EFFECTS OF WATER AND NUTRIENT AVAILABILITY ON PHYSIOLOGICAL-RESPONSESOF WOODY SPECIES TO ELEVATED CO2

The growth responses to elevated CO2 found in experiments are highly v ariable and depend on other experimental parameters such as irrigation , fertilization, light regime, etc. As yet, the strength or even the s ign of most interactions is all but impossible to predict from first p rinciples. Experiments in ambient and CO2-enriched ambient air (+250 p .p.m.) have been conducted in specially adapted greenhouses (Solardome s) 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 exp eriments are reviewed together with other published studies on interac tive effects of elevated CO2 and water and nutrient supply on physiolo gical processes, in particular gas exchange, in tree species. It is of ten assumed that drought tolerance will increase in elevated CO2 becau se of a suppression of stomatal conductance and an increase in instant aneous water use efficiency. There is, however, some evidence that suc h effects could be more than offset in beech by CO2-induced increases in leaf area. It is tentatively suggested that in beech, drought toler ance could already have been reduced by the increase in atmospheric CO 2 over the last century.