A Simple Diffusion Model of Transpiration Applied to a Thinned Douglas‐Fir Stand

Abstract

Values of forest transpiration rate, calculated from a vapor diffusion model that uses the vapor pressure defict (v.p.d.) of the canopy air, and measurements of the stomatal resistance (every 2—3 h) and leaf area index of the canopy, agreed well with those obtained from energy balance/Bowen ratio measurements. Stomatal resistance characteristics were also used in the diffusion model instead of actual stomatal resistance measurements to calculate transpiration rate. There was reasonable agreement between these transpiration values and energy balance measurements averaged over 3—h periods. Both the model, using stomatal resistance characteristics, and energy balance measurements showed that transpiration rate increased with increasing v.p.d. until a certain v.p.d. was reached after which the rate declined. The values of v.p.d. at which maximum transpiration rate occurred were 1.5, 1.3 and 1.1 kilopascals [=15, 13 and 11 mb] for soil water potential ranges of zero to —350 kilopascals [=0 to —3.5 bars], —350 to —950 kPa [=3.5 to —9.5 bars] and —950 to —1250 kPa [=9.5 to 12.5 bars] respectively. Calculations from the model, using stomatal resistance measurements (every 2—3 h) on 7 days, indicating that the fraction of transpiration from the thinned stand transpired by the salal undergrowth during daytime ours increased from approximately 45—70% during a 4—week drying period.