Forests (Chapter 17) of the Foundation document of Climate ... · The effects of climate change on...

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CHAPTER 17

POTENTIAL CONSEQUENCES OF CLIMATEVARIABILITY AND CHANGE FOR THEFORESTS OF THE UNITED STATESLinda Joyce1,2, John Aber3, Steve McNulty1, Virginia Dale4, Andrew Hansen5, LloydIrland6, Ron Neilson1, and Kenneth Skog1

Contents of this Chapter

Chapter Summary

Background

Climate and Forests

Key Issues

Forest Processes

Forests and Disturbance

Biodiversity

Socioeconomic Impacts

Adaptations: Forest Management Strategies under Climate Change

Crucial Unknowns and Research Needs

Literature Cited

Acknowledgments

1USDA Forest Service; 2Coordinating author for the National Assessment Synthesis Team; 3University of New

Hampshire, 4Oak Ridge National Laboratory; 5Montana State University, 6The Irland Group

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Potential Consequences of Climate Variability and Change

CHAPTER SUMMARY

Context

Forests cover nearly one-third of the US,providingwildlife habitat, clean air and water, cultural and aes-thetic values,carbon storage, recreational opportuni-ties such as hiking,camping, fishing,and autumnleaf tours,and products that can be harvested suchas timber, pulpwood,fuelwood,wild game, ferns,mushrooms,and berries. This wealth depends onforest biodiversity—the variety of plants,animals,and microbe species,and forest functioning—waterflow, nutrient cycling,and productivity. Theseaspects of forests are strongly influenced by climateand human land use.

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to predict; climate changes, changes in these distur-bances and their effects on forests are possible.

Analyses of the results of ecological models whendriven by several different climate scenarios indi-cate changes in the location and area of potentialhabitats for many tree species and plant communi-ties. For example,alpine and subalpine habitats andthe variety of species dependent upon them arelikely to be greatly reduced in the conterminous US.The ranges of some trees are likely to contract dra-matically in the US and largely shift into Canada.Expansion of potential habitats is possible foroak/hickory and oak/pine in the eastern US andPonderosa pine and arid woodland communities inthe West. How well plant and animal species adaptto or move with changes in their potential habitat isstrongly influenced by their dispersal abilities andthe disturbances to these environments. Introducedand invasive species that disperse rapidly are likelyto find opportunities in newly forming communi-ties.

The effects of climate change on socioeconomicbenefits obtained from forests will likely be influ-enced greatly by future changes in human demands,as determined by population growth,increases inincome, changing human values,and consumer pref-erences.

Outdoor recreation opportunities are very likely tobe altered by climate change. For example, warmerwaters would increase fish production and opportu-nities for some species;but decrease opportunitiesfor cold water species. Outdoor recreation opportu-nities in mountainous areas are likely to be altered.Summer recreation opportunities are likely toexpand in the mountainous areas when warmerlowland temperatures attract more people to higherelevations. Skiing opportunities are likely bereduced with fewer cold days and snow events. Inmarginal climate areas,the costs to maintain down-hill skiing opportunities are likely to rise whichwould possibly result in the closure of some areas.

Key Findings

Carbon storage in US forests is currently estimatedto increase from 0.1 to 0.3 Pg of carbon per yearand analyses suggest that carbon dioxide fertiliza-tion and land use have influenced this current stor-age. Within the next 50 years, forest productivity islikely to increase with the fertilizing effect of atmos-pheric carbon dioxide. Those productivity increasesare very likely to be strongly tempered by local envi-ronmental conditions (e.g.,moisture stress, nutrientavailability) and by human land use impacts such asforest fragmentation,increased atmospheric deposi-tion,and tropospheric ozone.

Economic analyses when driven by several differentclimate scenarios indicate an overall increase in for-est productivity in the US that is very likely toincrease timber inventory, subject to other externalforces. With more potential forest inventory to har-vest,the costs of wood and paper products to con-sumers are likely to decrease,as are the returns toowners of timberland. The changes in climate andconsequent impact on forests are likely to changemarket incentives to harvest and plant trees,andshift land uses between agriculture and forestry.These changes will likely vary within a region.Market incentives for forestry are likely to moderatesome of the climate-induced decline in the area ofnatural forests. International trade in forest prod-ucts could either accentuate or dampen priceeffects in the US,depending on whether forest har-vest activity increases or decreases outside the US.

Over the next century, changes in the severity, fre-quency, and extent of natural disturbances are possi-ble under future climate change. These changes innatural disturbances then impact forest structure,biodiversity, and functioning. Analyses of the resultsfrom climate and ecological models suggest that theseasonal severity of fire hazard is likely to increaseby 10% over much of the US,with possibly largerincreases in the southeastern US and Alaska andactual decreases in the Northern Great Plains.Although the interactions between climate changeand hurricanes,landslides,ice storms,wind storms,insects,disease,and introduced species are difficult

The remaining forestland is in various federal,state,county, and municipal ownerships. Over 50% of thefederal land in forests is managed by the US ForestService. The largest state owner of forestland isAlaska. County and municipal ownerships compriseless than 4% of the total forestland. However, over2.5 million acres of forestland are managed by theselocal governments in Minnesota alone. Over 80% ofthe forestland in federal ownership is found in thewest,while most of the forestland held by states isin the northern part of the US. Of the 472 millionacres in private ownership, over 60% is found in theeastern part of the US. Forest industry ownersaccount for 14% of the forestland in private owner-ship,and these lands are found mainly in the south-ern part of the US (54%).

Forests are an environment in which people recre-ate,such as the National Forests and Parks,and anenvironment in which people live,such as the NewEngland woods,and the conifer forests of RockyMountains. Forests provide recreational opportuni-ties such as hiking,camping, fishing,hunting,birdwatching,downhill and cross-country skiing,andautumn leaf tours. In addition,many rivers andstreams flowing through forests provide fishing,boating,and swimming recreational opportunities.Activities associated with recreation provide incomeand employment in every forested region of the USand Canada (Watson et al.,1998). Forests provideclean air and water, watershed and riparian buffers,moderate streamflow, and help to maintain aquatichabitats. New York City’s water supply is derivedfrom water collected within forested watersheds ina 2,000 square mile area. Forests provide wildlifehabitat. Flather et al.(1999) report that at least 90%of the resident or common migrant vertebratespecies in the US rely on forest habitats for part oftheir life requisites. Forests also provide culturaland aesthetic values,carbon storage,and productsthat can be harvested such as timber, pulpwood,fuelwood,wild game,edible plants,fruits and nuts,mushrooms,and floral products. This wealth fromforests depends on forest biodiversity – the varietyof plant and animal species – and forest function-ing—water flow, nutrient cycling and productivity.These aspects of forests are strongly influenced byclimate.


BACKGROUNDCovering nearly one-third of the US, forests are anintegral part of the vegetative cover of the nation’slandscape (Figure 1). The total area in forests in theUS is 747 million acres,which is about 7% of theforestland in the entire world. US forests are distrib-uted in the eastern and western parts of the US,with small stands of forests in the central part locat-ed mainly along rivers and streams. The white-red-jack pine forests of New England have supported atimber industry and the northeastern deciduousforests of maples,beeches,birches,and oaks providethe colorful autumn landscapes for tourists. TheMid-Atlantic region is rich in tree species from pineforests and coastal wetlands to northern uplandhardwoods such as oak-hickory and maple-beech-birch forest types. The southern forests are also amix of conifer and deciduous forests. Westernforests are primarily conifer forests such as Douglas-fir, fir-spruce, Ponderosa pine and piñon-juniper.

Ownership of forestland varies by region in the US.Over 63% of US forestland is in private ownership.

Current Distribution of Forests in the United States

douglas-firhemlock-sitka spruceponderosa pinewestern white pinelodgepole pinelarchfir-spruceredwoodchaparralpinyon-juniperwestern hardwoodsaspen-birch

white-red-jack pinespuce-firlongleaf-slash pineloblolly-shortleaf pineoak-pineoak-hickoryoak-gum-cypresselm-ash-cottonwoodmaple-beech-birchaspen-birch

Western Forests Eastern Forests

Figure 1. Map of forest vegetation types for theUnited States. Map is from the USDA Forest Service,http://www.srsfia.usfs.msstate.edu/rpa/rpa93.ht SeeColor Plate Appendix

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Climate change is one of several pressures onforests encompassed under the broader term,globalchange. Human activities have altered the vegeta-tion distribution of forests in the US. The arrival ofEuropeans along the eastern coast initiated the har-vest of forests. For example,eastern white pineswere highly prized as ship masts for the EnglishNavy. From 1600 to the mid 1800s,the nativeforests in the eastern US were extensively harvestedfor wood products as well as to clear land for agri-culture and urban uses (Figure 2). While total forest-land area has stabilized in the US since the early1900s,land use shifts still occur where forestland isconverted to agricultural or urban use and agricul-tural or pasture is planted to trees. In some parts ofthe eastern US,new forests have regrown on aban-doned agricultural lands,although forestland in thenortheastern part of the US is still less than 70% ofits original extent in the 1600s. Forestland in theSouth occupies less then 60% of the 1600s extent offorestland,and the establishment of pine plantationshas placed some 20% of the forestland in this regionunder intensive management. While western forestshave remained relatively constant in area since the1600s, recent expansion of urban areas and agricul-ture is fragmenting them. Across the Nation,urbanareas have continued to increase (Flather et al.,1999). This expansion of human influences into therural landscape alters disturbance patterns associat-ed with fire, flooding,and landslides. In addition,human activities can result in the dispersal of pollu-tants into forests. Increasing atmospheric concen-trations of ozone and deposition of nitrogen (N)compounds have profound effects on tree photosyn-thesis, respiration, water relations,and survival.

Human activities modify the species composition offorests and the disturbance regimes associated withforests. Fire suppression has altered southeastern,midwestern,and western forests. Harvesting meth-ods,such as clearcutting,shelterwood,or individualtree selection,have changed species composition innative forests. The average age of forest stands inthe East is less than the average for the West, reflect-ing the extensive harvesting as the eastern US wassettled. Intensive management along with favorableclimates in parts of the US has resulted in highlyproductive forests that are maintained for timberproduction,such as the southern pine plantations.Native and introduced insects and disease species,such as gypsy moth, chestnut blight,Dutch elm dis-ease,have altered US forests (Ayres and Lombardero,2000).Trees have been planted far outside their nat-ural ranges for aesthetic and landscaping purposesin urban and rural areas.

Population levels,economic growth,and personalpreferences influence the socio-economic valuesassociated with forests,and consequently theresources demanded from forests. Per capita con-sumption of wood in the US has been relatively sta-ble over the last several decades and the futuredemand for wood products is projected to followpopulation growth over the next 50 years.Technological development and consumer prefer-ences strongly affect the demand for specific woodand paper products. For example,consumer prefer-ence has influenced the increasing amount of recy-cled material used in fiber over the last severalyears. Though recreational hunting has been declin-ing,the economic impact is significant;Flather et al.(1999) estimated that, for all wildlife,hunters alonespent nearly 21 billion dollars on equipment andtravel in 1996. Other products harvested fromforests are more difficult to assign an economicvalue or to project future demand. Blatner (1997)estimates the harvest of mushrooms fromWashington,Oregon,and Idaho forests in 1992 to bevalued at about $40 million.

