings for confidence in the vulnerability assessment of ponderosa pine ecosystems in the Rocky 

Mountain Region.

Criterion 

(Reviewer 1) 

Score

(Reviewer 2) 

Score

(Reviewer 3) 

Score

(Reviewer 4) 

Score

(Reviewer 5) 

Score

Criterion rank 

confidence 

mean

Nonclimate

1. Extent 5 5 5 3 5 Very high

2. Human influences 3 5 3 1 3 Moderate

3. Resilience 3 3 3 5 5 High

4. Future trends 5 5 5 3 3 High

Total 16

High 

18

Very high

16

High 

12

Moderate 

16

High

16 

High

Climate

1. Ecosystem shift 5 1 3 3 5 Moderate

2. Species groups 3 5 5 3 3 High

3. Climatic events 3 5 5 5 3 High

4. Adaptive capacity 3 5 3 3 3 Moderate

5. Hydrology 5 5 5 5 5 Very high

6. Management 5 3 3 5 5 High

7. Interactions 5 5 5 5 5 Very high

Total 29

High

29

High

29

High 

29

High

 29 

High

29 

High

Overall confidence 

rank

23

High 

24

Very high

23

High

21

High

23

High

23

High
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Chapter 8. Great Plains Streams and Riparian 

Areas: Vulnerability to Nonclimate and Climate 

Stressors in the U.S. Forest Service Rocky 

Mountain Region

Janine R. Rice, Dave Winters, Claudia Regan, Rick Truex, and Linda A. Joyce

Quick Look: Great Plains Streams and Riparian Areas in the Rocky Mountain Region

Perennial and intermittent streams are important ecological features of the Great Plains landscape. Most of the Great Plains streams within the boundaries of national forests and grasslands 

have their headwaters on the Great Plains or east of the Rocky Mountains. They function very 

differently from mountain streams. Their hydrology is influenced primarily by local precipitation 

and the availability of groundwater; consequently, these streams tend to have highly variable 

streamflow. Intermittent streams are characterized by having periodic pools of standing water, 

remnants of streamflow, or input from springs. Great Plains intermittent and perennial streams 

are tributaries of larger perennial rivers on or outside of the national forests and grasslands; 

these rivers tend to have an east-west orientation on the Great Plains. 

Plant and animal species that live in riparian areas along these streams vary with moisture 

availability, as well as water quality. Most of the precipitation generally falls during spring and 

early summer. The seasonal flooding associated with this high precipitation can create beneficial 

conditions by connecting habitat, enabling animal movement, and supporting reproduction. 

Drought hinders plants and animals by fragmenting and reducing aquatic habitat temporarily 

until overland flow occurs. Highly variable and extreme temperatures, as well as highly variable 

streamflows, create harsh conditions that are near the tolerance limits of some aquatic species. 

Plants and animals inhabiting these harsh, highly dynamic environments have evolved life history strategies to successfully survive and reproduce. 

Quick Look: Vulnerability Assessment of Great Plains Streams and Riparian Areas in 

the Rocky Mountain Region

Vulnerability to nonclimate and climate stressors: Very high

Confidence: Very high

Exposure: Warming temperatures, reduced flows in streams, and potentially increased intermittent nature of streams. 

Current extent: Perennial and intermittent streams and rivers on national forests and grasslands 

within the Great Plains area of the Rocky Mountain Region include more than 13,000 mi, 

with most of these streams being relatively small first-order streams. 

Sensitivity and adaptive capacity to climate change: Plants and animals associated with Great 

Plains streams are adapted to a variable and extreme hydrological regime. However, increasing water temperatures and declining flow will fragment aquatic habitat and reduce 

the ability of plants and animals to move along riparian corridors. Drought and extreme 

temperatures increase stress on fish and aquatic insects, as well as on riparian plants and 

animals. Management options to mitigate the effects of climate change on aquatic systems 
180 USDA Forest Service RMRS-GTR-376. 2018

will require a landscape-scale approach, as these Great Plains streams are interconnected. 

Climate change will exacerbate the stressors caused by human uses of this landscape.

Nonclimate stressors: Streams and their associated riparian areas on national forests and grasslands may occur in watersheds where a mosaic of ownerships exist; thus, agriculture, planted 

pasture, and urban and energy development influence the dynamics of these intermittent and 

perennial streams. The legacy of past land use and the current land uses have hindered aquatic 

life and physical processes in Great Plains streams. Invasive and nonnative species compete 

with native plant and aquatic animal species. Future expansion of the urban area, natural 

gas and petroleum drilling, and water and energy development will continue to stress this 

ecosystem. 

Introduction___________________________________________________

Great Plains streams are highly dynamic and important features on the plains of 

the Forest Service, U.S. Department of Agriculture (USFS) Rocky Mountain Region. 

We focus this assessment on ephemeral, intermittent, and perennial streams that flow 

through USFS national grasslands in the Rocky Mountain Region (fig. 8.1). We separate 

streams and rivers by stream order (table 8.1). Most streams on national forests and 

grasslands in the Great Plains are of order 1 and 2. These streams typically have their 

headwaters on the Great Plains (Decker 2007); thus, their hydrology is influenced 

primarily by local precipitation and groundwater. These streams are tributaries of larger 

perennial rivers on or outside of the national grasslands. These rivers tend to have an 

east-west orientation on the Great Plains (figs. 8.1, 8.2). Many of these rivers have headwaters at higher elevation: the Niobrara (east-central Wyoming); Purgatoire, Arkansas, 

and Platte (Rocky Mountains); and Cheyenne (Black Hills). The riparian ecosystems 

associated with these streams are included in this assessment; the streams and relevant 

ecosystems together are henceforth referred to collectively as “Great Plains streams.”

The hydrological regimes that govern these Great Plains streams vary widely, from 

the relatively stable groundwater-driven flows in the Sandhills of Nebraska to the highly 

variable rain-driven flash floods in the intermittent and ephemeral streams on other parts 

of the Great Plains (Gage and Cooper 2013). Rain-driven hydrological regimes contribute to a tendency of highly variable discharge, often with intermittent flows, especially 

in headwater areas. Generally, Great Plains streams have highly variable and harsh 

conditions of temperature and highly variable discharge (Matthews 1988) with some 

organisms living very near lethal thermal limits (Matthews and Zimmerman 1990). 

The USFS defines ephemeral, intermittent, and perennial streams as follows 

(USDA FS 2015):

Ephemeral stream. A stream that flows only in direct response to precipitation in the 

immediate locality (watershed or catchment basin), and whose channel is at all other 

times above the zone of saturation.

Intermittent stream. A stream or reach of stream channel that flows, in its natural 

condition, only during certain times of the year or in several years, and is 

characterized by interspersed, permanent surface water areas containing aquatic flora 

and fauna adapted to the relatively harsh environmental conditions found in these 

types of environments. Intermittent streams are identified as dashed blue lines on 

USGS [U.S. Geological Survey] 7 1/2-inch quadrangle maps.
USDA Forest Service RMRS-GTR-376. 2018 181

Figure 8.1—Perennial rivers on USFS national forests and grasslands, in dark blue, and major tributaries, in light blue. 

Rivers are defined as having stream order equal to or greater than 5. (Data source: National Hydrography Dataset 

Plus (Horizon Systems Corporation n.d.). Note: Streams that were identified using crenulation may not be streams 

and these streams were removed or reduced. Intermittent streams are shown in figure 8.2.
182 USDA Forest Service RMRS-GTR-376. 2018

Perennial stream. A stream or reach of a channel that flows continuously or nearly so 

throughout the year and whose upper surface is generally lower than the top of the 

zone of saturation in areas adjacent to the stream. These streams are identified as 

solid blue on the USGS 7 1/2-inch quadrangle maps.

Most of the literature studies intermittent streams, and more often uses the term “intermittent”; however, some Great Plains publications (e.g., Friedman and Lee 2002) have 

focused on ephemeral streams. 

Riparian cover along streams can be limited to grasses and forbs in some areas, 

although cottonwoods (Populus spp.) and shrubs occur in areas with connected floodplains, saturated soils, or nearby spring-fed streams (Decker 2007; Gage and Cooper 

2013). Riparian and aquatic biota have adapted to the highly variable Great Plains 

hydrological regime, which receives the highest amounts of precipitation during the 

spring and early summer. Seasonal flooding can create beneficial conditions by connecting habitat, enabling the movement of organisms, and supporting reproduction, whereas 

drought hinders biota by fragmenting and reducing habitats. As a result, plants and 

animals inhabiting these harsh environments have evolved life history strategies to successfully survive and reproduce (Dodds et al. 2004).

Great Plains streams have been assessed for their vulnerability to climate change. 

As described in Chapter 1, the vulnerability framework has two major components, one 

related to nonclimate stressors and one related to climate stressors. Within the nonclimate 

component, four criteria were used to describe factors of vulnerability: current status of 

ecosystem extent, intrinsic resilience of the ecosystem to nonclimate stressors, human 

influences on the ecosystem, and future trends of nonclimate stressors on the ecosystem. 

Within the climate change component, seven criteria capture the vulnerability of individual species, sensitivity of ecosystem dynamics such as dependence of the ecosystem on 

the hydrological cycle, adaptive capacity of the ecosystem, potential for management to 

mitigate the effects of climate and nonclimate stressors, and interaction between climate 

change and nonclimate stressors. More information and the rationale for ranking ecosystem vulnerability by using these criteria are given in appendices A, B, and C. 

Table 8.1—Great Plains stream and river miles by stream order on National 

Forests and Grasslands lands in the Rocky Mountain Region. Streams are 

less than 5th order; rivers that are not highly influenced by groundwater 

being of 5th or higher order. Data derived from NHD stream segments that 

intersect National Grasslands. Stream segments can extend slightly outside 

National Grassland boundaries and thus stream miles are overestimated by 

a small amount in some cases.

