114

orests cover nearly one-third of the US,providing wildlife habitat, clean air and

water, cultural and aesthetic values,carbon storage,recreational opportunities

such as hiking,camping, fishing,and autumn leaf tours,and products that can be

harvested such as timber, pulpwood,fuelwood,wild game,ferns, mushrooms,and

berries. This wealth depends on forest biodiversity (the variety of plant and animal

species) and forest functioning (water flows,nutrient cycling,and productivity).

These aspects of forests are strongly influenced by climate. Native forests are adapted to their local climates; examples include the cold-tolerant boreal forests of

Alaska,the summer-drought tolerant forests of the Pacific Northwest,and the

drought-adapted piñon-juniper forests of the Southwest.

Human activities modify forests. Native forests have been converted to agricultural

and urban uses. In some cases, forests have regrown on abandoned agricultural

lands. Expansion of urban areas has fragmented forests into smaller, less-contiguous

patches. Fire suppression has changed the species found in southeastern,midwestern,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 and

landscaping purposes in urban and rural areas that are often far outside of the

species' natural range. Intensive management along with favorable climates in parts

of the US has resulted in highly productive forests,such as southern pine plantations. Human activities will continue to modify forests while forests are also experiencing the effects of climate change.

Key Issue: Effects on Forest Productivity 

everal environmental factors that control the water and carbon balances of

forests are changing rapidly and simultaneously. The global increases in atmospheric CO2

concentrations are the best-documented factor. However, in some

areas,other important atmospheric constituents are also increasing,including nitrogen oxides (a direct product of fossil fuel combustion that causes acid rain) and

ground-level ozone ("smog," a product of chemical reactions between hydrocarbons

and nitrogen oxides in the presence of sunlight).

A synthesis of laboratory and field studies and modeling indicates that forest productivity increases with the fertilizing effect of atmospheric CO2

(see box in

Agriculture section),but that these increases are strongly tempered by local conditions such as moisture stress and nutrient availability. Across a wide range of scenarios,it appears that modest warming could result in increased carbon storage in

most forest ecosystems in the conterminous US. Yet under some warmer scenarios,

forests,notably in the Southeast and the Northwest,could experience droughtinduced losses of carbon,possibly exacerbated by increased fire disturbance. These

potential gains and losses of carbon will be subject to changes in land-use,such as

the conversion of forests to agricultural lands.

S

FOREST SECTOR

F

KEY ISSUES

• Effects on Forest

Productivity

• Natural Disturbances

such as Fire and Drought

• Biodiversity Changes

• Socioeconomic Impacts

Modest warming could

result in increased carbon

storage in most 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 an

increased fire disturbance. 
115

Other components of environmental change,such as nitrogen deposition and

ground-level ozone concentrations,also affect forest processes. Models identify

a synergistic fertilization response between CO2and nitrogen enrichment,leading to further increases in productivity. Ozone,however, can suppress these

gains.Current ozone levels, for example,have likely decreased production by

10% in Northeast forests and 5% in southern pine plantations. Interactions

among these physical and chemical changes and other components of global

change are important in determining the future of US forests.

Intensive management along

with favorable climates in

parts of the US has resulted

in highly productive forests,

such as the southern pine

plantations. Human activities

will continue to modify

forests while forests are also

experiencing the effects of

climate change. Current Distribution of Forests in the United States

Forest land area has declined in

the East since 1600. Northern

forests have regrown somewhat

after being cleared for timber and

agriculture in the late 1800s.

Increases in the South reflect

plantations, as well as regrowth

from agricultural conversion.

Midwest (Heartland) forests have

been cleared for agriculture.

Forest land area is a much smaller percentage of the western

states, and has remained relative -

ly constant since the 1600s.

douglas-fir

hemlock-sitka spruce

ponderosa pine

western white pine

lodgepole pine

larch

fir-spruce

redwood

chaparral

pinyon-juniper

western hardwoods

aspen-birch

white-red-jack pine

spuce-fir

longleaf-slash pine

loblolly-shortleaf pine

oak-pine

oak-hickory

oak-gum-cypress

elm-ash-cottonwood

maple-beech-birch

aspen-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 Forests

Pacific NW

West

Alaska

Islands North

South

Heartland

Great 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 of

these disturbances. For example,some tree species have developed very thick

bark to protect them from repeated ground fires.

