TRANSPIRATION RATE OF DOUGLAS FIR TREES IN THINNED AND UNTHINNED STANDS

Abstract

A procedure is described for estimating the transpiration rate of trees in stands with understory vegetation. The procedure combines soil water balance measurements of stand evapotranspiration rate with a simple vapor diffusion model that requires occasional intensive measurements of stomatal conductance of the trees and understory vegetation. Weekly average transpiration rates of 22-yr-old Douglas fir trees in a thinned stand during sunny weather in July ranged from 23.6 L∙tree−1∙d−1 when θe the fraction of extractable soil water remaining in the root zone, was 0.79 to 4.9 L∙tree−1∙d−1 when θe was 0.20. The transpiration rate of trees in the thinned stand, which contained a salal understory, was very similar to that in a nearby unthinned stand with virtually no understory vegetation. As θe decreased from slightly more than 0.8 to slightly less than 0.2, the fraction of evapotranspiration from the thinned stand due to the salal understory increased from approximately 40 to 65%. Competition for soil water by the understory was considered to be a contributing reason why tree diameter growth in the thinned stand was only slightly greater than in the unthinned stand.