Promoting the O3 flux concept for European forest trees

Tropospheric ozone (O-3) levels are predicted to stay high, being a factor within global change with potential effects on the carbon sink strength of forest trees. Hence, new approaches to O-3 risk assessment and their validation are required, although appropriate databases for adult trees are scant. Approaches based on external O-3 exposure are presently being evaluated against the ones on O-3 flux into leaves, as the cumulative uptake has the capacity for deriving O-3 risk from cause-effect relationships. The effective dose, however, needs to account for the trees' O-3 defence and tolerance in addition to O-3 uptake. The current status of promoting the preferable mechanistic O-3 flux concept is highlighted for major regions of Europe, addressing refinements and simplifications needed for routine use. At the pan-European scale, however, the flux-based concept is ready for use in O-3 risk assessment and has the potential of meso-scale application at the forest ecosystem level. (c) 2006 Elsevier Ltd. All rights reserved.