Over millennia,local,regional,and global changes in temperature and precipitation have influenced the occurrence,frequency, and intensity of these natural

disturbances. These changes in disturbance regimes are a natural part of all

ecosystems. However, forests may soon be facing rapid alterations in the nature

of these disturbances as a consequence of climate change. For example,the

seasonal severity of fire hazard is projected to increase about 10% over the next

century over much of the US under both the Hadley and Canadian climate scenarios. Regionally,the Hadley scenario projects small decreases in fire hazard in

the northern Great Plains,and the Canadian scenario projects a 30% increase in

fire hazard for the southeastern US and Alaska.

The consequences of drought depend on annual and seasonal climate changes

and whether the current drought adaptations offer resistance and resilience to

new conditions. Under the Canadian and Hadley scenarios,the ecological models used in this Assessment indicate that increases in drought stresses will likely

occur in the Southeast,southern Rocky Mountains and parts of the Northwest

over the 21st century.

Dominant Forest Types

N

Current - 1960-1990

These maps show current and

projected forest types for the eastern US. The current distribution

of forest types reflects temperature and moisture gradients in this

part of the nation. The simulated

changes in forest types by the end

of the 21st century are in

response to the Hadley and

Canadian climate scenarios using

the DISTRIB model, a tree species

distribution model. Pine-dominated types decline in the Southeast

under both climate scenarios.

Oak-pine and oak-hickory forest

types are projected to expand

northward.

Regionally, the Hadley scenario projects small decreases in fire hazard in the northern Great Plains, and the

Canadian scenario projects a

30% increase in fire hazard

for the southeastern US and

Alaska. 

White-Red-Jack Pine

Spruce-Fir

Longleaf-Slash Pine

Loblolly-Shortleaf Pine

Oak-Pine

Oak-Hickory

Oak-Gum-Cypress

Elm-Ash-Cottonwood

Maple-Beech-Birch

Aspen-Birch

No Data
117

The interactions between climate

change and hurricanes,landslides,ice

storms,wind storms,insects,disease,

and introduced species are difficult

to predict. But as climate changes,

alterations in these disturbances and

in their effects on forests are possible.

Biodiversity Changes

hanges in the distribution and

abundance of plant and animal

species reflect the birth, growth,

death,and dispersal rates of individuals in a population. When aggregated,

these processes result in the local disappearance or introduction of a

species,and ultimately determine the

species’range. While climate and

soils exert strong controls on the

establishment and growth of plant

species,the response of plant and animal species to climate change will be

the result of many interacting and

interrelated processes operating over

several scales of time and space.

Migration rates, changes in disturbance regimes,and interactions within and between species will affect

the distribution of plants and animals.

In addition,human activities influence the occurrence and abundance

of species on the landscape.

Analyses of ecological models over

several climate scenarios indicate that

the location and area of the potential

habitats for many tree species and

communities are very likely to shift.

Potential habitats for trees favored by

cool environments are very likely to

shift north. Habitats of alpine and

sub-alpine spruce-fir could possibly

be eliminated. Aspen,and eastern

birch communities are likely to contract dramatically in the US and largely shift into Canada. Potential habitats that could possibly expand in the

US are oak/hickory and oak/pine in

the eastern US,and Ponderosa pine

and arid woodland communities in

the West.

How well these species track changes

in their potential habitats will be

strongly influenced by their dispersal

abilities and the disturbances to these

environments. Some native species

will have difficulty dispersing to new

habitats because of the rapid rate of

climate change and human land use

along migration routes. For example,

sagebrush and aspen communities

are currently being reduced by

conifer encroachment, grazing,invasive species,and urban expansion.

The effects of climate change on the

rate and magnitude of disturbance

(forest damage and destruction associated with fires,storms,droughts

and pest outbreaks) will be an important factor in determining whether

transitions from one forest type to

another will be gradual or abrupt. If

disturbances in New England, for

example,do not increase,there is a

possibility of a smooth transition

from the present maple,beech,and

birch tree species to oak and hickory.

Where disturbances increase,transitions are very likely to be abrupt.

Invasive (weed) species that disperse

rapidly are likely to find opportunities in newly forming communities.

Thus,the species composition of

these communities will likely differ

substantially from those occupying

similar habitats today.

