United States

Department of

Agriculture

Forest Service

Pacific Southwest

Research Station

General Technical 

Report

PSW-GTR-235 

June 2011

A Review of the Potential 

Effects of Climate Change 

on Quaking Aspen (Populus 

tremuloides) in the Western 

United States and a New Tool 

for Surveying Aspen Decline

Toni Lyn Morelli and Susan C. Carr
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Authors

Toni Lyn Morelli was a research ecologist, Pacific Southwest Research Station, 

800 Buchanan St., Albany, CA 94710; Susan C. Carr is a research ecologist, Rocky 

Mountain Research Station, 240 West Prospect St., Fort Collins, CO 80526. Morelli 

currently is located at the University of California, Berkeley

Cover Photo

Cover picture taken in 2008 in Terror Creek, on the Paonia Ranger District of the 

Grand Mesa Uncompahgre Gunnison National Forest, by Wayne Shepperd, Rocky 

Mountain Research Station, USDA Forest Service. 
Abstract

Morelli, Toni Lyn; Carr, Susan C. 2011. A review of the potential effects of 

climate change on quaking aspen (Populus tremuloides) in the Western United 

States and a new tool for surveying sudden aspen decline. Gen. Tech. Rep.

PSW-GTR-235. Albany, CA: U.S. Department of Agriculture, Forest Service,

Pacific Southwest Research Station. 31 p.

We conducted a literature review of the effects of climate on the distribution and 

growth of quaking aspen (Populus tremuloides Michx.) in the Western United 

States. Based on our review, we summarize models of historical climate determinants of contemporary aspen distribution. Most quantitative climate-based models 

linked aspen presence and growth to moisture availability and solar radiation. 

We describe research results pertaining to global climate change effects on aspen 

distribution and vigor. In addition, we present potential interactive effects related to 

climate change and natural disturbances and insect and pathogen outbreaks. Finally, 

we review the phenomenon of sudden aspen decline in western North America, 

which has been linked to drought and may be exacerbated by future climate change. 

Overall, research indicates a complex, unpredictable future for aspen in the West, 

where increased drought, ozone, and insect outbreaks will vie with carbon dioxide 

fertilization and warmer soils, resulting in unknown cumulative effects. Considering its positive moisture influence on the landscape, its economic impact, and its 

many benefits to the resilience of wildlife in terms of habitat and forage, aspen is a 

valuable, yet vulnerable, species in the face of global warming.

Keywords: Drought, forest health, global warming, Rocky Mountains, Sierra 

Nevada, sudden aspen decline. 
Summary

Given aspen’s desirable effects on the landscape in terms of maintaining biodiversity and conserving water, aspen stands may become increasingly valuable 

resources in the face of climate change. However, moisture stress and root damage 

resulting from a warmer climate may lead to the decline of aspen stands. These 

negative impacts of warmer and drier climates could be offset by the direct benefits 

of elevated carbon dioxide (CO2) on aspen growth and more frequent fires, which 

may increase aspen extent. On the other hand, if migration and regeneration or 

seedling establishment rates are sufficient to adapt to environmental change, aspen 

distribution may simply shift in response to future climate. Overall, research 

indicates a complex, unpredictable future for aspen in the West, where increased 

drought, ozone, and insect outbreaks will vie with CO2 fertilization and warmer 

soils, resulting in unknown cumulative effects. 
Content

1 Introduction

3 Climate Mitigates Aspen Extent

8 How Will Aspen Respond to Future Climates?

11 Sudden Aspen Decline (SAD)

13 Conclusion

14 Acknowledgments

14 Metric Equivalents

14 References

27 Appendix

 Introduction of a Sudden Aspen Decline (SAD) Survey Tool
GENERAL TECHNICAL REPORT PSW-GTR-235

f
A Review of the Potential Effects of Climate Change on Quaking Aspen (Populus tremuloides) in the Western United States

1

Introduction

Despite its wide range and economic and ecologic importance, little is known about 

the influence of climate on the growth and reproduction of quaking aspen (Populus 

tremuloides Michx.) in western North America. Studies related to climate effects 

on aspen distribution are typically models of correlative effects and are clustered 

in a few study regions (e.g., the central Rocky Mountains and Northwest Canada). 

Predicted effects of climate change on aspen are even more poorly understood. Here 

we summarize the results of climate effects studies related to aspen distribution 

and growth, with particular emphasis on the phenomenon of sudden aspen decline 

(SAD) and its connection with climate. Our literature review includes conclusions 

and predictions relative to climate change effects on aspen populations in western 

North America. We focus primarily on the relationship between climate determinants and aspen distribution, including the interaction of climate factors with 

disturbance effects. 

Aspen is the most widespread tree species in North America (Little 1971, 

Mueggler 1988). It is clonal, reproducing by root sprouting (Schier et al. 1985). 

Aspen clones thrive in high-resource environments, specifically high light and 

nutrient levels (Kinney et al. 1997). As a result, two-thirds of western aspen stands 

are seral, giving way to conifers that gradually overtop and shade out aspen starting after about 80 years (Mueggler 1985, Rogers 2002). In spite of this, distinctive 

features like clonality and shallow, widespread root systems make aspen a highly 

resilient species, as stems destroyed by pathogens, insects, or fire are replaced 

by root sprouts (Lieffers et al. 2001). Thus, in the absence of conifer competition, approximately 30 percent of aspen stands in the Western United States are 

hypothesized to be stable instead of seral, persisting in the absence of disturbance 

or climate change (Kay 1997, Sawyer and Keeler-Wolf 1995). Studies in the Rocky 

Mountain region support the idea that aspen regeneration can occur independently 

of natural disturbance (Elliot and Baker 2004, Kurzel et al. 2007, Larsen and 

Ripple 2003, Turner et al. 2003).