Stream order Stream miles River miles

1 8237 1*

2 2593 6*

3 1493

4 658

5 341

6 22

7

*The Dismal River is a 2nd order stream, and the Middle Loup River is a 3rd order 

stream. Both are in Nebraska and are fed by groundwater. They are defined as Rivers 

in this study. 
USDA Forest Service RMRS-GTR-376. 2018 183

Figure 8.2—Intermittent streams (light blue) and perennial streams (dark blue) on USFS national grasslands within 

the watersheds of Arkansas, Purgatoire, and Cimarron Rivers (black). Watershed boundaries (pink) represent 

the Hydrologic Unit Code 4. Note: National forest grasslands boundaries on the Purgatoire River are small 

and discontinuous and are not visible on the map. (Data source: National Hydrography Dataset Plus (Horizon 

Systems Corporation n.d.). Streams that were identified using crenulation may not be streams and were 

removed or reduced. Data errors may represent perennial streams as intermittent in some cases.
184 USDA Forest Service RMRS-GTR-376. 2018

In using the criteria to assess the vulnerability of Great Plains streams at the level of 

the Rocky Mountain Region, we drew on technical experts to evaluate our synthesis of the 

literature and the assigned vulnerabilities. Great Plains streams were ranked as very highly 

vulnerable overall; vulnerability to the nonclimate component was ranked as very high 

and the vulnerability to the climate component as high. The current nonclimate stresses 

on intermittent and perennial streams across the Great Plains have long been identified as 

endangering these streams (Dodds et al. 2004; Falke et al. 2011; Gido et al. 2010; Perkin 

et al. 2015; Worthington et al. 2014b); this assessment affirms those conclusions. 

Great Plains streams are increasingly being studied on national forests and 

national grasslands as well as in other areas throughout the Great Plains area of the 

Rocky Mountain Region. We draw from this literature where studies are within the 

Rocky Mountain Region. Even within this Region, these streams have great variation. 

Application of results to individual streams on individual national grasslands will 

require an understanding of local site characteristics and the site characteristics of the 

research studies reviewed here. 

Vulnerability of Great Plains Streams to Nonclimate Stressors_________

Summary of Key Vulnerabilities to Nonclimate Stressors

BOX 8.1

Current status of ecosystem extent

A very high vulnerability ranking is given as Great Plains streams have a limited extent in the Rocky Mountain 

Region.

Human influences on ecosystem

Great Plains streams are considered to have very high vulnerability to the legacy of past human influences and to 

the stressors associated with ongoing human influences. 

• Livestock grazing, row-crop agriculture, planted pasture, urban areas, roads, dams and reservoir development, 

and energy development have reduced riparian areas, compacted soils, altered biotic communities, 

destabilized channels, introduced undesirable plants, and resulted in runoff containing fertilizers, pesticides, 

and herbicides.

• Water diversions and groundwater pumping have reduced or completely removed water from streams, lowered 

water tables, and altered natural flows.

• Invasive and nonnative aquatic and plant species have altered ecological processes in Great Plains streams. 

Intrinsic resilience of ecosystem to nonclimate stressors

A moderate vulnerability ranking is given for the intrinsic resilience of Great Plains streams to nonclimate stressors.

• Factors that enhance resilience of Great Plains streams to nonclimate stressors:

○ Aquatic species have adaptive strategies to cope with short-term inhospitable conditions and tend to 

recover relatively fast when conditions allow. 

○ Springs, seeps, and groundwater connections can lower stress associated with water withdrawals to a 

limited extent.

• Factors that lower resilience of Great Plains streams to nonclimate stressors:

○ Some species are not resilient to long-term water loss, fragmentation, pollution, invasive species, or land 

uses.

Future trends of nonclimate stressors

Great Plains streams are considered to be very highly vulnerable to future nonclimate stressors associated with 

human population growth, increased demand for agricultural products, water and energy development, and invasive 

species. 
USDA Forest Service RMRS-GTR-376. 2018 185

Current Status and Human Influences______________________________

Great Plains streams (ephemeral, intermittent, perennial) occur on all Great Plains 

national grasslands in the Rocky Mountain Region. These national grasslands are a 

small portion of the land area in the Great Plains and are often surrounded by a mosaic 

of private and other Federal or State lands. 

Land uses in Great Plains watersheds include livestock grazing, row-crop agriculture, planted pasture, urban areas, roads, dams and reservoir development, and energy 

development (Brown and Froemke 2010; Gage and Cooper 2013; Reeves and Mitchell 

2012; Smith et al. 2014). The proportion of rangeland permanently converted to other 

uses varies from 8 percent in Wyoming to 56 percent in Nebraska; at the county level, 

loss of rangeland varies from 0 to 100 percent (see figure 11 in Reeves and Mitchell 

2012). Streams and their associated riparian areas on national forests and grasslands 

may occur in watersheds where a mosaic of ownerships exist; thus, agriculture, planted 

pasture, and urban and energy development may influence the dynamics of intermittent 

and perennial streams on these lands in national forests and grasslands. 

Uplands and riparian areas are grazed by resident wildlife, such as prairie dogs 

(Cynomys spp.), antelope (Antilocapra spp.), and, in some areas, bison (Bison bison). 

Domestic livestock grazing (cattle, sheep, and horses) continues on national forest lands. 

Generally, effects of grazing, both domestic and wild, vary with intensity and timing of 

grazing, type of grazing animal, grazing behavior of the animal, legacy of past grazing 

onsite, and current grazing management systems, including the time available for some 

recovery. Livestock grazing can have negative effects on aquatic and riparian resources 

when not properly managed (Wohl 2006); these negative effects can include reductions 

in riparian vegetation, compacted soils, altered biotic communities, lower water quality, destabilized channels, and introduction of noxious weeds (Kauffmann and Krueger 

1984). Fire and grazing in the tallgrass prairie have been important disturbances restricting the encroachment of trees under some circumstances (Veach et al. 2014). 

Streams and rivers are impacted by the activities of agriculture, urban and energy 

development, dams and reservoir development, and roads. All of these land uses, but 

especially agriculture, employ herbicides, pesticides, fertilizers, and many other chemicals. In a prairie watershed dominated by intermittent streamflow in first-order higher 

elevation reaches, Dodds and Oakes (2006) found greater total nitrogen, nitrate, and 

phosphorus concentrations in tributaries occupying the lower portions of the watershed, 

closely mirroring the increasing density of row crop agriculture from headwaters to 

lower elevation areas. Increased nitrogen inputs have caused slow-moving water, stagnant water, or lakes to become oxygen-depleted, killing animal life and degrading water 

quality for humans as well as Great Plains stream species (Dodds et al. 2009).

Roads are sources of sediment and contaminants; they can alter drainage patterns in watersheds, increase surface runoff, and contribute to the spread of invasive 

plants. Road crossings can alter natural streamflow by changing stream geomorphology, function, and habitat; Bouska et al. (2010) found that transport of water, sediment, 

and debris during bank-full flows was more like that of natural streams when box and 

corrugated culvert designs were used instead of low-water crossing designs. Studying 

first- and second-order streams, researchers found that fish communities in stream segments isolated by perched road crossings had lower species richness (alpha diversity) 
186 USDA Forest Service RMRS-GTR-376. 2018

relative to fish communities with connectivity to the entire stream network during summer and fall (Perkin and Gido 2012). They also found that fish communities with highly 

connected networks were more homogeneous than those without high connectivity. 

Energy development, particularly oil and gas development, across the Great Plains has 

increased (Reeves and Mitchell 2012), and this rapid development has raised concerns 

about spills and contaminants from wastewater and the associated impact on Great 

Plains streams and groundwater (Gallegos et al. 2015).

Water Use

Reservoirs, dams, and stream diversions, constructed to supply water for a variety 

of uses, regulate streamflow and flooding. Perkin et al. (2015) reported that Great Plains 

streams and rivers are impacted by more than 19,000 anthropogenic barriers. Flow 

alterations can have negative physical effects by reducing meandering loops, altering 

and reducing sediment transport, increasing erosion and down cutting, changing stream 

chemistry and nutrient transport, and narrowing channels (Merritt and Wohl 2006; Wohl 

and Cenderelli 2000). Flow alterations also impact plants and animals by altering composition and abundance of deepwater communities, reducing groundwater-dependent 

places of refuge, and modifying populations of algae, macroinvertebrates, fishes, and 

riparian plants (Falke et al. 2011, 2012; Friedman et al. 1998; Merritt and Wohl 2006; 

Perkin and Gido 2011; Strange et al. 1999; Wohl and Cenderelli 2000). Streams fragmented by water diversion structures can restrict upstream movement by fish. Altered 

flow patterns below dams and narrowing of river channels increase the presence of some 

invasive plant species (Friedman et al. 1998; Merritt and Wohl 2006). Decreased tributary flow increased salinity downstream in the Trans Pecos Region of the Rio Grande 

River with long-term consequences for fish assemblages (Miyazono et al. 2015). All of 

these process changes raise the risk of local extirpation for some aquatic species. 

Across the Great Plains, the effects of water development vary; Perkin et al. (2015) 

found fragmentation of streams to be most affected by water development barriers in 

the eastern parts of the Great Plains whereas stream desiccation was more common in 

the western parts, where extensive groundwater extraction has occurred. Over the last 

50 years, Gido et al. (2010) reported the greatest changes in fish communities across 

Kansas were found in basins in the western part of the State where intense groundwater 

withdrawals and fragmentation by reservoirs had occurred.

