What was done
Two-year old seedlings of Douglas fir (Pseudotsuga menziesii Mirb. Franco) were grown for four years in environmental chambers maintained at atmospheric CO2 concentrations of 350 and 530 ppm in combination with ambient and elevated (ambient plus 3.5°C) air temperatures to study the impact of elevated CO2 and temperature on stomatal conductance, transpiration and water-use efficiency in this coniferous species.

What was learned
During the final 21 months of treatment exposure (the period over which measurements were made), elevated CO2 reduced stomatal conductances and rates of needle transpiration by approximately 12%.  In contrast, elevated air temperature increased stomatal conductance and rates of needle transpiration by 17 and 37%, respectively.  In combination, however, exposure to elevated CO2 and elevated air temperature together had no significant impact on stomatal conductance and increased transpiration rate by only 19%.

What it means
Most climate models in use today would suggest that to obtain a 3.5°C increase in air temperature as a result of increasing the air's CO2 content implies a nominal doubling of atmospheric CO2 concentration, or twice the amount of CO2 enrichment provided in this experiment (360 as opposed to 180 ppm).  Under such circumstances, the temperature-induced 37% increase in needle transpiration could be expected to be reduced all the way to zero via the concomitant impact of the doubled CO2 concentration, since a 180-ppm increase in CO2 reduced the temperature-induced 37% increase in transpiration to only 19%.  Hence, with little to no increase in transpiration in a doubled-CO2 world, but with a modest CO2-induced increase in growth -- see Pseudotsuga menziesii (Mirbel) Franco [Douglas Fir] in the Plant Growth Data section of our website -- Douglas fir tree seedlings likely would experience an increase in water use efficiency, thereby allowing them to better deal with stressful conditions imposed by insufficient soil moisture.