Effects of water and nutrient availability on physiological responses of woody species to elevated CO2

The growth responses to elevated CO₂ found in experiments arehighly variable and depend on other experimental parameterssuch as irrigation, fertilization, light regime, etc. As yet,the strength or even the sign of most interactions is all butimpossible to predict from first principles. Experiments inambient and CO₂-enriched ambient air (+250 p.p.m.) have beenconducted in specially adapted greenhouses (Solardomes) at LancasterUniversity for the past four seasons on Sitka spruce (Piceasitchensis (Bong.) Carr.), wild cherry (Prunus avium L.), beech(Fagus sylvatica L.) and pedunculate oak (Quercus robur L.).These experiments are reviewed together with other publishedstudies on interactive effects of elevated CO₂ and water andnutrient supply on physiological processes, in particular gasexchange, in tree species. It is often assumed that drought tolerance will increase inelevated CO₂ because of a suppression of stomatal conductanceand an increase in instantaneous water use efficiency. Thereis, however, some evidence that such effects could be more thanoffset in beech by CO₂-induced increases in leaf area. It istentatively suggested that in beech, drought tolerance couldalready have been reduced by the increase in atmospheric CO₂over the last century.