Observations and Predictions

Although changes in tree species distributions may occur slowly, changes to regimes in forests are already being observed. This includes an increase in tree mortality from insect outbreaks and increases in the length and severity of fire seasons, exacerbated by increases in spring and summer temperatures and earlier snowmelts.

Approximately half of southern tree species will expand northward. One of the most notable changes is the predicted expansion of oak-hickory and oak-pine forests. Other species will decrease in importance as forest types such as aspen-birch, white-red-jack pine, maple-beech, and birch forests decrease in the United States and move north into Canada.

In the western United States, high-elevation chaparrals and grasslands are projected to increase at the expense of subalpine forests, alpine tundra, and Great Basin woodland communities. Plant and animal interactions may become uncertain as tree species shift in response to climate change potentially creating
new combinations of species that do not resemble current plant communities or habitats.

Overall, the boreal forest is likely to decrease in area, with major changes occurring along the southern boundaries as ranges of tree species shift northward.

Bird Species Vulnerability

Although only 2% of forest bird species show high vulnerability to climate change, more than half of the species with medium or high vulnerability were not previously considered species of conservation concern. Among species that are restricted to a single forest type, a higher proportion of birds in eastern forests (75%) show medium or high vulnerability than birds in western (57%), boreal (49%), or subtropical (27%) forests. 

Common Nighthawk and chicks by Pamela Hunt

Among the most vulnerable forest species are Bicknell’s Thrush because of its restricted breeding range in high elevation forests in the northeast United States, the endangered Kirtland’s Warbler because winter habitat on low-lying Caribbean islands is threatened, and specialized aerial insectivores such as Black Swift, Whip-poor-will, and Chuck-will’s-widow. Forest birds showing medium vulnerability include large flycatchers that feed on aerial insects and bird species in riparian or humid forests, mostly in the West, that are at risk from increased drought conditions and more frequent fires. 

Overall, only 2% of forest bird species show high vulnerability to climate change, and another 30% show medium vulnerability.

In general, because of their large ranges and high reproductive potential, forest birds are predicted to fare better in a changing climate than birds in other habitats. Important exceptions include species that are specialized on highly seasonal resources, such as aerial insects or nectar, or that are dependent on high-elevation, extremely humid, or riparian forests. 

The ranges of many forest birds will proabably shift as ranges of tree species shift, generally northward or to higher elevations. Some species may become less common in the United States as their ranges increase in Canada. Conversely, species currently occurring primarily in Mexico may become more common in the United States. Increased drought and frequency of fire in western forests may also alter forest bird communities. 

More than a third of forest birds are Neotropical migrants. These long-distance migrating birds may experience mismatches in the timing of breeding with the availability of seasonal food resources, causing ecological disruption of bird communities or reproductive failure.

Key Steps

Short-term actions should focus on managing forests to increase resistance to change and promote resilience. Managers can help forests resist climate change by protecting forests with high ecological integrity by protecting forests with high ecological integrity such as National Forest roadless areas and by improving forest health and reducing undesirable (or extreme) effects of fires, insects, and diseases. We can increase the resilience of forests to accommodate gradual changes by emphasizing process rather than structure and composition, such as restoring natural fire regimes where possible, and restoring natural hydrology to maintain fragile riparian forests. 

Hooded Warbler courtesy USFWS

Long-term management practices will enable forests to respond to change. Examples include forest management to assist tree species transitions and range shifts and connecting landscapes by protecting large forest blocks and creating corridors, especially along latitudinal and elevation gradients.

Preservation of forests with the highest carbon stores, such as the moist mature and old-growth forests of the Pacific Northwest would prevent vast amount of carbon from reaching the atmosphere if these forests were logged. Additionally, preservation of these forests provides habitat protection for the threatened and declining Northern Spotted Owl and Marbled Murrelet.

Better monitoring of aerial insect eating birds, especially swifts and nightjars, may provide a sensitive barometer to environmental change, especially close to urban areas.

Conservation in Action

Extensive forests have huge potential to lessen human impacts of climate change and are part of the solution to addressing climate change. Greenhouse gas reduction through carbon sequestration can be achieved by avoiding deforestation, promoting reforestation, managing forests to sequester and retain carbon, and sequestering carbon after harvest in wood products.

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