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

Starting in 1996, individual trees of Scots pine (Pinus sylvestrisL.) aged 30 years, were grown in closed-top chambers and exposedto 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 temperatureduring the ‘growing season’ and ‘off season’,respectively; Elev. T) or a combination of elevated CO₂and warmertemperature (Elev. CT). Sap flow was monitored simultaneouslyby the constant-power heat balance method in a total of 16 trees,four for each treatment, over a 32 d period in summer 1998 (afterthe completion of needle expansion and branch elongation). Toquantify the contributions of crown and physical environmentalvariables to total crown transpiration, a ‘sun/shade model’was developed and used to partition the changes in transpirationto different sources. The results of the sap flow measurementsindicate that (1) total daily sap flow (E_tree.d) varied from0.15–3.41 kg per tree; (2) the treatment effect on E_tree.ddependedgreatly on the weather conditions; (3) the cumulative E_tree.dforthe 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 withthe sap flow results. The model computations showed that, ona typical sunny day in summer, the effect of treatment on crownstomatal conductance was responsible for approx. 80% of thechange inE_tree.d , while the increase in needle area and theeffect on total radiation absorption contributed only a smallpercentage. Furthermore, sunlit needles were responsible forover 60% of change in transpiration. The effect of the treatmentson E_tree.dwas larger at high temperature and vapour pressuredeficit but was not sensitive to incident daily radiation. Copyright2000 Annals of Botany Company Transpiration model, sap flow, CO₂and temperature elevation, environment-controlled chamber, Pinus sylvestris L.