© 2004 Heron Publishing—Victoria, Canada
A comparison of three approaches to modeling leaf gas exchange in annually drought-stressed ponderosa pine forests
Laurent Misson (1, 2), Jeanne A. Panek (1, 3) and Allen H. Goldstein (1)
1. Department of Environmental Science, Policy and Management, 151 Hilgard Hall, University of California, Berkeley, Berkeley,
CA 94720-3110, USA / 2. Corresponding author (lmisson@nature.berkeley.edu) / 3. Panek & Associates Consulting, 2311 Webster Street, Berkeley, CA 94705, USA / Received April 28, 2003; accepted October 10, 2003; published online March 1, 2004
Summary
We tested, compared and modified three models of stomatal conductance at the leaf level in a forest ecosystem where drought
stress is a major factor controlling forest productivity. The models were tested against 2 years (1999 and 2000) of leaf-level
measurements on ponderosa pine (Pinus ponderosa Dougl. ex Laws.) growing in the Mediterranean climate of California, USA. The Ball, Woodrow and Berry (1987) (BWB) model
was modified to account for soil water stress. Among the models, results of the modified BWB model were in the closest agreement
with observations (r2 = 0.71). The Jarvis (1976) model showed systematic simulation errors related to vapor pressure deficit (r2 = 0.65). Results of the Williams, Rastetter, Fernandes et al. (1996) (SPA) model showed the poorest correlation with empirical
data, but this model has only one calibration parameter (r2 = 0.60). Sensitivity analyses showed that, in all three models, predictions of stomatal conductance were most responsive
to photosynthetically active radiation and soil water content. Stomatal conductance showed little sensitivity to vapor pressure
deficit in the Jarvis model, whereas in both the BWB and SPA models, vapor pressure deficit (or relative humidity) was the
third most important variable. Parameterization of the SPA model was in accordance with the parameterization of the modified
BWB model, although the two models differ greatly. Measured and modeled results indicate that stomatal behavior is not water
conservative during spring; however, during summer, when soil water content is low and vapor pressure deficit is high, stomatal
conductance decreases and, according to the models, intrinsic water- use efficiency increases.
Keywords:
models, PAR, soil water content, stomatal conductance, vapor pressure deficit, water-use efficiency.