Invasive and Nonnative Species

Invasive plant species have expanded considerably, with 20 species affecting the 

Rocky Mountain Region (Pearson et al. 2011). Tamarisk (Tamarix spp.) and Russian 

olive (Elaeagnus angustifolia) are aggressive nonnative species that have altered ecological processes in Great Plains riparian areas (Decker 2007). Nonnative forb species 

are widely established in western Great Plains riparian areas and include (Anderson et 

al. 2006; Decker 2007): 

• creeping bentgrass (Agrostis stolonifera),

• cheatgrass (Bromus tectorum), 

• Canada thistle (Cirsium arvense), 
USDA Forest Service RMRS-GTR-376. 2018 187

• Mexican fireweed (Bassia scoparia [= Kochia scoparia]), 

• purple loosestrife (Lythrum salicaria), 

• sweetclover (Melilotus spp.), 

• common dandelion (Taraxacum officinale), and 

• yellow salsify (Tragopogon dubius). 

Aquatic ecosystems have been invaded by: 

• Asian carp (bighead carp: Hypophthalmichthys nobilis; silver carp: 

Hypophthalmichthys molitrix), and

• American bullfrog (Bufo americana).

More information on manual and mechanical treatments, biological controls, and pesticides to control invasive spread of aquatic species can be found at the National Invasive 

Species Center (2016). 

Aquatic invasive and nonnative species have been intentionally or unintentionally 

introduced to many aquatic ecosystems in the Rocky Mountain Region. Nonnatives, 

such as largemouth bass (Micropterus salmoides), are placed in streams and ponds for 

recreational fishing and there is evidence that they can have negative effects on native 

species of concern (Turek et al. 2013, 2014). Invasive species have caused a decline in 

native populations, altered nutrient cycling, exacerbated disease spread, and disrupted 

food webs (Anderson et al. 2006; Lodge et al. 2000; Rahel et al. 2008; Stromberg et al. 

2007). 

The effects of land use, water use, and the introduction of invasive species are 

often interdependent and interacting. Changes in channel morphology associated with 

nonnative species may contribute to the success or failure of fishes as related to egg and 

larvae transport time in fragmented stream reaches. Another area of concern is evaluating the effects of invasive species on hydrology, especially as a function of their growth 

forms and evapotranspiration potentials (Hall and Rus 2013; Huddle et al. 2011). 

Intrinsic Resilience of Ecosystem to Nonclimate Stressors____________

Factors That Enhance Resilience to Nonclimate Stressors

Great Plains streams have high physical and chemical variability, variations in 

flow, and wide ranges in temperature and precipitation. Species are adapted to stream 

conditions ranging from floods that maintain stream channels and connect habitat, to 

dry conditions that exacerbate stressful abiotic conditions and limit habitat (Dodds et 

al. 2004). Plant and animal species have adapted to these widely varying conditions and 

tend to recover relatively fast when conditions allow (Fritz and Dodds 2004; Matthews 

and Zimmerman 1990). Many species can withstand periods of poor water quality 

(Labbe and Fausch 2000); this ability helps increase resilience to water loss or degraded 

water quality caused by human land uses. Functioning riparian areas enhance aquatic 

species resilience as riparian plants can stabilize banks and provide organic matter to 

aquatic species (Dwire and Kauffmann 2003; Naiman et al. 2005). Springs, seeps, and 

groundwater connections in some Great Plains streams can, on a local level, mitigate 

drier conditions (Burk and Kennedy 2013; Dodds et al. 2004). 
188 USDA Forest Service RMRS-GTR-376. 2018

By building dams, beaver (Castor canadensis) can increase hydrological and 

ecological complexity: The numbers of different species increase, productivity and 

abundance increase, retention and distribution of water and sediment across valley bottoms increase, and floodplains can grow in complexity and size (Baker and Hill 2003; 

Kemp et al. 2012; Westbrook et al. 2011). 

Factors That Lower Resilience to Nonclimate Stressors 

Watershed function is inhibited in the Great Plains as water withdrawals exceed 

the historical dynamics, and transport of sediment or chemicals increases, affecting 

stream animals and plants (Gido et al. 2010; Perkin et al. 2014; Strange et al. 1999). 

Migration of aquatic species can be limited by culverts and other barriers (Perkin and 

Gido 2011). Stream and groundwater withdrawal for human uses alters the variation in 

and amount of streamflow and reduces aquatic habitat (Perkin and Gido 2011). Water 

management can also homogenize the flow regime; the more uniform flow along the 

major river corridors has led to the expansion of forested riparian corridors that facilitate 

species migration westward, resulting in the loss of native riparian and grassland species 

(Knopf 1986). 

Great Plains stream species are not resilient to persistent or continuous degradations to stream habitat and water quality beyond their tolerance levels (Meador and 

Goldstein 2003). Tilled agricultural land that lacks adequate riparian buffers contributes 

increased sediment with high nutrient and pollution inputs to Great Plains streams 

(Dodds and Oakes 2006). Widespread grazing has increased nutrient and sediment levels, altered riparian vegetation, and, indirectly, changed temperature regimes by altering 

stream channel dimensions and reducing shading from riparian shrubs and trees (Belsky 

et al. 1999).

Great Plains stream ecosystems are not resilient to invasions by nonnative aquatic 

or riparian species, or diseases such as viral hemorrhagic septicemia. Invasive species 

may extend their range farther into Great Plains streams (Decker 2007), disrupting ecological processes and replacing native species (Dukes and Mooney 1999; Rahel 2002).

Future Trends of Nonclimate Stressors_____________________________

Future population growth will increase the demand for domestic drinking water 

as well as water associated with land uses such as agriculture and energy. Using projections of population and climate from the 2010 Resources Planning Act Assessment, Foti 

et al. (2012) reported that increased population growth and potential changes in climate 

would increase the vulnerability of water supply in several river basins in the Great 

Plains. These analyses did not consider management to specifically address water use 

efficiency or any climate change mitigation actions. Where urbanization and other land 

uses have led to a modification of natural flow regimes, the associated changes in the 

abundance and composition of native plant and animal communities affect ecosystem 

services, such as water storage and nutrient cycling, that are dependent on particular 

species or functional groups (Strange et al. 1999). These stressors—population growth, 

land use development, and energy development—and the associated air and water quality challenges will continue to affect Great Plains streams.
USDA Forest Service RMRS-GTR-376. 2018 189

BOX 8.2

Key Vulnerabilities

A very high vulnerability ranking is given for Great Plains stream ecosystem capacity for network shift. 

• Opportunities for shifts to cooler northward latitudes are limited because of the general east-west orientation 

of Great Plains streams. 

• Projected drying and warming may further fragment these streams, potentially making range shift responses 

more difficult for aquatic species.

Native fish population declines may continue, and local extirpations may occur 

because of competition with nonnative populations (Rahel and Olden 2008). Between 

2009 and 2012, Asian carps, such as bighead carp and silver carp, invaded three prairie stream tributaries to the Missouri River in South Dakota: Big Sioux, James, and 

Vermillion. These carp may expand their numbers by outcompeting native species. 

Population expansions of invasive plant species, such as tamarisk and cheatgrass, 

are likely to continue (Bradley et al. 2009). New introductions of invasive species may 

occur in currently uninfected areas, and these species may spread more easily because 

of roads, land use changes, urban areas, and agriculture (Bradley et al. 2010), as well as 

streamflow changes (Merritt and Poff 2010).

Vulnerability of Great Plains Streams to Climate Stressors_____________

Ecosystem Capacity for Network Shift

Elevational and Latitudinal Shift Potential

The potential for elevational or latitudinal shifts for species in Great Plains streams 

is very limited. Most intermittent and perennial streams flow into rivers that are generally oriented east-west on the Great Plains landscape (figs. 8.1, 8.2); thus, migration to 

northern latitudes via stream corridors will not be possible. Fish species in these streams 

are not adapted to the environmental conditions of streams at higher elevations, for 

example, in the Rocky Mountains with steeper channel slopes (Brunger Lipsey et al. 

2005). As a result, these species are unlikely to migrate to upstream habitat. 

Some Great Plains riparian plants may be able to migrate to more climatically 

suitable upstream or upslope areas and to more northern latitudes by wind or animal 

dispersal (Perry et al. 2012). Downstream drift in rivers, which is the primary way in 

which seeds are dispersed over long distances (Merritt and Wohl 2002), would not help 

riparian plants migrate upslope. 

Fragmentation 

Increased fragmentation of Great Plains streams associated with changes in 

climate will limit range shift responses to climate change. Great Plains streams are 

fragmented because of highly variable streamflow, which fluctuates seasonally from periods of very high to low or nonexistent flow (Costigan et al. 2015; Lake 2000; Larson 

et al. 2013). Plants and animals have developed adaptive responses to these dynamic 

shifts in habitat, quickly recovering from dry periods (Falke et al. 2012; Spurgeon et al. 
190 USDA Forest Service RMRS-GTR-376. 2018

2014). Agriculture, roads, groundwater pumping, water diversion structures, and water 

withdrawal for human uses have caused more fragmentation (See Human Influences on 

Ecosystem section).

Large, connected areas are necessary for Great Plains watersheds to provide 

habitat for aquatic species (Dodds et al. 2004; Fausch et al. 2002). Flooding or bankfull runoff is an important and natural process that enhances stream connectivity and 

habitat complexity in Great Plains watersheds. Flooding also promotes plant and animal 

diversity and productivity in riparian areas (Friedman et al. 1998; Poff et al. 1997). Fish 

species whose eggs develop while floating downstream, including several species of 

minnows (family Cyprinidae), require more than 200 mi of continuous stream length to 

achieve population stability (Perkin and Gido 2011; Perkin et al. 2010). Eggs of pelagic 

broadcast spawning minnows drift in suspension during their entire development and 

larvae continue to drift until they become free-swimming. Worthington et al. (2014a) 

documented interactions between discharge, a factor in driving transport times of eggs, 

and higher dispersion of habitat patches, a factor in retention of eggs within the river 

network. At low flow velocities, eggs are likely to drop out of suspension, suggesting 

that low flow velocities may override habitat complexity. At high velocities, habitat 

complexity may help keep eggs in suspension. Worthington et al. (2014b) reported a 

higher probability of Arkansas River shiner (Notropis girardi) presence when freeflowing length was greater than 230 mi. Stream connectivity promotes greater diversity 

and higher population densities in fish communities (Perkin and Gido 2011). 

