Assessment of ozone risk to Central European forests: Time series indicates perennial exceedance of ozone critical levels

In this study, the stomatal ozone (O₃) fluxes were investigated at five low-elevation forest sites in Western Germany (Rhineland Palatinate) over the time period 1998-2019. The Phytotoxic Ozone Dose with an hourly threshold of uptake (Y), to represent the detoxification capacity of trees (POD1 in mmol m^-2 per leaf area, with Y = 1 nmol O₃ m^-2 s^-1), and the number of exceedances of the O₃ critical level of 5.2 mmol O₃ m^-2 per leaf area for European beech and 9.2 mmol O₃ m^-2 per leaf area for Norway spruce were calculated by using the DO₃SE model. A Principal Component Analysis revealed strong correlations between daily O₃ concentrations, daytime O₃ (for hours with global radiation exceeding 50 W m^-2), POD1, global radiation, vapor pressure deficit and air temperature. Moreover, a significant correlation was obtained between POD1 and soil water content (SWC) at all sites (r = 0.51-0.74). The Random Forests Analysis confirmed that the SWC is the most important predictor of stomatal O₃ fluxes. The soil water supply is very important for POD1 estimation, because drought decreases stomatal conductance, leading to a reduction of transpiration, as well as to lower O₃ uptake through stomata. Between 1998 and 2019, the drier and warmer climate induced a soil drought (on average, SWC - 0.15 % per year) leading to lower stomatal O₃ uptake by forests (- 0.36 mmol O₃ m^-2 per year). Hence, during growing seasons with sufficient water supply and often lower O₃ levels compared to hot and dry periods, forests are at higher O₃ risk than during hot and dry periods when the drought stress is more significant than O₃ stress despite relatively higher O₃ levels. Irrespective of these differences in O₃ uptake between relatively cool and humid as compared to relatively hot and dry years in the study region, the Critical Level for O₃ was exceeded in late spring/early summer (May/June) during all 22 years. Risk assessment for the protection of European forests, which is urgently needed due to the forests current critical state after several successive years of drought and exceedance of the O₃ critical level in large areas of Europe, should therefore become flux-based to account for the inter-twined effects of drought and O₃ on the physiology and health of forest trees in the region. For stomatal O₃ fluxes estimation, a better soil water and leaf parameterization is needed e.g., by taking into account both O₃- and drought-induced effects.