Ozone Risk to Forests and Crops under Drought Modulation: A 15 years Flux-Based and Economic Loss Assessment for Saxony, Germany

Abstract. Tropospheric ozone (O₃) at ground level is a phytotoxic pollutant that affects vegetation and reduces crop productivity, with implications for forest ecosystems, agriculture, and food security. The present study presents a 15-year assessment (2006–2020) of O₃ risk in Saxony, Germany, using the stomatal flux-based metric PODySPEC. POD₁SPEC was applied to forests (spruce and beech) and grasslands, while POD₆SPEC was used for croplands (wheat). Risk estimations were conducted under two scenarios: a worst-case, assuming unrestricted irrigation and a best-case incorporating modelled soil water content (SWC). Given Saxony’s extensive forest cover and the sensitivity of high-elevation ecosystems, a detailed forest evaluation was performed. POD₁SPEC in spruce and beech frequently exceeded critical levels, with values up to 70 % higher at mountain than rural sites. While stomatal O₃ uptake declined in dry years at rural sites, likely due to drought-induced closure, forests at mountain sites sustained O₃ stomatal uptake even during prolonged droughts, reflecting drought tolerance. The number of dry days, used as a proxy for drought duration, helped explain these contrasting responses. Grasslands were also consistently in the high-risk zone, with POD₁SPEC exceeding critical levels throughout the time series. Under worst-case assumptions, potential reductions reached ~9 % for above-ground biomass and ~16 % for flower numbers, with impacts about 20 % higher at mountain than rural sites. These findings suggest that meteorological conditions strongly modulate O₃ uptake in grassland systems. For wheat, estimations under worst-case conditions indicate yield reductions of up to 14 % at mountain sites and 7 % at rural sites, corresponding to average annual economic losses of about €66 million and €34 million, respectively, based on the 2016–2020 producer price. Assuming similar losses under the 2025 wheat price, the economic loss increases by 13 %. These results highlight the importance of site-specific, flux-based O₃ risk assessments for guiding air quality and land-use policies in Saxony. More broadly, the approach offers a framework for evaluating O₃ impacts in other mountain regions where agriculture is essential and adaptive capacity is limited.