Interactions between Drought and O 3 Stress in Forest Trees

Temperature increase and altered precipitation are facets of “Global Change”, along with enhanced tropospheric ozone (O 3 ) and CO 2 levels. Both O 3 and drought may curtail the probably limited capacity of “extra” carbon fixation in forest trees under a CO 2 ‐enriched atmosphere. In view of the exceptionally dry year of 2003 in Central Europe, this mini‐review highlights O 3 /drought interactions in biochemical and ecophysiological responses of trees. Such interactions appear to vary, depending on the genotype and factorial scenarios. If O 3 perturbs stomatal regulation, tolerance to both drought and persisting O 3 exposure may be weakened, although drought preceding O 3 stress may “harden” against O 3 impact. Stomatal closure under drought may shield trees against O 3 uptake and injury, which indeed was the case in 2003. However, the trees “tuning” between O 3 uptake and defence capacity is crucial in stress tolerance. Defence may be constrained due to limited carbon fixation, which results from the trade‐off with O 3 exclusion upon stomatal closure. Drought may cause a stronger reduction in stem growth than does ozone on an annual basis.