Aspen forests can be managed for multiple uses because they are associated 

with water conservation, livestock and wildlife forage, aesthetic value, and economic benefits through increased tourism (Bartos and Campbell 1998, DeByle and 

Winokur 1985). Moreover, aspen stands have been shown to be hotspots of biodiversity (Stohlgren et al. 1999), with aspen considered a keystone species (Bartos 

2001). Aspen forests have higher vascular plant species richness than other forest 

communities of the southern Boreal region (Reich et al. 2001) and support some of 

the highest diversity in the Sierra Nevada (Potter 1998).

Aspen forests can be 

managed for multiple 

uses because they 

are associated with 

water conservation, 

livestock forage, 

aesthetic value, 

economic benefits, 

and are hotspots of 

biodiversity. 
GENERAL TECHNICAL REPORT PSW-GTR-235

2

Aspen stands provide wildlife habitat in the form of structural diversity, 

cavities for nesting, decay among live stems, and a dense understory. Over 100 vertebrate and invertebrate herbivore species can be found in aspen forests (Lindroth 

2008). Aspen stands support some of the highest bird diversity in the United States 

(De-Byle and Winokur 1985, Griffis-Kyle and Beier 2003) and the greatest number 

of bird species in the Sierra Nevada specifically (Richardson and Heath 2004). A 

Colorado study showed that aspen habitat contains more plant and butterfly species 

per area than any of the other major vegetation types (Chong et al. 2001), although 

soil moisture has an effect (Weixelman et al. 1999). Aspen is also important beaver 

(Castor canadensis) habitat (Shepperd et al. 2006).

Aspen stands can help conserve water on the landscape as well. Net water 

consumption by aspen trees, in terms of ground water and surface waterflow, is 

considerably less than that of conifers (Jaynes 1978, LaMalfa and Ryel 2008). 

Researchers have reported decreases of 7.62 to 17.78 cm (Gifford et al. 1984) in 

water yield to the watershed when conifers replace aspen. Aspen’s water conservation is mostly due to its low water efficiency and also to greater snow accumulation 

under aspen (LaMalfa and Ryel 2008); tower-based monitoring of Canadian boreal 

forest sites (Amiro et al. 2006) showed greater annual evapotranspiration from 

aspen forest than from coniferous forests (black spruce [Picea mariana (Mill.) 

Britton, Sterns & Poggenburg] and jack pine [Pinus banksiana Lamb.]). Because 

of their higher moisture content and associated herbaceous understory, aspen 

stands also act as fire breaks (Fechner and Barrows 1976, Peet 2000, van Wagner 

1977); aspen-dominated landscapes are less likely to ignite from lightning fire than 

spruce-dominated landscapes (Krawchuk et al. 2006). In fact, aspen stands have 

been found to be 200 times less likely to burn than spruce-fir stands (Bigler et al. 

2005). Finally, soil under aspen stands retains more nutrients, such as nitrogen, 

potassium, and calcium than soil under conifers (St. Clair 2008).

An ongoing debate is whether aspen populations in the Western United States 

are decreasing outside the range of recent natural variability. Many researchers have 

shown evidence of substantial declines in aspen extent since the mid-19th century 

(Bartos 2001, Gallant et al. 2003, Potter 1998, Wirth et al. 1996), although not in all 

areas (Brown et al. 2006, Manier and Laven 2001). Some researchers hypothesize 

that the discrepancy between current and late 19th -century aspen extent reflects not 

unusual decline in recent decades but merely a return to historical levels after an 

exceptionally large pulse of aspen regeneration from around 1850 to 1920 as a result 

of extensive logging, mining, grazing, and burning in the Western United States 

(Kulakowski et al. 2004, 2006; Shepperd et al. 2006; Smith and Smith 2005). 
A Review of the Potential Effects of Climate Change on Quaking Aspen (Populus tremuloides) in the Western United States

3

Another factor in aspen distributon shifts may have been climate change. Cooler 

and wetter climate conditions at the end of the Little Ice Age appear to have 

increased aspen extent in the Sierra Nevada (Shepperd et al. 2006). The climate 

trends over the last several decades of increasing temperature and reduced moisture 

may explain some of the more recent decline in aspen extent (Hogg et al. 2008, 

Worrall et al. 2008). Although the different hypotheses of human disturbance and 

changing climate are difficult to separate (Millar and Woolfenden 1999), photographic and other evidence confirms that aspen were generally more prevalent 

throughout the Sierra Nevada in the recent past than they are now (Shepperd et al. 

2006), similar to other areas of the West.

Climate Mitigates Aspen Extent

General environmental conditions: aspen growth and distribution in western 

North America—

Climate is a strong determining factor for the growth and distribution of aspen in 

western North America. In general, long-term trends in temperature, precipitation, 

and solar radiation, coupled with environmental conditions, affect the availability 

of aspen habitat. Although aspen can tolerate extremely cold air temperatures, 

cold soils (6 °C or less) stress aspen plants, leading to inhibited root growth and 

decreased water intake (Landhäusser and Lieffers 1998, Wan and Zwiazek 1999). 

Moreover, despite the size and motion of the leaves that prevent overheating and 

stomatal closure and give aspen its name (Roden and Pearcy 1993), aspen trees 

function poorly in hot, dry conditions (Jones et al. 1985b). Photosynthesis declines 

at temperatures greater than 25 °C (Lawrence and Oechel 1983), especially when 

humidity is low (Dang et al. 1997).

Aspen trees in western North America typically inhabit areas where annual 

precipitation exceeds annual evapotranspiration (Jones 1985). In general, stands in 

the Rocky Mountain region occur where total annual precipitation exceeds 38 cm 

per year (Jones 1985, Jones and DeByle 1985). Similarly, aspen stands in Canada 

occur where total precipitation exceeds potential evapotranspiration (PET, a climate 

moisture index) (Chen et al. 2002, Hogg 1994, Hogg and Hurdle 1995). Aspen 

distribution is also related to growing-season precipitation and moisture deficit in 

the Great Lakes region of the United States and Canada (Gustafson et al. 2003, 

Iverson and Prasad 1998). 

