Ozone phytotoxicity in the Czech Republic: Insights from exposure- and flux-based metrics and visible foliar injury in European beech and Norway spruce

This study evaluated the phytotoxic potential of ground-level ozone (O3) on native forest tree species at eight intensive forest monitoring plots in the Czech Republic. O3 risk was assessed using the exposure-based AOT40F (accumulated O3 concentration above 40 ppb for forest protection) and the flux-based POD1 (phytotoxic ozone dose above a flux threshold of 1 nmol O3 m-2⋅s-1 per leaf area) indices, together with data on visible foliar O3 injury. The analysis was based on data from the 2021-2023 vegetation seasons. AOT40F exceeded the critical level of 5 ppm.h at all plots. The critical levels of POD1 (5.2 mmol.m-2 for Fagus sylvatica and 9.2 mmol.m-2 for Picea abies) were exceeded at all sites in all seasons, except for one plot in 2022. Exceedance of POD1 generally occurred earlier than that of AOT40F for F. sylvatica, while for P. abies it occurred later in the season. POD1 showed a significant positive relationship with altitude, but AOT40F did not. Air temperature, soil moisture, and the O3 concentration were identified as the main predictors of stomatal O3 flux. Visible O3 injury was absent in P. abies and limited in F. sylvatica, with a weak relationship to the O3 metrics. The results emphasise the greater relevance of flux-based approaches for O3 risk assessment and indicate that visible foliar O3 injury alone provides only a limited indicator of O3 impacts under Czech conditions.