Tropospheric concentrations of phytotoxic ozone (O₃) have undergone a great increase from preindustrial 10–15 ppbv to a present-day concentration of 35–40 ppbv in large parts of the industrialised world due to increased emissions of O₃ precursors including NO_x, CO, CH₄ and volatile organic compounds. The rate of increase in O₃ concentration ranges between 1 ppbv per decade in remote locations of the Southern hemisphere and 5 ppbv per decade in the Northern hemisphere, where largest sources of O₃ precursors are located. Molecules of O₃ penetrating into the leaves through the stomatal apertures trigger the formation of reactive oxygen species, leading thus to the damage of the photosynthetic apparatus. Accordingly, it is assumed, that O₃ increase reduces the terrestrial carbon uptake relative to the preindustrial era. Here we summarise the results of previous manipulative experiments in laboratory growth cabinets, field open-top chambers and free-air systems together with O₃ flux measurements under natural growth conditions. In particular, we focus on leaf-level physiological responses in trees, variability in stomatal O₃ flux and changes in carbon fluxes and biomass production in forest stands. As the results reported in the literature are highly variable, ranging from negligible to severe declines in photosynthetic carbon uptake, we also discuss the possible interactions of O₃ with other environmental factors including solar radiation, drought, temperature and nitrogen deposition. Those factors were found to have great potential to modulate stomata openness and O₃ fluxes.

Future ground-level concentrations of phytotoxic ozone are projected to grow in the Northern Hemisphere, at a rate depending on emission scenarios. We explored the likely changes in net ecosystem production (NEP) due to the increasing concentration of tropospheric ozone by applying a Generalized Additive Mixed Model based on measurements of ozone concentration ([O3]) and stomatal ozone flux (FsO3), at a mountainous Norway spruce forest in the Czech Republic, Central Europe. A dataset covering the growing period (May-August 2009) was examined in this case study. A predictive model based on FsO3 was found to be marginally more accurate than a model using [O3] alone for prediction of the course of NEP when compared to NEP measured by the eddy covariance technique. Both higher [O3] and FsO3 were found to reduce NEP. NEP simulated at low, pre-industrial FsO3 (0.5 nmol m-2 s-1) was higher by 24.8% as compared to NEP assessed at current rates of FsO3 (8.32 nmol m-2 s-1). However, NEP simulated at high FsO3 (17 nmol m-2 s-1), likely in the future, was reduced by 14.1% as compared to NEP values at current FsO3. The interaction between environmental factors and stomatal conductance is discussed in this paper.

In order to understand the effect of phytotoxic tropospheric ozone (O₃) on terrestrial vegetation, we quantified the impact of current O₃ concentration ([O₃]) on net ecosystem production (NEP) when compared to the conditions of the pre-industrial era. We compared and tested linear mixed-effects models based on [O₃] and stomatal O₃ flux (F_sto). The managed ryegrass–clover (Lolium perenne and Trifolium pratense) mixture was grown on arable land in the Czech Republic, Central Europe. Values of [O₃] and F_sto were measured and calculated based on resistance analogy, respectively, while NEP was calculated from eddy covariance CO₂ fluxes. We found the F_sto-based model more precise when compared to measured NEP. High F_sto was found even at low [O₃], while broad summer maximum of [O₃] was not necessarily followed by significant NEP decline, due to low soil water content leading to a low stomatal conductivity and F_sto. Comparing to low pre-industrial O₃ conditions, current levels of O₃ resulted in the reduction of cumulative NEP over the entire growing season, up to 29.7 and 13.5% when the [O₃]-based and F_sto-based model was applied, respectively. During the growing season, an O₃-induced reduction of NEP ranged between 13.1% in May and 26.2% in July when compared to pre-industrial F_sto levels. Looking to the future, high [O₃] and F_sto may lead to the reduction of current NEP by approximately 13.3% on average during the growing season, but may increase by up to 61–86.6% in autumn, indicating further O₃-induced acceleration of the senescence. These findings indicate the importance of F_sto and its inclusion into the models estimating O₃ effects on terrestrial vegetation. The interaction between environmental factors and stomatal conductance is therefore discussed in detail.