The influence of topography and landscape orientation on aspen distribution 

further underscores the effects of temperature and moisture availability. Elevation, 

aspect, and slope affect local climate environment, including length of the frostfree period and temperature extremes. Aspen is restricted to higher elevations and 
GENERAL TECHNICAL REPORT PSW-GTR-235

4

more northerly aspects in the warmer southern regions of its distribution (i.e., the 

southern Rocky Mountains and Baja California), whereas aspen stands commonly 

inhabit south-facing slopes at higher elevations and in the colder parts of their 

extent (Jones et al. 1985b). For example, in the Rocky Mountains, aspen stands 

typically occupy elevations between 1828 and 3048 m (Jones 1985), whereas in 

the boreal and prairie transition regions of northwest Canada, aspen inhabits lower 

elevations (less than 1828 m) (Perala 1990).

Aspen distribution appears related to local edaphic conditions as well as 

climate. In western North America, aspen stands inhabit soils that are well-drained, 

loamy, high in organic matter, and have soil water tables between 0.6 and 2.5 m 

(Perala 1990). In the central and northern Rocky Mountain region, aspen stands 

occur on soils derived from basic igneous rock or neutral to calcareous shales and 

limestones. However, aspen stands also inhabit riparian and other poorly drained 

areas, likely owing to the consistent water supply as well as the lack of conifer 

competition there. 

Regional ecological models of aspen distribution and growth—

Ecological models from specific regions in the West underscore the influence of 

climate on aspen extent (table 1). Two regions where aspen-climate relationships 

have been well-studied are the northern Rocky Mountains and the boreal and 

prairie regions of western Canada. The resulting correlative models differ with 

respect to which climate variables were considered and which climate variables 

were found to be related to aspen presence and growth. However, in general, studies 

show that aspen distribution appears to be related to temperature, precipitation, and 

solar radiation. In the Greater Yellowstone Ecosystem (GYE) of the northern Rocky 

Mountains, growing season shortwave radiation was the primary factor correlated 

with aspen presence, coinciding with a major north-south split in aspen distribution 

and abundance (Brown 2003, Brown et al. 2006). Brown (2003) found that GYE 

aspen occurs in warmer and wetter sites compared to coniferous species, with 

higher amounts of solar radiation, snowfall, and temperatures, and lower values of 

PET. Specifically, GYE aspen stands occurred at an average elevation of 2300 m 

(range = 1559 to 2921 m), with an annual precipitation of 70.6 cm (range = 33.8 to 

153.4 cm) and mean annual temperature of 2.1 °C (range = 2.2 to 6.1 °C). Similarly, 

GYE aspen growth rates were positively correlated with temperature (annual 

maximum temperature 7 to 12 °C) (Brown 2003, Brown et al. 2006). The authors 

suggested that earlier onset of aspen spring growth in the GYE is associated with 

higher spring temperatures and precipitation.
A Review of the Potential Effects of Climate Change on Quaking Aspen (Populus tremuloides) in the Western United States

5

Table 1—Publications that include numerical models of aspen performance related to climate 

predictorsa

Significant

Study Model/response climate 

Reference region variable(s) predictors Comments

Brown 2003, GYE Model 1: GS shortwave GS shortwave radiation

 Brown et al. Classification tree radiation, GS correlated with geographic

 2006 regression of aspen PET, annual separation (North and 

 presence/absence snowfall, GS South). GS PET most

 min tempera- important in southern 

 ture, slope stands

GYE Model 2: Annual max Highest aspen growth

 Multiple regression temperature, on sites with warmer

 of aspen growth GS PET, slope temperatures and 

 (determined from intermediate GS 

 tree ring cores) precipitation

GYE Model 3: Total annual Aspen cover decrease

 Multiple regression snowfall, related to increased

 of percentage of percentage of snowfall and conifer cover, 

 aspen cover change conifer change, aspen cover increase

 between 1956 and GS shortwave positively correlated with

 2001 radiation GS shortwave radiation

Brandt et al. Canada Prairie Multiple regression CMI, years Percentage of dead stems

 2003 region of climate and pest pest infestation negatively correlated with

 predictors on (root disease CMI, percentage of living

 percentage of dead and tent stems positively correlated

 and living stems caterpillar), with CMI

 tree age

Elliott and Baker SW Colorado; Multiple subset Mean spring, Aspen seedling

 2004 treeline in San Juan regression of aspen: summer, establishment occurred in

 Mountains year of seedling and annual cooler years with higher

 establishment precipitation spring precipitation; 

 vegetative growth in 

 drier, warmer years

Gustafson et al. Upper Great Lakes Multiple regression Topographic Statistical model includes

 2003 of site and climate moisture index, predictor variables of aspen

 predictors over a GS precipitation, growth 

 large region: annual number of 

 growth estimates optimum grow-

 from tree ring ing days, June 

 analysis moisture deficit, 

 soil drainage 

 class
GENERAL TECHNICAL REPORT PSW-GTR-235

6

Table 1—Publications that include numerical models of aspen performance related to climate 

predictorsa

 (continued)

Significant

Study Model/response climate 

Reference region variable(s) predictors Comments

Hessl and Graumlich GYE Chi-square tests of None Only climate predictors

 2002 predictor variables considered were total

 across aspen age precipitation and Palmer

 classes estimated drought stress index. 

 from tree cores Statistical tests and predictor

 variables very general

Hogg et al. 2005 Western Canada Multiple regression Current year CMI was the best predictor

 of annual aspen CMI, CMI of aspen growth over

 growth estimated from previous a 50-year period

 from tree ring 4 years, canopy 

 analysis defoliation, 

 growing season 

 degree days 

 April–July

Hogg et al. 2008 Western Canada Model 1: Minimum Aspen growth and mortality

 Multiple regression annual climate during a 5-year period

 of climate and moisture index was most affected by

 defoliation 2000–2004, drought

 predictors percentage 

 on aspen growth of canopy 

 (net and total defoliation, 

 biomass change, mean stand 

 2000–2005) age, percentage 

 of silt

Model 2:

