Modelling and measuring transpiration from Scots pine with increased temperature and carbon dioxide enrichment

Abstract

Starting in 1996, individual trees of Scots pine (Pinus sylvestris L.) aged 30 years, were grown in closed-top chambers and exposed to either normal ambient conditions (CON), elevated CO2 (approx. 700 μmol mol-1; Elev. C), elevated temperature (approx. 2 °C and approx. 6 °C above the outside ambient temperature during the 'growing season' and 'off season', respectively; Elev. T) or a combination of elevated CO2 and warmer temperature (Elev. CT). Sap flow was monitored simultaneously by the constant-power heat balance method in a total of 16 trees, four for each treatment, over a 32 d period in summer 1998 (after the completion of needle expansion and branch elongation). To quantify the contributions of crown and physical environmental variables to total crown transpiration, a 'sun/shade model' was developed and used to partition the changes in transpiration to different sources. The results of the sap flow measurements indicate that (1) total daily sap flow (E(tree.d)) varied from 0·15-3·41 kg per tree; (2) the treatment effect on E(tree.d) depended greatly on the weather conditions; (3) the cumulative E(tree.d) for the 32 d dropped significantly by 22% relative to CON (P = 0·038) under Elev. C and increased significantly by 21% (P = 0·043) and 16% (P = 0·048) under Elev. T and Elev. CT, respectively. In general, the modelled transpiration gave good agreement with the sap flow results. The model computations showed that, on a typical sunny day in summer, the effect of treatment on crown stomatal conductance was responsible for approx. 80% of the change in E(tree.d), while the increase in needle area and the effect on total radiation absorption contributed only a small percentage. Furthermore, sunlit needles were responsible for over 60% of change in transpiration. The effect of the treatments on E(tree.d) was larger at high temperature and vapour pressure deficit but was not sensitive to incident daily radiation. (C) 2000 Annals of Botany Company.