Dominant Forest Types

C

Hadley Scenario - 2070-2100 Canadian Scenario - 2070-2100
118

FORESTS KEY ISSUES

Socioeconomic Impacts 

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

and some types of paper. However,the US also imports forest products,including

35% of its softwood lumber and more than half of its newsprint from Canada. The

market for wood products in the US is highly dependent upon the future area in

forests,species composition of forests,future supplies of wood,technological

change 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 indicate that forest productivity gains are very likely to increase timber inventories over

the next 100 years. Under these scenarios,the increased wood supply leads to

reductions in log prices that,in turn,decrease producers’profits. At the same time,

lower forest-product prices mean that consumers generally benefit. The projected

net effect on the economic welfare of participants in both timber and agricultural

markets increases about 1% above current values. Land will likely shift between

forestry and agricultural uses as these economic sectors adjust to climate-induced

changes in production. Although US total forest production generally increases,

hardwood output is higher in all scenarios but softwood output increases only

under moderate warming. Timber output increases more in the South than in the

North.Sawtimber volume increases more than pulpwood volume.

It is very likely that outdoor recreation will be altered by climate change. Changes

in 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 are

likely to shift summer recreation activities,such as hiking,northward or to higher

elevations. In winter, downhill skiing opportunities will very likely decrease with

fewer cold days and reduced snowpack. Costs to maintain skiing opportunities are

likely to rise in marginal climate areas. Effects on fishing will likely vary; warmer

waters will increase fish production and opportunities for some warm water

species,but decrease habitat and

opportunities for cold water species.

N

Land will likely shift between

forestry and agricultural uses

as these economic sectors

adjust to climate-induced

changes in production.

2000 2010 2020 2030 2040 2050

Canadian-Century

Canadian-TEM

Hadley-Century

Hadley-TEM

Base

1.2

1.25

1.3

1.35

1.4

1.45

1.5

1.55

Projected Average Price for Standing Timber in US Forests

Prices for standing timber under all climate

change scenarios remain lower than in a

future without climate change (base).

Prices under the Canadian scenario remain

higher than prices under the Hadley scenario when either the TEM or the Century

model is used.
119

Adaptation Strategies 

hile projected climate changes are likely to alter forests,the motivation for

adaptation strategies will be strongly influenced by the level of economic

activity in the US,population growth,tastes,and preferences including society’s perceptions about these changes in forests. Market forces are powerful when it comes

to land use and forestry, and as such,influence adaptation on private lands.

However, for those forests valued for their current biodiversity,strategies to maintain these plant and animal species under climate change remain to be developed.It

is possible that such strategies will be unavailable or impractical.

Markets for forest products adjust through altering prices for timber, wood,and

paper products. The changes in climate and the consequent impact on forests will

very likely change the market incentives for investment in intensive forest management (such as planting,thinning, genetic conservation,and tree improvement) and

the incentive to develop and invest in wood-conserving technologies.Although

these price changes are likely to alter consumption patterns (for example,substitution between wood and non-wood products), overall increase in the consumption

of wood products very likely will still be predominantly influenced by population

growth,the level of economic activity in the US and internationally, and personal

preferences.

Timber producers could possibly adjust and adapt to climate change under the scenarios used here, if new technologies and markets are recognized in a timely manner. Adaptations could include salvaging dead and dying timber and replanting

species appropriate to a new climate. The extent and pattern of timber harvesting

and prices in the US will also be influenced by the global changes in forest productivity and prices of overseas products.

Potential climate-induced changes in forests must be put into the context of other

human-induced pressures,which will undoubtedly change significantly over future

decades. While the potential for rapid changes in natural disturbances could challenge current management strategies,these changes will co-occur with human

activities such as agricultural and urban encroachment on forests, multiple use of

forests,and air pollution.

W

Timber producers could

possibly adjust and adapt

to climate change under

the scenarios used here,

if new technologies and

markets are recognized in

a timely manner.

Adaptations could include

salvaging dead and dying

timber and replanting

species appropriate to a

new climate. 

-10

-8

-6

-4

-2

0

2

Forest product consumers

Change in Timber Product Welfare

from 2000 to 2100

Increased forest growth

overall leads to

increased wood supply;

reductions in log prices

decrease producers’

welfare (profits), but

generally benefit consumers through lower

wood-product prices.

Welfare is present

value of consumer and

producer surplus discounted at 4% for 2000-

2100.

Timber product consumers

Timber product producers

Timber total welfare