Coupling of environment to plant response: a simulation model of transpiration

A low-resolution simulation model of transpiration was developed and run, withdata from field studies in southwestern Oregon. The output of the model served as a means ofrelating environmental variables to plant response. This relation was used to define an ordinatewhich, in conjunction with previously developed ecosystem ordinates, proved helpful for compar-ing ecosystems, predicting community composition and, in special cases, growth. The datarequirements of the model are modest; we deliberately developed a model that can be used ondata obtained from field studies where electric power is unavailable and use of sophisticatedinstrumentation is impossible. The model requires inputs of air and soil temperature, atmo-spheric humidity, seasonal plant water potential (expressed as plant moisture stress, the absolutevalue of plant water potential), and a model of stomatal behavior. Where it was impossible toobtain accurate data, stochastic models were used to provide the necessary input.
The model simulated both potential and actual transpiration, the ratio of which is the mostvaluable single index of the seasonal moisture regime. Where no measurable stomatal controlwas exerted by Douglas-fir, the ratio was 1.0, indicating that adequate water was available tomeet the transpiration demand. The ratio approached 0.3 on the drier locations. Significantchanges in vegetation and growth were associated with this index.
Key words: Atmospheric humidity; conifers; ecosystem ordination; environment; models;simulation; stomatal behavior; temperature; transpiration; water potential.