Crown conductance and tree and stand transpiration in a second-growthAbies amabilisforest

Abstract

We measured whole-tree sap flow in a 43-year-old Abies amabilis (Dougl. ex Loud.) Dougl. ex J. Forbes n Tsuga heterophylla (Raf.) Sarg. forest in western Washington, U.S.A. We calculated whole-tree crown conductance to water vapor (gcrown) by substituting the sap flow data and meteorological measurements into the inverted PenmannMonteith equation. Individual tree sap flow and crown conductance varied widely with tree size, with the smallest tree sampled having average gcrown (on a ground-area basis) of 0.57 mms n1 and transpiring up to 4.9 kgday n1 , while the largest tree measured had an average gcrown of 7.20 mms n1 and lost as much as 98 kgday n1 . Crown conductance responded linearly and positively to radiation, and had a negative exponential response to vapor pressure deficit. These response patterns were utilized to construct an empirical model of gcrown that explained from 52 to 73% (average 66%) of the variation in gcrown. The dominant and codominant trees in the stand transpired for longer periods during the day than trees in the smaller size classes, and contributed disproportionately to total stand transpiration. Daily total sap flow for individual trees was strongly correlated with tree basal area; we used the relationship between these variables to estimate daily stand transpiration. Stand transpiration calculated for 28 days between August 6 and October 12, 1993, ranged from 0.01 to 3.52 mmday n1 . Stand transpiration increased curvilinearly with increasing average daily vapor pressure deficit, reflecting the negative response of gcrown to vapor pressure deficit.