Are Bavarian Forests (southern Germany) at risk from ground-level ozone? Assessment using exposure and flux based ozone indices

Exposure and flux-based indices of O-3 risk were compared, at 19 forest locations across Bavaria in southern Germany from 2002 to 2005; leaf symptoms on mature beech trees found at these locations were also examined for O-3 injury. O-3 flux modelling was performed using continuously recorded O-3 concentrations in combination with meteorological and soil moisture data collected from Level II forest sites. O-3 measurements at nearby rural open-field sites proved appropriate as surrogates in cases where O-3 data were lacking at forest sites (with altitude-dependent average differences of about 10% between O-3 concentrations). Operational thresholds of biomass loss for both O-3 indices were exceeded at the majority of the forest locations, suggesting similar risk under long-term average climate conditions. However, exposure-based indices estimated higher O-3 risk during dry years as compared to the flux-based approach. In comparison, minor O-3-like leaf injury symptoms were detected only at a few of the forest sites investigated. Relationships between flux-based risk thresholds and tree response need to be established for mature forest stands for validation of predicted growth reductions under the prevailing O-3 regimes. (C) 2009 Elsevier Ltd. All rights reserved.