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

Upcoming decades will experience increasing atmospheric CO 2 and likely enhanced O 3 exposure which represents a risk for the carbon sink strength of forests, so that the need for cause–effect related O 3 risk assessment increases. Although assessment will gain in reliability on an O 3 uptake basis, risk is co-determined by the effective dose, i.e. the plant's sensitivity per O 3 uptake. Recent progress in research on the molecular and metabolic control of the effective O 3 dose is reported along with advances in empirically assessing O 3 uptake at the whole-tree and stand level. Knowledge on both O 3 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 O 3 risk assessment can be implemented. To this end, perspectives are derived for validating and promoting new O 3 flux-based modelling tools. Clarifying and linking mechanisms of O 3 uptake and effective dose are research challenges highlighted in view of recent progress and perspectives towards cause–effect based risk assessment.