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, cham ber and micrometeorological techniques, respectively. At all three sca les, instantaneous water use efficiency exhibited a similar dependence on vapor pressure deficit (delta e) with values of 21, 13 and 10 g CO 2 per kg H2O for delta e of 0.5, 1.0 and 1.5 kPa, respectively. The fl uxes of CO2 and H2O plotted against incident light above the stand dif fered in magnitude according to scale, with highest and lowest values being obtained for leaf- and stand-level measurements, respectively. A t the tree level, the fluxes of CO2 and H2O reflected the social posit ion of the sample trees with the dominant tree having higher values th an the subdominant tree. Average long-term water use efficiency, estim ated from stand-level measurements, was 6.3 g dry biomass per kg of tr anspired water. This value is high compared with values for other tree species and may be associated with a high foliar nitrogen concentrati on. We conclude that water availability will be a critical factor in s hort-rotation willow forestry despite the relatively high water use ef ficiency of the species.