Synopsis of the CASIROZ case study: Carbon sink strength of Fagus sylvatica L. in a changing environment - Experimental risk assessment of mitigation by chronic ozone impact

Databases are needed for the ozone (O₃) risk assessment on adult forest trees under stand conditions, as mostly juvenile trees have been studied in chamber experiments. A synopsis is presented here from an integrated case study which was conducted on adult Fagus sylvatica trees at a Central-European forest site. Employed was a novel free-air canopy O₃ fumigation methodology which ensured a whole-plant assessment of O₃ sensitivity of the about 30 m tall and 60 years old trees, comparing responses to an experimental 2 x ambient O₃ regime (2 x O₃, max. 150 nl O₃ l^-1) with those to the unchanged 1 x ambient O3 regime (1 x O₃ = control) prevailing at the site. Additional experimentation on individual branches and juvenile beech trees exposed within the forest canopy allowed for evaluating the representativeness of young-tree and branch-bag approaches relative to the O₃ sensitivity of the adult trees. The 2 x O₃ regime did not substantially weaken the carbon sink strength of the adult beech trees, given the absence of a statistically significant decline in annual stem growth; a 3% reduction across five years was demonstrated, however, through modelling upon parameterization with the elaborated database. 2 x O₃ did induce a number of statistically significant tree responses at the cell and leaf level, although the O ₃ responsiveness varied between years. Shade leaves displayed an O₃ sensitivity similar to that of sun leaves, while indirect belowground O₃ effects, apparently mediated through hormonal relationships, were reflected by stimulated fine-root and ectomycorrhizal development. Juvenile trees were not reliable surrogates of adult ones in view of O₃ risk assessment. Branch sections enclosed in (climatized) cuvettes, however, turned out to represent the O₃ sensitivity of entire tree crowns. Drought-induced stomatal closure decoupled O₃ intake from O₃ exposure, as in addition, also the "physiologically effective O₃ dose" was subject to change. No evidence emerged for a need to lower the "Critical Level for Ozone" in risk assessment of forest trees, although sensitive tree parameters did not necessarily reflect a linear relationship to O₃ stress. Exposure-based concepts tended to overestimate O₃ risk under drought, which is in support of current efforts to establish flux-related concepts of O₃ intake in risk assessment.