Water use efficiency of short-rotation Salix viminalis at leaf, tree and stand scales.

Fluxes of water and carbon dioxide from short-rotation Salix viminalis L. were measured at leaf, tree and stand scales using porometer, chamber and micrometeorological techniques, respectively. At all three scales, instantaneous water use efficiency exhibited a similar dependence on vapor pressure deficit (deltae) with values of 21, 13 and 10 g CO(2) per kg H(2)O for deltae of 0.5, 1.0 and 1.5 kPa, respectively. The fluxes of CO(2) and H(2)O plotted against incident light above the stand differed in magnitude according to scale, with highest and lowest values being obtained for leaf- and stand-level measurements, respectively. At the tree level, the fluxes of CO(2) and H(2)O reflected the social position of the sample trees with the dominant tree having higher values than the subdominant tree. Average long-term water use efficiency, estimated from stand-level measurements, was 6.3 g dry biomass per kg of transpired water. This value is high compared with values for other tree species and may be associated with a high foliar nitrogen concentration. We conclude that water availability will be a critical factor in short-rotation willow forestry despite the relatively high water use efficiency of the species.