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orests cover nearly one-third of the US,providing wildlife habitat, clean air andwater, cultural and aesthetic values,carbon storage, recreational opportunities

such as hiking,camping, fishing,and autumn leaf tours,and products that can beharvested such as timber, pulpwood,fuelwood,wild game, ferns, mushrooms,andberries. This wealth depends on forest biodiversity (the variety of plant and animalspecies) and forest functioning (water flows, nutrient cycling,and productivity).These aspects of forests are strongly influenced by climate. Native forests are adapt-ed to their local climates; examples include the cold-tolerant boreal forests ofAlaska,the summer-drought tolerant forests of the Pacific Northwest,and thedrought-adapted piñon-juniper forests of the Southwest.

Human activities modify forests. Native forests have been converted to agriculturaland urban uses. In some cases, forests have regrown on abandoned agriculturallands. Expansion of urban areas has fragmented forests into smaller, less-contiguouspatches. Fire suppression has changed the species found in southeastern,midwest-ern,and western forests. Harvesting methods,where all trees or a few trees are cut,have also changed species composition. Trees have been planted for aesthetic andlandscaping purposes in urban and rural areas that are often far outside of thespecies' natural range. Intensive management along with favorable climates in partsof the US has resulted in highly productive forests,such as southern pine planta-tions. Human activities will continue to modify forests while forests are also experi-encing the effects of climate change.

Key Issue: Effects on Forest Productivity

everal environmental factors that control the water and carbon balances offorests are changing rapidly and simultaneously. The global increases in atmos-

pheric CO2 concentrations are the best-documented factor. However, in someareas,other important atmospheric constituents are also increasing,including nitro-gen oxides (a direct product of fossil fuel combustion that causes acid rain) andground-level ozone ("smog," a product of chemical reactions between hydrocarbonsand nitrogen oxides in the presence of sunlight).

A synthesis of laboratory and field studies and modeling indicates that forest pro-ductivity increases with the fertilizing effect of atmospheric CO2 (see box inAgriculture section),but that these increases are strongly tempered by local condi-tions such as moisture stress and nutrient availability. Across a wide range of scenar-ios,it appears that modest warming could result in increased carbon storage inmost forest ecosystems in the conterminous US. Yet under some warmer scenarios,forests,notably in the Southeast and the Northwest,could experience drought-induced losses of carbon,possibly exacerbated by increased fire disturbance. Thesepotential gains and losses of carbon will be subject to changes in land-use,such asthe conversion of forests to agricultural lands.

S

FOREST SECTOR

F

KEY ISSUES

• Effects on ForestProductivity

• Natural Disturbancessuch as Fire and Drought

• Biodiversity Changes

• Socioeconomic Impacts

Modest warming couldresult in increased carbonstorage in most forestecosystems in the conter-minous US. Yet undersome warmer scenarios,forests, notably in theSoutheast and theNorthwest, could experi-ence drought-inducedlosses of carbon, possiblyexacerbated by anincreased fire disturbance.

115

Other components of environmental change,such as nitrogen deposition andground-level ozone concentrations,also affect forest processes. Models identifya synergistic fertilization response between CO2 and nitrogen enrichment,lead-ing to further increases in productivity. Ozone,however, can suppress thesegains.Current ozone levels, for example,have likely decreased production by10% in Northeast forests and 5% in southern pine plantations. Interactionsamong these physical and chemical changes and other components of globalchange are important in determining the future of US forests.

Intensive management alongwith favorable climates in

parts of the US has resultedin highly productive forests,

such as the southern pineplantations. Human activities

will continue to modifyforests while forests are also

experiencing the effects ofclimate change.

Current Distribution of Forests in the United States

Forest land area has declined inthe East since 1600. Northernforests have regrown somewhatafter being cleared for timber andagriculture in the late 1800s.Increases in the South reflectplantations, as well as regrowthfrom agricultural conversion.Midwest (Heartland) forests havebeen cleared for agriculture.Forest land area is a much small-er percentage of the westernstates, and has remained relative -ly constant since the 1600s.

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

Forest Land Percentages

0

10

20

30

40

50

60

70

80

90

100

1600 1700 1800 1900 2000

Year Eastern ForestsPacific NWWestAlaskaIslands North

SouthHeartlandGreat Plains

Western Forests

Dashed lines indicate estimates.

116

FORESTS KEY ISSUES

Natural Disturbances Such as Fire and Drought

atural disturbances having the greatest effects on forests include insects,disease,introduced species, fires,droughts,hurricanes,landslides,wind

storms,and ice storms. Tree species have developed adaptations to some ofthese disturbances. For example,some tree species have developed very thickbark to protect them from repeated ground fires.

