What was done
Two-year-old seedlings of Douglas fir (Pseudotsuga menziesii Mirb. Franco) were grown for three years in environmental chambers maintained at atmospheric CO2 concentrations of either 350 or 550 ppm and ambient or elevated (ambient plus 4°C) air temperatures to study the impacts of elevated CO2 and temperature on needle morphology and stomatal function in this coniferous species.

What was learned
Neither elevated CO2 nor elevated air temperature, acting alone or together, significantly affected needle stomatal density.  Likewise, the 200-ppm increase in atmospheric CO2 concentration and the 4°C increase in air temperature, when applied together, did not significantly affect either stomatal conductance or transpiration.  However, the extra CO2 alone reduced stomatal conductance and transpiration by 8 and 12%, respectively; while the elevated air temperature alone increased stomatal conductance and transpiration by 100 and 66%, respectively.  Nevertheless, as noted above, their combined influence produced no significant change in either parameter.

What it means
If air temperature alone was to increase in the future, transpirational water losses from Douglas fir seedlings likely would increase too.  However, if the air's CO2 concentration rose concurrently, it would likely nullify the temperature-induced increases in water loss (since there was no significant change in water loss when CO2 and temperature varied together in this experiment, and since the air temperature increase employed was considerably larger than what climate models typically predict for a 200 ppm increase in atmospheric CO2 concentration).  Consequently, the CO2 vs. temperature tug-of-war for water ends up being pretty much a draw, if climate model predictions of global warming are largely correct.  If, as we believe, however, the models greatly over-predict CO2-induced warming, the CO2 effect prevails in the fight for water, and Douglas fir seedlings conserve water and are able to grow and successfully reproduce where it has been too dry for them to do so in the past.