 Multiple regression

 of climate and

 defoliation 

 percentage of

 stem mortality, 

 2000–2005

Hogg 1994, Hogg Western Canada Overlay of climate CMI Current aspen range

 and Hurdle 1995 Parkland region isoclines with coincident with CMI 

 current aspen > -15 cm (-5.9 in) isocline;

 range; prediction predicted northern shift

 of future isoclines in CMI isoclines under

 and corresponding 2 x carbon dioxide (CO2) 

 aspen distribution model, reduction of boreal

 (aspen) forest by one half,

 resembling contemporary

 parkland region
A Review of the Potential Effects of Climate Change on Quaking Aspen (Populus tremuloides) in the Western United States

7

Table 1—Publications that include numerical models of aspen performance related to climate 

predictorsa

 (continued)

Significant

Study Model/response climate 

Reference region variable(s) predictors Comments

Note: Some publications present more than one model (listed singularly by row).

P = precipitation, PET = potential evapotranspiration, GS = growing season, GYE = Greater Yellowstone Ecosystem, CMI = climate moisture index, 

equal to the monthly precipitation minus PET.

a

 Model types and response variables are summarized, as well as climate variables found to be significant in the model.

Mean annual temperture

 most influential at coarse

 scale (< 4.4 °C/7.9 °F), 

 followed by flat topography,

 PET < 60 mm/mo (<2.4

 in/mo) and prevalence of

 sandy soils. Prediction of 

 extreme range reduction

 around Great Lakes region

 under 2 x CO2 climate

 change

N/A

Mean annual

 temperature,

 heterogeneity

 of county

 elevation,

 mean annual

 PET, soil

 texture

September

 PET most 

 discriminating

 variable of

 aspen vs. conifer

 presence

Regression tree

 model of aspen

 importance values

 derived from

 Forest Inventory

 and Analysis

 data by county;

 predictive model of

 future distribution

 based on same

 model

Geographic

 information 

 system model of

 predicted aspen

 presence as

 function of PET

 and conifer cover

Eastern United

 States

Eastern Utah

Iverson and

 Prasad 1998

Sexton et al.

 2006

A recent indepth analysis (Rehfeldt et al. 2009) of aspen climate space tested 

a 34-variable model to identify the predictors of current aspen extent in the U.S.

 Forest Service’s Rocky Mountain Region, based on 118,000 U.S. Forest Inventory

and Analysis (FIA) plots. A subset model that included eight climate variables 

successfully predicted most of the current extent of aspen. The annual dryness 

index (a ratio of growing-degree-days to annual precipitation) was the strongest 

climate predictor. Their general conclusion was that aspen distribution limits at the 

xeric fringe of its range are dictated mostly by moisture stress (Rehfeldt et al. 2009).

Ecological models in other regions suggest the influence of moisture and temperature on aspen distribution as well. Aspen stands in Canada exist where moisture availability is not limiting, as measured by total annual precipitation exceeding 

annual PET (Chen et al. 2002, Hogg 1994, Hogg and Hurdle 1995). Similarly, aspen 

distribution is related to growing-season precipitation and moisture deficit in the 

Eastern United States and Great Lakes region (Gustafson et al. 2003, Iverson and 

Prasad 1998). 
GENERAL TECHNICAL REPORT PSW-GTR-235

8

Moisture availability appears to affect western aspen growth as well as distribution. Models of aspen growth are typically based on retrospective analysis 

of tree ring patterns, which is correlated with historical climate patterns (Brown 

2003, Brown et al. 2006, Gustafson et al. 2003, Hessl and Graumlich 2002), and 

suggest that extreme drought conditions impose the greatest limits to aspen growth 

and survival (Brandt et al. 2003, Hogg et al. 2005, 2008). Studies of net ecosystem 

production in aspen support these findings (Barr et al. 2007). Climate moisture 

indices from the period of a recent severe drought in western Canada explained the 

most variation in recent aspen growth and mortality (Hogg et al. 2008). A study in 

Manitoba, Canada, found that a hot June reduced radial growth of quaking aspen, 

and aspen trees do not depend on early season water availability, for growth to 

the same extent as bur oak. The study also found that the temperature in the previous October does not necessarily affect aspen tree growth (Quercus macrocarpa

Michx.) (Boone et al. 2004).

How Will Aspen Respond to Future Climates?

There are some common expectations for how the climate of western North 

America will change as atmospheric carbon dioxide (CO2 ) increases. Overall, 

climatologists project that the Western United States will see increased summer 

temperatures, more precipitation in the form of rain and less snow, lower total 

annual precipitation in most areas, and increasing extreme weather prompting 

more frequent natural disturbances (Cayan et al. 2008, Dettinger 2005, Knowles 

and Cayan 2004, Mastrandrea et al. 2009, Moser et al. 2009). Here we summarize 

predictions for aspen stand dynamics for the next century in light of generalized 

climate projections.

Temperature and precipitation—

Higher temperatures and increased moisture stress are predicted to affect aspen 

mortality and regeneration in western North America (Brandt et al. 2003, Elliott 

and Baker 2004, Worrall et al. 2008). Aspen is a water-limited, drought-intolerant 

species (Niinemets and Valladares 2006); thus, severe droughts can cause death 

or decline of aspen. Such drought impacts have been seen in Canada (Hogg et al. 

2002, Zoltai et al. 1991), where increased temperatures and changes in precipitation 

patterns coincided with reduced aspen presence in Canadian boreal forests (Hogg 

1994, Hogg and Hurdle 1995). Increased evapotranspiration and decreased moisture 

have been implicated in the conversion of Canadian aspen parklands to grassland 

(Zoltai et al. 1991). Decreased moisture availability is predicted to disfavor aspen 

in eastern Utah, because of its higher water demands compared to co-occurring 

conifer species (Sexton et al. 2006). Similar evidence from the Eastern United 

Climatologists 

project that the 

Western United 

States will see 

increased summer 

temperatures, more 

precipitation in the 

form of rain and less 

snow, lower total 

annual precipitation 

in most areas, and 

increasing extreme 

weather prompting 

more frequent natural 

disturbances.
A Review of the Potential Effects of Climate Change on Quaking Aspen (Populus tremuloides) in the Western United States

9

States has prompted predictions of the eventual disappearance of aspen from that 

region (Iverson et al. 2008a, 2008b). A modeling analysis in Wisconsin predicted 

that warming climates could cause aspen to decline in the boreal forest of the Great 

Lakes region (He et al. 2002).

