Abstract. Ozone (O 3 ) is a toxic air pollutant that can damage plant leaves and substantially affect the plant's gross primary production (GPP) and health. Realistic estimates of the effects of tropospheric anthropogenic O 3 on GPP are thus potentially important to assess the strength of the terrestrial biosphere as a carbon sink. To better understand the impact of ozone damage on the terrestrial carbon cycle, we developed a module to estimate O 3 uptake and damage of plants for a state-of-the-art global terrestrial biosphere model called OCN. Our approach accounts for ozone damage by calculating (a) O 3 transport from 45 m height to leaf level, (b) O 3 flux into the leaf, and (c) ozone damage of photosynthesis as a function of the accumulated O 3 uptake over the lifetime of a leaf.A comparison of modelled canopy conductance, GPP, and latent heat to FLUXNET data across European forest and grassland sites shows a general good performance of OCN including ozone damage. This comparison provides a good baseline on top of which ozone damage can be evaluated. In comparison to literature values, we demonstrate that the new model version produces realistic O 3 surface resistances, O 3 deposition velocities, and stomatal to total O 3 flux ratios. A sensitivity study reveals that key metrics of the air-to-leaf O 3 transport and O 3 deposition, in particular the stomatal O 3 uptake, are reasonably robust against uncertainty in the underlying parameterisation of the deposition scheme. Nevertheless, correctly estimating canopy conductance plays a pivotal role in the estimate of cumulative O 3 uptake. We further find that accounting for stomatal and non-stomatal uptake processes substantially affects simulated plant O 3 uptake and accumulation, because aerodynamic resistance and non-stomatal O 3 destruction reduce the predicted leaf-level O 3 concentrations. Ozone impacts on GPP and transpiration in a Europe-wide simulation indicate that tropospheric O 3 impacts the regional carbon and water cycling less than expected from previous studies. This study presents a first step towards the integration of atmospheric chemistry and ecosystem dynamics modelling, which would allow for assessing the wider feedbacks between vegetation ozone uptake and tropospheric ozone burden.

Abstract. Regional estimates of the effects of ozone pollution on forest growth depend on the availability of reliable injury functions that estimate a representative ecosystem response to ozone exposure. A number of such injury functions for forest tree species and forest functional types have recently been published and subsequently applied in terrestrial biosphere models to estimate regional or global effects of ozone on forest tree productivity and carbon storage in the living plant biomass. The resulting impacts estimated by these biosphere models show large uncertainty in the magnitude of ozone effects predicted. To understand the role that these injury functions play in determining the variability in estimated ozone impacts, we use the O-CN biosphere model to provide a standardised modelling framework. We test four published injury functions describing the leaf-level, photosynthetic response to ozone exposure (targeting the maximum carboxylation capacity of Rubisco (Vcmax) or net photosynthesis) in terms of their simulated whole-tree biomass responses against data from 23 ozone filtration/fumigation experiments conducted with young trees from European tree species at sites across Europe with a range of climatic conditions. Our results show that none of these previously published injury functions lead to simulated whole-tree biomass reductions in agreement with the observed dose–response relationships derived from these field experiments and instead lead to significant over- or underestimations of the ozone effect. By re-parameterising these photosynthetically based injury functions, we develop linear, plant-functional-type-specific dose–response relationships, which provide accurate simulations of the observed whole-tree biomass response across these 23 experiments.

Abstract. Tropospheric ozone (O3) and nitrogen deposition affect vegetation growth and, thereby, the ability of the land biosphere to take up and store carbon. However, the magnitude of these effects on the contemporary and future terrestrial carbon balance is insufficiently understood. Here, we apply an extended version of the O–CN terrestrial biosphere model that simulates the atmosphere to canopy transport of O3, its surface and stomatal uptake, the O3-induced leaf injury, and the coupled terrestrial carbon and nitrogen cycles. We use this model to simulate past and future impacts of air pollution against a background of concurrent changes in climate and carbon dioxide concentrations (CO2) for two contrasting representative concentration pathway (RCP) scenarios (RCP2.6 and RCP8.5). The simulations show that O3-related damage considerably reduced northern hemispheric gross primary production (GPP) and long-term carbon storage between 1850 and the 2010s. The simulated O3 effect on GPP in the Northern Hemisphere peaked towards the end of the 20th century, with reductions of 4 %, causing a reduction in the northern hemispheric carbon sink of 0.4 Pg C yr−1. During the 21st century, O3-induced reductions in GPP and carbon storage are projected to decline, through a combination of direct air pollution control methods that reduce near-surface O3 and the indirect effects of rising atmospheric CO2, which reduces stomatal uptake of O3 concurrent with increases of leaf-level water use efficiency. However, in hot spot regions such as East Asia, the model simulations suggest a sustained decrease in GPP by more than 8 % throughout the 21st century. O3 exposure reduces projected carbon storage at the end of the 21st century by up to 15 % in parts of Europe, the US, and East Asia. Our simulations suggest that the stimulating effect of nitrogen deposition on regional GPP and carbon storage is lower in magnitude compared to the detrimental effect of O3 during most of the simulation period for both RCPs. In the second half of the 21st century, the detrimental effect of O3 on GPP is outweighed by nitrogen deposition, but the effect of nitrogen deposition on land carbon storage remains lower than the effect of O3. Accounting for the stimulating effects of nitrogen deposition but omitting the detrimental effect of O3 may lead to an overestimation of projected carbon uptake and storage.

<p><strong>Abstract.</strong> Regional estimates of the effects of ozone pollution on forest growth depend on the availability of reliable damage functions that estimate a representative ecosystem response to ozone exposure. A number of such damage functions for forest tree species and forest functional types have recently been published and subsequently applied in terrestrial biosphere models to estimate regional or global effects of ozone on forest tree productivity and carbon storage in the living plant biomass. The resulting impacts estimated by these biosphere models show large uncertainty in the magnitude of ozone effects predicted. To understand the role that these damage functions play in determining the variability of estimated ozone impacts, we use the O-CN biosphere model to provide a standardised modelling framework. We test four published damage functions describing the leaf-level, photosynthetic response to ozone exposure (targeting V<sub>cmax</sub> or net photosynthesis) in terms of their simulated whole-tree biomass responses against field data from 23 ozone filtration/fumigation experiments conducted with European tree species at sites across Europe with a range of climatic conditions. Our results show that none of these previously published damage functions lead to simulated whole-tree biomass reductions in agreement with the observed dose-response relationships derived from these field experiments, and instead lead to significant over- / or underestimations of the ozone effect. By re-parameterising these photosynthetic based damage functions we develop linear, plant functional type specific dose-response relationships, which provide accurate simulations of the observed whole-tree biomass response across these 23 experiments.</p>

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