Ozone causes substantial reductions in the carbon sequestration of managed European forests

Abstract. The annual, accumulated stomatal ozone uptake during the vegetation season, i.e. the species-specific Phytotoxic Ozone Dose above a threshold of 1.0 nmol m^-2 s^-1, POD₁SPEC, was estimated for European forest tree plant functional types for the years 2008–2012. These POD₁SPEC estimates were based on ozone concentrations simulated with the EMEP CTM model in combination with stomatal ozone uptake estimated with the DO₃SE (Deposition of Ozone for Stomatal Exchange) model. POD₁SPEC -based dose-response relationships for impacts of ozone on forest growth rates were constructed based on results from multi-year experiments with young trees generated within the framework of the UNECE LRTAP Convention. Official information on forest gross growth rates as well as natural and harvest removals for different European countries for the years 2008–2012 were used to estimate annual changes in forest living biomass carbon (C) stocks under two different scenarios, with and without the negative impacts of ozone on forest gross growth rates, estimated using the POD₁SPEC -based dose-response relationships for impacts. This resulted in estimates of the annual gap between forest gross growth and the total removals, i.e. the annual forest stock changes, under current negative ozone impacts, as well as in the absence of negative ozone impacts. Estimates were made by collating any species-specific information into broad European coniferous and deciduous forest types for consistency with forest statistics. The default IPCC methodology was used to convert estimates of the impacts of ozone on the annual changes in forest living biomass C stocks. The results showed that the critical level for negative impacts on forests suggested by the UNECE LRTAP Convention, based on POD₁SPEC, was exceeded in large parts of Europe during 2008–2012, except for inland areas in the Mediterranean and for small parts of Continental Europe as well as for and the Fennoscandian mountain range. The highest POD₁SPEC was estimated for the coastal regions of mid-latitude Europe including the UK, limited to the north by mid-Sweden and south Norway and Finland. To the south, lower values for POD₁SPEC were estimated for most of the Iberian Peninsula as well as parts of the Mediterranean coastal regions. It was estimated that reduced ozone exposure, similar to pre-industrial conditions, would increase European forest stem volume growth rates by 9 %, but it would increase European forest annual net changes in standing stocks by 28 %. The difference in gross forest stem volume growth with and without ozone impacts was relatively similar in for example Germany and France. However, since the gap between gross growth and total removals was much smaller for Germany, the enhanced growth in the absence of ozone had a much larger relative impact on the forest standing stock changes in Germany, compared to France. Summarized for all European forests, the C sequestration to the living biomass C stock was estimated to increase by 31 % in the absence of ozone exposure. A thorough review of the literature resulted in the conclusion that mature trees under field conditions cannot be assumed to be less sensitive to ozone exposure compared to young trees under experimental conditions strongly suggesting these results are credible for European forest stands of different age classes.