C O M M E N T A R Y

Biodiversity in US Forests underGlobal Climate Change

Andrew Hansen1* and Virginia Dale2

1Ecology Department, Montana State University, Bozeman, Montana 59717, USA; and 2Oak Ridge National Laboratory,Oak Ridge, Tennessee 37830-6036, USA

There is increasing evidence that humans are alter-ing the climate of the Earth (Watson and others1997; Kerr 2000). Among the challenges to scien-tists concerned with climate change are predictingthe consequences of global climate change andevaluating strategies for mitigating the negative ef-fects of these changes. To date, biodiversity has notbeen well integrated into global change studies.

Biodiversity refers to the “species, genetic andecosystem diversity in an area” (Swingland 2000).We believe that biodiversity is highly relevant toboth understanding global change and valuing itsconsequences. The diversity of organisms on theEarth provide a plethora of goods and services tohumans, including foods, medicines, ecological ser-vices, and spiritual well-being (Pimental and others1997). Biodiversity also influences how ecologicalsystems respond to climate change. The responsesof individual organisms to climate begins the cas-cade of ecological processes that are manifest aschanges across landscapes, biomes, and the globe.The dynamics initiated by organisms often providefeedbacks to the atmosphere and oceans and fur-ther modify climate. Thus, the consideration ofbiodiversity is important for understanding ecolog-ical response to global change, predicting futureresponses, valuing these changes for humans, anddesigning strategies to mitigate negative effects.

The following four papers project the potentialresponses of species, communities, and biomes tothe changes in climate predicted under an anthro-pogenic doubling of atmospheric carbon dioxide(CO2) by general circulation models (GCM). They

focus on the forest vegetation in the conterminousUnited States as reflected by changes in the distri-bution of biomes, community types, and tree spe-cies. Species richness of trees and terrestrial verte-brates is also analyzed. The studies were donewithin the context of the Forest Sector of the Na-tional Assessment of Climate Change and Variabil-ity (http://www.nacc.usgcrp.gov).

Predictions about climate change vary among theseveral GCM that have been developed. Climatescenarios from both the older equilibrium andnewer transient GCM simulations (McNulty andAber 2001) were used to drive the biodiversityanalyses reported in these papers. We put greaterconfidence in biodiversity outcomes that were inagreement under several climate scenarios. Dis-agreement among biodiversity predictions wastaken as an indication of uncertainty either in thepredictions of the climate or biodiversity models.Thus, the authors report major findings where mostof the models agree but also point out certain areasof disagreement.

The suite of papers uses three approaches toproject climate change effects on forest diversity.The opening paper by Bachelet and others (2001)uses the biogeography models MAPSS and MC1 toproject the potential distributions of plant biometypes under climate change. These models deter-mine leaf area based on climate, available soil wa-ter, and atmospheric CO2. Biomes are determinedby rules that consider climatic thresholds and leafarea of trees, shrubs, and grasses. Wildfire is simu-lated internally by these models. Potential distribu-tions of individual tree species are simulated byIverson and Prassad (2001) and Shafer and others(2001) using statistical models. Current relation-

Received: 12 May 2000; Accepted: 5 January 2001*Corresponding author; e-mail: hansen@montana.edu

Ecosystems (2001) 4: 161–163DOI: 10.1007/s10021–001–0001-8 ECOSYSTEMS

© 2001 Springer-Verlag

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ships among tree distribution, climate, and soils areused to develop predictive statistical functions oftree distribution under future climate. Shafer andothers (2001) focus on western tree and shrub spe-cies, whereas Iverson and Prassad (2001) focus ontrees in the eastern United States. Iverson andPrassad (2001) also group the responses of individ-ual species to predict changes in forest communi-ties. Finally, Currie (2001) uses energy theory (Cur-rie 1991) as a basis for projecting the potentialspecies richness of trees, mammals, birds, reptilesand amphibians under changing climate. The meth-ods of each of the papers are based on sets ofsimplifying assumptions. The authors carefully de-scribe these assumptions so that the reader canmake better judgments about the reliability of theresults.

