Predicting the canopy resistance of a mixed hardwood forest

Abstract

Extensive measurements of stomatal resistance using a porometer in a maple-beech hardwood forest over two growing seasons and at two different canopy levels are described. The analysis permits a generalization that yields a final equation relating overall canopy resistance to solar irradiance and soil moisture content. Threshold irradiance initiating stomatal opening in both the maple and beech leaves was around 0.01–0.02 cal. cm −2 min −1, rising to about 0.15 cal. cm −2 min −1, when the leaves were touched by frost and senescing. Extrapolated values of minimum leaf resistance varied between 1.0 and about 7.0 sec cm −1, and were strongly dependent on soil moisture deficit. Using measured leaf area indices, overall canopy resistance was then derived from the weighted values of leaf resistance of the sunlit and shaded regions of the canopy for the whole summer season.