Figure 2. Land area changes in forestland. Data are from ForestService Resource Bulletin PNW-RB-168, Forest Resource ReportNo. 23, No. 17, No. 14, the Report of the Joint Committee onForestry, 77th Congress 1st Session, Senate Document No. 32.Data for 1850 and 1870 were based on information collected duringthe 1850 and 1870 decennial census; data for 1907 were also basedon the decennial census modified by expert opinon, reported byR.S. Kellogg in Forest Service Circular 166. Data for 1630 wereincluded in Circular 166 as an estimate of the original forest areabased on the current estimate of forest and historic land clearinginformation. These data are provided here for general referencepurposes only to convey the relative extent of the forest estate inwhat is now the US at the time of European settlement. See ColorPlate Appendix

Forest Land Coverage over the Past 400 Years

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ty development. Temperature affects fruit and seedyields and quality by influencing factors such as flow-ering,bud dormancy, and ripening of fruit and cones(Kozlowski and Pallardy, 1997). Changes in air tem-perature in autumn and spring can also affect harden-ing and dehardening of tree needles (Guak et al.,1998).Temperature affects ecosystem-level processessuch as soil decomposition and mineralization.Indirect effects associated with warming could belarger than the direct effect on plant growth in sub-polar biomes where warming of permafrost is likely(Mooney et al.,1999).

Shortages or excesses of water offset the rates ofmost important processes controlling the biogeo-chemistry of the major nutrients. In particular, forest-ed wetlands are sensitive to changes in hydrologicregimes. While forest ecosystems generally occupythose regions with low annual water deficits, waterlimitation in space and time is still critical for overallcarbon balances,and is one of the major driversembedded in the models used to predict globalchange effects.

KEY ISSUESThe vulnerability of forests to climate variability andchange is examined by looking at four key issues: for-est processes,disturbance,biodiversity, and socioeco-nomic benefits.

• Forest processes regulate the flux and apportion-ment of carbon, water, nutrients,and other con-stituents within a forest ecosystem. These process-es operate at spatial scales from leaf to landscapeand control responses of forest ecosystems,suchas forest productivity, to environmental factorssuch as temperature,precipitation,and atmospher-ic concentrations of CO2.

• Forests are subjected to disturbances that arethemselves strongly influenced by climate. Thesenatural disturbances include fire,hurricanes,land-slides,ice storms,wind storms,drought,insects,disease,and introduced species.

• Biodiversity refers to the variety of populations,species,and communities. Climate influences thedistribution and abundance of plant and animalspecies through food availability, habitat availabili-ty, and survivorship.

• Forest processes and forest biodiversity areuniquely capable of providing goods such as woodproducts,wild game,and harvested plants,ecologi-cal services such as water purification,and ameni-ties such as scenic vistas and wilderness experi-ences—the socioeconomic benefits of forests.

At the global scale,population,economic growth,and personal preferences influence the demandsfrom forests. Wood consumption rose 64% globallysince 1961. This increase was strongly influencedby rising wood per capita consumption for fuel-wood,paper products,and industrial fiber(Matthews and Hammond,1999). More than half ofthe wood fiber produced globally is consumed asfuel in contrast to the US,where fuelwood compris-es around 14% of total wood fiber consumed(Brooks,1993;Matthews and Hammond,1999). Thedemands on forests globally are expected to changeas the world’s populations become increasinglyurban. More industrial products and environmentalservices are likely to influence the management ofthe world’s forests (Brooks,1993).

CLIMATE AND FORESTSClimate influences the composition,structure,andfunction of forest ecosystems,the amount and quali-ty of forest resources,and the social values associat-ed with forests. Native forests are adapted to localclimatic features. Where summer drought is typicalin the Pacific Northwest,native conifer and hard-wood forests have water-conserving leaves (Shrineret al.,1998). Black spruce and white spruce arefound in the cold-tolerant boreal forests where win-ter temperature extremes can reach –62˚ to –34˚C(-79˚ to –30˚F) (Burns and Honkala,1990). Thepiñon-juniper forests of the Southwest are drought-adapted.

Changes in the distribution and abundance ofplant or animal species reflect the birth, growth,death,and dispersal rates of individuals in a popu-lation. Climate and soil are strong controls on theestablishment and growth of plants. Climate influ -ences the distribution and abundance of animalspecies through changes in resource availability,fecundity, and survivorship (Hansen and Rotella,1999). Changes in disturbance regimes,and com-petitive and cooperative interactions with otherspecies also affect the distribution of plants andanimals. In addition,human activities influencethe occurrence and abundance of species on thelandscape.

Temperature and Precipitation. The spatial and tem-poral distribution of water and temperature are theprimary determinants of woody plant distributionsover the Earth. Air temperature affects physiologicalprocesses of individual plants,and over the long-term,the environmental conditions for seed devel-opment and germination,population,and communi-

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of nutrient cations (calcium,magnesium,and potas-sium). Low availability of N in the soil limits forestproduction. Increases in forest growth in responseto N deposition have been reported both inScandinavia and the US,although the responsevaries between deciduous and coniferous species(Magill et al.,1997;Aber et al.,1998;Magill et al.,1999).

Ozone is a highly reactive gas. Closely associatedwith the combustion of fossil fuels,ozone is formedin the lower atmosphere (ground-level ozone)through chemical reactions between nitrogenoxides and hydrocarbons in the presence of sun-light. High levels of ozone occur, generally in sum-mer, when warm,stagnant air masses over denselypopulated and highly industrialized regions accumu-late large quantities of nitrogen oxides and hydro-carbons. Thus, the distribution of ozone concentra-tions is very ir regular in space and time.

Ozone leaves the atmosphere through reactionswith plants and soil surfaces along a number ofphysical and chemical degradation pathways.Ozone concentrations can dissipate in a matter ofdays when air masses move away from urban areasthroughout the eastern US,or from western citiesacross more remote forested areas,such as from LosAngeles to the San Bernardino and San GabrielMountains,California. Ozone tends to remain in theatmosphere when it cannot react with material (e.g.vegetation or soils). Thus,ozone concentrations ineastern Maine can be as high as over urban Bostonbecause ozone-bearing air masses have reachedthese remote areas with little loss of ozone as theypassed over the ocean. High concentrations canoccur in relatively remote mountaintop locationsbecause ozone also accumulates at the top of theatmospheric boundary layer where contact withvegetated surfaces is minimal.

U n l i ke nitrogen and S deposition,w h i ch by theirn a t u re are slow and cumu l a t i ve , the effects of ozoneon ve getation are direct and immediate, as the pri-m a ry mechanism for damage is through direct plantu p t a ke from the atmosphere through stomata (smallopenings in the plant leaf through which water andgases pass into and out of the plant). Ozone is as t rong oxidant that damages cell membra n e s ;t h eplant must then expend energy to maintain theses e n s i t i ve tissues. The net effect is a decline in netphotosynthetic ra t e . The degree to which photosyn-thesis is reduced is a function both of dose andspecies conductance ra t e s , the rate at which leave sex ch a n ge gases with the atmosphere (Musselmanand Massman, 1 9 9 9 ) . A n a lyses suggest that curre n t

Changes in these goods,services,and amenitiesare influenced by changes in factors that deter-mine their supply — land area,productivity, man-agement,production technology, quality ofamenities — and their demand — human popu-lation leve l s , economic grow t h , and personal pre f-e re n c e s .

1. Forest Processes

Current Environmental Changes Include Depositionof Nitrogen and Ozone.These environmental changes influence the abilityof forests to respond to changes in climate (Aber etal.,2001). Tree species have been shown to be sen-sitive to air pollutants such as ozone and sulfur (S)(Fox and Mickler, 1996;Taylor et al.,1994;US EPA,1996) and nitrogen (N) deposition has been linkedto soil acidification and cation depletion in forests(Aber et al.,1995;Aber et al.,1998). Total depositionof N and associated acidity in the US have increasedas much as 10-fold over global background levels asa result of human activity (Galloway, 1995;Vitouseket al.,1997;Matthews and Hammond,1999).Combustion of fossil fuels injects N and S into theatmosphere as simple oxides. The N and S oxidesare retained in the atmosphere only for days toweeks,whereas carbon dioxide (CO2) is retained fordecades. Some N and S compounds can be re-deposited on the forest surface either in a dry formor, by dissolution into water, in a wet form (“acidrain”) (Boubel et al.,1994). The shorter residencetime of N and S in the atmosphere results in anintensely regional distribution of deposition,withthe eastern US,and especially the Northeast, experi-encing the highest levels of both S and N(NADP/NTN, 1997).

With the large reduction in S deposition in the lastdecade from the controls imposed by the Clean AirAct of 1990,the importance of the acids in precipi-tation has been reduced. Sources of N vary region-ally. Urban areas contribute to increased N deposi-tion in pine forests in southern California (Fenn etal.,1996). Nitrogen deposition as ammoniumoccurs where fertilizer is applied intensively orwhere livestock are concentrated in feedlots(Lovett,1994). Agricultural lands and feedlots alongthe Front Range of the Rocky Mountains inColorado contribute to increased N deposition inthe alpine and forest ecosystems in Rocky MountainNational Park (Baron et al.,1994;Musselman et al.,1996;Williams et al.,1996).

High levels of N deposition can result in negativeeffects such as soil acidification,causing depletion

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ozone levels have decreased production 10% inn o rt h e a s t e rn fo rests (Ollinger et al., 1997) and 5% ins o u t h e rn pine plantations (Weinstein et al., 1 9 9 8 ) .Wa rming of surface air, a consistent fe a t u re of theH a d l ey and Canadian scenarios used in this assess-m e n t , is like ly to increase ozone and other air-qualityp ro blems (see Watson et al., 1996 for analysis of pre-vious climate scenari o s ) ,f u rther increasing the stre s son fo rests in areas where air quality is compro m i s e d .

Impacts of Elevated Atmospheric Carbon Dioxideand Climate Change on Forest Processes

Experimental exposure of trees to elevated atmos-pheric CO2 has shown significant changes in physi-ological processes, growth,and biomass accumula-tion (Aber et al.,2001;Mooney et al.,1999). Over awide range of CO2 concentrations,photosynthesishas been increased in plants representative of mostnorthern temperate forests (Eamus and Jarvis,1989;Bazzaz,1990;Mohren et al.,1996;Long et al.,1996;Kozlowski and Pallardy, 1997). Reviews of theextensive CO2-enrichment studies have shown vari-able but positive responses in plant biomass accu-mulation (Ceulemans and Mousseau,1994;Saxe etal.,1998;Mooney et al.,1999). In a review of stud-ies not involving environmental stress,biomass accu-mulation was greater for conifers (130%) than fordeciduous species (only 49% increase) under elevat-ed CO2 (Saxe et al.,1998). The wide range of plantresponses reflects,in part,the interaction of otherenvironmental factors and the CO2 response(Mooney et al.,1999;Stitt and Krapp,1999;Morisonand Lawlor, 1999; Johnson et al.,1998;Curtis andWang,1998). A recent field-scale experiment pro-duced significant (25%) increases in forest growthunder continuously elevated concentrations of CO2

for loblolly pine in North Carolina (Delucia et al.,1999). While positive tree responses to elevatedCO2 are likely to be overestimated if abiotic andbiotic environmental factors are not considered,most ecosystems responded positively in terms ofnet carbon uptake to increases in atmospheric CO2

above the current ambient level.

