Overview of the Effects of Climatic Variability and Change on Forest Ecosystems of Northwest United States

The Global Change Research Act of 1990 requires the U.S. Global Change Research Program (USGCRP) to produce the National Climate Assessment (NCA) for the President and Congress every four years, analyzing the effect of global change on multiple sectors and regions in the United States and projecting major trends forward for up to 100 years. Effects on Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector (PNW-GTR-870) serves as the U.S. Forest Service sector technical report for the NCA and includes descriptions of key regional issues and examples of a risk-based framework for assessing climate-change effects.

USFS researchers state that “projected changes in climate (temperature and precipitation means and extreme events), increased atmospheric carbon dioxide (CO2), and increased nitrogen deposition are likely to affect U.S. forests throughout this century. Effects will be both direct (e.g., effects of elevated CO2 on forest growth and water use) and indirect (e.g., altered disturbance regimes), and will differ temporarily and spatially across the United States. Some of these effects may already be occurring. For example, large insect outbreaks and large wildfires during the past decade (Bentz et al. 2009, Turetsky et al. 2010) are a wake-up call about the potential effects of a rapidly changing climate on forest ecosystems.

The Northwest can anticipate the following:

  • Based on projections of distribution of tree species and forest biomes, widespread changes in the distribution and abundance of dominant forest species are expected. Forest cover will change faster via disturbance and subsequent regeneration responses, rather than through slow adjustment to gradual warming.

  • Climate is projected to become unfavorable for Douglass-fir (Pseudotsuga menziesii (Mirb.) Franco) over 32 percent of its current range in Washington, and up to 85 percent of its range of some pine species may be outside the current climatically suitable range.

  • Area burned and biomass consumed by wildfire will greatly increase, leading to changes in ecosystem structure and function, resource values in the WUI, and expenditures for fire suppression and fuels management.

  • A combination of higher temperature and dense, low-vigor stands have increased vulnerability to bark beetles and other insects, and mortality is currently high in some dry forests.

References Cited:

Bentz, B.; Allen, C.D.; Ayres, M. [et al.]. 2009. Beetle outbreaks in western North America: Causes and consequences. Salt Lake City, UT: University of Utah Press. 44p.

Turetsky, M.R.; Kane, E.S.; Harden, J.W. [et al.]. 2010. Recent acceleration of biomass burning and carbon losses in Alaskan forests and peatlands. Nature Geoscience. 4:27-31.