Changes in winter precipitation may have negative impacts on aspen as well. 

Snow cover mediates soil temperature, providing insulation for roots in extreme 

cold (Frey et al. 2004), and inhibiting ungulate browsing in winter (Martin 2007). 

Thus, reduced snow accumulation may contribute to damaged roots and retard 

regeneration. However, the relationships are unclear: one analysis of aspen extent 

showed that mild winters and warmer wetter summers favored aspen, and snowy 

cold winters and dry bright summers were detrimental to aspen, leading to grasslands and conifer succession (Brown et al. 2006).

Increased atmospheric CO2—

As CO2 increases, longer roots and thus better nutrient uptake should increase 

aspen productivity (Pregitzer et al. 2000). One experimental study in Wisconsin 

showed that aspen growth increased 39 percent with elevated atmospheric CO2, 

with a faster rate under increased moisture (Norby et al. 2005). However, benefits 

may decrease over time, and increased ozone may negate the positive effects of 

elevated CO2 (Kubiske et al. 2006). One study modeled that aspen in the Canadian 

boreal will increase productivity for the next 200 years, acting as a large carbon 

sink. However, prolonged (6-year) droughts would eventually cause severe dieback 

(Grant et al. 2006). Therefore, some researchers stress that the long-term effects of 

elevated atmospheric CO2 on aspen will be complex and difficult to predict (Hogg 

2001, Lindroth et al. 2001). 

Climate interactions with natural disturbances—

Future climate changes may increase the frequency of physical disturbances (e.g., 

floods and wildfire), which alone would be expected to increase aspen on the 

landscape. However, interactions between different factors make the net effect of 

extreme weather difficult to predict. If the climate warms and dries, and if there are 

other stressors present such as heavy ungulate browsing, aspen may be unable to 

resprout or establish new seedlings (Romme et al. 2001).

Changes in fire frequency are predicted to affect aspen distribution and growth. 

Many authors have argued that increased temperatures and decreased precipitation 

would lead to more frequent fires (Spracklen et al. 2009, Westerling et al. 2006), 

which would favor aspen regeneration through suckering (Elliot and Baker 2004, 

Graham et al. 1990, Jones and DeByle 1985, Rogers 2002, Schier et al. 1985). In 

fact, in high-elevation forests with long fire intervals, the natural succession of 
GENERAL TECHNICAL REPORT PSW-GTR-235

10

aspen stands to conifers may be reset by future stand-replacing fires, especially if 

the area burned by such fires increases because of climate change (Dale et al. 2001).

Insects and other interaction effects—

Climate change may induce indirect effects on aspen productivity via increased 

frequency of and vulnerability to pathogens and herbivores, which interact with 

environmental stress to cause tree mortality (Frey et al. 2004; Hogg et al. 2005, 

2008; Jones et al. 1985a). For instance, drought conditions in the spring and following summer or deep late spring snowpacks plus summer drought may increase the 

susceptibility of aspen to death through canker infections (Cryer and Murray 1992, 

Johnston 2001). Moreover, drier, warmer conditions may favor gypsy moth (Lymantrai dispar) invasions in Utah and possibly elsewhere in the West (Logan 2008) and 

forest tent caterpillar (Malacosoma disstria Hubner) outbreaks in western Canada 

(Hogg et al. 2002). Drought could also reduce sprouting after a disturbance because 

of higher susceptibility to insects and pathogens (Sexton et al. 2006). 

Mammal herbivory can exacerbate drought effects on aspen growth and 

distribution. For example, chronic heavy browsing by elk (Cervus canadensis) in 

the interior Western United States (e.g., Rocky Mountain National Park), in combination with drought and fire suppression, seems to be leading to aspen decline 

(Romme et al. 2001). Climate change may have the strongest effect on areas where 

aspen are patchily distributed on marginal habitat and ungulate browsing is heavy 

(Romme et al. 2001). The Book Cliffs in Utah exemplify the combination of drying 

climates, displacement by conifers through shading, soil and microclimate effects, 

and ungulate browsing that could decrease aspen cover in the future (Sexton et al. 

2006).

The role of seedling regeneration—

Aspen seedling regeneration may become increasingly important in a changing 

climate, providing the genetic diversity needed for the population to adapt to rapidly 

changing conditions. There is debate over the average age of aspen clones, but if 

current aspen stands were established in centuries past, they may be genetically 

adapted to cooler climates such as occurred during the Little Ice Age (Barnes 

1966, Tuskan et al. 1996). A rare quantitative analysis showed that aspen seedling 

establishment at treeline in southwestern Colorado may have occurred in years with 

lower than normal mean maximum summer temperatures (21 to 22 °C) and higher 

mean spring precipitation (5 to 6 cm). Conversely, accelerated asexual reproduction 

was correlated with lower spring precipitation (3 to 4 cm) and warmer maximum 

summer temperatures (23 to 24 °C) (Elliott and Baker 2004). The researchers 
A Review of the Potential Effects of Climate Change on Quaking Aspen (Populus tremuloides) in the Western United States

11

speculated that future aspen seedling regeneration may be limited to higher elevations and latitudes where the requisite cooler and wetter temperatures prevail.

Sudden Aspen Decline (SAD)

An ongoing phenomenon, the rapid death of some or all of a mature aspen stand 

with little or no regeneration, dubbed sudden aspen decline (SAD), may be an indicator of the response of aspen to climate change. It was brought into focus in Utah 

and Arizona starting in 2002, and soon after in Colorado (Shepperd 2008, Worrall 

et al. 2008). However, unusual aspen mortality has occurred periodically over 

the last four decades in the Great Lakes region, Canada, and the interior Western 

United States.

