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 spruce trees were measured for one growing season, under three water regimes, giving a seasonal water input of 696 mm (irrigated), 579 mm (control) and 270 mm (drought). Potential transpiration was calculated from the Penman-Monteith equation, using maximal canopy conductance as a function of vapour pressure deficit. Sap flow was measured by a tree-trunk heat balance method and the data were expressed per unit ground surface. This allowed calculation of canopy conductance ( g c) for the three treatments. The median of the daytime g c was about 1.1 cm s −1 for non-limiting water conditions, and about 0.5 cm s −1 for the conditions of a moderate water deficit. Pre-dawn water potential did not decrease until daily transpiration rates were substantially reduced, with corresponding g c of 0.2 cm s −1. The type of control which limits transpiration 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 differences between the treatments. Seasonal transpiration reached 380 mm, 325 mm and 209 mm for the irrigated, control and drought treatment, respectively.