A significant question is how long these increasedresponses can be sustained. The observed acclima-tion or down-regulation of photosynthetic rates(Long et al.,1996;Lambers et al.,1998;Rey andJarvis,1998) has been ascribed to a physiologicalresponse (accumulation of photosynthetic reserves,Bazzaz,1990),or a morphological response (changesin trees,Pritchard et al.,1998;Tjoelker et al.,1998b).Declining photosynthetic rates have also beenascribed to the result of a water or nutrient stressimposed on pot-grown seedlings where root growth

is limited (Will and Teskey, 1997a;Curtis and Wang,1998). Down-regulation has been shown in low-temperature systems such as the Arctic tundra,where the initial enhancement of net carbon uptakedeclined after 3 years of elevated CO2 exposure(Shaver et al.,1992;Mooney et al.,1999). Down reg-ulation is likely in areas where nutrient availabilitydoes not increase along with carbon dioxide. Arecent review of large-scale field exposures to car-bon dioxide suggested that,though variable,treeresponse to CO2 was sustained over these short-term studies (Norby et al.,1999). They also exam-ined the CO2 response of trees growing near surfacevents of deep geothermal springs,and concludedthat a basal area increase of 26% was sustainedthrough 3 decades of elevated carbon dioxide.

Under enriched CO2 conditions, water use efficien-cy (WUE) has been shown to increase,which resultsin higher levels of soil moisture. These increasedlevels of soil moisture have been shown to be a sig-nificant factor in increased carbon uptake in water-limited ecosystems (Mooney et al.,1999). Whilethere is still uncertainty as to whether stomatal con-ductance decreases under elevated CO2 (Long et al.,1996;Will and Teskey, 1997b;Saxe et al.,1998;Curtisand Wang,1998),WUE increases either with or with-out changes in stomatal conductance (Aber et al.,2001). With constant conductance,the higheratmospheric CO2 concentration results in faster car-bon (C) uptake with constant water loss. If conduc-tance is reduced,a tradeoff is established betweenincreased C gain (which is partially reduced bydecreased conductance) and decreased water loss(also reduced by decreased conductance).Experimental studies have emphasized leaf-levelresponses. A physiological response observed atone scale (i.e.,leaf) does not necessarily imply that aresponse will be observed at the larger scale (i.e.,canopy, watershed). There is some evidence thatthe reduction in stomatal conduction of treeseedlings is not seen in mature trees (Ellsworth,1999;Mooney et al.,1999;Norby et al.,1999).

In the near-term, changes in the physiology of plantsare likely to be the dominant response to elevatedcarbon dioxide,strongly tempered by local environ-mental conditions such as moisture stress, nutrientavailability, as well as by individual species respons-es (Egli and Korner, 1997;Tjoelker et al.,1998a;Berntson and Bazzaz,1998;Crookshanks et al.,1998;Kerstiens,1998). Over the long term,plant specieschanges within forests will have a large influence onthe response of forests (Mooney et al.,1999).

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Interactions of Multiple Stresses on Forests

It is crucial to understand not only the direct effectsof CO2, ozone,temperature,precipitation,and N andS deposition on forests,but also the interactiveeffects of these stresses. For example,if canopyconductance in forests is reduced in response toCO2 enrichment,then ozone uptake is reduced andthe effects of this pollutant mitigated. Droughtstress has a similar effect by reducing stomatal con-ductance. However, if N deposition leads toincreased N concentrations in foliage and hencehigher rates of photosynthesis and increased stom-atal conductance,then the positive effect ofincreased photosynthesis is partially offset byincreased ozone uptake. Reductions in productionfrom ozone damage could possibly speed the onsetof N saturation in ecosystems and the attendantdevelopment of acidified soils and streams. To proj-ect the effects of climate change and other stresseson forests,their interactive effects on forest process-es must be understood and integrated into ecologi-cal models.

Carbon Sequestration

Globally, an estimated 62-78% of the terrestrial car-bon is in forests (Shriner et al.,1998). NorthAmerican (Canada and US) forests hold 14% of thisglobal total,with large amounts contained in theboreal forests. Estimates of the carbon sequesteredannually as a result of climate and carbon dioxidefertilization in US forests were analyzed recently(Schimel et al.,2000) at a value of 0.08 Pg (Pg=Petagrams where 1 Pg = 1015 grams) of carbon peryear. This analysis focused on the 1980-1993 period.Estimates of carbon sequestration based on forestinventory data were 0.28 Pg of carbon per year, andmost of this increase in carbon storage was estimat-ed to be on private timberland (Birdsey and Heath,1995). Houghton et al.(1999) estimated theincrease in carbon stored per year ranged from 0.15to 0.35 Pg of carbon per year. Schimel et al.(2000)suggest that the larger inventory-based estimatesimply that the effects of intensive forest manage-ment and agricultural abandonment on carbonuptake in the US are probably equal to or largerthan the effects of climate and atmospheric carbondioxide. A comprehensive approach to account forcarbon fluxes to and from the atmosphere is thefocus of the recent IPCC Special Report on LandUse,Land-Use Change,and Forestry (Watson et al.,2000).

Projecting the Impact of Climate Change on Forest Processes

A number of studies have examined the climate con-trols on the distribution and productivity of forestsusing different types of models (Goudriaan et al.,1999;McGuire et al.,1992;VEMAP members,1995).In this assessment,three types of models were usedto project the impact of climate change on forestprocesses. Biogeochemistry models simulate thegain,loss and internal cycling of carbon, nutrients,and water;with these models,the impact of changesin temperature,precipitation,soil moisture,atmos-pheric carbon dioxide,and other climate-related fac-tors can be examined for their influence on suchprocesses as ecosystem productivity and carbonstorage.Biogeography models examine the influ-ence of climate on the geographic distribution ofplant species or plant types such as trees, grasses,and shrubs. Dynamic global vegetation models inte-grate biogeochemical processes with dynamicchanges in vegetation composition and distribution.

An earlier analysis using biogeochemistry modelsand four climate scenarios (different from thisassessment) shows increased net primary productiv-ity (NPP) at the continental scale (VEMAP members,1995). Carbon storage results vary with the mod-eled sensitivity to changes in water availability.When the direct effects of carbon dioxide are notincluded,several analyses indicate a reduction inproductivity (biogeochemistry models:VEMAP mem-bers,1995) or in vegetation density (biogeographymodel:Neilson et al.,1998).

In this assessment,three biogeochemistry models(TEM,Century, and Biome-BGC) and one dynamicglobal vegetation model (MC1) show increases intotal live vegetation carbon storage (3 to 11 Pg C)for forest ecosystems within the conterminous USunder the Hadley scenario (Table 1). Under theCanadian scenario,the models project changes inlive vegetation carbon for forests from a reductionof 1.6 Pg to an increase of 11 Pg C (Pg= Petagramswhere 1 Pg = 1015 grams). The MC1 model simu-lates a decline in vegetation carbon of about 2 Pg(Bachelet et al.,2001;Daly et al.,2000). Results forlive vegetation in all ecosystems and for live anddead vegetation in both forest and all ecosystemsparallel these results. Modeling the changes inspecies groups and fire disturbance are the primaryreasons for the carbon responses in MC1.

In the MC1 projections, regional changes in carbonstorage vary greatly across the conterminous US,andreflect the likelihood of disturbances such as

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hurricanes,landslides,wind storms,and icestorms. Over geologic time,local, regional,andglobal changes in temperature and precipitationhave influenced the occurrence,frequency, andintensity of these natural disturbances.

Impacts of disturbances are seen over a broadspectrum of spatial scales,from the leaf and treeto the forest and forested landscape. Disturbancescan result in:leaf discoloration and reduction of leaffunction;deformation of tree structure such as bro-ken branches or crown losses;tree mortality orchronic stress resulting in tree death;altered regen-eration patterns including losses of seed banks;dis-ruption of physical environment including soil ero-sion;alterations in biomass and nutrient turnover;impacts on surface soil organic layers and theunderground plant root and reproductive tissues;and increased landscape heterogeneity (patchinessof forest communities). Introduced species (inva-sives) can affect forest ecosystems through her-bivory, predation,habitat destruction,competition,loss of gene pools through hybridization with nativespecies,and disease (either causing or carrying dis-ease). Outbreaks of native insects and disease canresult in similar impacts on forests.

At the ecosystem scale,introduced species as wellas outbreaks of native insects and diseases can alternatural cycles and disturbance regimes,such asnutrient cycles,and fire frequency and intensity(Mack and D’Antonio,1998). Some tree specieshave developed adaptations to survive repeatedoccurrences of certain disturbances over time.Thick bark on some trees allows their survival in

drought or fire (Figure 3). Under the Hadley sce-nario about 20% of current forest area experiencesome level of carbon loss,while the remaining 80%experience increased storage. Under the muchwarmer and generally drier Canadian scenario, closeto 80% of current forest area experiences a drought-induced loss of carbon. Reductions in carbon stor -age are projected in MC1 to be especially severe inthe eastern and southeastern US,where losses exac-erbated by drought or fire approach 75%.

In summary, synthesis of laboratory and field studiesand recent simulation experiments indicate that for-est productivity increases with the fertilizing effectof atmospheric carbon dioxide and that these pro-ductivity increases are strongly tempered by localconditions such as moisture stress and nutrientavailability. It is likely that modest warming couldresult in carbon storage gains in some forest ecosys-tems in the conterminous US. Under even warmerconditions,it is likely that drought-induced losses ofcarbon would occur in some forests,notably in theSoutheast and the Northwest. These losses of car-bon would possibly be enhanced by increased firedisturbance. These potential gains and losses of car-bon are very likely to be subject to changing land-use patterns,such as the conversion of forests toother uses.

2. Forests and Disturbance

Natural Disturbances Impacted by Climate

Natural disturbances,impacted by climate,includeinsects,disease,introduced species, fires,droughts,

Table 1. Changes in Carbon Storage for the Conterminous US

These results are based on simulations by the VEMAP models (TEM,BIOME-BGC,CENTURY, MC1) using thetwo transient climate change scenarios described in the text. Baseline period is 1961-1990. Changes aregiven at decades centered on 2030 and 2095,and for forest ecosystems,and all ecosystems. Values areexpressed in Pg (billions of metric tons).

Hadley Canadian 2030 2095 2030 2095

Forest Ecosystems Live Vegetation 0.4 to 4.0 3.0 to 11.1 -1.5 to 2.9 -1.6 to 10.5 Total (Live + Dead) 0.3 to 3.3 3.2 to 10.5 -2.9 to 2.0 -0.6 to 9.4

All Terrestrial EcosystemsLive Vegetation 0.4 to 4.7 3.3 to 13.2 -1.2 to 3.8 -1.8 to 13.9 Total (Live + Dead) 0.2 to 4.8 3.4 to 14.5 -0.2 to 4.4 -1.9 to 15.0

ground-level fires. In western forests, repeatedground-level fires reduce intermediate height vegeta-tion that can serve as fuel between the surface andthe crown. Thus,these repeated ground-level firesreduce the occurrence of stand-killing crown fires.

Disturbances can be regional or widespread.Landslide processes exhibit very strong geographicpatterns. Pacific coastal mountains are particularlyprone to sliding because of weak rocks,steepslopes,and high precipitation from frontal storms inthese tectonically active areas. Other disturbances,such as insects,pathogens,or introduced species,are widespread across the US. Many disturbancescascade into others. For example,drought oftenleads to insect outbreaks,disease,or fire. Insectsand disease can also create large fuel loads andthereby contribute to increased fire frequency.

Disturbances can be of either natural or anthro-pogenic origin (e.g., fire). Many large forest areashave been affected by past human activities,andcurrent disturbance regimes are profoundly differ-ent from historical regimes. For example, fire sup-pression in the fire-adapted western forests has ledto increased forest density and biomass, changes inforest composition,and increased outbreaks ofinsects and disease (Flannigan et al.,2000;Ayres andLombardero,2000). Some forest types have evolved

to depend on the periodic occurrence of the distur-bance. Long-leaf pine forests are a fire-climaxecosystem that would not exist if fire were not apart of the Southeast.Nearly 1.5 million acres ofprescribed burning and other fuel reduction treat-ments in 1998 were used to enhance forest healthand diversity by restoring fire-dependent ecosys-tems that have been affected by long-term fire sup-pression (USDA Forest Service,1998). Natural dis-turbances interact with human activities such as airpollution,harvest, agricultural and urban encroach-ment,and recreation.

