Modeling Carbon and Water Vapor Fluxes and Carbon Isotope Discrimination at the Canopy Scale in a Semi-arid Pine Forest.

Water, energy, and carbon exchange between the biosphere and the atmosphere in forest ecosystems are strongly coupled and affected by stomatal conductance and photosynthesis, which in turn respond to environmental factors such as air humidity, temperature, radiation, and soil water content. In this study, we test biochemical models of photosynthesis, stomatal conductance, and carbon isotope discrimination at canopy scales, using eddy covariance and isotopic data from the AmeriFlux and BASIN networks. Carbon and water vapor fluxes were simulated with an ecophysiologically based model (ISOLSM) driven by half hourly meteorology at the Old Ponderosa flux tower in Metolius (OR). The model was parameterized with half hourly eddy covariance data of carbon, latent and sensible heat fluxes, and foliar carbon isotope ratios (\delta¹³C) from the same site. Carbon isotope discrimination was sensitive to stomatal conductance parameters that also affect estimates of carbon and water vapor fluxes, reducing the parameter space obtained from the eddy flux data. The variability of simulated \delta¹³C was similar to that of observed \delta¹³C, and the relation between vapor pressure deficit (VPD) and simulated \delta¹³C of assimilated carbon was similar to that between VPD and observed \delta¹³C of ecosystem respiration. These simulations support hypotheses of tight atmospheric controls of stomatal conductance, photosynthesis, and carbon isotope discrimination at short time scales, which affect biosphere-atmosphere carbon and water exchange at large spatial scales.