Climate changes that reduce streamflow or reduce the amount of beneficial flooding may magnify the natural level of fragmentation common in Great Plains streams, 

further restricting aquatic species migration and survival (Lake 2000). Climate models 

project reduced streamflow and potentially greater fragmentation in the central and 

southern Great Plains (Perkin et al. 2010). Warmer temperatures may fragment perennial 

streams by creating areas of warmer water that would be unsuitable habitat for some 

aquatic species (Rahel et al. 1996). 

Vulnerability of Foundation or Keystone Species to Climate Change____

BOX 8.3

Key Vulnerabilities

A high vulnerability ranking is given for foundation and keystone species of Great Plains streams.

• Great Plains native fish have a very high vulnerability to higher temperatures, reduced streamflows, and 

habitat fragmentation. 

• Benthic macroinvertebrates are moderately vulnerable to the expected changes in climate, as life history 

behaviors reduce their susceptibility to warming and drying. However, tolerance levels potentially could be 

exceeded.

• Foundation species of riparian plants and animals are highly vulnerable to extreme temperatures and 

drought, which may cause stress, hamper reproduction, or cause local extirpations.

We selected a subset of Great Plains species for assessment of vulnerability to 

capture a range of environmental tolerances and roles in ecosystem function. We chose 

native fish as foundation species; they have a substantial influence on aquatic food 
USDA Forest Service RMRS-GTR-376. 2018 191

sources. Native or nonnative trout rarely occur in Great Plains streams, and are excluded 

from discussion here (see Chapter 6). We assessed benthic macroinvertebrates as keystone species. These small animals, which live in the substrates of the stream channel 

bed and benthos of ponds and lakes, can serve as environmental indicators and are an 

important food source. These animals can be insects, mayflies, stoneflies, caddisflies, 

beetles, midges, crane flies, and dragonflies. Other members of the benthic macroinvertebrate community are snails (class Gastropoda), clams (class Pelecypoda), aquatic 

worms (class Oligochaeta), and crayfish (Cambarus spp.). We also assessed riparian 

plant species, as they have properties associated with foundation species: They substantially influence ecosystem processes by cycling nutrients, dampening environmental 

disturbances, and regulating and filtering runoff. Animals use these riparian habitats for 

breeding, feeding, and shelter. 

Fish 

Researchers found a connection among declines and local extirpations of some 

native Great Plains fishes and higher temperatures, reduced streamflows, and stream 

fragmentation. Local extirpations occur during drier conditions, especially in streams 

that are not fed by groundwater (Falke et al. 2012). Researchers saw a low level of 

genetic diversity when streams had less water (Fitzpatrick et al. 2014). The genetic 

adaptation of fishes to projected warmer temperatures is not likely to keep pace with 

the rate of warming in the Great Plains (Matthews and Zimmerman 1990). Great Plains 

fish are expected to have a very high vulnerability to the warming and drying associated 

with climate change. 

Benthic Macroinvertebrate Species

Benthic macroinvertebrate species can survive in stream habitats that may periodically have little to no flow, as well as in areas where flow is perennial. Some immature 

and adult benthic macroinvertebrates also burrow into the hyporheic zone under the 

stream bed, where adequate water and oxygen are present to sustain them when surface 

flow is absent. Although crayfish and many other benthic macroinvertebrates survive 

drought and flooding by adapting their life history behaviors and movement, Fritz and 

Dodds (2004) reported that recolonization from unaffected areas was a primary means 

of recovery in northeastern Kansas. Burk and Kennedy (2013) noted the importance of 

groundwater-dependent refugia for macroinvertebrate communities in the recent drought 

in Texas. Recovery of communities under future warming and drought will depend 

on the ability of species to adapt, and also the availability of habitat along the streams 

that species may recolonize after drying periods. Consequently, we expect many Great 

Plains benthic macroinvertebrates will be moderately vulnerable to the effects of climate 

change.

Riparian Plant Communities and Riparian-Dependent Animals

Riparian plant communities that grow in hot and sometimes dry conditions during the growing season may suffer intensified heat stress and drying related to climate 

change (Perry et al. 2012). Many riparian-dependent animals (such as birds, insects, 

reptiles, and amphibians) cannot survive in temperatures exceeding 107 to 115 °F (Perry 
192 USDA Forest Service RMRS-GTR-376. 2018

et al. 2012), a threshold which may be exceeded with future climate warming (Kunkel 

et al. 2013a,b). Less water, warmer temperatures, and higher evaporation rates also 

increase the potential for animals (such as bats and birds) to suffer from dehydration 

and lower reproduction rates (Perry et al. 2012). Cottonwoods and willows (Salix spp.) 

are drought-intolerant and may also suffer considerable stress with lower or intermittent 

streamflows from climate change (Rood et al. 2003). We expect riparian plant and animal communities will be highly vulnerable to the warmer and drier conditions that may 

result from climate change. 

Sensitivity to Extreme Climatic Events______________________________

BOX 8.4

Key Vulnerabilities

A high vulnerability ranking is given for the sensitivity of Great Plains streams to extreme climatic events.

• Great Plains streams are very sensitive to droughts, which can reduce aquatic habitat and limit water 

availability in riparian areas. 

• Great Plains streams are moderately sensitive to flooding. Natural floods maintain stream habitat complexity, 

creating connections that enable fish to reproduce and find food resources. However, floods can restructure 

stream channels and riparian areas, and cause short-term disruption of plant and animal communities.

• Aquatic species in Great Plains streams are very sensitive to extreme heat that exceeds species tolerance 

and reduces habitat quality.

Sensitivity to Drought

Many streams on national forest lands in the Great Plains have headwaters that 

originate in the Great Plains. These streams may be more sensitive to drought because 

they are dependent on local precipitation and, possibly, connections with groundwater, 

in contrast to perennial streams and rivers, which receive snowmelt input to runoff. 

Thus, any change in precipitation will have consequences for Great Plains streams. 

Precipitation projections are highly variable, but with warming temperature and no 

change in precipitation, water demands by terrestrial plants could result in increased 

water stress of Great Plains streams.

Drying during summer can result in intermittent or no flows in these streams; 

drought can exacerbate summer drying and fragmentation of the fish habitat (Falke et al. 

2011). Maintaining a base flow throughout the length of a stream may require groundwater discharge to the stream along much of its length (Winter 2007). For intermittent 

streams on the Konza Prairie in Kansas, Costigan et al. (2015) concluded that local 

water-table fluctuations and soil moisture conditions control hydrological responses 

more than do local weather patterns. Burk and Kennedy (2013) noted the importance 

of groundwater-dependent places of refuge for aquatic species. Using 12 different 

future climate projections, Perkin et al. (2010) reported that climate change effects on 

stream fragments will vary regionally; the northern Great Plains may see increases in 

streamflow whereas Nebraska and Kansas in the southern Great Plains may see declines. 

They noted the potential for increased imperilment of pelagic spawning fish because of 
USDA Forest Service RMRS-GTR-376. 2018 193

reductions in streamflow in the central and southern Great Plains. Given the potential 

for the increased harm that drought can bring to Great Plains species, Great Plains 

streams are considered to be very sensitive to drought under climate change.

Sensitivity to Floods

Researchers observed more-intense precipitation in Kansas; in contrast, Colorado, 

Wyoming, South Dakota, and Nebraska did not have a statistically significant trend 

in precipitation intensity over the 20th century (see figure 8 in Groisman et al. 2004). 

Researchers project Wyoming and South Dakota will see statistically significant increases in intense precipitation by the mid-21st century (Kunkel et al. 2013a,b). Increased risk 

of flooding is likely in the northern Great Plains (Shafer et al. 2014), but the picture is 

less clear for most of the Great Plains area of the Rocky Mountain region.

Flooding of Great Plains streams is mostly driven by local storms and typically occurs in spring and early summer (Dodds et al. 2004; Matthews 1988), with 

detrimental or beneficial effects. Great Plains stream flooding can restructure habitats, 

causing increased sediment in streamflows, which may cause short-term disruption of 

communities; flooding also offers benefits in the long term, by maintaining stream and 

riparian habitat complexity (Miller et al. 2003; Poff et al. 1997). Flooding can inhibit 

reproduction, causing eggs to drift and larval fish to be buried under sediment and die 

(Durham and Wilde 2006). Flooding can increase connections and provide migrating 

species with opportunities to migrate off-channel, where they may find habitat with rich 

resources (Dodds et al. 2004; Fausch et al. 2002). Post-flooding connections also help 

fish reproduction, which in turn promotes greater diversity and higher population densities in fish communities (Dodd et al. 2004; Franssen et al. 2006). Post-flood conditions 

stimulate riparian plains cottonwood (Populus deltoides W. Bartram ex Marshall ssp. 

monilifera (Aiton) Eckenwalder), which establish on moist, bare, high areas when flooding subsides (Auble and Scott 1998; Friedman and Lee 2002). Great Plains streams are 

sensitive to flooding, but moderately so, as flooding can provide benefits. 