Impact of Climate Change on Forest Disturbances

Climate variability and climate changes alter the fre-quency, intensity, timing,and/or spatial extent of dis-turbances. Many potential consequences of futureclimate change will possibly be buffered by theresilience of forest communities to natural climaticvariation. However, the extensive literature on thissubject suggests that new disturbance regimesunder climate change will likely result in significantperturbations to US forests,with lasting ecologicaland socioeconomic impacts (Dale et al.,2001).

Hurricanes. Hurricanes seriously impact forestsalong the eastern and southern coasts of the US aswell as the Caribbean Islands (Lugo,2000). They 499


Figure 3. Change in live vegetation car-bon density from the historical period(1961-1990) to the 2030s (2025-2035) andthe 2090s (2090-2099) under two climatescenarios. Change in the biomass con -sumed by natural fires between the 20th

century (1895-1993) and the 21st century1994-2100) as simulated by the dynamicvegetation model MC1 under two climatescenarios (bottom two panels) (Bacheletet al. 2001) See Color Plate Appendix

Patterns of Live Vegetation for Different Times and Climate Scenarios

Hadley Canadian

near 20% for the Northeast and small decreases forthe northern Great Plains,with increases less than10% over the rest of the continent. The warmer anddrier Canadian scenario produces a 30% increase infire severity for the Midwest,Alaska,and sections ofthe Southeast,with about 10% increases elsewhere.These results suggest a possible increase of 25-50%in the area burned in the US. Temperature and pre-cipitation are not the only climate-related factorsthat influence fire regimes; for example,lightningstrike frequency was estimated to increase 44%under the Goddard Institute for Space Studies gener-al circulation model scenario (Price and Rind,1994).Other factors such as length of the fire season,weather conditions after ignition,and vegetationcharacteristics also influence the fire regimes.Wotton and Flannigan (1993) found that the fire sea-son would be on average 30 days longer in a doublecarbon dioxide climate as compared to the currentclimate for Canada. Wildfire severity was at least assensitive to changes in wind as to changes in tem-perature and precipitation in a climate change sensi-tivity analysis for California (Torn and Fried,1992).Human activities such as fire policies and land usewill likely also influence fire regimes in the future(Keane et al.,1998). For example,wildfires on alllands in the western US increased in the 1980s after30 years of aggressive fire suppression that had ledto increases in forest biomass.

The second modeling study examined the influenceof climate change on vegetation and the interactionwith natural fires (Bachelet et al.,2001). Theamount of biomass consumed in fire increasedunder future climates in analyses with the dynamicglobal vegetation model,MC1 (Figure 4). In thismodel, fire occurrence,severity, and size are simulat-ed as a direct function of fuel and weather condi-tions (Lenihan et al.,1997;Daly et al.,2000,Bacheletet al.,2001). In the western US,increased tempera-ture,steady to increased precipitation,CO2 fertiliza-tion,and increased water use efficiency enhanceecosystem productivity, resulting in more biomass.The highly variable climate of dry years interspersedwith wet years and the fuel buildup contributes tomore and larger fires in the western landscape. Inthe eastern US under the Canadian scenario, firesare projected to increase in the Southeast as a resultof increased drought stress in forests.

Both approaches suggest the potential for anincreased area to be burned with a changing cli-mate.These analyses are based on the physical andbiological factors influencing potential fire hazard.Factors such as current land management,land use,and ownership are not considered. Harvest activi-

can inflict sudden and massive tree mortality, com-plex patterns of tree mortality including delayedmortality, and altered patterns of forest regeneration(Lugo and Scatena,1996). Because most hurricanedamage is from floods,effects can be removed fromthe actual hurricane in distance (heavy precipitationwell inland from the coast) and in time (delaysinvolved in water movement,and in mortality result-ing from excessive water).Hurricanes can lead toshifts in successional direction,higher rates ofspecies turnover, opportunities for forest specieschange,increasing landscape heterogeneity, fasterbiomass and nutrient turnover, and lower above-ground biomass in mature vegetation (Lugo andScatena,1995).

Hurricane location,size,and intensity are influencedby sea surface temperatures and by regional weath-er features (Emanuel,1999). Sea surface tempera-tures (SSTs) are expected to increase,with warmerSSTs expanding to higher latitudes (Royer et al.,1998;Walsh and Pittock,1998). Climate change isalso likely to influence the frequency of regionalweather events conducive to hurricane formation,although it is not yet possible to say whether hurri-cane frequency increases or decreases (Royer et al.,1998;Henderson-Sellers et al.,1998;Knutson et al.,1998;Knutson and Tuleya,1999).

Fire. Fire frequency, size,intensity, seasonality, type,and severity are highly dependent on weather andclimate. An individual fire results from the interac-tion of ignition agents (such as lightning,fuel condi-tions,and topography) and weather (including airtemperature, relative humidity, wind velocity, andthe amount and frequency of precipitation). Overthe 1989-1998 period,an average of 3.3 millionacres burned annually in the US, varying from 1 mil-lion acres a year to over 6 million,mostly in thewest and southeast. Most of the burned acres result-ed from human-caused fires.

Two modeling approaches were used to look at theimpact of climate change on fire:1) fire severity rat-ings estimated from future climate (Flannigan et al.,2000),and 2) the interaction of vegetation biomassand climate in establishing conditions for fire(Bachelet et al.,2000).

In the analysis by Flannigan et al.(2000),the futurefire severity is projected to increase over much ofNorth America under both climate scenarios andthese results are consistent with earlier analyses(Flannigan et al.,1998). The results show great vari-ation for the US and Canada. The warmer and wet -ter Hadley scenario suggests fire severity increases500


ties and the conversion of forest land to other useswould also alter the amount and kind of fuel.

The rapid response of fire regimes to changes in cli-mate is well established (Flannigan et al.,1998;Stocks et al.,1998),so this response has the poten-tial to overshadow the direct ef fects of climatechange on species distribution,migration,andextinction within fire-prone areas. This possibility ofincreased fire poses challenges to the managementof protected areas such as national parks (Malcolmand Markham,1998) and to the management offorests for carbon storage.

Drought. Droughts occur in nearly all forest ecosys-tems (Hanson and Weltzin,2000). The primaryimmediate response of trees to drought is to reducenet primary production (NPP) and water use,whichare both driven by reduced soil moisture and stom-atal conductance. Under extended severe droughtconditions,plants die. Seedlings and saplings usuallydie first and can succumb under moderate droughtconditions. Deep rooting,stored carbohydrates,andnutrients in large trees make them susceptible onlyto longer, more severe droughts. Secondary effectsalso occur. When reductions in NPP are extreme orsustained over multiple growing seasons,increasedsusceptibility to insects or disease is possible,espe-cially in dense stands (Negron,1998). Drought canalso reduce decomposition processes leading to abuildup of organic matter on the forest floor.

The consequences of a changing drought regimedepend on annual and seasonal changes in climateand whether a plant’s current drought adaptationsoffer resistance and resilience to new conditions.Forests tend to grow to a maximum leaf area thatuses nearly all available growing-season soil water(Eagleson,1978;Hatton et al.,1997; Kergoat,1998;Neilson and Drapek,1998). A small increase ingrowing-season temperature could increase evapora-tive demand and trigger moisture stress.Using thisassumption about leaf area, results from MAPSS, abiogeography model,and MCI,a dynamic global veg-etation model,suggest that increased evaporativedemands will likely cause future increases indrought stress in forests of the Southeast,southernRocky Mountains,and parts of the Northwest(Neilson and Drapek,1998;Bachelet et al.,2000).While earlier forest models,often known as ‘Gap’models (Shugart,1984) also suggested forestdeclines,all of a tree’s current drought adaptations,such as adjusting growth rates or aboveground-belowground allocations of carbon,had not beenincorporated into the analysis (Loehle and LeBlanc,1996). These adaptations buffer the impact of cli -

mate,including drought,on individual trees and for-est stands. The current generation of ecologicalmodels,such as MAPSS,MC1 and others,haveimproved upon these process algorithms,includingthe suggestions from Loehle and LeBlanc (1996).

Wind events. Small-scale wind events,such as torna-does and downbursts,are products of mesoscaleweather circumstances (Peterson,2000). These dis-turbances can create very large areas of damage.For example,an October 25,1997 windstorm flat-tened nearly 13,000 acres of spruce-fir forest in theRoutt National Forest of Colorado (USDA ForestService Routt National Forest,1998),and a July 4,1999 windstorm flattened roughly 250,000 acres offorest in the Boundary Waters Canoe Area ofMinnesota (Minnesota Dept.of Natural Resourcespress release 7/12/99). Although small-scale windevents occur throughout the US,the highest con-centration of tornadoes occurs across the CentralGreat Plains states of Oklahoma, Texas,Kansas,andNebraska.

If climate change increases intensity of all atmos-pheric convective processes,this change will accel-erate the frequency and intensity of tornadoes andhailstorms (Berz,1993). Karl et al.(1995a) foundthat the proportion of precipitation occurring inextreme weather events increased in the US from 501


Figure 4. Simulated total biomass consumed by fire over the con-terminous US under historic and two future climates; Hadley(HADCM2SUL) and Canadian (CGCM1) scenarios. The fire simula-tions are for potential vegetation and do not consider historic firesuppression activities. However, grid cells with more than 40%agriculture have been excluded from the calculations (Lenihan et.al., 1997, Daly et. al., 2000, Bachelet et. al., 2001). See Color PlateAppendix

Biomass Consumed under Two Scenarios of Future Climate

affect Fraser fir (Abies fraseri) survival (Dale et al.,1991).

A key feature of most invasive species is that theyhave the capacity to thrive in disturbed environ-ments through their high reproductive rates, gooddispersal abilities,and rapid growth rates (Vitouseket al.,1996). If climate change results in increaseddisturbances such as fire or drought,these disrup-tions to ecosystems create just the type of environ-ments in which invasive species are likely to expandrapidly. The interactions among introduced species,native communities,intensively managed forests,human activities that fragment ecosystems,increased atmospheric deposition,and climatechange might positively affect the prevalence ofinvaders,but forecasting specific impacts of inva-sions remains problematic (Dukes and Mooney,1999;Williamson,1999).

Insect and Pathogen Outbreaks Outbreaks ofinsects and pathogens can adversely affect recre-ation,wildlife habitat, wood production and ecologi-cal processes.Over the 1986 to 1995 period,these 4native insect species damaged annually the follow-ing acreages: over 4 million acres for western sprucebudworm;less than 1 million acres for easternspruce budworm;less than 2 million acres formountain pine beetle;and nearly 12 million acresfor southern pine beetle (The Heinz Center, 1999).Within this time period,the acreage affected by anyone of these insects could vary from less than halfof the long-term average to three times the long-term average. Nearly 13 million acres of southernforests are affected by a single disease,fusiform rust,and 29 million acres of western forests are affectedby a parasitic plant, dwarf-mistletoe (The HeinzCenter, 1999). Disturbances such as drought andfire influence these outbreaks.