Collectively, the results suggest substantialchange in the potential habitats of several speciesand communities. Forest area in the US is projectedto decrease by an average 11% under global change(range, 123% to –45%), with the lost forest re-placed by savanna and arid woodland biome types.Community types predicted to increase includeoak/hickory and oak/pine in the East and pon-derosa pine and arid hardwoods in the West. Sev-eral important community types, however, are pro-jected to decrease greatly in area or disappearentirely from the conterminous US. These commu-nities include alpine habitats, sagebrush, subalpinespruce/fir forests, and the aspen–birch and maple–beech–birch types. These predictions for commu-nity types reflect changes in potential habitat forindividual tree and shrub species. Seven of the 80eastern species modeled are predicted to be reducedin suitable habitat by at least 90%, including big-tooth aspen (Populus grandidentata), quaking aspen(P. tremuloides), sugar maple (Acer saccharum),northern white cedar (Thuja occidentalis), balsam fir(Abies balsamea), red pine (Pinus resinosa), and paperbirch (Betula papyrifera). Regional importance isprojected to increase by 100% or more for 12 spe-cies, including four oaks and one hickory. In thewestern US, the potential habitats for dominantrainforest conifers such as western hemlock (Tsugaheterophylla) are simulated to decrease west of theCascade Mountains and expand into mountainranges throughout the interior West. Potential hab-itat for several subalpine conifers, including Engle-mann spruce (Picea engelmannii), mountain hem-lock (Tsuga mertensiana), and several species of truefir (Abies) are simulated to contract substantially inthe western conterminous US. The potential habitatfor big sagebrush (Artemisia tridentata) is simulatedto shift largely from the US into Canada. It is re-

placed in the US by potential habitat for shrubs,such as creosote bush (Larrea tridentata), now foundin the Southwest. The potential habitat for Pon-derosa pine (Pinus ponderosa) is simulated to expandin the western US, including the area west of theCascade and Sierra mountain ranges.

The potential habitats for most eastern species areprojected to move to the north, several species by100–530 km. While the ranges of many taxa in theWest shift northward, topographic complexity re-sults in some conifer species associated with mesicclimates shifting south and east along the RockyMountains. The complex topography in the Westresults in many current tree populations being dis-junct. Consequently, dispersal to new habitats un-der climate change will be more difficult in theWest than in the East, where populations are morecontinuous and topography and microclimate areless variable.

Potential species richness is projected to increasefor trees, reptiles, and amphibians, particularly inthe coldest portions of the US. Potential bird andmammal species richness is projected to decrease inthe southern US but increase to the north.

The extent to which biodiversity tracks suchchanges in potential habitats will depend on manyfactors not included in the analyses. Most importantamong these is organism dispersal. The pace of cli-mate change is likely to exceed the natural dispersalrates of several species. Thus, these species are notlikely to reach newly suitable habitats without hu-man intervention. Rapidly dispersing weedy speciesmay dominate these new habitats, leading to en-tirely new community types. Actual patterns of dis-persal are likely to be influenced by factors thatinteract with climate, including disturbance regimesand human land use.

In summary, these analyses indicate that climatechange is likely to exert a strong influence on biodi-versity in the US. The relatively high level of un-certainty and simplifying assumptions of these anal-yses, however, are indication that much moreresearch is needed before we fully understand theconsequences of climate change for biodiversity andthe implications of these changes for human soci-ety. Strategies for mitigating such changes or copingwith them are currently underdeveloped (Hansenand others 2001). Ecologists have a great deal ofwork to do to help society understand and copewith the impact of climate change on biodiversity inthe 21st century.

ACKNOWLEDGMENTS

Louis Iverson, Sarah Shafer, and Patrick Bartleinprovided helpful comments on the manuscript. We

162 A. Hansen and V. Dale

also thank John Aber and Steve McNulty for chair-ing the Forest Sector. Funding was provided by theUS Global Change Research Program’s National As-sessment of Climate Change, the USDA Forest Ser-vice Global Change Research Program, the NASALand Cover Land Use Change Program, and theDepartment of Energy. The National Assessmentwas mandated by the US Congress to provide de-tailed understanding of the consequences of climatechange for the nation and to examine possible cop-ing mechanisms to adapt to climate change. OakRidge National Laboratory is managed by the Uni-versity of Tennessee–Battelle LLC for the US De-partment of Energy under contract DE-AC05-00OR22725. This paper is Environmental SciencesDivision publication number 5063.

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