The challenge of making ozone risk assessment for forest trees more mechanistic

Upcoming decades will experience increasing atmospheric CO₂ and likely enhanced O₃ exposure which represents a risk for the carbon sink strength of forests, so that the need for cause-effect related O₃ risk assessment increases. Although assessment will gain in reliability on an O₃ uptake basis, risk is co-determined by the effective dose, i.e. the plant's sensitivity per O₃ uptake. Recent progress in research on the molecular and metabolic control of the effective O₃ dose is reported along with advances in empirically assessing O₃ uptake at the whole-tree and stand level. Knowledge on both O₃ uptake and effective dose (measures of stress avoidance and tolerance, respectively) needs to be understood mechanistically and linked as a pre-requisite before practical use of process-based O₃ risk assessment can be implemented. To this end, perspectives are derived for validating and promoting new O₃ flux-based modelling tools.