An extensive body of scientific literature suggestsmany pathways through which elevated carbondioxide and climate change could significantly alterpatterns of disturbance from insects and pathogens(Ayres and Lombardero,2000). Elevated carbondioxide and climate change could possibly increaseor decrease the disturbances of insects andpathogens through direct effects on the survival,reproduction,and dispersal of these organisms. Forexample,an increase in the interannual variation inminimum winter temperatures could possibly favormore northerly outbreaks of southern pine beetles,while decreasing more southerly outbreaks(Ungerer et al.,1999). It is also possible that climatechange would alter insect and pathogen distur-bances indirectly through changes in the abundance

1910 to 1990. Karl et al.(1995b) further suggestthat the US climate has become more extreme (interms of temperature and precipitation anomalies)in recent decades. Further, Etkin (1995) found apositive correlation between monthly tornado fre-quency in western Canada and mean monthly tem-perature,and inferred that this relationship suggestsincreased tornado frequency under a warmer cli-mate. Despite the above indirect inferences abouttornado frequencies,and the direct data on thunder-storm trends,there is still inadequate understandingof tornado genesis to directly forecast how climatechange will affect the frequency or severity of wind-storms in the next century (Peterson,2000).

Ice Storms. Ice storms,also known as glaze events,result when rain falling through subfreezing airmasses close to the ground is supercooled so thatraindrops freeze on impact (Irland,2000). TheNational Weather Service (NWS) defines an icestorm as an occurrence of freezing precipitationresulting in either structural damage or at least 0.25inch of ice accumulation. Ice accumulation canvary dramatically with topography, elevation,aspect,and areal extent of the region where conditionsfavor glaze formation. While ice storms can occur asfar into the southern US as northern Mississippi andTexas,the frequency and severity of ice stormevents generally increases toward the northeast(Irland,2000).

Depending on forest stand composition,amount andextent of ice accumulation,and stand history, dam-age can range from light and patchy to total break-age of all mature stems (Irland,1998). Even thoughthe weather conditions producing ice storms arewell understood,it is not known how changes in cli-mate will affect the frequency, intensity, location,orareal extent of ice storms.

Introduced species. Climate,as well as human activi-ties,largely determine the potential and realized dis-tributional ranges of introduced species (Simberloff,2000). Unsuitable climate at points of arrivalrestricts the survival of a great majority of intro-duced species (Williamson,1999). In warmer partsof the US,introduced species comprise a larger frac-tion of the biota (Simberloff, 1997). Where climatecurrently restricts invasives, changes in temperatureor precipitation may facilitate increased growth,reproduction,or expansion of their ranges. Forexample,laboratory studies of balsam woolly adel-gid (Adelges piceae), growing under various temper-ature conditions,provided the basis for simulationssuggesting that temperature-induced changes in thepopulation dynamics of the insect significantly502


of their natural enemies and competitors. In addi-tion, climate,and elevated carbon dioxide,influ-ence the susceptibility of trees to insects andpathogens through changes in the chemistry ofplant tissues (Ayres and Lombardero,2000).

The short life cycles,high mobility, reproductivepotential,and physiological sensitivity to tempera-ture suggest that even modest climate change willpossibly have rapid impacts on the distribution andabundance of many forest insects and pathogens.Beneficial impacts could possibly result wheredecreased snow cover increases winter mortality.Detrimental impacts could possib ly result whenwarming accelerates insect development and facili-tates dispersal of insects and pathogens into areaswhere tree resistance is less. Detrimental impactscould also possibly result from interactions of dis-turbances. For example, warming could increaseoutbreaks in boreal forests that would tend toincrease fire frequency (Ayres and Lombardero,2000). Already, the impact of insects and disease inforests is widespread. In 1995, over 90 millionacres of forestland in the US were affected by afew species:southern pine beetle,mountain pinebeetle,spruce budworm (eastern and western),spruce beetle, dwarf mistletoe, root disease,andfusiform rust. Thus,there are potentially impor-tant ecological and socio-economic consequencesto these beneficial and detrimental impacts (Ayresand Lombardero,2000;Ayres and Reams,1997).

3. Biodiversity

Land Use Impacts Species, Communities, and Biomes

Global ch a n ge encompasses a number of eve n t so c c u rring at the continental scale, i n cluding cl i m a t ech a n ge , land use ch a n ge , species inva s i o n , and airp o l l u t i o n . Global ch a n ge has and will like ly contin-ue to affect the abundance and distribution ofplants and animals which , in turn , will have consid-e rable ra m i fications for human economics, h e a l t h ,and social we l l - b e i n g . O rganisms provide go o d si n cluding material pro d u c t s , fo o d s , and medicines.In addition, the number and kinds of species pre s-ent affect how ecosystems respond to globalch a n ge . It is the responses of individual org a n i s m sthat begin the cascade of ecological processes thatthen manifest themselves as ch a n ges across land-s c a p e s ,b i o m e s , and the globe (Hansen et al., 2 0 0 1 ,Wa l ker et al., 1 9 9 9 ) .

Humans modify the quality, amount,and spatialconfiguration of habitats. A number of natural

community types now cover less than 2% of theirpre-settlement ranges (Noss et al.,1994).Examplesinclude:spruce-fir forests in the southernAppalachians;Atlantic white-cedar in parts ofVirginia and North Carolina; red and white pine inMichigan;longleaf pine forests in the southeasterncoastal plains;slash pine rockland habitat in south-ern Florida;loblolly/shortleaf pine forests in thewest gulf coastal plains;and oak savannas inOregon. For the species dependent upon ecosys-tems that have declined in area,such habitat losscan reduce effective population sizes, genetic diver-sity, and the ability of species to evolve adaptationsto new environments (Gilpin,1987). The areainvolved in land use shifts can dwarf the land areainvolved in natural disturbances. For example,thearea of harvested cropland went from 292 millionacres in 1964 to 347 million acres in 1982,andthen down to 293 million in 1987. For compari-son,the total area burned in fires at 1 to 6 millionacres annually is much less than these land areashifts in and out of agriculture. Though the forestremains standing,only five species of insects areestimated to defoliate about 21 million acres eachyear, on average (The Heinz Center, 1999).

Land use change alters the spatial pattern of habi-tats by creating new habitats that are intensivelyused by humans or by reducing the area and frag-menting natural habitats. These changes increasethe distance between habitat patches and reduceoverall habitat connectivity. Native forests havebeen converted to agricultural and urban uses,notably in the eastern and midwestern parts of theUS. In some cases, forests have regrown on aban-doned agricultural lands. Recent expansion ofurban areas and agriculture are fragmenting west-ern forests. Nationally, urban areas have doubled inarea between 1942 and 1992 (Flather et al.,1999).While urban areas increased most rapidly in theSouth and Pacific Coast regions,the most influen-tial land use change has been the increase inhuman population density in rural areas,particular-ly in the Rocky Mountain and Pacific Coast regions.This expansion alters disturbance patterns associat-ed with fire, flooding,and landslides.Roadways andexpansion of urban areas have fragmented forestsinto smaller, less-contiguous patches.

Loss of habitat and degradation of habitat qualitycan reduce population size and growth rates,andelevate the chance of local extinction events(Pulliam,1988). The steadily increasing number ofspecies listed as threatened and endangered in theUS is currently at 1,232 (USDI, Fish and WildlifeService,2000). Factors contributing to species 503


association with climate warming (Abraham andJefferies,1997). The breeding dates of both amphib-ians and birds in Great Britain have shifted one tothree weeks earlier since the 1970s in associationwith increasing temperatures (Beebee,1995;Cricket al.,1997).

The primary focus for this analysis is on the conti-nental-scale response of forest vegetation as reflect-ed in climate-induced changes in the distributions ofbiomes,community types,species richness,and indi-vidual tree and shrub species. Vegetation responsesto projected future climate change were assessed byreviewing the available literature and by using theclimate predictions of global climate models asinput to a set of different vegetation simulationmodels. Paleoecological studies of climate changeimpacts on forests provide information about pastresponses. However, their results are limited withrespect to the future because the current size, age,and species composition of temperate forests hasbeen strongly impacted by human activities,andsecond,global temperatures are predicted toincrease at an unprecedented rate (Dale,1997).Although vegetation models incorporating land-usedynamics are under rapid development at localscales,the current state of knowledge does notallow for integrating the effects of land use at thecontinental scale.

The vegetation models used in this analysis includethe following. Two statistical models project the dis-tribution of individual tree species with resultsaggregated into community types. For the easternUS,the DISTRIB model,which projects potentialchanges in suitable habitat of 80 tree species, wasdeveloped from 33 environmental variables and thecurrent distribution of each tree species (Iverson etal.,1999). The results for 80 species under each cli-mate change scenario were also aggregated to exam-ine potential changes in forest types in the easternUS (Iverson and Prasad,2001). Shafer et al.(2001)developed local regression models that estimate theprobability of occurrence of 51 tree and shrubspecies across North America based on 3 bioclimaticvariables:mean temperature of the coldest month,growing degree days,and a moisture index.Changes in species richness of trees and terrestrialvertebrates based on energy theory (Currie,1991)were also analyzed under the different climate sce-narios. Species interactions and the physiologicalresponse of species to carbon dioxide are notincluded in these statistical models. In contrast,theMAPSS biogeography model (Neilson,1995) projectsbiome response to climate as change in vegetationstructure and density based on light,energy, and

endangerment include habitat conversion, resourceextraction,and exotic species (Wilcove et al.,1998).The spatial distribution of such factors results inspecies at risk being concentrated in particularregions of the US,especially the southernAppalachians,the arid Southwest,and coastal areas(Flather et al.,1998),which can be traced back tohuman population growth and attendent land-useintensification. Land-use intensification has alsobeen found to affect animal communities broadly.Along a gradient of increasing land-use intensifica-tion in forested regions of the eastern US,nativespecies of breeding birds are increasingly under-rep-resented and exotic species over-represented(Flather, 1996).

Species that take advantage of anthropogenic habi-tats,such as some deer, goose,and furbearer species,have greatly expanded in recent years (Flather et al.,1999). Some species of deer (e.g.,white-tailed deer)are now so abundant that the primary concern iscontrolling their populations. In addition, exoticspecies have become established and greatlyexpanded their ranges in the US (Drake et al.,1989).

Impact of Climate Change on Biodiversity

Changes in the distribution and abundance of plantor animal species reflect the birth, growth,death,and dispersal rates of individuals in a population.When aggregated,these changes manifest as localextinction and colonization events,which are themechanisms that determine a species’ range. Whileclimate and soils are strong controls on the estab-lishment and growth of plants,the response of plantand animal species to climate change will be theresult of many interrelated processes operating overseveral scales of time and space. Migration rates,changes in disturbance regimes,and competitiveand cooperative interactions with other species willaffect the distribution of plants and animals.Because of the individualistic response of species,biotic communities are not expected to respond asintact units to climate change. Community compo-sition responds to a complex set of factors includingthe direct effects of climate,differential species dis-persal,and indirect effects associated with changesin disturbance regimes,land use,and interspecificinteractions (Peters,1992).

Some of the best evidence that species’distributionsare correlated with changing climates is found forplants (Webb,1992). Recent observations of somespecies suggest a response to historical changes inclimate. Breeding ranges of some mobile species,such as waterfowl,have expanded northward in504


water limitations (VEMAP Members,1995). Thepotential natural vegetation is coupled directly toclimate and hydrology, and rules are applied to clas-sify vegetation into biome types. The model con-siders the effects of altered carbon dioxide onplant physiology. The analyses for species,commu-nities,and biomes used an equilibrium climate sce-nario based on the transient Canadian and transientHadley scenarios. The baseline scenario was theaverage climate for the 1961-1990 period. The “cli-mate change”scenario was the average of the pro-jected climate for 2070 to 2100. The results ofthese analyses are given below. The implications ofspecies dispersal and land use change are discussedin the context of these analyses.

