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 POD0.5, 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 POD0.5, 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 PODy is able to reconcile the effects of O3 and soil water availability on species-specific oak productivity.