The Influence of Ozone on Net Ecosystem Production of a Ryegrass-Clover Mixture under Field Conditions

In order to understand the effect of phytotoxic tropospheric ozone (O-3) on terrestrial vegetation, we quantified the impact of current O-3 concentration ([O-3]) 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 [O-3] and stomatal O-3 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 [O-3] 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 [O-3], while broad summer maximum of [O-3] 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 O-3 conditions, current levels of O-3 resulted in the reduction of cumulative NEP over the entire growing season, up to 29.7 and 13.5% when the [O-3]-based and F-sto-based model was applied, respectively. During the growing season, an O-3-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 [O-3] 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 O-3-induced acceleration of the senescence. These findings indicate the importance of F-sto and its inclusion into the models estimating O-3 effects on terrestrial vegetation. The interaction between environmental factors and stomatal conductance is therefore discussed in detail.