Tree Species. The potential distribution of treespecies under climate change is modeled usingboth statistical models (Iverson and Prasad,2001;Shafer et al.,2001) and both climate scenarios usedin the National Assessment. These statisticalapproaches assume that there are no barriers tospecies migration. Results should be viewed asindications of the potential magnitude and direc-tion of range shifts under a changed climate andnot as predictions of change.

For many of the eastern tree species,their possibleranges shift north (Iverson et al.,1999;Iverson andPrasad,2001). Under both climate scenarios,therange of sugar maple (Acer saccharum) shifts outof the United States entirely (Figure 5). White oak(Quercus alba) remains within its current rangebut is reduced in importance in the southern partsand increases in the northern parts of its range. Atotal of 7 of the 80 eastern tree species are project-ed to decline in regional importance by at least90%:bigtooth aspen (Populus grandidentata),aspen (Populus tremuloides),sugar maple,north-ern white-cedar (Thuja occidentalis),balsam fir(Abies balsamea), red pine (Pinus resinosa),andpaper birch (Betula papyrifera). The ranges ofmost species are projected to move to the north,with the ranges of several species moving north by60 to 330 miles (100 to 530 km). For somespecies,such as aspen,paper birch,northern white-cedar, balsam fir, and sugar maple,the optimum lati-tude for their occurrence moves north of the US-Canadian border.

When integrated into community types,southernforest types expand while higher elevation andnorthern forest types decline in area (Figure 6).The oak-hickory type is projected to expand inarea by 34% primarily to the north and east(Iverson and Prasad,2001). The oak-pine type is

projected to expand in area by 290% throughoutthe Southeast. Area of spruce-fir and aspen-birchtypes is projected to decline by 97% and 92%respectively. These types are replaced mainly byoak-hickory and oak-pine forests. The loblolly-short-leaf pine type is also projected to be reduced by32% and shifts north and west,being replaced in itscurrent zone by the oak-pine type. The longleaf-slash type is projected to be reduced by 31% inarea.

In the western US,the potential future ranges formany tree species are simulated to change,withsome species’ ranges shifting northward into Canada(Shafer et al.,2001). Simulated future ranges forWestern hemlock (Tsuga heterophylla) andDouglas-fir (Pseudotsuga menziesii) (Figure 7) areprojected to decrease west of the Cascade

505


Projected Changes in Distribution of Sugar Maple

Figure 5. Projected distribution for sugar maple under current cli-mate and the Hadley climate scenario and for the eastern UnitedStates, using statistical models developed by Iverson et al. (1999).The Predicted Current map is the current distribution and impor-tance value of sugar maple, as modeled from the regression treeanalysis. Importance value is an index based on the number ofstems and basal area of both the understory and the overstory.Predicted Hadley is the potential suitable habitat for sugar mapleunder the Hadley climate scenario. These potential maps imply nobarriers to migration. The Difference map represents the differ-ence between Modeled Current and Predicted Hadley maps. ThePotential Shifts map displays the modeled current distribution,along with predicted potential future distribution (using the Hadleyscenario) and the overlap where the species is now and is project-ed to be in the future. See Color Plate Appendix

Mountains with, for example,species typical todayof Glacier National Park expanding to the southeastinto Yellowstone National Park (Bartlein et al.,1997). Contrasts between eastern and westernrange shifts can be seen in the simulated futurerange of Paper birch (Betula papyrifera) whoseeastern US range limit contracts northward butwhose western US range limit expands southward(Figure 7).

Community Richness. The relationship betweenspatial patterns of climate and richness of trees andterrestrial vertebrate species were used to predictchanges in species richness under climate changeacross North America (Hansen et al.,2001;Currie,1991;Currie, 2001). The climate relationships withspecies richness were stronger for temperature thanprecipitation. Because the climate-richness relation-ships,including where maximal richness occurs,dif-fer across taxonomic groups,the impacts of climatechange will likely also vary across the groups.

Across all scenarios,tree richness is projected toincrease in the cooler regions:northern US,thewestern mountains,and near the Canadian Border(Currie,2001). Because species richness of birdsand mammals is currently highest in moderatelywarm areas and decreases in hotter areas,the modelprojected species richness for these taxa declines asair temperatures increase. Those scenarios wherewarming is projected to occur results in a decreaseof bird and mammal richness—-a 25% decrease inlow-elevation areas in the Southeast, for example.Warming in colder areas (such as mountainousareas) is projected to result in highly variableincreases in richness (11% to >100%). Cold-bloodedanimals such as reptiles and amphibians would ben-efit from increased air temperatures. Consequently,under the warming climate scenarios,species rich-ness for these taxa is projected to increase fromabout 11% to 100% over the entire conterminousUS.

Flather et al.(1998) identified regions in the US thathave high concentrations of the currently designat-ed threatened and endangered species. Species rich-ness was used to explore the potential impact of cli-mate change on taxonomic groups within thesehotspots of species endangerment. The projectedrichness results were overlaid on maps of currenthot spots for threatened and endangered species,and the species richness changes within each taxo-nomic group were evaluated. Within thesehotspots,species richness for reptiles and amphib-ians increased,whereas bird and mammal richnessappeared to be much reduced in many of them,especially in the East.

Mountains. The potential future range of Westernhemlock extends into mountain ranges throughoutthe interior west while Douglas-fir expands east ofthe Cascades and Sierras as well as northward alongthe west coast of Canada into Alaska. The potentialfuture ranges for subalpine conifers such asEngelmann spruce (Picea engelmannii),Mountainhemlock (Tsuga mertensiana),and several speciesof fir (Abies species) are much reduced in the west-ern parts of the US;however these subalpinespecies expand to the north along the west coast ofCanada and into Alaska. While many of the moremesic and higher elevation species shift northwardinto Canada,the potential future range of Ponderosapine (Pinus ponderosa) expands within the interiorwestern US.

The complex topography of the western US,com-bined with its seasonal and regional variations in cli-mate,strongly influences potential future shifts inthe ranges of tree species and their likely future abil-ities to successfully disperse to new habitat (Hansenet al.,2001;Shafer et al.,;2001). Species range shiftsin the western US are simulated to occur in alldirections whereas in the eastern US,the shifts tendto be northward as temperature increases and west-ward as precipitation increases. In the western US,several conifer species associated with moderatelymoist climates shift south and east along the Rocky

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Figure 6. Projected forest communities under (a) current climate, (b) the Hadley climatescenario, and (c) the Canadian climate scenario, based on the results of individualanalyses of 80 tree species shifts (see Prasad and Iverson, 1999-ongoinghttp://www.fs.fed.us/ne/delaware/atlas/index.html) See Color Plate Appendix

Current and Projected Forest Communities in the Eastern US

a. b.

c.

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Biome Shifts. The impact of potentialclimate change on the biogeography ofthe US is examined using six scenariosthat varied in degree of warming(Bachelet et al.,2001). Across the sce-narios, forest area is projected todecrease by an average of 11%,with arange of +23% under the moderatelywarming scenarios to –45% under thehottest scenarios. Northeast mixedforests (hardwood and conifer) are pro-jected to decrease by 72% in area in theUS,on average,as they shift into Canada(Neilson et al.,1998). The range of east-ern hardwoods is projected to decreaseby an average of 34%. Although thesedeciduous forests shift north, replacingNortheast mixed forests,they aresqueezed from the south by Southeastmixed forests or from the west by savan-nas and grasslands,depending on thescenario. Southeast mixed forests areprojected to increase under all scenarios(average 37%). This biome would remain intactunder moderately warm scenarios,but would beconverted to savannas and grasslands in the Southunder the hotter scenarios.

Alpine ecosystems are projected to all but disappearfrom the western mountains,being overtaken byencroaching forests. Under the climate scenariosstudied, wet coniferous forests in the northwestdecrease in area by 9% on average,while the extentof interior western pines change little. Responses inboth wet conifer and interior pine forests showwide variations among scenarios,with expansionsunder newer transient scenarios (Bachelet et al.,2001).Arid woodlands also are projected to expandin the interior West and Great Plains,encroachingon some grasslands.

Likely Species Shifts, Dispersal, and Land UseImpacts. The locations and areas of potential habi-tats for many plant and animal species are likely toshift as climate changes. Potential habitats for treesfavored by cool environments are likely to shiftnorthward. The habitats of alpine,subalpinespruce/fir, and aspen communities are likely to con-tract dramatically in the US and largely shift intoCanada. Potential habitats that are likely to increasein the US are oak/hickory, oak/pine,ponderosa pine,and arid woodland communities. Most of theseresults were evident in the results from three inde-pendent models:the mechanistic model MAPSS andtwo statistical models. These results were also rela-tively robust across the several climate scenarios

analyzed,including the Hadley and the Canadianscenarios. The statistical models are highly defensi-ble for this application because of the strong andextremely well-documented relationship betweentree species performance and environmental condi-tions,especially climate and soil.However, statisticalmodels do not incorporate a direct CO2 effect,which enhances water-use efficiency. Even so,it iswell accepted that climate and soils are strong con-trols on the establishment, growth,and reproduc-tion of many tree species.

How well these species actually track changes intheir potential habitats will very likely be stronglyinfluenced by their dispersal abilities and the distur-bances to these environments. Good analyticalmodels for dispersal are few. Some native specieswill very likely have difficulty dispersing to new

Figure 7. Simulated distributions and scenario agreement for Betulapapyrifera and Pseudotsuga menziesii (after Hansen et al., 2001).Estimated probabilities of occurrence for each taxon simulated withobserved modern climate (left panel). Comparison of the observeddistributions with the simulated future distributions under future cli-mate conditions as generated by the Hadley (HADCM2) andCanadian (CGCM1) scenarios for 2090-2099 (middle panels). Grayindicates locations where the taxon is observed today and is simu-lated to occur under future climate conditions; red indicates loca-tions where the taxon is observed today but is simulated to beabsent under future climate conditions; and blue indicates loca-tions where the taxon is absent today but is simulated to occurunder future climate conditions. Scenario agreement (right panel).Light purple indicates locations where the species is simulated tobe present under the future climate of either the HADCM2 orCGCM1 scenario; dark purple indicates locations where the speciesis simulated to be present under both future climate scenarios. SeeColor Plate Appendix

Paper Birch and Douglas Fir Tree Distributions under Future Climate Change Scenarios

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habitats because of the rapid rate of climate changeand the varying land uses along alternate migrationroutes. For example,aspen communities are cur-rently being reduced by conifer encroachment, graz-ing,invasive species,and urban expansion. Weedspecies that disperse rapidly will likely to be well-represented in these new habitats. Hence,thespecies composition of newly forming communitieswill likely differ substantially from those occupyingsimilar habitats today.

4. Socioeconomic Impacts

Current Supply and Demand for Amenities, Services,and Goods from Forests

Forests and the forestry sector provide theresource base and flow of amenities, goods,andservices that provide for needs of individuals,communities, regions,the nation,and export cus-tomers. Harvested products include timber, pulp-wood,fuelwood,wild game, ferns, mushrooms,floral greenery, and berries. Recreational opportu-nities in forests range from skiing, swimming,hik-ing and camping to birding,and autumn leaf tours.Forest land is also managed to provide clean waterand habitat for wildlife.

