TRANSPIRATION FROM COPPICED POPLAR AND WILLOW MEASURED USING SAP-FLOWMETHODS

Transpiration rates from poplar (Beaupre, Populus trichocarpa x deltoi des) and willow (Germany, Salix burjatica) clones, grown as short-rota tion 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 measure d 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 flu x often exceeded the daily net radiation flux. Similarly high transpir ation was estimated for the willow. The transpiration rates were highe r than any reported rates from agricultural or tree crops grown in the UK and arose because of high aerodynamic and stomatal conductances. T he high stomatal conductances were maintained even when atmospheric hu midity 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 o f poplar or willow short-rotation coppice will result in reduced drain age to stream flow and aquifer recharge. (C) 1998 Elsevier Science B.V .