The growth responses to elevated CO2 found in experiments are highly variable and depend on other experimental parameters such as irrigation, fertilization, light regime, etc. As yet, the strength or even the sign of most interactions is all but impossible to predict from first principles. Experiments in ambient and CO2-enriched ambient air (+250 p.p.m.) have been conducted in specially adapted greenhouses (Solardomes) at Lancaster University for the past four seasons on Sitka spruce (Picea sitchensis (Bong.) Carr.), wild cherry (Prunus avium L.), beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.). These experiments are reviewed together with other published studies on interactive effects of elevated CO2 and water and nutrient supply on physiological processes, in particular gas exchange, in tree species.

It is often assumed that drought tolerance will increase in elevated CO2 because of a suppression of stomatal conductance and an increase in instantaneous water use efficiency. There is, however, some evidence that such effects could be more than offset in beech by CO2-induced increases in leaf area. It is tentatively suggested that in beech, drought tolerance could already have been reduced by the increase in atmospheric CO2 over the last century.