THE EFFECTS OF WATER AVAILABILITY ON TRANSPIRATION, WATER POTENTIAL AND GROWTH OF PICEA-ABIES DURING A GROWING-SEASON

Transpiration, pre-dawn shoot water potential and growth of Norway spr uce trees were measured for one growing season, under three water regi mes, giving a seasonal water input of 696 mm (irrigated), 579 mm (cont rol) and 270 mm (drought). Potential transpiration was calculated from the Penman-Monteith equation, using maximal canopy conductance as a f unction of vapour pressure deficit. Sap flow was measured by a tree-tr unk heat balance method and the data were expressed per unit ground su rface. This allowed calculation of canopy conductance (g(c)) for the t hree treatments. The median of the daytime g(c) was about 1.1 cm s-1 f or non-limiting water conditions, and about 0.5 cm s-1 for the conditi ons of a moderate water deficit. Pre-dawn water potential did not decr ease until daily transpiration rates were substantially reduced, with corresponding g(c) of 0.2 cm s-1. The type of control which limits tra nspiration is discussed. The basal-area growth was related closely to the amount of water infiltrated into the soil. The estimated long-term tree water-use efficiency (the total dry matter produced per unit of water transpired) was on average 4.8 g kg-1, with insignificant differ ences between the treatments. Seasonal transpiration reached 380 mm, 3 25 mm and 209 mm for the irrigated, control and drought treatment, res pectively.