Characteristics—

Sudden aspen decline occurs rapidly and simultaneously across a grove, in 1 to 3 

years (Peterson and Peterson 1992, Worrall et al. 2008). It appears on the landscape 

as white defoliated trees that are still standing with their bark intact, indicating that 

they died recently. Large trees appear to die first and effects may start at the edge of 

a grove (Ciesla 2008). Younger cohorts are often not affected (Shepperd and Guyon 

2006).

One can distinguish SAD from insect defoliation or frost damage because of 

complete defoliation in addition to dieback of tree branches (Worrall et al. 2008). 

There is great concern among researchers that roots are dying first (Worrall et 

al. 2008), resulting in the lack of regeneration and other stereotypical SAD signs, 

although the response may be delayed for a season (Campbell et al. 2008). A 2007 

study found up to 90 percent of root volume dead in several stands in Colorado 

(Worrall et al. 2008). With complete root death, the aspen grove will eventually 

revert to a nonaspen vegetation type. 

Incidence—

Sudden aspen decline has occurred recently and most noticeably in southwestern 

Colorado, northern Arizona, and parts of Utah and Canada, but it has also been 

seen in Idaho, Nevada, Montana, and Wyoming. Data from aerial detection surveys of permanent plots indicated that the average mortality rate of aspen in Utah, 

Nevada, and western Wyoming in 2006 and 2007 was 31 percent; two-thirds of all 

dead trees died between 2005 and 2007 (Hoffman et al. 2008). A 2006 aerial survey 

across Colorado spotted 56 091 ha of SAD (Worrall et al. 2008). An estimated 13 

percent of aspen cover in Colorado showed effects of SAD by 2007 (Rodebaugh 

2008), and a 2008 aerial survey revealed that 216 000 ha were noticeably affected 

(http://www.aspensite.org/SAD/sad_faqs.pdf).

An ongoing 

phenomenon, the 

rapid death of some 

or all of a mature 

aspen stand with little 

or no regeneration, 

dubbed sudden aspen 

decline (SAD), may 

be an indicator of the 

response of aspen to 

climate change.
GENERAL TECHNICAL REPORT PSW-GTR-235

12

Outside of the intermountain West and the Rocky Mountains, the extent of 

SAD is unclear. A recent survey in eastern Washington showed no sign of SAD in 

two national forests (Hadfield and Magelssen 2004). Very little survey work has 

been done to explore the incidence of SAD in California. To aid survey efforts, we 

have developed a new SAD survey tool to help federal employees obtain baseline 

data from aspen stands and identify SAD events as they emerge (see appendix).

Causes—

Sudden aspen decline appears to have a strong climate correlation, as most occurrences can be related to high temperatures and drought (Worrall et al. 2008). In 

addition, SAD-like events seem to occur earlier in areas with higher annual temperatures and drier climates (Hogg and Hurdle 1995, Shields and Bockheim 1981). 

One explanation for SAD in the intermountain West is that drought and hot weather 

in the early 2000s stressed aspen stands. A related cascade of events was seen at 

the same time in western Canada (Hogg et al. 2008). Similarly, a drought in 1961 

caused ubiquitous aspen mortality in the grasslands of western Canada a few years 

later, causing direct deaths or secondary deaths from Cytospora canker (Zoltai et 

al. 1991). A study by Rehfeldt et al. (2009), using several general circulation models 

and climate scenarios, suggested that most SAD events in the Rocky Mountain 

region occurred within areas that they project may no longer be viable aspen habitat 

by 2060. 

Researchers have developed a decline disease hypothesis for SAD (Frey et 

al. 2004, Worrall et al. 2008): stand and site factors such as age, slope, and aspect 

predispose aspen to decline; defoliation or a severe drought and high summer temperatures incite decline among those aspen predisposed; and finally, opportunistic 

insects and pathogens contribute to the death of the aspen. Research has implicated 

other factors, including herbivore impacts and freeze-thaw events (Cayford et 

al. 1959, Cox and Malcolm 1997, Frey et al. 2004). Fine root damage caused by 

extreme winter freeze followed by drought could cause SAD by reducing water and 

nutrient uptake (Frey et al. 2004). A comparable winter exposure mechanism has 

been implicated in the sudden decline of yellow-cedar (Chamaecyparis nootkatensis (D. Don) Spach) in southeastern Alaska (Beier et al. 2008).

Some stands and sites in the Western United States are particularly vulnerable 

to SAD (Baker and Shaw 2008, Brandt et al. 2003, Worrall et al. 2008): those at (1) 

low elevation (e.g., 2100 to 2500 m in the Colorado Rocky Mountains), (2) south and 

southwest aspects, and (3) flatter slopes. Over 90 percent of aspen stems have died 

on some low-elevation sites in Arizona, with 16 to 43 percent mortality in mid- and 

high-elevation sites (Fairweather and Geils 2008). Because low-elevation sites and 
A Review of the Potential Effects of Climate Change on Quaking Aspen (Populus tremuloides) in the Western United States

13

southern aspects are generally drier and warmer in the summer and more prone 

to additional stress by freezing and drying soil in the winter, these observations 

suggest a climate causation. Aspen stands on sloped areas may be better adapted 

to moisture stress and thus not as affected by acute drought events (Worrall et al. 

2008). Further, with changing climates low-elevation sites may be receiving less 

snow and thus may be increasingly less insulated and more vulnerable to freezethaw events during the winter. There may also be a correlation with conifer 

competition, as conifers may not be as abundant at high elevations. 

