About Pacific Northwest Climate

Climate Impacts in Brief

Climate Impacts on Pacific Northwest Forests

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Impacts of Climate Variability

Climate variations associated with the Pacific Decadal Oscillation (PDO) affect Pacific Northwest (PNW) forests both directly and indirectly. We have examined the effects of PDO on forest growth and on forest fire activity. There is no discernible effect of the El Niño/Southern Oscillation (ENSO).

Effects of climatic variability on tree growth: Forests at upper (cold) and lower (dry and/or hot) timberlines are most likely to show strong direct effects of climatic variation on tree growth, since they are closer to their physiological limits and, therefore, more prone to stress at these locations. At upper timberline, tree ring analyses showed that mountain hemlock (Tsuga mertensiana) growth correlates well with climatic variations associated with the PDO (Figure 1). This correlation occurs because the cool phase of the PDO tends to result in greater snowpack, a limiting factor for growth at upper timberline. At lower treeline, forests are in severe competition for water with grasses and shrubs. The extended periods of drought associated with the warm phase of the PDO intensify this competition, limiting growth rates for ponderosa pine (Pinus ponderosa) near the lower timberline. (Figure 1, Peterson and Peterson 2001).

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Low frequency variability in PNW tree growth at upper and lower treeline compared to the PDO

Figure 1 Low frequency variability in PNW tree growth at upper and lower treeline compared to the PDO. Mountain hemlock growth patterns are smoothed (five-year means) radial growth indices derived from factor analysis of tree ring chronologies from 31 sites across the PNW (from southern Oregon to the Canadian border) (Peterson and Peterson 2001). The ponderosa pine growth pattern is the first principal component of radial growth chronologies from forests in eastern Washington (Brubaker, L. B. 1980. Spatial patterns of tree growth anomalies in the Pacific Northwest. Ecology 61:798-807). The PDO time series is a five-year running mean of the average winter (November to March) standardized PDO index (Zhang et al. 1997; Mantua et al. 1997).

Effects of climatic variability on forest fires: The PDO may influence broader fluctuations in forest structure, composition, and function through drought-related forest fire activity, which is generally higher during warm phases of the PDO. Forest fires were more extensive across the Pacific Northwest (PNW) during the 1925-45 PDO warm phase than during the cool phases before and after that (Figure 2, Mote et al. 1999). The resurgence of fire activity in the late 1980s was consistent with the warm-dry phase of the PDO. Some of the decline during the cool phase after 1945 and some of the increase in the most recent warm phase may be related to effects of fire suppression programs.

PDO and PNW forest fires.

Figure 2 PDO and PNW forest fires. For the period 1922-1995, the number of forest fires burning more than 1,000 acres was higher during warm phase PDOs (blue bar) than cool phase PDOs (white bar) in nearly all of the region’s national forests.

Climate Change Impacts

Biophysical inference and the results of vegetation change modeling suggest that a number of different vegetation change scenarios are possible for the PNW as a result of climate change. These scenarios differ dramatically, ranging from projections of forest expansion to forest dieback, as a result of uncertainty regarding how projected temperature and precipitation changes will interact to affect drought stress in trees or otherwise modify total annual productivity. Other major uncertainties are whether increased levels of carbon dioxide (CO2) in the atmosphere would increase primary productivity or help trees withstand reduced soil moisture. The likeliest scenario seems to be that increased forest growth could occur during the next few decades, but that at some point temperature increases would overwhelm the ability of trees to make use of higher winter precipitation and higher CO2.

In any case, the changes in climate are likely to cause plant communities to undergo shifts in their species composition and/or experience changes in densities. Species range shifts are expected to be individualistic rather than primarily as collections of currently associated species. In other words, species won't all move together. Extinction of local populations and, potentially, species are expected with climate change. Species with poor dispersal ability may have particular difficulty in shifting their spatial distributions in response to climatic changes. Loss of biological diversity is likely if environmental shifts outpace species migration rates and interact with population dynamics to cause increased rates of local population extinction.

For More Information

Selected References

For more publications on climate impacts on PNW forest ecosystems, please see CIG Publications.

Fagre, D. B., and D. L. Peterson. (in press). Taking the pulse of mountains: Ecosystem responses to climatic variability. To appear in Climatic Change.

Keeton, W. S., J. F. Franklin, and P. W. Mote. (in review). Climate variability, climate change, and forest ecosystems in the Pacific Northwest. Chapter 8 in E.L. Miles, A. K. Snover and The Climate Impacts Group, Rhythms of Change: An Integrated Assessment of Climate Impacts on the Pacific Northwest. Cambridge, Massachusetts: MIT Press.

McKenzie, D., Z. M. Gedalof, D. L. Peterson, and P. W. Mote. (in press). Climatic change, wildfire, and conservation. To appear in Conservation Biology.

Mote, P., D. Canning, D. Fluharty, R. Francis, J. Franklin, A. Hamlet, M. Hershman, M. Holmberg, K. Gray-Ideker, W.S. Keeton, D. Lettenmaier, R. Leung, N Mantua, E. Miles, B. Noble, H. Parandvash, D.W. Peterson, A. Snover, and S. Willard. 1999. Climate Variability and Change, Pacific Northwest. National Atmospheric and Oceanic Administration, Office of Global Programs, and JISAO/SMA Climate Impacts Group, Seattle, WA. 110 pp.

Peterson, D.W. and D.L. Peterson. 2001. Mountain hemlock growth responds to climatic variability at annual and decadal scales. Ecology 82(12):3330-3345.