Use of a simulation model and ecosystem flux data to examine carbon-water interactions in ponderosa pine

Drought stress plays an important role in determining both the structure an d function of forest ecosystems, because of the dose association between th e carbon (C) and hydrological cycles. We used a detailed model of the soil- plant-atmosphere continuum to investigate the links between carbon uptake a nd the hydrological cycle in a mature, open stand of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) at the Metolius river in eastern Oregon over a 2 -year period (1996-1997). The model was parameterized from local measuremen ts of vegetation structure, soil properties and meteorology, and tested aga inst independent measurements of ecosystem latent energy (LE) and carbon fl uxes and soil water content. Although the 2 years had very different precip itation regimes, annual uptake of C and total transpiration were similar in both years, according to both direct observation and simulations. There we re important differences in ratios of evaporation to transpiration, and in the patterns of water abstraction from the soil profile, depending on the f requency of summer storms. Simulations showed that, during periods of maxim um water limitation in late summer, plants maintained a remarkably constant evapotranspirative flux because of deep rooting, whereas changes in rates of C accumulation were determined by interactions between atmospheric vapor pressure deficit and stomatal conductance. Sensitivity analyses with the m odel suggest a highly conservative allocation strategy in the vegetation, f ocused belowground on accessing a soil volume large enough to buffer summer droughts, and optimized to account for interannual variability in precipit ation, The model suggests that increased allocation to leaf area would grea tly increase productivity, but with the associated risk of greater soil wat er depletion and drought stress in some years. By constructing sparse canop ies and deep rooting systems, these stands balance reduced productivity in the short term with risk avoidance over the long term.