Validation of the stomatal flux approach for the assessment of ozone effects on young forest trees. A summary report of the TOP (Transboundary Ozone Pollution) experiment at Curno, Italy

This paper summarises the main results of a two-year experiment carried out in an Open-Top Chambers facility located in Northern Italy. Three ozone treatments (charcoal-filtered; non-filtered; and open plots) and two soil moisture treatments (watered and non-watered plots) were performed on seedlings of Populus nigra, Fagus sylvatica, Quercus robur and Fraxinus excelsior. Stomatal conductance models were drawn up and parameterised for the four species in South Alpine environmental conditions, allowing the calculation of ozone stomatal fluxes. The flux-based approach provided a better performance than the AOT40 in predicting the onset of foliar visible injuries. Critical flux levels for three of the four species are proposed (ranging between 30 and 33 mmol O3 m-2). Soil water stress delayed visible injury appearance and development by limiting ozone uptake. Data from charcoal-filtered treatments suggest the existence of an hourly flux threshold (0.12-5.77 nmol m-2 s-1), below which complete ozone detoxification may be ensured by plant s response