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Impacts on Forest Growth and ProductivityMany aspects of projected climate change will likely affect forest

growth and productivity. Three examples are described below:

increases in carbon dioxide (CO2), increases in temperature, and

changes in precipitation.

View enlarged image

Projected shifts in forest types.[3]The maps show current and projected

forest types. Major changes are projected for many regions. For example,

in the Northeast, under a mid-range warming scenario, the currently

dominant maple-beech-birch forest type (red shading) is projected to be

completely displaced by other forest types in a warmer future.

Source:USGCRP (2009)

Carbon dioxide is required for photosynthesis, the processby which green plants use sunlight to grow. Given sufficient

water and nutrients, increases in atmospheric CO2may

enable trees to be more productive.[2]Higher future CO2levels

could benefit forests with fertile soils in the Northeast.However, increased CO2may not be as effective in promoting

growth in the West and Southeast, where water is limited.[3]

Warming temperatures could increase the length of thegrowing season. However, warming could also shift the

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geographic ranges of some tree species. Habitats of some

types of trees are likely to move northward or to higher

altitudes. Other species may be at risk locally or regionally if

conditions in their current geographic range are no longersuitable.[2]For example, species that currently exist only on

mountaintops in some regions may die out as the climate

warms since they cannot shift to a higher altitude.

Climate change will likely increase the risk of drought insome areas and the risk of extreme precipitation and

flooding in others. Increased temperatures would alter the

timing of snowmelt, affecting the seasonal availability ofwater. Although many trees are resilient to some degree of

drought, increases in temperature could make future

droughts more damaging than those experienced in the

past. In addition, drought increases wildfire risk, since dry

trees and shrubs provide fuel to fires. Drought also reduces

trees' ability to produce sap, which protects them from

destructive insects such as pine beetles.[3]

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Impacts of isturbances

The brown and red trees in this picture have been infested by mountain

pine beetles near Winter Park, Colorado in May 2007. Source:CCSP

(2008) (PDF)

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Climate change could alter the frequency and intensity of forest

disturbances such as insect outbreaks, invasive species, wildfires,

and storms. These disturbances can reduce forest productivity and

change the distribution of tree species. In some cases, forests canrecover from a disturbance. In other cases, existing species may

shift their range or die out. In these cases, the new species of

vegetation that colonize the area create a new type of forest.

Insect outbreaks often defoliate, weaken, and kill trees. Forexample, pine beetles have damaged more than 1.5 million

acres of forest in Colorado and spruce beetles have damaged

more than 2.5 million acres in Alaska.[3]The hemlock woolly

adelgid, an invasive species that is sensitive to cold weather

and destroys Eastern hemlock, will likely extend its habitat

north as the climate warms.[4]A lack of natural controls, such

as predators, or pathogens, or inadequate defenses in trees,

can allow insects to spread. Climate change could contribute

to an increase in the severity of future insect outbreaks.

Rising temperatures may enable some insect species to

develop faster and expand their ranges northward. Invasive

plant species can displace important native vegetation

because the invasive species often lack natural predators.

Climate change could benefit invasive plants, since they are

generally more tolerant to a wider range of environmental

conditions than are native plants.[3]

In recent years, wildfires consumed more than 6.25 millionacres of forest in Alaska (roughly equal to the area of

Massachusetts). Warm temperatures and drought conditions

during the early summer contributed to this event. [5]Climate

change is projected to increase the extent, intensity, and

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frequency of wildfires in certain areas of the country. Warmer

spring and summer temperatures, coupled with decreases in

water availability, dry out woody materials in forests and

increase the risk of wildfire. Fires can also contribute toclimate change, since they can cause rapid, large releases of

carbon dioxide to the atmosphere.[2]

View enlarged image

The size of wildfires in the United States increased from 1983 to 2008.

Source:USGCRP (2009)

Hurricanes, ice storms, and wind storms can cause damageto forests. Hurricanes Rita and Katrina in 2005 damaged a

total of 5,500 acres of forest. The amount of carbon released

by these trees as they decay is roughly equivalent to the net

amount of carbon sequestered by all U.S. forests in a single

year.[2]

Disturbances can interact with one another, or with changes in

temperature and precipitation, to increase risks to forests. For

example, drought can weaken trees and make a forest more

susceptible to wildfire or insect outbreaks. Similarly, wildfire can

make a forest more vulnerable to pests.[2][3]The combination of

drought and outbreaks of beetles has damagedpion pine forests

in the Southwest.

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