IMPACTS OF OZONE ON FORESTS - A EUROPEAN PERSPECTIVE

In comparison with the effects of extended drought periods or severe n utrient stress, those of ozone are generally much milder, at least wit h respect to growth. However, there is substantial evidence from exper iments, in the main using young saplings, that O-3 does impose a stres s on forest trees under European conditions. Decreased chlorophyll con tents and photosynthetic rates, changes in carbon allocation, increase d antioxidant activity, and reductions in biomass due to O-3 have ofte n been recorded, particularly in fast-growing species. Furthermore, O- 3 appears to weaken the trees' resilience to a range of biotic and abi otic stresses. Interactions between O-3 and climatic stress, in partic ular drought and frost hardiness, are likely to result in potentially detrimental effects. A link between the occurrence of O-3 and forest d amage is not unequivocally established in Europe, and the problem rema ins of extrapolating and/or scaling up from studies on seedlings to pr edict responses to O-3 of mature trees and forest stands, because we k now so little about acclimation to O-3. An accurate assessment is also lacking of the magnitude of the O-3 effect on European trees both in terms of the forest areas affected and its extent. In this review we s uggest that C allocation is the key factor underlying the responses of trees to O-3. Stomata also play a key role, since the acquisition of C must be achieved while an effective control over water consumption i s retained.