Modelling ozone fluxes to forests for risk assessment: status and prospects

• Risk assessment of ozone effects on forests is gradually moving from concentration-based exposure metrics to a more complicated approach that requires modelling of ozone fluxes to trees. • This study reviews the status of the DO 3 SE stomatal flux model employed within the Convention on Long-range Transboundary Air Pollution, describing a range of applications and identifying major research needs, especially in the context of support that could be provided by the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests. • The most urgent development need for DO 3 SE is the modelling of the soil moisture status and its effect on stomatal conductance. Furthermore, the data related to the physical characteristics and the seasonal dynamics of physiological activity of vegetation continue to pose problems. • There is a clear need for more extensive validation of models and risk estimates using more rigorous statistical procedures and comparisons with flux networks and satellites. • The current large-scale forest monitoring activities provide only limited possibilities for flux modelling, but could be enhanced by introducing a new monitoring strategy outlined here.