Ozone risk assessment in three oak species as affected by soil water availability

To derive ozone (O₃) dose-response relationships for three European oak species (Quercus ilex, Quercus pubescens, and Quercus robur) under a range of soil water availability, an experiment was carried out with 2-year-old potted seedlings exposed to three levels of water availability in the soil and three levels of O₃ pollution for one growing season in an ozone free-air controlled exposure (FACE) facility. Total biomass losses were estimated relative to a hypothetical clean air at the pre-industrial age, i.e., at 10 ppb as daily average (M24). A stomatal conductance model was parameterized with inputs from the three species for calculating the stomatal O₃ flux. Exposure-based (M24, W126, and AOT40) and flux-based (phytotoxic O₃ dose (POD)_0-3) dose-response relationships were estimated and critical levels (CL) were calculated for a 5% decline of total biomass. Results show that water availability can significantly affect O₃ risk assessment. In fact, dose-response relationships calculated per individual species at each water availability level resulted in very different CLs and best metrics. In a simplified approach where species were aggregated on the basis of their O₃ sensitivity, the best metric was POD_0.5, with a CL of 6.8 mmol m^-2 for the less O₃-sensitive species Q. ilex and Q. pubescens and of 3.5 mmol m^-2 for the more O₃-sensitive species Q. robur. The performance of POD₀, however, was very similar to that of POD_0.5, and thus a CL of 6.9 mmol m^-2 POD₀ and 3.6 mmol m^-2 POD₀ for the less and more O₃-sensitive oak species may be also recommended. These CLs can be applied to oak ecosystems at variable water availability in the soil. We conclude that POD_y is able to reconcile the effects of O₃ and soil water availability on species-specific oak productivity.