Transpiration from coppiced poplar and willow measured using sap-flow methods.

Abstract Transpiration rates from poplar (Beaupre, Populus trichocarpa × deltoides ) and willow (Germany, Salix burjatica ) clones, grown as short-rotation coppice (three-year-old stems on four-year-old stools) at a site in south-west England, were measured through the summer of 1995. Area-averaged transpiration was estimated by scaling sap-flow rates measured in individual stems to a stand area basis using measurements of leaf area and stem diameter distribution. Sap flow in poplar was measured using the stem heat balance, heat pulse velocity and deuterium tracing techniques; in willow only the stem heat balance method was used. In June and early July the mean daily transpiration from the poplar was 6±0.5 mm day −1 , stomatal conductances averaged 0.33 mol m −2  s −1 for leaves in the upper layer of the canopy and daily latent heat flux often exceeded the daily net radiation flux. Similarly high transpiration was estimated for the willow. The transpiration rates were higher than any reported rates from agricultural or tree crops grown in the UK and arose because of high aerodynamic and stomatal conductances. The high stomatal conductances were maintained even when atmospheric humidity deficits and soil water deficits were large. Much lower rates (1±1 mm day −1 ) from both clones were recorded in August at the end of a drought period. These results suggest that extensive plantation of poplar or willow short-rotation coppice will result in reduced drainage to stream flow and aquifer recharge.