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This program takes advantage of the Sierra Nevada's substantive climatic gradients as "natural experiments," allowing us to evaluate climatic mechanisms controlling forest composition, structure, and dynamics (e.g., Halpin 1995, Kern 1996, Stephenson 1998, Stephenson et al. 1998 and in prep.). A fortuitous combination of extreme environmental gradients and physiographic complexity makes the Sierra Nevada mountain range an ideal laboratory for such an approach. Elevation rises from near sea level to 4,418 m in less than 100 km horizontal distance -- one of the most extreme elevational gradients in temperate North America. A steep temperature gradient -- from warm mediterranean to cold alpine -- parallels the elevational gradient, and in turn, is overlain by a gradient of decreasing precipitation from west to east. These climatic gradients combine with highly variable soils and topography to create a physical template that includes an extraordinary range of local site water balances (Stephenson 1998).
The figures below demonstrate the range of environmental conditions experienced at this site. In addition to exhibiting great spatial variability in climate, the Sierra Nevada holds rich paleoecological records of past climate change (see figures below).
Tree-ring reconstructions of (A) summer temperature and (B) winter precipitation anomalies in the southern Sierra Nevada since A.D. 800 (dashed lines), expressed as departures from the mean of the observational record (1928 - 1988). The smoothed series (solid lines) emphasize the frequency of variation shown to be most important in spectral analyses of the reconstructed climatic record (>100 yr for temperature; >14.5 yr for precipitation). (Redrawn from Graumlich 1993, courtesy of L. Graumlich.)
Across scales from local Sierra Nevada forest stands to continents, water balance equations have been used successfully to explain vegetation distribution (Stephenson 1990, 1998). Factors that affect site water availability (e.g., soil depth) and evaporative demand (e.g., slope aspect) have intrinsically different effects on site water balances (Stephenson 1998). These differences are evident in forest patterns. Spatial hydrology (e.g., topographic convergence, lateral hydrologic fluxes) represents an additional parameter with predictive power for explaining forest distribution (Halpin 1995).
Future Directions:
Two independent models suggest that local effects of soils and topography can profoundly alter site water balances in the Sierra, sometimes with an effect equivalent to a halving or doubling of regional precipitation. These models suggest that effects of slope aspect and soil water holding capacity on site water balance should be of comparable magnitude, but of fundamentally different effect on forest pattern. Yet, actual forest patterns suggest that slope aspect may have much less effect than soil water holding capacity -- e.g., models predict that the elevation of a given forest type should be > 500 m higher on a south-facing slope than on a north-facing slope, yet the observed difference is < 200 m. Given the profound influence local conditions are likely to have on site sensitivity to climatic change, it is important that we reconcile this apparent contradiction in order to have confidence in our model projections. To do so, we will gather micro-meteorological and soil moisture data from a network of sites.
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This page last updated: Thursday, March 22, 2007