Flux-based ozone risk assessment for adult beech forests

Tropospheric ozone (O₃) is a critical threat to forest ecosystems. A stomatal flux-based risk evaluation methodology at the leaf level was established recently in the context of the Convention on Long-Range Transboundary Air Pollution (LRTAP). This study demonstrates improvement and validation of the stomatal flux-effect approach for adult beech with results from the 8-year free-air O₃ enrichment experiment at "Kranzberger Forst" (Germany). The risk assessment module of the SVAT model FO₃REST, being under development for local scale O₃-risk assessment of adult beech stands, was parameterized according to the LRTAP Convention's Mapping Manual. Mean maximum stomatal conductance for water vapour of 245 mmol H₂O m^-2 PLA s^-1, as suggested in the LRTAP Convention's Mapping Manual for beech, was affirmed by assessment at "Kranzberger Forst", resulting in 162 mmol O₃ m^-2 PLA s^-1 upon recommended adjustment of the O₃/water vapour diffusivity ratio to 0. 663. Based on this ratio, a provisional corrected flux-effect function was deduced. Modelled Phytotoxic O₃ Doses (POD₁) and potential O₃-caused losses in biomass formation estimated with a site-specific stomatal conductance algorithm differed slightly only from estimates by the original LRTAP parameterisation. Analysis-derived POD₁ target value within the meaning of Article 2 of the European Council Directive 2008/50/EC of 10 mmol O₃ m^-2 corresponded to potential loss in biomass formation of about 10 % in ambient air relative to "pre-industrial" conditions. However, exceedance occurred by about a factor of two during the study period, indicating high risk at "Kranzberger Forst" under ambient air. Assessment for doubled O₃ exposure indicated potential underestimation even of the O₃ risk because modelled losses in biomass formation are in the lower range of the standard deviation of the observed ones.