An epidemiological assessment of stomatal ozone flux-based critical levels for visible ozone injury in Southern European forests

Southern forests are at the highest ozone (O-3) risk in Europe where ground-level O-3 is a pressing sanitary problem for ecosystem health. Exposure-based standards for protecting vegetation are not representative of actual field conditions. A biologically sound stomatal flux based standard has been proposed, although critical levels for protection still need to be validated. This innovative epidemiological assessment of forest responses to O-3 was carried out in 54 plots in Southeastern France and Northwestern Italy in 2012 and 2013. Three O-3 indices, namely the accumulated exposure AOT40, and the accumulated stomatal flux with and without an hourly threshold of uptake (POD1 and POD0) were compared. Stomatal O-3 fluxes were modeled (DO3SE) and correlated to measured forest response indicators, Le. crown defoliation, crown discoloration and visible foliar O-3 injury. Soil water content, a key variable affecting the severity of visible foliar O-3 injury, was included in DO3SE. Based on flux-effect relationships, we developed species-specific flux based critical levels (CLef) for forest protection against visible O-3 injury. For O-3 sensitive conifers, CLef of 19 mmol m(-2) for Pinus cembra (high O-3 sensitivity) and 32 mmol m(-2) Pious halepensis (moderate O-3 sensitivity) were calculated. For broadleaved species, we obtained a CLef 01 25 mmol m(-2) Fagus sylvatica (moderate O-3 sensitivity) and of 19 mmol m for Fraxinas excelsior (high 03 sensitivity). We showed that an assessment based on PODY and on real plant symptoms is more appropriated than the concentration-based method. Indeed, POD0 was better correlated with visible foliar 03 injury than AOT40, whereas AOT40 was better correlated with crown discoloration and defoliation (aspecific indicators). To avoid an underestimation of the real 03 uptake, we recommend the use of POD0 calculated for hours with a non-null global radiation over the 24-h O-3 accumulation window. (C) 2015 Elsevier B.V. All rights reserved.