Light and water-use efficiencies of pine shoots exposed to elevated carbon dioxide and temperature.

An automatic gas exchange system was used to continuously measure water and carbon fluxes of attached shoots of Scots pine trees (Pinus sylvestris L.) grown in environment-controlled chambers for a 3-year period (1998-2000) and exposed to either normal ambient conditions (CON), elevated CO2 (+350 micro mol mol-1; EC), elevated temperature (+2-6 degrees C; ET) or a combination of EC and ET (ECT). EC treatment enhanced the mean daily total carbon flux per unit projected needle area (Fc.d) by 17-21 %, depending on the year. This corresponds to a 16-24 % increase in light-use efficiency (LUE) based on incident photosynthetically active radiation. The EC treatment reduced the mean daily total water flux (Fw.d) by 1-12 %, corresponding to a 13-35 % increase in water-use efficiency (WUE). The ET treatment increased Fc.d by 10-18 %, resulting in an 8-19 % increase in LUE, and Fw.d by 48-74 %, resulting in a reduction of WUE by 19-34 %. There was no interaction between CO2 and temperature elevation in connection with either carbon or water fluxes, as the carbon flux responded similarly in both ECT and EC, while the water flux in the ECT treatment was similar to that in ET. Regressions indicated that the increase in maximum LUE was greater with increasing air temperature, whereas changes in WUE were related only to high vapour pressure deficit. Furthermore, changes in LUE and WUE caused by ECT treatment displayed strong diurnal and seasonal variation.