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

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. To consider the impacts of ozone on forest growth rates, POD1SPEC-based dose-response relationships 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 and on natural and harvest removals for different European countries for the years 2008-2012 was used to estimate annual changes in forest living biomass carbon (C) stocks due to ozone. This was achieved using two different scenarios: with and without the negative impacts of ozone on forest gross growth rates, estimated using the POD1SPEC-based dose-response relationships as they relate to impacts. Results provided an estimate of the annual gap between forest gross growth and the total removals, i.e.The annual forest stock changes, both for current levels of ozone and their associated impacts and in the absence of ozone. Estimates were made by collating 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 ozone impacts on forests as suggested by the UNECE LRTAP Convention, based on POD1SPEC, was exceeded in large parts of Europe during 2008-2012, except in inland areas in the Mediterranean and in small parts of continental Europe and the Fennoscandian mountain range. The highest POD1SPEC was estimated for the coastal regions of mid-latitude Europe, including the UK and extending northwards to the middle of Sweden, the south of Norway, and Finland. To the south, lower values for POD1SPEC were estimated for most of the Iberian Peninsula and parts of the Mediterranean coastal regions. It was estimated that reduced ozone exposure, similarly to pre-industrial conditions, would increase European forest stem volume growth rates by 9 %, but this 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 in 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 that these results are credible for European forest stands of different age classes.