Environmental conditions and available technologyas well as social,political,and economic factorsinfluence the supply of and the demand for forestamenities, goods,and services. Factors that influ-ence supply of these services include forest area,forest productivity, forest management,productiontechnology, and the quality of amenities. Factorsthat influence demand include population,econom-ic growth and structure,and personal preferences.Changes in these factors also influence supply ofand demand for forest amenities, goods,and servic-es. Land-use pressures alter the amount and type ofland available for forest reserves, multiple-useforests,commercial recreation, forest plantations,and carbon storage (Alig,1986;Leemans et al.,1996;Solomon et al.,1993). Changes in forest speciescomposition, growth,and mortality alter the possi-ble supply of specific types of wood products,wildlife habitat,and recreation. Assumptions aboutchanges in human needs in the US and overseas alsoaffect the socioeconomic impacts from climatechange on US forests (Joyce et al.,1995; Perez-Garciaet al.,1997). Clearly, forest changes caused byhuman use of forests could exceed those impactsfrom climate change (Dale,1997).However, climatechange could impact many of the amenities, goods,and services from forests (Bruce et al.,1996),includ-

ing productivity of locally harvested plants such asberries or ferns;local economies through land useshifts from forest to other uses; forest real estate val-ues;and tree cover and composition in urban areas,and associated benefits and costs. In addition, cli-mate-change impacts on disturbances such as firecould increase fire suppression costs and economiclosses due to wildfires (Torn et al.,1999). In thisassessment,it was only possible to explore theimpact of climate change on wood products andrecreation in more detail.

North America is the world’s leading producer andconsumer of wood products. The US has substantialexports of hardwood lumber, wood chips,logs,andsome types of paper (Haynes et al.,1995). The USalso depends on Canada for 35% of its softwoodlumber and more than half of its newsprint. Past,current,and future land uses and management aswell as environmental factors such as insects,dis-ease, extreme climate events,and other disturbancesaffect the supply of forest products. These factorscan alter the cost of making and using wood prod-ucts,and associated jobs and income in an area.Changes in economic viability of wood productioninfluence whether owners keep land in forests orconvert it to other uses. Demand for wood prod-ucts,consumption,and trade is strongly influencedby US and overseas population,economicgrowth,and human values.While demand fortimber needed to make US wood products isprojected to grow at about the same rate aspopulation over the next 50 years,the demandsfor products and the kinds of timber harvestedwill be affected by technology change and con-sumer preferences. For example,use of recy-cled paper slows the increase in the amount oftimber harvest needed to meet increasingdemands for various paper products.

The combinations of resources,travel behavior,and population characteristics vary uniquelyacross the regions of the US. Participation in out-door activities is strongly related to age, ability anddisability, race,education,and income. These factorsinfluence the types of recreation opportunities avail-able and in which people participate.Approximately 690 million acres of federal lands areused for recreation,of which 95% are in the West.State and local governments manage over 54 millionacres,of which 30 million (55%) are in the East(Cordell et al.,1990). Most of the downhill skiingcapacity is in the western US while most of thecross-country skiing capacity is in the East.The num-ber of people participating in recreation is expectedto continue to increase for many decades. The

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Analyses of these particular scenarios indicate thatforest productivity gains increase timber inventoriesover the next 100 years. This increased wood sup-ply results in reductions in log prices,which,inturn,decrease producers’profits. At the same time,lower forest-product prices mean that consumersgenerally benefit (Figure 8). The net effect on the

importance of recreation opportunities near urbanareas is rising as US preference shifts from long-dis-tance vacations to frequent close-to-home trips(Cordell et al.,1990). A little over 14% of privatelands are open to recreation,but this amount isdeclining as lands are converted to other uses oraccess is restricted. Projected increases in per capi-ta income will likely contribute to higher demandfor snow-related recreation,although land- andwater-based activities will continue to dominatetotal recreation patterns (USDA Forest Service,1994).

Potential Impact of Climate Change on Timber andWood Product Markets, and Recreation

Adaptation in forest land management and timbermarkets. The possible degree and uncertainty inthe flow of value from forests — goods,services,and amenities — is influenced by the combined(and individual) uncertainty in changes in climate,forest productivity, and the economy. Comparisonsof earlier with more recent analyses indicate thatdifferences in assumptions,modeling structure,andscope of analysis such as spatial scale significantlyaffected the socioeconomic results. For example,an early study of Scandinavian boreal forests con-cluded that increased warming would benefit high-er latitude regions (Binkley, 1988); yet when theglobal trade patterns were analyzed,other regionswith lower production costs benefited most (Perez-Garcia et al.,1997). Early analyses (e.g.,Smith andTirpak,1989) that did not include economic forcesconcluded there would be significant damage to USforests. When active forest management was includ-ed in a previous economic analysis,timber marketsadapted to short-term negative effects of climatechange by reducing prices,salvaging dead anddying timber, and replanting species appropriate tothe new climate (Sohngen and Mendelsohn,1998).The total of consumer and producer benefit (sur-plus) remained positive.

For this assessment,the dynamic optimizationmodel FASOM (Adams et al.,1996,1997;Alig et al.,1997,1998) was used to evaluate the range of possi-ble projected changes in forest land area,timbermarkets,and related consumer/producer impactsassociated with climate change (Irland et al.,2001).The range of scenarios considered alternateassumptions about 1) climate (the Hadley and theCanadian scenarios),2) forest productivity (the TEMand CENTURY biogeochemistry models),and 3)timber and agricultural product demand (deter-mined by population growth and economicgrowth).

Figure 8. Prices for standing timber under all climate change sce-narios remain lower than a future without climate change (base-line). Prices under the Canadian scenario remain higher thanprices under the Hadley scenario when either the TEM or theCENTURYmodel are used. (Irland et al., 2001). See Color PlateAppendix

Average Price for Standing Timber in US Forests

Change in Timber Product Welfare from 2001 to 2100

Figure 9. Increased forest growth overall leads to increased woodsupply; reductions in log prices decrease producers’ welfare (prof-its), but generally benefit consumers through lower wood-productprices. Welfare is present value of consumer and producer surplusdiscounted at 4% for 2000-2100. (Irland et al., 2001). See ColorPlate Appendix

more tropical areas. Effects on fishing opportunitieswill likely vary with warming waters increasing fishproduction and opportunities for some specieswhile decreasing habitat and opportunities for coldwater species.

Recreation is likely to expand in mountainous areaswhere warmer temperatures attract more people tohigher elevations. Skiing is an important use offorested mountain landscape and is sensitive to theclimate in the mountains. Competition within theskiing industry is strong,with successful ski areasattracting customers by providing high-speed lifts,overnight accommodations,modern snowmakingand grooming equipment,and other amenities.Small ski areas often cannot compete and manyhave closed or have been annexed by adjacent larg-er areas (Irland et al.,2001). Climate change willlikely alter the primary factors influencing the abili-ty of a ski area to make snow;namely temperature,water availability, and energy. Higher winter temper-atures could possibly increase the amount of snowmelting,the number of rain events,and decrease theopportunities for snowmaking while increasing theneed for machine-made snow. The efficiency ofsnowmaking declines as temperature warms. Thecost of making snow is 5 times greater at 28˚F (–2˚C) as at 10˚F (–12 ˚C). The annual electricityusage at Maine’s Sunday River ski area is approxi-mately 26 million kilowatt-hours (Hoffman,1998),ata cost of nearly 2 million dollars per year and mostof this energy is used for snowmaking. On theother hand,higher winter temperatures and morerain events together could possibly result inincreased water availability for snowmaking andperhaps increased visitation with fewer extremelycold days. Changes in the geographic line of persist-ent subfreezing winter temperature could possiblyalter the location of winter recreation by affectingthe feasibility of snow-making,such as in the south-ern Appalachian Mountains. While the impacts ofclimate change on the US ski industry remain specu-lative,ski areas operating in marginal climates arelikely to be seriously affected.

ADAPTATIONS: FORESTMANAGEMENT STRATEGIES UNDER CLIMATE CHANGEA major challenge in developing strategies for cop-ing with the potential effects of climate change onforest processes and subsequent values is that themagnitude and direction of such changes at thelocal level remain highly uncertain. In addition,

economic welfare of participants in both timber andagricultural markets was projected to increase in allscenarios from between 0.4 to 0.7% above the cur-rent values (Figure 9). Land would likely shiftbetween forestry and agricultural uses as these eco-nomic sectors adjusted to climate-induced changesin production. Although US total forest productiongenerally is projected to increase in these analyses,hardwood output is higher in all scenarios whereassoftwood output increases only under moderatewarming. The extent of these changes varies byregion. In these analyses,timber output increasesmore in the South than in the North,and sawtimbervolume increases more than pulpwood volume.

While previous studies and this analysis differ in thedegree of market and human adaptation,one generalconclusion is that timber and wood product mar-kets will likely be able to adjust and adapt to climatechange (Irland et al.,2001). Assumptions aboutchanges in population,land use,trade in wood prod-ucts,consumption of wood products, recreation pat-terns,and human values are highly uncertain on acentury time scale. For example,if human needsfrom forests increase over the next 100 years andimports are limited,the socioeconomic impacts ofclimate change on forests would be greater than ifneeds are low or products can be imported fromareas where climate increases forest growth. Thus,assumptions about change in human needs in theUS and overseas,and about climate change effects inother parts of the world,are likely to be the majorfactors that determine socioeconomic impacts onthe US.

Recreation. Outdoor recreation will likely bealtered as a result of changes in seasonality of cli-mate,and air and water temperatures (Irland et al.,2001). Secondary impacts of environmentalchanges,such as increased haze with increased tem-peratures,and degraded aquatic habitats underchanging climates,will also likely affect outdoorrecreation opportunities. Because recreation isextremely broad and diverse in its environmentalrequirements (Cordell et al.,1999),it is difficult togeneralize about the impact of climate changeacross recreation as a whole (Wall,1998). Change inbenefits to consumers,as measured by aggregatedays of activities and economic value, vary by typeof recreation and location (Loomis and Crespi,1999;Mendelsohn and Mackowiki,1999). In some cases,recreation in one location will be substituted forrecreation in other locations. For example,tempera-ture increases will likely extend summer activitiessuch as swimming and boating in some forest areas,and substitute to some degree for such activities in510


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Mitigation measures,such as irrigation,mighttemporarily support specific gene pools untilnew and stable environments are identified.Recovery efforts can focus either on managingthe state of the system immediately after the dis-turbance,or managing the ongoing process ofrecovery. Recovery can be enhanced by addingstructural elements that create shade or othersafe sites necessary for reestablishing vegetationor that serve as perches for birds (and thusplaces where seeds would be dispersed).Alternatively, late successional species might beplanted to speed up succession.

There will likely be surprises in how changes inclimate alter the nature of forest disturbances andthe forests themselves. A monitoring programcould determine the influence of climate changeon forests and the natural disturbance regimes.Programs for monitoring the impact of forest dis-turbances currently exist for insects,pathogens,and fire. However, few surveys quantify the extentand severity of damage from wind,ice storms,andlandslides. Further, reserve areas such as wilder-nesses,are not currently monitored. Informationfrom monitoring programs could then be used toupdate risk assessments in management plans andprescriptions in an adaptive management sense(Walters,1997) or to assess the regional vulnerabil-ity of landscapes (O’Neill et al.,2000). A risk rank-ing system could identify aspects of the forestmost susceptible to severe repercussions from dis-turbance under a changing climate.

Human Adaptation to ForestChanges under an Altered Climate

While recent studies have suggested that humanactivities associated with markets will likely allowfor adaptation to ecological changes by changingland use practices,production and use technolo-gies,and consumption patterns,it may be impor-tant to examine the breadth of possible adapta-tions. As forest conditions change costs of goods,services,and amenities,investors,producers,andconsumers will likely shift investments and con-sumption decisions. However, it is possible thatseparate effects on producers and consumers oftimber products,especially in different regions,could be large and in opposite directions.

