Overview of the Effects of Climatic Variability and Change on Forest Ecosystems of the Southwest 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 Southwest can anticipate the following:

  • Both short- and long-term forest dynamics will the affected the disturbance processes facilitated by climatic extremes, primarily multiyear droughts

  • Although diebacks in species other than pinyon pine (Pinus edulis Engelm.) are not widespread, large fires and insect outbreaks appear to be increasing in frequency and spatial extent throughout the region

  • Increased disturbance from fire and insects, combined with lower forest productivity at most lower elevation locations, will result in lower C storage in most forest ecosystems. This may keep many low-elevation forests in younger age classes in perpetuity.

  • Increased fire followed by high precipitation (in winter in California, in early summer in much of the rest of the region) may result in increased erosion and downstream sediment delivery.


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.