Although there has been some debate (Frey et al. 2004), results indicate that 

SAD vulnerability does not increase with age once trees are physiologically mature 

(Brandt et al. 2003, Worrall et al. 2008). There is further uncertainty on whether or 

not tree size is correlated with SAD. Some researchers have shown large-diameter 

trees (stems greater than 30 cm) to be more susceptible (Worrall et al. 2008), 

whereas others hypothesize that tall thin trees in exposed xeric sites would be most 

vulnerable (Frey et al. 2004). If drought is the inciting factor for SAD, trees with 

small diameter at breast height should be most affected because water stress would 

increase water tension and xylem cavitation and cause dieback in the upper crown 

first, e.g., in cottonwoods (Populus spp.) (Rood et al. 2000). Further, conflicting 

results point to a potential effect of stand density (Hogg et al. 2002, Worrall et al. 

2008), although there is a potential confounding of latitude and insect preferences.

Various pathogens and insects appear to be more commonly associated with 

SAD than with other aspen mortality (Worrall et al. 2008). Although no single 

biotic factor appears to be responsible for SAD, five organisms were found to be 

commonly associated with SAD in Colorado (Worrall et al. 2008): Cytospora

canker, poplar borer (Saperda calcarata), bronze poplar borer (Agrilus liragus), and 

two aspen bark beetle species (Trypophloeus populi and Procryphalus mucronatus). 

All five are species that do not normally cause mortality in healthy aspen.

Conclusion

Climate change, through increased drought, ozone, and insect outbreaks, may cause 

aspen to become increasingly threatened (Nitschke and Innes 2008), exemplified 

by the SAD phenomenon. Conversely, elevated CO2 and more frequent fires could 

increase aspen extent (Shepperd et al. 2006, Zoltai et al. 1991). Alternatively, aspen 

distribution may simply shift in the future (Rehfeldt et al. 2009, Ryel and Bartos 

2008) if migration and regeneration or seedling establishment rates are sufficient 

to adapt to environmental change (Iverson et al. 2004). Changes in suitable aspen 

habitat will likely differ by region (Hogg 2001), necessitating decentralized 

approaches to research, monitoring, and management. Our review underscores the 
GENERAL TECHNICAL REPORT PSW-GTR-235

14

usefulness of local knowledge regarding aspen management, particularly in regard 

to predicting where aspen may thrive in the future. Finally, the survey we present 

here can help land managers track the SAD phenomenon throughout western North 

America, especially if SAD increases as a threat in a warmer, drier future.

Acknowledgments 

This review was conducted as part of a larger study of climate change effects on 

aspen distribution, sponsored by the USDA Forest Service, Rocky Mountain and 

Pacific Northwest Research Stations. Susan Frankel, Edward (Ted) Hogg, Patricia 

Manley, Michael Michaelian, and Wayne Shepperd provided extremely useful and 

timely comments on the manuscript. We thank Linda Joyce and Connie Millar 

for their contributions to this work. Morelli is grateful to John Guyon, Ted Hogg, 

Connie Millar, Paul Rogers, Wayne Shepperd, Jim Worrall, and especially David 

Burton for their contributions to the SAD survey. 

Metric Equivalents

When you know: Multiply by: To get:

Millimeters (mm) 0.0394 Inches

Centimeters (cm) 0.394 Inches

Meters (m) 3.28 Feet

Hectares (ha) 2.47 Acres

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

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Appendix: 

Introduction of a Sudden Aspen Decline (SAD) Survey Tool 

(developed by T.L. Morelli)

Toni Lyn Morelli

Pacific Southwest Research Station

USDA Forest Service

morellitlm@gmail.com

The purpose of this survey is to record baseline data for aspen stands in areas 

where sudden aspen decline (SAD) is not occurring, as well as to identify any 

stands where SAD is occurring.

The SAD Survey is designed to recognize potential symptoms of SAD as 

well as symptoms of general aspen decline and excessive browsing pressure. It is 

deliberately short (just 1 page printed double-sided) and does not require conducting transects. Moreover, the SAD survey does not require expertise in botany or 

forestry. If there are questions that go beyond your expertise, leave these sections 

blank.

The SAD survey is in a preliminary stage. Any federal employee interested 

in using the survey, or making revisions to it, can contact one of the coordinators 

above; we’d be happy to hear ideas.

Instructions for conducting survey— 

This survey should be conducted at the center of the aspen stand. It is best to 

conduct the survey after leaves have flushed and before they have fallen (late spring 

to early fall, depending on your location).

Answer the questions from what you know of the stand. Thus, if the stand is 

small enough that you are able to see/walk through the whole stand, answer the 

questions about the entire stand. If the stand is large, answer the questions about 

what you can see from the center of the stand. Question 12 addresses the issue of 

whether the edge of the stand is different from the center.

Your contact information will be useful in case there is anything on the sheet 

that we cannot read or is otherwise unclear. 

Use the Stand ID that makes sense for your federal agency. U.S. Forest Service 

should use a Forest Code and District Code, plus the Stand Code when possible.

Check the box or boxes for primary stand type that seems most fitting. If there 

is another stand type that you think would be more appropriate (e.g., snowpocket), 

you can write it in, but please also check at least one of the four present boxes. 

David Burton

Aspen Delineation Project

peregrines@prodigy.net
GENERAL TECHNICAL REPORT PSW-GTR-235

28

Primary stand type is defined as follows (Shepperd et al. 2006): Mark “slope” if 

the aspen stand is found on a hill. Mark “lithic” if the aspen are found in or next to 

a talus or other rocky field. Mark “meadow fringe” if the aspen stand occurs on the 

edge of a meadow. Mark “riparian” if the aspen stand occurs in an area with permanent or seasonal standing or moving water.

“Primary aspen form” is to distinguish the more common tree form from the 

shrub-like krummholz form.

Questions:

1. Estimate the percentage of the canopy that shows recent crown loss, which is 

thinning of the foliage or branch dieback. The categories are 0–33%, 34–66%, 

and 66–100%.

2. Estimate the percentage of the canopy that has died and fallen. The categories 

are 0–33%, 34–66%, and 67–100%.

3. Estimate the percentage of the stand that has died and is still standing. Then 

estimate the percentage of those dead and still standing trees that died recently. 