Helping human communities to adapt to changingforest conditions could include reducing potentialsocioeconomic impacts by mitigating carbonbuildup in the atmosphere. Carbon buildup in theatmosphere can be slowed by changing forest man-

potential climate-induced changes in forest process-es must also be put into context of other human-induced pressures on forests,which will likelychange significantly over future decades and cen-turies. Finally, such strategies must be considered interms of their economic viability. Strategies for cop-ing with climate change could include:1) active for-est management to promote forest adaptation to cli-mate change,and 2) assistance to urban and ruralcommunities to adapt to changing forest conditions.

Active Forest Management underClimate Change

Current forest management capabilities provide ini-tial guidance on how to manage forests under afuture changing climate. The value of the forest,anychanges in natural disturbance regimes,and theavailable environmentally and economically accept-able management options influence the copingstrategies for forests. One way forests may be aidedin adapting to climate change is to take steps todecrease other forest stresses,such as atmosphericdeposition. Strategies for coping with disturbancesin forests will vary regionally.

If climate change results in alteration of such distur-bance regimes as fire,drought,or insects and dis-ease,managers could try to cope with these impactsby influencing forest ecosystems prior to the distur-bance,mitigating the forest disturbance itself, manip-ulating the forest after the disturbance,or facilitatingthe recovery process. Prior to the disturbance,theecosystem could be managed in ways that alter itsvulnerability or ability to enhance recovery from adisturbance. For example,trees susceptible to ice orwind storms could be removed,as is common incities. Density and spacing of tree planting could bealtered to reduce susceptibility to drought. Speciescomposition could be changed to reduce vulnerabil-ity of forests to fire,drought,wind,insects,orpathogens. Management could be designed toreduce the opportunity for disturbance to occur.Examples include:limiting the introductions of non-native species;burning restrictions;and prescribedfires to reduce fuel loads. While manipulations offuel type,load,and arrangement could protect localareas of high value,fuel management may not becomplicated for larger landscapes. Some distur-bances,such as fire,insects,disease,and drought canbe managed through preventive measures,orthrough manipulations that affect the intensity orfrequency of the disturbance. For example, fire,insect,and disease controls are examples of manag-ing to reduce the impact of a disturbance for high-valued forests.

Integration of Climate and Land Use Change intoBiodiversity, Ecosystem Structure, and HigherTrophic Level Models. Climate and land use inter-act in ways that influence biodiversity, implying thatthese factors cannot be considered in isolation(Hansen et al.,2001). Climate and land use jointlyinfluence species distributions through alteration ofdispersal routes and changes in habitat throughchanges in disturbances such as wildfire, flooding,and landslides. Land use may modify the local cli-mate through changes in transpiration, cloud forma-tion and rainfall,and increased levels of drying.Land use models include socio-economic variablessuch as human population size,affluence,and cul-ture (Alig,1986); however, the influence of climateon land use and land use impacts on climate are notconsidered. Global and local climate projectionssimplify the feedbacks between land use, vegetation,and climate. Integrated models of land use and cli-mate are needed to predict the interactions of thesetwo influences on biodiversity.

Understanding the Interactions of Current ForestDisturbances and Climate. Basic information on thedisturbance — frequency, intensity, spatial extent —and the climatological conditions that initiate thesecurrent forest disturbances are currently not avail-able for a number of forest disturbances. For exam-ple,the genesis of ice storms is well-understood,butthe nature of the ice accumulation in relation tostorm characteristics is not well-understood.Similarly, the understanding of what specific clima-tological conditions lead to the formation andoccurrence of small-scale wind events is inadequate.Analyses suggest that future climate variability maypush fire and hurricanes outside of the well-studiedhistorical conditions that initiate them and controltheir intensity and frequency. For most distur-bances,there is a need for better understanding ofthe interactions between climate variability, distur-bance frequency, and spatial patterns.Once the rela-tionships between weather, climate,and forest dis-turbances have been quantified,the occurrences ofsuch disturbances could be predicted to help mini-mize their impact.

Quantifying the Impact of Disturbances on ForestStructure and Function. A key aspect of managingthe forest before,during,or after disturbances is anunderstanding of the disturbance impacts onforests. For disturbances such as fire,the impact hasbeen studied extensively. For others such as icestorms,insects,diseases,and introduced species, amore complete understanding of impacts is neces-sary before we can manage for the disturbance,or

agement and forest industry technology toincrease net carbon sequestration. Carbon storagein forests could be increased by reducing the con-version of forests to other land uses,setting asideexisting forests from harvest,or reducing forestfires (Birdsey et al.,2000;Sampson and Hair, 1996).Carbon storage could also be increased by con-

verting other land uses to forests and by enhanc-ing forest management. Improvements in forestindustry technology could enhance sequestrationby allowing substitution of wood products whereappropriate for products requiring more energyand carbon emissions in production and use,andincreasing the use life of products and recycling ofproducts allowing more carbon accumulation inforests (Sampson and Hair, 1996).

CRUCIAL UNKNOWNSAND RESEARCH NEEDS Linkages between Forest Processes, Air Quality,and Climate Change. It is crucial to begin tounderstand not only the direct effects of CO2,ozone,temperature,precipitation,and nitrogen andsulfur deposition on forests,but also the interac-tive effects of these stresses. Physiological impactspropagate through forest ecosystems by alteringcompetitive interactions among individuals andspecies,litter characteristics,and soil processes.These impacts and interactions feed back to affectphysiological processes, nutrient cycling,andhydrology. These interactions stress the need forintegrated ecological research to improve ourunderstanding of these forest dynamics.

Responses of Terrestrial Animals to ChangingClimate, and Trace Gases. The response of treesto climate,soils,and other biophysical controls isrelatively well-documented,in comparison to otherplants or animals. For native species,and the inter-action of introduced species and natives,under-standing the combined ef fects of climate,carbondioxide concentration,and nutrients such as nitro-gen on terrestrial vertebrates and invertebrates isseverely lacking. The response of vertebrates andinvertebrates to soils and other biophysical con-trols is also lacking. Research is needed to com-bine models of impacts of climate change andother factors on forest pests (insects andpathogens,introduced species) with models ofrange and abundance changes of host species.

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explore the interactions of multiple disturbances.Research might identify herbivores and pathogensthat are likely to be key agents of forest disturbancein the next 50 years. Integrated continental surveyswould help to determine the sensitivity of differenttypes of pests and diseases to environmental changeand their potential for increased outbreaks at themargins of their existing ranges.

Trees and forests have developed adaptations by co-evolving with disturbances. Research exploring theutility of these adaptations under a changing climatewould be valuable. For example,more work inunderstanding the inherent genetic variation withinparticular species is needed to assess the resilienceof both unmanaged and managed forests. Researchon how genetic traits can be manipulated toincrease resistance to specific stresses throughbiotechnology, expanded tree breeding programs,and seed banks would also be useful for adapting tochange in areas where extensive planting is used.

Information to Manage Forests under MultipleDisturbances. Capabilities to manage forests cur-rently, as well as under future potential climatechange, rest on the understanding of how forestsrespond to multiple disturbance events,and howone event affects the response to subsequent distur-bance. Fire and biological disturbances such asinsects,pathogens,and introduced species have sig-nificant interactions. A better understanding ofthese interactions would improve the ability tomake long-range predictions about the fate of forestsuccession and ecosystem dynamics,and would leadto better predictions of conditions under which oneevent affects the response to a subsequent one.

Human Adaptation in Response to Climate ChangeImpacts on Forests. Research needs include betterlinks between ecological models and economicmodels,and improvements in ecological models tocapture human adaptation activities. Enhancingthese linkages may require additional informationabout the interface between ecological processesand management objectives. Continued evaluationis needed of climate change impacts on a widerange of forest goods and services such as watersupply, carbon storage,nonwood forest products,aswell as timber and recreation. For example,animproved understanding is needed of how extremeweather events affect timber yields,salvage activity,and timber markets. When stakeholders associatedwith forest operations in the Mid-Atlantic regionwere surveyed,more than 20% reported majorimpacts on their forests from heavy rains,high

winds,or ice storms (Fisher et al.,2000).Increased forest operation costs associated withextreme weather events were noted by all man-agers regardless of their forest management objec-tive. New generations of combinedvegetation/economic models should incorporate arange of adaptation mechanisms – for exampleinvestments in new/altered management strategiesand in new technologies.

Research needs also include more appliedresearch on how adaptation could occur based onhow it has occurred in the recent and historicalpast (McIntosh et al,2000). Previous marketingand recreational studies typically examined humanbehavior against many attributes including a set ofenvironmental attributes. These studies haveshown that people adapt behavior to environmen-tal events such as fish advisories,smog alerts,beach closing, fish catch rates,and nitrogen depo-sition. This wealth of information could be usedto examine market and recreational choices to cli -mate change phenomena. Further, this informa-tion would be the basis to begin testing the sensi-tivity of human adaptation to different climate sce-narios.

US Trade Flows. The US forest sector operateswithin a global market. Information is needed onhow climate change will alter US trade flows inwood and harvest in the US, versus harvest over-seas. As mitigation alternatives are identified,thecosts,benefits,and the distribution of costs andbenefits must be evaluated. Trade flows can besensitive to fairly modest shifts in exchange ratesand price levels,to a degree that can be importantfor regions within a nation.

Longer-term Socioeconomic Impacts on Consumerand Producer Benefit/Costs. Improvements inmethods are needed that value long-term changesin resource conditions and consumer and produc-er benefits/costs. While it has been shown thathuman migration patterns are sensitive to climateand climate change,a larger social questionremains: will climate change phenomena causelarger socioeconomic changes than demographicor macroeconomic phenomena?

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Paul J. Hanson,Oak Ridge National LaboratoryMark Hutchins,Sno-Engineering,IncLouis Iverson,US Department of Agriculture,

Forest ServiceLloyd Irland,The Irland GroupLinda Joyce,US Department of Agriculture,

Forest ServiceMaría Lombardero,Universidad de Santiago,Lugo,

SpainJames Lenihan,Oregon State UniversityAriel E.Lugo,US Department of Agriculture,

Forest ServiceBruce McCarl,Texas A&M UniversityRon Neilson,US Department of Agriculture,

Forest ServiceChris J. Peterson,University of GeorgiaSarah Shafer, University of OregonDaniel Simberloff, University of TennesseeKen Skog,US Department of Agriculture,

Forest ServiceBrent L.Sohngen,Ohio State UniversityBrian J. Stocks,Canadian Forest ServiceFrederick J. Swanson,US Department of Agriculture,

Forest ServiceJake F.Weltzin,University of TennesseeB.Michael Wotton,Canadian Forest Service

* signifies Assessment team chairs/co-chairs

ACKNOWLEDGMENTSMany of the materials for this chapter are based on

contributions from participants on and thoseworking with the

Forest Sector Assessment TeamJohn Aber*,University of New HampshireSteven McNulty*,US Department of Agriculture,

Forest ServiceDarius Adams,Oregon State UniversityRalph Alig,US Department of Agriculture,

Forest ServiceMatthew P.Ayres,Dartmouth CollegeDominique Bachelet,Oregon State UniversityPatrick Bartlein,University of OregonCarter J. Betz,US Department of Agriculture,

Forest ServiceChi-Chung Chen,Texas A&M UniversityRosamonde Cook,Colorado State UniversityDavid J. Currie,University of Ottawa,CanadaVirginia Dale,Oak Ridge National LaboratoryRaymond Drapek,Oregon State UniversityMichael D. Flannigan,Canadian Forest ServiceCurt Flather, US Department of Agriculture,

Forest ServiceAndy Hansen,Montana State University

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