CANOPY TRANSPIRATION FROM A BOREAL FOREST IN SWEDEN DURING A DRY YEAR

Estimation of areal evapotranspiration is crucial for the parameteriza tion of the soil-vegetation-atmosphere interface in climate models and for the assessment of land-use changes on water resources. Present kn owledge on how areal forest evapotranspiration depends on forest speci es composition and age is insufficient. In this study, transpiration o f 50- and 100-year-old coniferous stands was estimated on the basis of sap-flow measurements on 24 trees, 12 in each stand. The measured sam ples represented the size distribution of Pinus sylvestris and Picea a bies trees. Daily canopy transpiration (E-Q) was scaled from individua l tree flow rates using the quotients of stem circumferences of the sa mple trees to those of the stands. E-Q was used in a rearranged Penman equation to deduce a potential canopy conductance, valid for non-limi ting soil-water conditions, from a period when soil-water storage was not limiting transpiration. This enabled quantification of the seasona l transpiration deficit, which in both stands reached at least one fif th of the total potential transpiration over the growth season. The es timated fluxes of E-Q were low with a maximum daily value of about 2.8 mm in the 50-year-old stand. For dry-weather days, E-Q was well corre lated to daily sums of stand evapotranspiration estimated from eddy-co rrelation measurements. Responses to drought were species specific. Tr anspiration in pines from the 50-year-old stand was less affected by d rought relative to spruce or older pine trees, which was also reflecte d by stem increment during the season. (C) 1997 Elsevier Science B.V.