These trees can be distinguished by their white color as they will still have most 

of their bark. The categories are 0–33%, 34–66%, and 67–100%.

4. Estimate the size class for the majority (greater than 50%) of aspen that are 

standing and alive. The categories are less than 1 in (2.54 cm) dbh, 1–8 in 

(2.54–20.32 cm) dbh, and greater than 8 in (20.32 cm) dbh.

5. Estimate the size class for the majority (greater than 50%) of mortality that has 

occurred recently (indicated by their white color as they will still have most of 

their bark). The categories are less than 1 in (2.54 cm) diameter at breast height 

(dbh), 1–8 in (2.54–20.32 cm) dbh, and greater than 8 in (20.32 cm) dbh.

6. Compare the absolute number of young aspen of 1–5 in (2.54–12.7 cm) dbh 

in the stand with the absolute number of mature aspen trees (greater than 8 

in/20.32 cm). For example, if you estimate 60 young aspen and 10 large mature 

aspen, approximate your answer by checking the “5x” box. If you can see 2 

young aspen and 17 mature aspen, check the “<1x” box.

7. Compare the number of sprouts (less than 1 in/2.54 cm dbh) in the stand with 

the number of mature aspen trees (greater than 8 in/20.32 cm). For example, if 

you can see approximately 500 sprouts and 30 large aspen stems, answer “10x”. 

If you can see no or very few sprouts, answer “<1x”.

8. Estimate the percentage of sprouts (less than 1 in dbh) that show evidence 

of herbivory. Evidence includes any chewed leaves or stems. Also note if the 

herbivory appears light, moderate, or heavy. Thus, in an area with consistent 
A Review of the Potential Effects of Climate Change on Quaking Aspen (Populus tremuloides) in the Western United States

29

but low-density deer use, you might see one chewed leaf from 80% of sprouts. 

In this case, you should mark “76–99%” and “light.”

9. Describe the pathogen or insect damage that you see, including the species if 

you can identify them. Note any beaver damage observed.

10. Note any evidence of fire in the stand.

11. Note the presence of conifers, which section and portion they make up of the 

forest, and which species predominates, if you know.

12. Now walk to the edge of the stand and note whether sprouting is uniform across 

the stand. For example, in some stands there are more sprouts at one edge than 

other edges or in the center. Also note if there are patches of mortality in different parts of the stand.

Finally, there is a section to make notes if you run out of room on the form or have 

anything to add. 

If you can take pictures, they will help supplement this information, and will be 

much appreciated. 

Comments, questions, pictures, or completed surveys?

Contact David Burton at 916-663-2574, peregrines@prodigy.net

Aspen Delineation Project, P.O. Box 348, Penryn, CA 95663
GENERAL TECHNICAL REPORT PSW-GTR-235

30

Sudden Aspen Decline (SAD) Survey

***CONDUCT THIS SURVEY IN CENTER OF STAND DURING SPRING OR SUMMER***

 DATE:

Surveyor’s Name: Surveyor’s Phone #

Stand Center GPS Coordinates: N W units:

 Stand ID: Stand size: ha

Environmental context: Elevation: m Aspect: ° Slope: %

Primary stand type: Slope Lithic Meadow Fringe Riparian

Primary aspen form: Upright tree Shrub

1. How much recent crown loss (thinning of the foliage and/or branch dieback) across the canopy?

 <34% 34–66% >66%

2. What percentage of the stand is down and dead aspen?

 <34% 34–66% >66%

3. What percentage of the stand is standing and dead aspen?

0–25% 26–50% 51–75% 76–100%

 Do they appear recently dead (bark still intact)?

 <34% 34–66% >66%

4. The Majority (>50%) of live aspen is in which size class?

<1 inch dbh 1–8 inch dbh >8 inch dbh

5. The majority of current aspen mortality is located in which size classes?

<1 inch dbh 1–8 inch dbh >8 inch dbh

6. How many more young established aspen (1–5 inch dish) are present than mature aspen (>8 inches)?

<1x 2x 5x >10x

7. How many more aspen sprouts (<1 inch dbh) are present than mature aspen (>8 inches)?

<1x 10x 100x 1000x

8. What percentage of sprouts (<1 inch dbh) show any evidence of ungulate (e.g., elk, deer, cattle, sheep) herbivory?

0–25% 26–50% 51–75% 76–100%

 Is the sprout herbivory light moderate heavy

9. Is there evidence of pathogens or insect damage? Is there evidence of beaver damage? Describe

10. Is there any evidence of past fire in the stand?

Yes No

11. What is the size of conifers within this stand?

no conifers conifer in understory mixed in canopy majority of canopy

Primarily which species of conifer?

12. Walk to the edge of the stand. Is sprouting uniform across the stand?

Yes No Explain:

 Is morality uniform across the stand?

Yes No
A Review of the Potential Effects of Climate Change on Quaking Aspen (Populus tremuloides) in the Western United States

31

If possible, please take at least three pictures: the environmental context (broader view), a stand shot, and, where 

appropriate, a closeup showing environmental damage.

Sudden Aspen Decline (SAD) Survey (continued)

GENERAL NOTES: .

Comments, questions, or completed surveys/pictures?

Contact David Burton at 916-663-2574, peregrines@prodigy.net

Aspen Delineation Project, P.O. Box 348, Penryn, CA 95663

Developed by Toni Lyn Morelli, Pacific Southwest Research Station, and David Burton, Aspen Delineation Project.

Thanks to John Guyon, Ted Hogg, Connie Millar, Paul Rogers, Wayne Shepperd, and Jim Worrall for comments.

Pacific Northwest Research Station

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Telephone (503) 808-2592

Publication requests (503) 808-2138

FAX (503) 808-2130

E-mail pnw_pnwpubs@fs.fed.us

Mailing address Publications Distribution

Pacific Northwest Research Station

P.O. Box 3890

Portland, OR 97208-3890
U.S. Department of Agriculture

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333 SW First Avenue

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