PARENT SESSION
Oral Session #63: Plant Ecology: Water Relations. Presiding: J. Cleverly.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Hall of Ideas H.
Topographically modulated water and energy balance and the spatial distribution of plants at a multi-watershed scale.
Kennedy, Jeff1, 1
ABSTRACT- A digital terrain analysis on California's Big Sur Coast tests the hypothesis that current plant-climate relationship models perform poorly in seasonally dry mountainous terrain (Neilson, et al. 1994), because they are: (1) too temporally coarse, using annual climate statistics that ignore seasonal asynchrony between available energy and available water and (2) too spatially coarse, using grid cells too large to resolve topographic facets (i.e., plants grow in topoclimates that differ markedly from regional climatic norms). I model mean monthly actual evapotranspiration and soil moisture deficit (Stephenson 1990, Eagleman 1976) to account for energy-water asynchrony. I use weather balloon mean monthly temperature and RH profiles to calculate highly nonlinear, monthly-varying lapse rates, which I apply to Willmott and Matsura's (1993) "smart" temperature interpolation algorithm. Temperatures are adjusted for topographically modulated solar radiation (TAPES-G/SRAD solar radiation module, Wilson and Gallant 2000) applied to a 30 m DEM. I document a more-than-six-fold range of summertime vapor pressure deficits over a 1180-meter elevation gradient, and how a smoothed Fast-Fourier-Transformed DEM can improve precipitation-elevation regression/interpolation in mountainous terrain (R2=0.869 vs 0.736 unsmoothed). The resulting Thornthwaite-style climate diagrams for selected topo-facets graphically illustrate the degree of climatic departure from regional norms. CCA direct gradient analysis of ground plot vegetation data (n=90) versus modeled climate parameters documents the plant-climate-water-balance relationship. This approach is broadly applicable to other Mediterranean climate and seasonally dry ecosystems.
KEY WORDS: water balance, energy balance, climate modeling, vegetation direct gradient analysis