Stomatal responses of Douglas-fir seedlings to elevated carbon dioxide and temperature during the third and fourth years of exposure

Two major components of climate change, increasing atmospheric CO₂] and increasing temperature, may substantially alter the effects of water availability to plants through effects on the rate of water loss from leaves. We examined the interactive effects of elevated CO₂] and temperature on seasonal patterns of stomatal conductance (g_s), transpiration (E) and instantaneous transpiration efficiency (ITE) in Douglas‐fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings. Seedlings were grown in sunlit chambers at either ambient CO₂ (AC) or ambient + 180 µmol mol^?1 CO₂ (EC), and at ambient temperature (AT) or ambient + 3·5 °C (ET) in a full‐factorial design. Needle gas exchange at the target growth conditions was measured approximately monthly over 21 months. Across the study period and across temperature treatments, growth in elevated CO₂] decreased E by an average of 12% and increased ITE by an average of 46%. The absolute reduction of E associated with elevated CO₂] significantly increased with seasonal increases in the needle‐to‐air vapour pressure deficit (D). Across CO₂ treatments, growth in elevated temperature increased E an average of 37%, and did not affect ITE. Combined, growth in elevated CO₂] and elevated temperature increased E an average of 19% compared with the ACAT treatment. The CO₂ supply and growth temperature did not significantly affect stomatal sensitivity to D or the relationship between g_s and net photosynthetic rates. This study suggests that elevated CO₂] may not completely ameliorate the effect of elevated temperature on E, and that climate change may substantially alter needle‐level water loss and water use efficiency of Douglas‐fir seedlings.