Sensitivity to Extreme Heat

Climate warming or extreme heat events may exceed the thermal limits for species. These events may reduce growth and reproductive success of riparian plants, 

mammals, and birds, and in some cases result in local extirpations (Perry et al. 2012; 

Whitney et al. 2016). Temperature can affect riparian animals through increased heat 

stress, reducing midday activities, changing sex ratios, and resulting in earlier spring 

phenology (Perry et al. 2012). 

Summer temperatures of some Great Plains streams are already close to the upper 

thermal limits of fishes such as minnows and darters (family Percidae) (Matthews and 

Zimmerman 1990). Studies of fish temperature tolerance have used the critical thermal 

method (CTM), where fish are acclimated to a specific temperature and then exposed to 

gradually rising temperatures until either loss of equilibrium or onset of muscle spasms. 

This endpoint temperature is the CT (critical thermal) maximum. If the fish at this point 

are quickly returned to their pretest acclimation temperature, survival of 100 percent 

is expected (Beitinger et al. 2000). This method may overestimate long-term safe 
194 USDA Forest Service RMRS-GTR-376. 2018

temperatures; fish may suffer native effects at temperatures below these CTmaxima, especially if subjected for prolonged periods (days, weeks). We give ranges of CTmaxima, 

reflecting the different acclimation temperatures used in the studies (table 8.2).

Given the occurrence of high summer temperatures, fish assemblages in Great 

Plains streams would be negatively affected by relatively minor increases in summer 

water temperatures. Extreme heat events may be coupled with stream drying, reducing 

the opportunities for fish to move to cooler areas. Extended periods of extreme heat may 

also indirectly cause water quality degradation, which could exceed species tolerance 

levels (Murdoch et al. 2000). 

High daytime maximum temperatures increase riparian plant heat stress, can 

reduce growth, and may also reduce germination, flowering, fruit ripening, and seed set 

(Perry et al. 2012). Temperatures above 113 °F damage or kill the leaf tissue in most 

plant species (Perry et al. 2012). The western box turtle (Terrapene ornata Agassiz) and 

mud turtles (Kinostermon spp.) are reptiles with temperature-dependent sex determinations, for which small increases in temperature during incubation can shift the sex ratios, 

with consequences to their population dynamics (Perry et al. 2012). High temperatures 

may also affect the ability of birds and mammals to use evaporative cooling and may 

increase mortality from heat stress. 

Table 8.2—Critical thermal maxima (CTmaxima) reported by Beitinger et al. 2000.

Common name (Species name) CTmaximum

Arkansas darter (Etheostoma cragini) 35.0–38.4 °C (95.0–101.1 °F)

Arkansas River shiner (Notropis girardi) 38.6 °C (101.5 °F)

Central stoneroller (Campostoma anomalum) 28.8–37.2 °C (83.8–99.0 °F)

Creek chub (Semotilus atromaculatus) 35.7 °C (96.3 °F)

Fathead minnow (Pimephales promelas) 32.4–40.4 °C (90.3–104.7 °F) 

Johnny darter (Etheostoma nigrum) 30.1–37.4 °C (86.2–99.3 °F)

Plains topminnow (Fundulus sciadicus) 37.0 °C (98.6 °F)

Plains minnow (Hybognathus placitus) 31.8–40.0 °C (89.2–104.0 °F)

Red shiner (Cyprinella lutrensis) 32.0–39.0 °C (89.6–102.2 °F)
USDA Forest Service RMRS-GTR-376. 2018 195

BOX 8.5

Key Vulnerabilities

A high vulnerability ranking is given for the intrinsic adaptive capacity of Great Plains streams to climate change.

• Factors that strengthen adaptive capacity:

○ Species are adapted to the highly variable hydrological regime, and can quickly recover from 

disturbances such as flooding. 

○ Riparian areas stabilize banks, provide organic matter to aquatic and riparian animals, and help 

modulate flows. 

○ Some streams may receive groundwater contributions from springs and seeps, which can lessen the 

effects of warming and drying.

○ Beaver dams can enhance aquatic habitat.

• Factors that weaken adaptive capacity:

○ Intensified drought (warmth and drying) may exceed tolerances while reducing or degrading the habitat 

of Great Plains stream species. 

Intrinsic Adaptive Capacity to Climate Change_______________________

Factors That Strengthen Adaptive Capacity to Climate Change

Great Plains stream species are adapted to the variability in seasonal hydrological 

patterns, where peak flow or flooding typically occurs during spring and early summer 

(Matthews 1988). Macroinvertebrates, fishes, birds, mammals, and reptiles have different strategies to recover from these seasonal patterns and from flooding. Recovery of 

microbial processes (algae) is typically within weeks, as populations are colonized by 

upstream or airborne propagules (Dodds et al. 2004). Macroinvertebrate species may 

move from places of refuge occupied during the dry period or flood. Those with slower 

life cycles may move 1 to 2 months after the disturbance. The diversity of benthic 

macroinvertebrate species, as well as population numbers, recovers from flooding twice 

as quickly in streams that dry out seasonally compared to perennial streams (Fritz and 

Dodds 2004). Riparian plants can have a relatively fast recovery period after a disturbance (Dwire and Kauffmann 2003). As they recover, they stabilize banks, provide 

organic matter to aquatic and riparian animal species, and maintain cool stream temperatures (Naiman et al. 2005). Riparian areas intercept sediment and retain and process 

nutrients, enhancing adaptive capacity for plants and animals (Naiman and Decamps 

1997). Human activities can increase or decrease adaptive capacity of Great Plains 

streams (see Human Influences on Ecosystem section).

Great Plains fish have adapted to historical drought conditions by migrating to 

pools that maintain water during low flows, reproducing quickly, and withstanding 

periods of marginal water quality and relatively high temperatures (Dodds et al. 2004; 

Matthews and Zimmerman 1990). Some fish species have high genetic diversity, and are 

adapted to the highly variable hydrological regime (Matthews and Zimmerman 1990). 

For example, some Great Plains minnows, chub, and shiners spawn in open water, 

releasing semibuoyant eggs that disperse. This reproductive strategy is an adaptive response to the changeable streamflows that are common in the Great Plains (Durham and 

Wilde 2006). 
196 USDA Forest Service RMRS-GTR-376. 2018

Groundwater connections, springs, and seeps can be important sources of moisture 

during drier conditions. Spring-fed streams or groundwater, along with riparian vegetation, can help maintain suitable aquatic temperatures as air temperatures increase and 

warm these habitats under climate change (Burk and Kennedy 2013; Falke et al. 2012). 

Some Great Plains streams dominated by cottonwoods and willows can support beaver 

populations (Robel et al. 1993). Beavers can increase hydrological and ecological 

complexity by their dam building, which distributes water and sediment across valley 

bottoms and extends floodplains (Baker and Hill 2003; Westbrook et al. 2011). Beavers 

help fish populations by increasing habitat complexity, which in turn promotes species 

diversity and increases population sizes (Kemp et al. 2012). Beaver dams can have a 

negative impact on the movement of fish in some situations, such as in narrow tributary 

streams and under low flows (Kemp et al. 2012). 

Factors That Weaken Adaptive Capacity to Climate Change

Although plant and animal species have developed traits and behaviors for surviving drought conditions common in Great Plains streams, intensified drought may exceed 

species tolerance levels while reducing or degrading habitat (Dodds et al. 2004; Falke et 

al. 2012). Researchers found that species such as minnows, which reproduce by releasing eggs in open water, did not reproduce when streamflow stopped and pools were 

isolated (Durham and Wilde 2006). Great Plains streams can have large channel changes 

during flooding (Dodds et al. 2004), which can bury and kill drifting eggs and larval fish 

(Durham and Wilde 2006). 

Although Great Plains stream species can be highly tolerant to temperature changes, temperature limits do exist, and future climate warming may exceed species limits 

(see Sensitivity to Extreme Climatic Events section). Species could adapt genetically to 

warmer temperatures, but given the expected rapid rate of temperature increases, genetic 

adaption is unlikely (Matthews and Zimmerman 1990).

Dependence on Specific Hydrological Regime_______________________

The hydrological regime for most Great Plains streams on national forests and 

grasslands is rain-driven. Streamflow for most streams occurs in association with 

precipitation events, which have high variability temporally and spatially. Flooding is 

flashy and challenging to predict. Streams may dry seasonally depending on connections with the groundwater table (Hansen 2001). Dodds et al. (2004) described Great 

Plains streams as “life on the edge”; they exist in a precarious balance between flood 

and drying. Species have developed adaptive strategies to tolerate these highly variable conditions: eggs that proceed through their lifecycle development while floating 

BOX 8.6

Key Vulnerabilities

A high vulnerability ranking is given for Great Plains streams, as these ecosystems are dependent on a highly 

variable hydrological regime that may become even more variable as climate changes.
USDA Forest Service RMRS-GTR-376. 2018 197

BOX 8.7

Key Vulnerabilities

Some management options are available to lessen the impacts of climate change on Great Plains streams, but 

they may not always be economically, politically, socially, or ecologically feasible. Consequently, the vulnerability 

ranking is high.

downstream, macroinvertebrates that survive in subsurface water in dry periods, fish 

that can withstand low-quality water for periods of time (Labbe and Fausch 2000; 

Perkin and Gido 2011; Worthington et al. 2014b). 

Although aquatic species in Great Plains streams have evolved in this variable 

hydrological environment, there are limits or thresholds to their tolerances (Burk and 

Kennedy 2013; Fitzpatrick et al. 2014). Increased drought conditions could further 

fragment aquatic habitat, dry permanent pools, or, in extreme conditions, lower the 

water table such that the connection to groundwater is lost (Falke et al. 2011). Increased 

flooding, as well as increased drying or drought, could exceed species tolerance levels 

(Bouska et al. 2015). Thus, Great Plains streams are highly vulnerable to changes in 

hydrological regime. 