Over millennia,local, regional,and global changes in temperature and precipita-tion have influenced the occurrence,frequency, and intensity of these naturaldisturbances. These changes in disturbance regimes are a natural part of allecosystems. However, forests may soon be facing rapid alterations in the natureof these disturbances as a consequence of climate change. For example,theseasonal severity of fire hazard is projected to increase about 10% over the nextcentury over much of the US under both the Hadley and Canadian climate sce-narios. Regionally, the Hadley scenario projects small decreases in fire hazard inthe northern Great Plains,and the Canadian scenario projects a 30% increase infire hazard for the southeastern US and Alaska.

The consequences of drought depend on annual and seasonal climate changesand whether the current drought adaptations offer resistance and resilience tonew conditions. Under the Canadian and Hadley scenarios,the ecological mod-els used in this Assessment indicate that increases in drought stresses will likelyoccur in the Southeast,southern Rocky Mountains and parts of the Northwestover the 21st century.

Dominant Forest Types

N

Current - 1960-1990

These maps show current andprojected forest types for the east-ern US. The current distributionof forest types reflects tempera-ture and moisture gradients in thispart of the nation. The simulatedchanges in forest types by the endof the 21st century are inresponse to the Hadley andCanadian climate scenarios usingthe DISTRIB model, a tree speciesdistribution model. Pine-dominat-ed types decline in the Southeastunder both climate scenarios.Oak-pine and oak-hickory foresttypes are projected to expandnorthward.

Regionally, the Hadley sce-nario projects small decreas-es in fire hazard in the north-ern Great Plains, and theCanadian scenario projects a30% increase in fire hazardfor the southeastern US andAlaska.

White-Red-Jack PineSpruce-FirLongleaf-Slash PineLoblolly-Shortleaf PineOak-PineOak-HickoryOak-Gum-CypressElm-Ash-CottonwoodMaple-Beech-BirchAspen-BirchNo Data

117

The interactions between climatechange and hurricanes,landslides,icestorms,wind storms,insects,disease,and introduced species are dif ficultto predict. But as climate changes,alterations in these disturbances andin their effects on forests are possi-ble.

Biodiversity Changes

hanges in the distribution andabundance of plant and animal

species reflect the birth, growth,death,and dispersal rates of individu-als in a population. When aggregated,these processes result in the local dis-appearance or introduction of aspecies,and ultimately determine thespecies’ range. While climate andsoils exert strong controls on theestablishment and growth of plantspecies,the response of plant and ani-mal species to climate change will bethe result of many interacting andinterrelated processes operating overseveral scales of time and space.Migration rates, changes in distur-bance regimes,and interactions with-in and between species will affect

the distribution of plants and animals.In addition,human activities influ-ence the occurrence and abundanceof species on the landscape.

Analyses of ecological models overseveral climate scenarios indicate thatthe location and area of the potentialhabitats for many tree species andcommunities are very likely to shift.Potential habitats for trees favored bycool environments are very likely toshift north. Habitats of alpine andsub-alpine spruce-fir could possiblybe eliminated. Aspen,and easternbirch communities are likely to con-tract dramatically in the US and large-ly shift into Canada. Potential habi-tats that could possibly expand in theUS are oak/hickory and oak/pine inthe eastern US,and Ponderosa pineand arid woodland communities inthe West.

How well these species track changesin their potential habitats will bestrongly influenced by their dispersalabilities and the disturbances to theseenvironments. Some native specieswill have difficulty dispersing to newhabitats because of the rapid rate of

climate change and human land usealong migration routes. For example,sagebrush and aspen communitiesare currently being reduced byconifer encroachment, grazing,inva-sive species,and urban expansion.

The effects of climate change on therate and magnitude of disturbance(forest damage and destruction asso-ciated with fires,storms,droughtsand pest outbreaks) will be an impor-tant factor in determining whethertransitions from one forest type toanother will be gradual or abrupt. Ifdisturbances in New England, forexample,do not increase,there is apossibility of a smooth transitionfrom the present maple,beech,andbirch tree species to oak and hickory.Where disturbances increase,transi-tions are very likely to be abrupt.

Invasive (weed) species that disperserapidly are likely to find opportuni-ties in newly forming communities.Thus,the species composition ofthese communities will likely dif fersubstantially from those occupyingsimilar habitats today.

