Modelling and Measuring Transpiration from Scots Pine with Increased Temperature and Carbon Dioxide Enrichment

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 CO 2 (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 CO 2 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 ( Etree.d ) varied from 0.15–3.41 kg per tree; (2) the treatment effect on Etree.d depended greatly on the weather conditions; (3) the cumulative Etree.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 Etree.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 Etree.d was larger at high temperature and vapour pressure deficit but was not sensitive to incident daily radiation.