Likelihood of Managing Climate Change Effects______________________

Management options for lessening the effects of climate change on aquatic habitats include reducing current stressors, enhancing ecosystem resilience and, as climate 

continues to change, helping plant and animal species adapt without substantial loss 

of soil, soil nutrients, and plant cover (Millar et al. 2007). Managers have successfully 

used methods to reduce current stressors on aquatic communities and enhance resilience 

to climate change across the western United States (Luce et al. 2012; Rieman and Isaak 

2010). These management practices include the following (Fitch et al. 2003; Palmer et 

al. 2009; Perkin et al. 2015; Rahel 2013, Rieman and Isaak 2010; Wohl et al. 2005): 

• maintain or restore natural hydrological processes by removing stock ponds; 

• maintain and restore streamside plants to reduce flooding impacts and sediment 

inputs; 

• maintain or restore riparian, floodplain, and wetland areas, and their connection to 

streams; 

• reintroduce beaver; 

• protect or restore critical or unique habitats that buffer species survival when 

conditions are unfavorable; 

• limit or stop the introduction of nonnative species; 

• eliminate or control the input of pollutants or contaminants to surface water and 

groundwater; 

• remove or modify barriers to the movement of native fish, and intentionally add 

barriers to prevent expansion of invasive species; 

• maintain or reconnect large networks of habitat; 
198 USDA Forest Service RMRS-GTR-376. 2018

• manage domestic livestock grazing to improve condition of riparian vegetation and 

floodplains;

• increase riparian plant cover to provide shade and maintain stream temperature; 

• allow natural changes in stream channels; and 

• minimize soil compaction by limiting roads, trails, and grazing.

As conditions continue to warm and potentially dry further, it may not be possible 

to maintain habitat and species in their current locations. Management options may 

include transporting individuals to otherwise inaccessible habitat or places of refuge to 

maintain genetic diversity and to allow new species to colonize when native fish species 

can no longer survive (Rieman and Isaak 2010). 

Although many management options exist to reduce stressors, they are somewhat 

limited and may not completely lessen the effects of severe flooding, warming, and 

drought. Riparian vegetation can maintain stream temperatures by providing shade, 

but temperature maintenance may or may not be enough to sustain suitable habitats for 

plants and animals. Withdrawing less water for human use can help maintain stream 

habitat, but can conflict with water demands from agriculture and cities, uses that are 

expected to increase in the future. Development of management strategies would need 

to incorporate new information quickly. For example, if some stream habitats where 

species are targeted for restoration develop unsuitably high temperatures, management 

strategies would have to change (Auerbach et al. 2012). 

Potential for Climate Change to Exacerbate Effects of Nonclimate 

Stressors, or Vice Versa_______________________________________

BOX 8.8

Key Vulnerabilities

Climate change may exacerbate the magnitude, intensity, and effect of nonclimate stressors on Great Plains 

streams. Consequently, the vulnerability ranking is very high.

• Climate change may exacerbate the spread of invasive aquatic and terrestrial species that may outcompete 

and replace native species.

• Climate change may increase the demand for surface and subsurface water resources for human activities, 

with potential effects on streamflows and riparian plant communities.

Warmer Temperatures and Invasive Species

Invasive or noxious weed species grow in aquatic and riparian areas of Great 

Plains streams. Nonnative shrubs, forbs, and grasses are widely established in Great 

Plains riparian areas, where they have altered ecological processes (See Human Impacts 

on Ecosystem section). Carp, mollusks, and other nonnative aquatic and semiaquatic 

animals have invaded streams and disrupted ecosystem function. Researchers expect 

that many invasive plant species will expand their range as temperatures warm, displacing native species and disrupting ecological function (Bradley et al. 2009, 2010; Rahel 

and Olden 2008). Tamarisk and cheatgrass will expand their range, according to projections (Bradley et al. 2009). Researchers predict these invasive and nonnative plants and 
USDA Forest Service RMRS-GTR-376. 2018 199

animals will invade more watersheds under climate change, and potentially outcompete 

and replace native species (Rahel and Olden 2008). 

Drought, Warmer Temperatures, and Human Water Use

Great Plains streams can be sensitive to changes in groundwater dynamics. 

Changes in precipitation and longer periods of drought would reduce streamflows unless 

these systems have connections to groundwater. Groundwater pumping has lowered 

the water table in many places in the Great Plains in the past and continues to do so. 

Increased need for irrigation water or water for livestock is likely under climate change. 

Further groundwater pumping would exacerbate drought. Diverting Great Plains 

stream water for human use has various impacts: It reduces habitat connections, alters 

flood regimes, increases or decreases sediment transport, decreases connections with 

riparian floodplains, and alters aquatic and riparian food webs (see Human Influences 

on Ecosystem section). Increasing demand for water resources could result in less 

streamwater and disrupt ecological processes in Great Plains streams. There will also be 

increased pressure to develop reservoir storage capacity; reservoirs have many negative 

ecological effects on stream ecosystems (Franssen and Tobler 2013).

Summary of the Regional Vulnerability of Great Plains Streams and 

Riparian Areas_______________________________________________

The overall regional vulnerability ranking for the ecosystem is a combination of 

the vulnerability to nonclimate stressors and the vulnerability to climate stressors. For 

Great Plains streams and riparian areas, the mean overall vulnerability ranking was very 

high with mean expert reviewer rankings in the high or very high category (fig. 8.3, 

table 8.3). Mean vulnerability rankings were very high for both the nonclimate stressor 

vulnerability and climate vulnerability. Expert reviewer rankings ranged from the high 

to the very high category for both vulnerabilities (fig. 8.3). 

The expert reviewers had very high overall confidence in the ranking of the overall 

regional vulnerability (fig. 8.4). The four experts’ individual rankings were in the high 

or very high category (table 8.4). Mean confidence was also high for the climate vulnerability ranking, but individual rankings ranged from the moderate to the very high 

category (fig. 8.4a). Mean confidence was very high for the nonclimate stressor vulnerability ranking, with individual confidence rankings in the high or very high category 

(fig. 8.4b). Calculations of mean vulnerability and confidence included only the four 

complete reviewer rankings.
200 USDA Forest Service RMRS-GTR-376. 2018

Figure 8.3—Overall regional vulnerability ranking and 

confidence ranking for Great Plains streams in the 

Rocky Mountain Region. The large arrow points to 

the mean score for vulnerability and for confidence. 

Bar represents the range of scores. Only four reviewer 

scores are included in mean calculations here and in 

tables 8.3 and 8.4.

Table 8.3—Original and reviewer rankings for the regional vulnerability assessment of Great Plains streams and riparian areas 

in the Rocky Mountain Region. Underline indicates a reviewer score change from the original assigned score. 

Criterion 

Original 

score

(Reviewer 1) 

Score

(Reviewer 2) 

Score

(Reviewer 3) 

Score

(Reviewer 4) 

Score *

(Reviewer 5) 

Score

Reviewer 

criterion 

rank mean

Nonclimate

1. Extent 5 5 5 5 5 5 Very high

2. Human influences 5 5 5 5 5 5 Very high

3. Resilience 3 3 3 3 5 3 Moderate

4. Future trends 5 5 5 3 5 5 Very high

Total 18

Very high

18

Very high

18

Very high

16

High

20

Very high

18

Very high

18

Very high

Climate

1. Ecosystem shift 5 5 5 5 5 5 Very high

2. Species groups 5 3 5 3 * 5 High

3. Climatic events 5 3 5 3 3 5 High

4. Adaptive capacity 3 3 3 3 3 5 High

5. Hydrology 5 3 3 5 5 5 High

6. Management 3 3 5 3 3 5 High

7. Interactions 5 5 5 5 5 5 Very high

Total 31

Very high

25

High 

31

Very high

27

High

* 35

Very high

30

Very high

Overall vulnerability 

rank

25

Very high 

22

Very high

25 

Very high

22

High

* 27

Very high

24

Very high

* Calculation of mean vulnerability and confidence (table 8.4) included only the four complete reviewer rankings.
USDA Forest Service RMRS-GTR-376. 2018 201

Table 8.4—Reviewer rankings for confidence in the vulnerability assessment of Great Plains streams and riparian areas in 

the Rocky Mountain Region.

Criterion 

(Reviewer 1) 

Score

(Reviewer 2) 

Score

(Reviewer 3) 

Score

(Reviewer 4) 

Score *

(Reviewer 5) 

Score 

Criterion rank 

confidence 

mean

Nonclimate

1. Extent 5 5 5 5 5 Very high

2. Human influences 5 5 5 5 5 Very high

3. Resilience 3 5 5 5 5 Very high

4. Future trends 3 5 3 5 5 High

Total 16

High 

20

Very high

18

Very high 

20

Very high

20

Very high

19 

Very high

Climate

1. Ecosystem shift 5 5 5 5 5 Very high

2. Species groups 3 5 3 * 5 High

3. Climatic events 3 5 3 5 5 High

4. Adaptive capacity 3 5 5 3 3 High

5. Hydrology 3 3 5 5 3 High

6. Management 1 5 5 1 5 High

7. Interactions 5 5 5 5 5 Very high

Total 23

Moderate

33

Very high

31

Very high 

* 31 

Very high

30 

Very high

Overall confidence 

rank

20

High 

27

Very high

25

Very high

* 26

Very high

25

Very high

* Calculation of mean vulnerability (table 8.3) and confidence included only the four complete reviewer rankings.

Figure 8.4—Regional vulnerability of Great Plains streams to nonclimate (a) and climate (b) stressors in the Rocky Mountain 

Region. The large arrow points to the mean score for vulnerability and for confidence. Bar represents the range of scores. 

