Transpiration by two poplar varieties grown as coppice for biomass production

Fast-growing tree clones selected for biomass plantations are highly produc tive and therefore likely to use more water than the agricultural crops the y replace. We report field measurements of transpiration through the summer of 1994 from two poplar clones, Beaupre (Populus trichocarpa Torr. & A. Gr ay x P. deltoides Bartr. ex Marsh.) and Dorschkamp (P. deltoides x P. nigra L.), grown as unirrigated short-rotation coppice in southern England. Stan d transpiration was quantified by scaling up from sap flow measurements mad e with the heat balance method in a sample of stems. Leaf conductances, lea f area development, meteorological variables and soil water deficit were al so measured to investigate the response of the trees to the environment. Hi gh rates of transpiration were found for Beaupre. In June, when soil water was plentiful, the mean (+/- SD) transpiration rate over an 18-day period w as 5.0 +/- 1.8 mm day(-1), reaching a maximum of 7.9 mm day(-1). Transpirat ion rates from Dorschkamp were lower, as a result of its lower leaf area in dex. High total leaf conductances were measured for both Beaupre (0.34 +/- 0.17 mor m(-2) s(-1)) and Dorschkamp (0.39 +/- 0.16 mol m(-2) s(-1)). Leaf conductance declined slightly with increasing atmospheric vapor pressure de ficit in both clones, but only in Beaupre did leaf conductance decrease as soil water deficit increased.