Ozone effects on European forest growth—Towards an integrative approach

The phytotoxic impacts of tropospheric ozone on cellular and molecular processes are increasingly understood and quantified. They generally lead to a decrease in plant carbon uptake that is mainly detectable at leaf scale. At larger scales, most of the empirical and modelling studies reported negative ozone effects on productivity, but the latest empirical studies and model developments temper these observations and simulations. Ozone impacts on European forest growth are controversial and seem negligible because of (1) differences among tree sizes and species, sites, data sources and methodological approaches, (2) the moderate link between adult tree growth and carbon uptake, (3) the presence of several confounding factors such as drought or nitrogen fertilization, and of legacy effects of past land use and management on stand development, and (4) the presence of compensatory processes such as tree acclimation, population adaptation and changes in competition intensity and species composition. We provide a comprehensive review of the empirical and modelling approaches available for ozone risk assessment by detailing their respective main outputs, advantages and limits, and examine research gaps. Synthesis . To disentangle between the forcing factors and to better understand ozone impacts at each ecosystem level and feedbacks across levels, and to reinforce the strength of ozone impact predictions, we recommend to combine ozone‐controlled experiments and long‐term monitoring data with physiological and forest succession process‐based models.