Only four reviewer scores are included in mean calculations here and in tables 8.3 and 8.4.

a) b)
202 USDA Forest Service RMRS-GTR-376. 2018

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Appendix A: Assumptions and Structure of the Rocky Mountain 

Climate Change Vulnerability Assessment

Ecosystem vulnerability is assessed in a theme-based approach across the Rocky 

Mountain Region. Factors that increase or reduce vulnerability are described in a 

spatially nonexplicit manner, such as along elevational gradients or areas of different 

disturbance regimes. Biological conditions are the primary focus of this assessment, and 

physical conditions, such as topography, geomorphic conditions, and soils are addressed 

as factors that affect biological conditions. This assessment also assumes that if a goal 

of management is to maintain the priority ecosystem, the identification of different components of ecosystem processes and their vulnerability can aid in adaptation planning 

that helps the production of ecosystem services.

This assessment determines the vulnerability to nonclimate stressors and climate 

change stressors separately by summarizing relevant literature for individual criteria. 

For example, factors for intrinsic adaptive capacity to climate are assessed separately 

from past human impacts and resilience to nonclimate stressors such as fire. The interactions between nonclimate and climate stressors are accounted for in one criterion. Four 

nonclimate criteria are used to rank the effects of nonclimate stressors on the priority 

ecosystem as high, moderate, or low (table A.1). Seven criteria are used to rank climate 

change effects on the priority ecosystem (table A.2). See Appendix B for details on the 

rationale for the criteria, and Appendix C for calculation of the final regional vulnerability ranking for the priority ecosystems.

Table A.1—Criteria to assess ecosystem vulnerability to nonclimate stressors.

Criterion to assess vulnerability to 

nonclimate stressors Assessment

Current status of ecosystem extent Determines the amount and distribution of the ecosystem

Human influences on ecosystem Assesses past human activities such as land use for the 

degree of impacts on the ecosystem; assess stressors 

associated with ongoing human influences

Intrinsic resilience of ecosystem to nonclimate 

stressors

Assesses the degree of resilience to nonclimate stressors 

such as fire or anthropogenic influences

Future trends of nonclimate stressors Assesses the likely trend of nonclimate stressors such as fire 

or anthropogenic influences to determine the future degree 

of influence on the ecosystem
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Table A.2—Criteria to assess ecosystem vulnerability to climate stressors.

Criterion to assess vulnerability to 

climate stressors Assessment

Capacity for network shift (aquatic ecosystems)/Capacity 

for range shift (terrestrial ecosystems)

Assesses availability of upstream or upslope area for a 

network or range shift, proximity of terrestrial ecosystem 

to southern range limit, and amount of fragmentation or 

ecosystem connectivity

Vulnerability of cold-adapted, foundation, or keystone 

species to climate change 

Assesses the vulnerability of species that contribute to 

the function of the ecosystem

Sensitivity to extreme climatic events Differentiates the sensitivity of ecosystem components to 

extreme climatic events such as drought, floods, extreme 

heat

Intrinsic adaptive capacity to climate change Identifies factors such as biodiversity or physical 

characteristics that serve to strengthen or weaken 

adaptive capacity to climate change

Dependence on specific hydrological regime Determines the degree of dependence on a narrow and 

specific hydrological regime

Likelihood of managing climate change effects Determines the feasibility of management approaches 

that are available to facilitate adaptation to climate 

change

Potential for climate change to exacerbate effects of 

nonclimate stressors, or vice versa 

Determines the degree of exacerbating interactions 

between climate and nonclimate stressors
210 USDA Forest Service RMRS-GTR-376. 2018

Appendix B: Criteria and Rationale for Assessing Ecosystem 

Vulnerability to Nonclimate and Climate Stressors

A detailed rationale is given for each of the four criteria for nonclimate stressors 

(table B.1) and each of the seven criteria for climate stressors (table B.2). Each criterion 

was ranked as high (score = 5), moderate (score = 3), or low (score = 1). The final 

regional vulnerability ranking for each priority ecosystem is calculated as the mean of 

reviewers’ numerical rankings for nonclimate and climate stressors (table B.3).
USDA Forest Service RMRS-GTR-376. 2018 211

Table B.1—Ecosystem vulnerability to nonclimatic stressors: criteria and detailed rationale.

Current status of ecosystem extent

Rationale for ranking:

Ecosystems that are rarer and less widespread (e.g., alpine lakes confined to isolated mountaintops) are 

less likely to adapt or persist in the future when subjected to nonclimate stressors. But ecosystems that 

are widespread throughout an area or have a widespread and not necessarily continuous distribution 

throughout an area may have a higher ability to adapt.

Rank (Score)

Limited extent (<0 percent) or not widespread in the Rocky Mountain Region High (5)

Moderate extent (10–50 percent) or moderately widespread in the Rocky Mountain Region Moderate (3)

Widespread extent (>50 percent) or widespread in the Rocky Mountain Region Low (1)

Human influences on ecosystem

Rationale for ranking:

Human influence and past land use can negatively or positively affect ecosystems, making them more or 

less susceptible to the effects of nonclimate stressors. For example, overgrazing can displace native plants 

or damage riparian vegetation and cause bank instability. Examples of beneficial influences are silvicultural 

practices that lessen negative impacts to ecosystems, or fuels treatments that reduce severe fire risk.

Rank (Score)

Human influences have had an overall negative effect on ecosystem function

(restoration projects, water management, fire management, vegetation management, logging or fuels 

treatments, introduction or spread of invasive species, transportation, grazing, and recreation management)

High (5)

Human influences have had an overall moderately negative effect on ecosystem function (restoration 

projects, water management, fire management, vegetation management, logging or fuels treatments, 

introduction or spread of invasive species, transportation, grazing, and recreation management)

Moderate (3)

Human influences have had little effect or an overall positive effect on ecosystem function (restoration 

projects, water management, fire management, vegetation management, logging or fuels treatments, 

control of invasive species, transportation, grazing, and recreation management)

Low (1)

Intrinsic resilience of ecosystem to nonclimate stressors 

Rationale for ranking:

Some ecosystems may be intrinsically more resilient to nonclimate stressors because they are adapted to 

a wide range of environmental conditions. This flexibility makes them better able to withstand or recover 

from, for example, fire disturbance or agricultural land uses such as grazing, or fragmentation from logging 

or road building. Ecosystems that have high connectivity and biodiversity also have a higher resilience to 

negative effects from nonclimate stressors.

Rank (Score)

Low ability of ecosystem to be resilient against nonclimate stressors: fire, insects and pathogens, invasive 

species, water use, land use (roads, urbanization, agriculture, recreation, grazing, mining) High (5)

Moderate ability of ecosystem to be resilient against nonclimate stressors: fire, insects and pathogens, 

invasive species, water use, land use (roads, urbanization, agriculture, recreation, grazing, mining) Moderate (3)

High ability of ecosystem to be resilient against nonclimate stressors: fire, insects and pathogens, invasive 

species, water use, land use (roads, urbanization, agriculture, recreation, grazing, mining) Low (1)

Future trends of nonclimate stressors

Rationale for ranking:

Ecosystems that are likely to experience stabilization or a decrease in nonclimate stressor intensities in 

the future are likely to be able to persist better than ecosystems where such stressor trends are likely to 

increase. For example, extensive expansion of the wildland-urban interface, and an increased demand for 

water, forest products, and recreation in some areas, may increase future stresses.

Rank (Score)

Large increase in frequency or intensity (or both) of nonclimate stressors negatively affecting ecosystem: 

fire, invasive species, land use (inholdings, roads, agriculture, urbanization, recreation, grazing, mining, 

mineral extraction, water quality) 

High (5)

Moderate increase in frequency or intensity (or both) of nonclimate stressors negatively affecting 

ecosystem: fire, invasive species, land use (inholdings, roads, agriculture, urbanization, recreation, grazing, 

mining, mineral extraction, water quality) 

Moderate (3)

Little or no increase or lessening of frequency or intensity (or both) of nonclimate stressors negatively 

affecting ecosystem: fire, invasive species, land use (inholdings, roads, agriculture, urbanization, 

recreation, grazing, mining, mineral extraction, water quality) 

Low (1)
212 USDA Forest Service RMRS-GTR-376. 2018

Table B.2—Ecosystem vulnerability to climate change: Criteria and detailed rationale.

1. Capacity for network shift (aquatic ecosystems)

Capacity for range shift (terrestrial ecosystems)

Rationale for scoring aquatic ecosystems:

Stream ecosystems that exist at high elevations are likely to be highly vulnerable to climate change 

(because an upslope migration is not possible). For example, many cold-adapted species in headwater 

streams would not be able to migrate upslope if temperatures became too warm. Upstream migration 

from middle elevation streams may also be constrained, but less so. Low-elevation stream migration 

may be least affected, as low-elevation stream ecosystems have the most potential to extend their 

ranges upslope. Streams that are connected are more likely to support populations and persist under a 

warmer climate. Stream fragmentation inhibits migration and can isolate populations.

Rationale for scoring terrestrial ecosystems: 

Ecosystems that are close to the southern extremes of their biogeographic distributions in the USDA 

Forest Service, Rocky Mountain Region (e.g., whitebark pine) and that may be close to the southern 

edges of their range of climatic tolerances, may be more vulnerable to a warming climate than 

ecosystems that are farther north of these bioclimatic edge zones. Ecosystems closer to the northern 

edge of their current limit in the Rocky Mountain Region (e.g., pinyon pine) may benefit by being able 

to extend northward.

