Gas-exchange and sap flow measurements of Salix viminalis trees in short-rotation forest

Abstract

Water use efficiency was estimated for Salix viminalis (L.) trees growing in the field under non-limiting nutrient and soil water conditions. Gas exchange measurements were used to calculate conductances to water vapour and CO 2 fluxes. Sap flow was measured using a tree-trunk heat balance technique. Transpiration and CO 2 uptake were estimated using an open-top ventilated chamber enclosing the whole foliage of a tree. Mesophyll conductance to CO 2 (g m ') was modelled as a non-linear function of global radiation (R g ) and air temperature. Stomatal conductance (g s ) was modelled as a function of R g and vapour pressure deficit (δe). The models explained 92 and 88% of the variation of g m ' and g s , respectively. The net photosynthetic capacity of S. viminalis was high compared to a number of broad-leaved species. The optimum temperature for photosynthesis was found to be 20.5 °C. The instantaneous water use efficiency (WUE i ) was high: it reached a maximum of 15 g (CO 2 ) kg -1 (H 2 O) for small rates of E, and decreased down to 9 g kg -1 for peak rates of E. Water use efficiency calculated from above-ground biomass measurements and from net carbon assimilation revealed a specific carbon allocation pattern during the season. The combination of gas exchange and dendrometric measurements was found to be a promising approach of nondestructive estimation of root/shoot allocation of carbon.