Author(s):
Jeremy Littell - University of Washington
David L. Peterson - USDA Forest Service PNW Research Station
Abstract:
Despite a well-developed body of research focusing on tree-growth / climate relationships, there is considerable uncertainty associated with climate change impacts to montane forests in western North America. At least a substantial fraction of this uncertainty is due to a combination of a lack of structured sampling along climate gradients and the pattern-process approach of relating local responses to extra-regional drivers. In this study, we employed a biophysically- and biogeographically-informed sampling structure over several mountain ecosystems (from the maritime Olympic Mountains in Washington to the Crown of the Continent Ecosystem in continental western Montana) to account for this uncertainty. We used landscape modeling to inform sampling of tree-ring chronologies along biophysical gradients in four protected areas. The chronologies were then analyzed for the dominant climate / growth relationships associated with different portions of elevation and continentality gradients. Distinctly hierarchical growth/climate relationships emerged, with coherent patterns in sensitivity to environmental variables sometimes emerging at the plot scale and other times at the watershed or physiographic province scale. During the 20th century, water balance figures prominently in growth-climate models. In addition, the conceptual approach we used illustrates the utility of favoring mechanism-response, regional-scale strategies for climate impacts research. By relating Douglas-fir growth to the local, physical water/energy manifestation of dominant Pacific modes of climate variability, we eliminated a source of uncertainty that allowed us to compare growth/climate relationships across ecosystems. In this context, our relationships can probably be expanded into quantitative growth models allowing linkages of productivity with predictions of future climate.
