Forest Processes

During the twenty-first century, tree mortality from forest disturbances may switch the United States from a current carbon sink (offsetting 13 % of U.S. fossil fuel greenhouse gas emissions) to a source. Carbon losses from disturbances in western U.S. forests (insects, wildfire) may be partially offset by increased growth in the East, where water is sufficient and elevated atmospheric carbon dioxide (CO₂) and N deposition promote tree growth. Habitat for some tree species will likely move northward and upward in elevation, and the movement of suitable habitat may be faster than species can disperse to the new habitats. Direct and indirect effects of climate change will affect the hydrologic cycle. The effects of elevated CO₂ on transpiration will likely be less than ± 10 %, a relatively small change compared to the effects of precipitation variability on transpiration. More frequent droughts will probably reduce streamflow, and concentrating precipitation in intense storms will likely increase the risk of erosion and landslides. Tree mortality from disturbances will likely increase runoff, and decreased snow cover depth, duration, and extent will advance the timing of runoff. Some effects, like the response of mature trees to elevated CO₂, are difficult to project because current empirical data and modeling are inadequate. The effects of climate change on forest ecosystems can be projected with some confidence at regional scales, although projecting changes at smaller spatial scales (e.g., watersheds) or for individual tree species will be challenging because of the complexity and variability of biophysical interactions at specific locations.