Ozone impacts on forests.

Ozone is the air pollutant of major concern for forests, and is also recognized as a significant greenhouse gas. Current background O<sub>3</sub> levels in the Northern Hemisphere have increased around 2-4.5 times since pre-industrial times. Although many questions remain, we now know a great deal about how O<sub>3</sub> affects forests at the cell/leaf level (by inducing a cascade of biochemical, physiological and morphological responses) and at the tree/canopy level (by affecting allocation and carbon strength, reproduction, hydrology and the response to co-occurring stressors). The most significant O<sub>3</sub> impact is on the regulatory capacity of resource allocation rather than on productivity. However, forest ecosystems are much complex. Gas exchange measurements carried out at steady state suggest that O<sub>3</sub> reduces stomatal conductance and thus increases protection from drought and additional O<sub>3</sub>. Ozone, however, may impair stomatal control and predispose trees to drought stress under dynamic conditions, as those in forests. Ozone-induced increases in sap-flow have been shown to result in stem growth losses, depletion of soil moisture in the rooting zone and reduced streamflow, with implications for all the forest hydrological processes. In addition, species-specific and individual-specific responses to O<sub>3</sub> affect forest competition and biodiversity. There is a need to scale from the cell and leaf level up to the ecosystem and regional level, and to develop a biologically significant and usable standard to protect forests from O<sub>3</sub>. Further work is needed for understanding the cause/effect-based relationships between O<sub>3</sub> and other stresses at the stand and ecosystem level. Modelling may provide new insights into O<sub>3</sub> impacts on forests, even though scalar and conceptual uncertainties still limit the current understanding of basic physiological mechanisms.