Branch transpiration of pine and spruce scaled to tree and canopy using needle biomass distributions

Branch water exchange and total tree water uptake were measured in a mixed Norway spruce and Scots pine stand in central Sweden during the 1995 and 19 96 growing seasons. Branch transpiration was scaled to canopy level on the basis of a branch conductance model, using vertical needle-area distributio ns obtained by destructive sampling. Comparison with total tree water uptak e scaled to canopy level showed agreement within 10%, for periods when the canopy was not affected by climatically induced stress. Comparison of scale d fluxes on individual trees showed that measurements of transpiration at b ranch level provide information on the direct response of transpiration to variations in weather, and furthermore that the time-lag between transpirat ion and tree water uptake was as much as 3 h. The vertical needle-area dist ribution of Scots pine was similar to that found by other authors. Needle-a rea distribution on Norway spruce, which has not been described before, sho wed that it has its largest needle area at the top of the crown. Specific n eedle area varied considerably both within trees and between trees. For spr uce, mean specific needle area (+/-SD) varied from 2.4+/-0.5 mm(2) mg(-1) a t the top of the crown to 7.1+/-1.9 mm(2) mg(-1) at the base. Corresponding figures for Scots pine were 3.4+/-2.0 and 9.1+/-2.1.