Ozone Amplifies Water Loss from Mature Trees in the Short Term But Decreases It in the Long Term

We measured whole-tree transpiration of mature Fagus sylvatica and Picea abies trees exposed to ambient and twice-ambient O₃ regimes (1xO₃ and 2xO₃ free-air fumigation). After eight years, mean daily total transpiration did not vary with the O₃ regime over the 31 days of our study, even though individual daily values increased with increasing daily O₃ peaks in both species. Although the environmental parameters were similar at 1xO₃ and 2xO₃, the main factors affecting daily transpiration were vapour pressure deficit in 2xO₃ spruce and O₃ peaks in beech. For a mechanistic explanation, we measured O₃-induced sluggish stomatal responses to variable light (sunflecks) by means of leaf-level gas exchange measurements only in the species where O₃ was a significant factor for transpiration, i.e., beech. Stomata were always slower in closing than in opening. The 2xO₃ stomata were slower in opening and mostly in closing than 1xO₃ stomata, so that O₃ uptake and water loss were amplified before a steady state was reached. Such delay in the stomatal reaction suggests caution when assessing stomatal conductance under O₃ pollution, because recording gas exchange at the time photosynthesis reached an equilibrium resulted in a significant overestimation of stomatal conductance when stomata were closing (ab. 90% at 1xO₃ and 250% at 2xO₃). Sun and shade leaves showed similar sluggish responses, thus suggesting that sluggishness may occur within the entire crown. The fact that total transpiration was similar at 1xO₃ and 2xO₃, however, suggests that the higher water loss due to stomatal sluggishness was offset by lower steady-state stomatal conductance at 2xO₃. In conclusion, O₃ exposure amplified short-term water loss from mature beech trees by slowing stomatal dynamics, while decreased long-term water loss because of lower steady-state stomatal conductance. Over the short term of this experiment, the two responses offset each other and no effect on total transpiration was observed.