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The Climate Impacts Group's (CIG) research on climate and Pacific Northwest (PNW) forest ecosystems has made important contributions to our understanding of climate influences on forest productivity, forest fires, and forest resource management. The CIG's use of tree-ring chronologies has also been key in identifying past climate variations and trends in the PNW. Key findings from this research include the following.
- In a statewide assessment of the responses of Washington's forests to projected shifts in temperature and precipitation, Littell et al. (2010) quantifies the potential declines in Douglas-fir growth, the shifts in species composition, the increases in area burned (Figure 1), and the rise in mountain pine beetle outbreaks at higher elevations.
- In a comprehensive geographical synopsis of the effects of a warmer temperatures in western North America, McKenzie et al. (2009) and McKenzie et al. (2007) describe the changes in forests from the Southwest to Alaska in response to water stress, air pollution, insect outbreaks and fire management. The southern extent of western forests are highly susceptible to elevated drought stress, increasing their vulnerability to insect attack. Throughout the forested range of the West, the unprecedented proliferation of bark beetles combined with a projected increase in drought stress, creates conditions conducive to the spread of wildfires. The responses of the forests to these stresses will vary in terms forest growth, regeneration and carbon dynamics.
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Figure 1 Changes in the distribution of annual area burned for the historical and three future time periods, under the A1B ("medium") emissions scenario for two climate models (Echam5 and CGCM_t47). White dashed lines in each box indicates the median area burned. To estimate actual area burned, use the value on the vertical axis as a power of 10 (e.g., 10^Y).
- The climate has proven to have a significant influence over the area burned by wildfire in the American West in the last century (Littell et al. 2009 and Peterson et al. 2008) and over millenial timescales (Prichard et al. 2009). Historical evidence suggests that fire-prone conditions often built up in the year concurrent with the fire and were highly correlated to other large-scale climatic patterns, such as ENSO and PDO (Heyerdahl et al. 2008). Wildfire trends were shown to vary with vegetation type, precipitation, drought conditions and temperatures. These results have strong implications for forest management practices, such as fire supression and fuel treatment, and for adaptation measures in anticipation of climate change (Keeton et al. 2007).
- Adopting forest management strategies that take into consideration the potential impacts of climate change is a novel challenge for land managers in the U.S. national forests. CIG researchers have formed partnerships with the managers at the Olympic National Forest (WA) and the Tahoe National Forest (CA) to incorporate science into the operations of these forests (Littell et al. (in review)). The role of scientists is to provide a comprehensive understanding the projected effects of climate change and to participate in the development of options for adaptation to increase the forests' resiliency (Blate et al. 2009).
- The U.S. Government's Climate Change Science Program was establish a platform for scientific research and dissemination of climate change impacts, associated risks and possible adaptation strategies for various sectors of national importance. One of these sectors included federally managed lands and waters to determine their sensitivity to climate-induced changes. Joyce et al. (2008) examines six systems and explores the potential adaptation options available to natural resource managers within the context of legislative and administrative authority.
Other forest ecosystem/climate-related research findings at the University of Washington include:
- Analysis of lake sediments in the North Cascade Range reveal good correlations between charcoal deposits, pollen, and macrofossils during the past 10,000 years. Fire has been highly variable over this time although the mean frequency has not varied greatly during different climates. Vegetation distribution and abundance has varied considerably, with pulses of establishment keyed to individual large fires. These data provide an empirical baseline for the natural range of variability in large-fire occurrence (Prichard, S.J. 2003. Spatial and Temporal Dynamics of Fire and Vegetation Change in Thunder Creek Watershed, North Cascades National Park, Washington. PhD dissertation, University of Washington, Seattle).
- Carbon distribution in subalpine forest ecosystems of the Cascade Range is highly variable at small spatial scales, with much larger carbon storage in tree islands than in adjacent meadows. Carbon stored in subalpine forests likely has a long residence time in cold conditions. Clearcut logging in these forests can affect the distribution of soil carbon, depending on the post-harvest treatment (burning versus no burning). In general, leaving logging slash and minimizing burning will encourage long-term storage of carbon (Sanscrainte et al. 2003; Sanscrainte et al. 2003)