Dominant Forest Types

C

Hadley Scenario - 2070-2100 Canadian Scenario - 2070-2100

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FORESTS KEY ISSUES

Socioeconomic Impacts

orth America is the world’s leading producer and consumer of wood prod-ucts. The US has substantial exports of hardwood lumber, wood chips,logs,

and some types of paper. However, the US also imports forest products,including35% of its softwood lumber and more than half of its newsprint from Canada. Themarket for wood products in the US is highly dependent upon the future area inforests,species composition of forests,future supplies of wood,technologicalchange in production and use, availability of substitutes such as steel and vinyl,demands for wood products,and competitiveness among major trading partners.

Analyses of the forest and agriculture sectors for a range of climate scenarios indi-cate that forest productivity gains are very likely to increase timber inventories overthe next 100 years. Under these scenarios,the increased wood supply leads toreductions in log prices that,in turn,decrease producers’profits. At the same time,lower forest-product prices mean that consumers generally benefit. The projectednet effect on the economic welfare of participants in both timber and agriculturalmarkets increases about 1% above current values. Land will likely shift betweenforestry and agricultural uses as these economic sectors adjust to climate-inducedchanges in production. Although US total forest production generally increases,hardwood output is higher in all scenarios but softwood output increases onlyunder moderate warming. Timber output increases more in the South than in theNorth.Sawtimber volume increases more than pulpwood volume.

It is very likely that outdoor recreation will be altered by climate change. Changesin benefits,as measured by aggregate days of activities and total economic value,will vary by type of recreation and location. In some areas,higher temperatures arelikely to shift summer recreation activities,such as hiking,northward or to higherelevations. In winter, downhill skiing opportunities will very likely decrease withfewer cold days and reduced snowpack. Costs to maintain skiing opportunities arelikely to rise in marginal climate areas. Effects on fishing will likely vary; warmerwaters will increase fish production and opportunities for some warm water

species,but decrease habitat andopportunities for cold water species.

N

Land will likely shift betweenforestry and agricultural usesas these economic sectorsadjust to climate-inducedchanges in production.

2000 2010 2020 2030 2040 2050

Canadian-CenturyCanadian-TEMHadley-CenturyHadley-TEMBase

1.2

1.25

1.3

1.35

1.4

1.45

1.5

1.55Projected Average Price for Standing Timber in US Forests

Prices for standing timber under all climatechange scenarios remain lower than in afuture without climate change (base).Prices under the Canadian scenario remainhigher than prices under the Hadley sce-nario when either the TEM or the Centurymodel is used.

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Adaptation Strategies

hile projected climate changes are likely to alter forests,the motivation foradaptation strategies will be strongly influenced by the level of economic

activity in the US,population growth,tastes,and preferences including society’s per-ceptions about these changes in forests. Market forces are powerful when it comesto land use and forestry, and as such,influence adaptation on private lands.However, for those forests valued for their current biodiversity, strategies to main-tain these plant and animal species under climate change remain to be developed.Itis possible that such strategies will be unavailable or impractical.

Markets for forest products adjust through altering prices for timber, wood,andpaper products. The changes in climate and the consequent impact on forests willvery likely change the market incentives for investment in intensive forest manage-ment (such as planting,thinning, genetic conservation,and tree improvement) andthe incentive to develop and invest in wood-conserving technologies.Althoughthese price changes are likely to alter consumption patterns (for example,substitu-tion between wood and non-wood products), overall increase in the consumptionof wood products very likely will still be predominantly influenced by populationgrowth,the level of economic activity in the US and internationally, and personalpreferences.

Timber producers could possibly adjust and adapt to climate change under the sce-narios used here, if new technologies and markets are recognized in a timely man-ner. Adaptations could include salvaging dead and dying timber and replantingspecies appropriate to a new climate. The extent and pattern of timber harvestingand prices in the US will also be influenced by the global changes in forest produc-tivity and prices of overseas products.

Potential climate-induced changes in forests must be put into the context of otherhuman-induced pressures,which will undoubtedly change significantly over futuredecades. While the potential for rapid changes in natural disturbances could chal-lenge current management strategies,these changes will co-occur with humanactivities such as agricultural and urban encroachment on forests, multiple use offorests,and air pollution.

WTimber producers could

possibly adjust and adaptto climate change underthe scenarios used here,if new technologies and

markets are recognized ina timely manner.

Adaptations could includesalvaging dead and dying

timber and replantingspecies appropriate to a

new climate.

-10

-8

-6

-4

-2

0

2

Forest product consumers

Change in Timber Product Welfarefrom 2000 to 2100

Increased forest growthoverall leads toincreased wood supply;reductions in log pricesdecrease producers’welfare (profits), butgenerally benefit con-sumers through lowerwood-product prices.Welfare is presentvalue of consumer andproducer surplus dis-counted at 4% for 2000-2100.

Timber product consumers

Timber product producers

Timber total welfare