Ecosystems that exist at high elevations (within 1,000 ft of the summits) are likely to be highly 

vulnerable to climate change (because they may not be able to simply migrate upslope). For example, 

many alpine areas in Colorado and Wyoming that exist above elevations of 10,500 to 11,500 ft may 

have an area of less than 3,000 ft for upslope migration. Middle elevation ecosystems may also be 

adversely affected, but less so, and low elevation ecosystems may be least affected, as they have the 

potential to extend their ranges upslope. 

Ecosystems that are biogeographically homogeneous are comparatively free to shift latitudinally or 

upslope across landscapes. This connectivity reduces vulnerability to the changing climate compared to 

ecosystems that are constrained by heterogeneity at a biogeographic scale. Examples of the latter might 

be ecosystems that are separated by mountain ranges or lowlands, large water bodies, or extensive 

urban areas. Rank (Score)

Aquatic:

1) Shifts highly constrained by elevation

2) Highly constrained by fragmentation

Terrestrial:

1) Located close to southern limit (<125 mi) 

2) Located close to highest elevation (<1,000 ft)

3) Highly constrained by lack of connectivity at the biogeographic scale

High (5)

Aquatic: 

1) Shifts moderately constrained by elevation 

2) Moderately constrained by fragmentation

Terrestrial:

1) Located moderately far from the southern limit of distribution (125–250 mi) 

2) Located a moderate distance from highest elevation (1,000–2,000 ft) 

3) Moderately constrained by lack of connectivity at the biogeographic scale

Moderate (3)

Aquatic:

1) Shifts not constrained by elevation 

2) Not constrained by fragmentation

Terrestrial:

1) Located farther from the southern limit of distribution (>250 mi)

2) Located a distance below highest elevation (>2,000 ft)

3) Not constrained by lack of connectivity at the biogeographic scale

Low (1)
USDA Forest Service RMRS-GTR-376. 2018 213

2. Vulnerability of cold-adapted, keystone, or foundation species to climate change

Rationale for ranking: 

Foundation species are those that have substantial influences on community structure as a consequence 

of high population presence. 

Keystone species are those that exert strong effects on the structure of their community, despite a low 

population presence. If there is reason to believe that either foundation or keystone species in an 

ecosystem are particularly vulnerable to climate change, the whole ecosystem type may be in jeopardy.

The elevational range in the USFS Rocky Mountain Region is such that several ecosystems occur in 

colder environments and have species that are cold-adapted or require snow conditions for survival, 

or both. Cold-adapted species, which may or may not necessarily be a foundation or keystone species, 

may also be particularly vulnerable to temperature warming (e.g., ecosystem habitats that depend 

on snow cover in winter). Species tolerant of warmer temperatures may be less vulnerable to climate 

warming. Rank (Score)

Cold-adapted, foundation, or keystone species that are likely to be particularly vulnerable to climate 

change

High (5)

Cold-adapted, foundation, or keystone species unlikely to be vulnerable to climate change Low (1)

3. Sensitivity to extreme climatic events (e.g., drought, floods, extreme heat, freeze dates, windstorms)

Rationale for ranking:

Some ecosystems may be more sensitive than others to extreme climatic events. For example, prolonged 

droughts may eliminate wetlands, while some lakes would persist. Ice storms or windstorms may level 

forests. Extreme heat events may cause mortality or inhibit some biota more than others. Rank (Score)

Highly sensitive to extreme climatic events

Ecosystems that risk being greatly reduced in extent (>50-percent reduction) or eliminated entirely from 

an area by higher frequencies or intensities of extreme events

High (5)

Less sensitive to extreme climatic events

Ecosystems that risk significant reductions in extent (20- to 50-percent reduction) due to an increased 

frequency or severity of extreme events, but that are unlikely to be eliminated from an area

Moderate (3)

Not sensitive to extreme climatic events

Ecosystems that are unlikely to be affected to any significant extent by an increased frequency of 

extreme events

Low (1)

4. Intrinsic adaptive capacity to climate change

Rationale for ranking: 

The diversity within which an ecosystem exists may affect its resilience and adaptive capacity. Ecosystems 

with diverse biotic, physical, and topographic characteristics (high biodiversity; variety in aspects, slopes, 

geologies and soil types, elevations) may be more able to survive climate change than ecosystems that 

are less varied, because the former, by existing across widely differing conditions, may be at lower risk of 

elimination by any future climatic conditions.

Some ecosystems may be intrinsically more resistant to climate stressors because, for example, they 

have more rapid recovery times. Ecosystems in which the recovery period from the impacts of stressors 

is shorter (<20 years) may have greater intrinsic adaptive capacities than slower developing ecosystems 

(recovery times of >20 years). For example, riparian forests (e.g., cottonwood) may take many years to 

recover from a mortality event. This may render the riparian area intrinsically more vulnerable to the 

potential intervening effects of climate change than are ecosystems that have shorter recovery periods 

(e.g., grasslands or willow riparian areas). Rank (Score)

Unlikely to be significant

Climate resilience is lower, rendering it less able to withstand climate change impacts 

High (5)

Moderately significant

Climate resilience is moderate, enabling it to moderately withstand climate change impacts

Moderate (3)

Likely to be significant

Climate resilience of ecosystem type is high, enabling it to better withstand climate change impacts. 

Low (1)
214 USDA Forest Service RMRS-GTR-376. 2018

5. Dependence on a specific hydrological regime

Rationale for ranking:

Some ecosystems are confined to areas with specific and relatively narrow hydrological conditions. For 

example, glaciated valley wetlands are confined to areas where previous glacial activity has created 

areas of moisture collection in valleys. Abundant winter snow cover and cold temperatures create 

a snowmelt- driven hydrological regime. Forested or other vegetated areas are less dependent on a 

specific hydrological regime and tolerate a wider variety of precipitation conditions. Rank (Score)

Ecosystems that are dependent on specific hydrological regime High (5)

Ecosystems less dependent on specific hydrological regime Low (1)

6. Likelihood of managing climate change effects 

Rationale for ranking:

How we are able to manage ecosystems is likely to become an important factor in conserving resources 

under climate change. However, some ecosystems may be less easy to manage than others. For example, 

managing the impacts of climate change on early seral or riverine ecosystems may be easier (e.g., through 

using fire, plantings, water level control) than managing ecosystems that are more intrinsically vulnerable 

to climate change (e.g., high elevation ecosystems). 

Our current understanding of adaptation or mitigation options is theoretical, and limited in application in 

some cases. For some ecosystem types, the management community has developed restoration techniques 

that may be effective management tools for adapting ecosystems to a changing climate. For example, 

techniques to restore the historical fire regime may facilitate adaptation to a different fire regime, provided 

that some understanding about potential shifts in fire regimes is available. Rank (Score)

Not feasible

No known feasible management approaches that could be employed to mitigate effects of climate change

High (5)

Moderately feasible

Management approaches that could moderately mitigate the effects of changing climate already exist and 

have been shown to be effective

Moderate (3)

Feasible

Management approaches that could mitigate effects of changing climate already exist and have been 

shown to be effective

Low (1)

7. Potential for climate change to exacerbate effects of nonclimate stressors, or vice versa 

Rationale for ranking:

For some ecosystems and species, it is likely that significant impacts of climate change will be expressed 

through their exacerbating effects on current or future nonclimate stressors, or conversely, nonclimate 

stressors will exacerbate the effects of climate change. This indicator seeks to capture the potential effects 

of the interaction between climate change and nonclimate stressors. For example, warmer temperatures 

may increase the effects of drought, fire, or insect outbreak (e.g., warmer temperatures, greater drought, 

more stress to trees, greater likelihood of insect outbreak). Conversely, nonclimate stressors such as 

expansion of the wildland-urban interface or people’s increased demand for water or recreation may 

exacerbate the effects of a warmer or drier climate. Rank (Score)

Potential for large increase in stressor impacts

There is a high risk (>50-percent probability) that climate change may exacerbate the impacts of 

nonclimate stressors, or vice versa 

High (5)

Potential moderate

There is a moderate risk (10- to 50-percent probability) of climate change exacerbating the effects of 

nonclimate stressors, or vice versa

Moderate (3)

Potential low

There is only a low risk (<10-percent probability) of climate change exacerbating the effects of nonclimate 

stressors, or vice versa

Low (1)
USDA Forest Service RMRS-GTR-376. 2018 215

Table B.3—Calculation of regional vulnerability score and confidence score for summary of vulnerability. Vulnerability is 

assigned to one of five categories: Very High, High, Moderate, Low, or Very Low vulnerability. 

Vulnerability score: 

[Nonclimate vulnerability sum + Climate vulnerability sum] = Regional vulnerability 

 2

Confidence score: 

[Nonclimate confidence sum + Climate confidence sum] = Regional confidence 

 2
216 USDA Forest Service RMRS-GTR-376. 2018

Appendix C: Vulnerability Scoring Categories

Ranges of scores and the corresponding vulnerability categories for ranking the 

vulnerability of the priority ecosystems are shown in table C.1.

Table C.1. Range of scores and vulnerability categories for mean reviewers’ ranking of ecosystem vulnerability 

to nonclimate stressors, vulnerability to climate change, and overall regional vulnerability.

Nonclimate stressors Vulnerability category

4–6 Very Low Vulnerability

7–10 Low Vulnerability

11–13 Moderate Vulnerability

14–17 High Vulnerability

18–20 Very High Vulnerability

Climate change Vulnerability category

7–12 Very Low Vulnerability

13–18 Low Vulnerability

19–23 Moderate Vulnerability

24–29 High Vulnerability

30–35 Very High Vulnerability

Overall regional vulnerability: Mean of nonclimate 

stressor and climate stressor rankings Vulnerability category

5–9 Very Low Vulnerability

10–14 Low Vulnerability

15–18 Moderate Vulnerability

19–23 High Vulnerability

24–28 Very High Vulnerability
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