Modelling impacts of ozone on gross primary production across European forest ecosystems using JULES

Abstract. This study investigates the effects of tropospheric ozone (O3), a potent greenhouse gas and air pollutant, on European forests, an issue lacking comprehensive analysis at the site level. Unlike other greenhouse gases, tropospheric O3 is primarily formed through photochemical reactions, and it significantly impairs vegetation productivity and carbon fixation, thereby affecting forest health and ecosystem services. We utilise data from multiple European flux tower sites and integrate statistical and mechanistic modelling approaches to simulate O3 impacts on photosynthesis and stomatal conductance. The study examines six key forest sites across Europe – Hyytiälä and Värriö (Finland), Brasschaat (Belgium), Fontainebleau-Barbeau (France), and Bosco-Fontana and Castelporziano 2 (Italy) – representing boreal, temperate and Mediterranean climates. These sites provide a diverse range of environmental conditions and forest types, enabling a comprehensive assessment of O3 effects on gross primary production (GPP). We calibrated the Joint UK Land Environment Simulator (JULES) model using observed GPP data to simulate different O3 exposure sensitivities. Incorporating O3 effects improved the model's accuracy across all sites, although the magnitude of improvement varied depending on site-specific factors such as vegetation type, climate and ozone exposure levels. The GPP reduction due to ozone exposure varied considerably across sites, with annual mean reductions ranging from 1.04 % at Värriö to 6.2 % at Bosco-Fontana. These findings emphasise the need to account for local environmental conditions when assessing ozone stress on forests. This study highlights key model strengths and limitations in representing O3–vegetation interactions, with implications for improving forest productivity simulations under future air pollution scenarios. The model effectively captures the diurnal and seasonal variability of GPP and its sensitivity to O3 stress, particularly in boreal and temperate forests. However, its performance is limited in Mediterranean ecosystems, where pronounced O3 peaks and environmental stressors such as high vapour pressure deficit exacerbate GPP declines, pointing to the need for improved parameterisation and representation of site-specific processes. By integrating in situ measurements, this research contributes to developing targeted strategies for mitigating the adverse effects of O3 on forest ecosystems.