Measuring, modelling and testing ozone exposure, flux and effects on vegetation in southern European conditions - What does not work? A review from Italy

Ozone (O-3) exposure at Italian background sites exceeds UN/ECE concentration-based critical levels (CLe(c)), if expressed in terms of AOT40. Yet the occurrence of adverse effects of O-3 on forests and crops is controversial. Possible reasons include (i) ability of response indicators to provide an unbiased estimate of O-3 effects, (ii) setting of current CLe(c) in terms of cut-off value and accumulation level, (iii) response functions adopted to infer a critical level, (iv) environmental limitation to O-3 uptake and (v) inherent characteristics of Mediterranean vegetation. In particular, the two latter points suggest that critical levels based on accumulated stomatal flux (CLe(f)) can be a better predictor of O-3 risk than CLec. While this concept is largely acknowledged, a number of factors may limit its applicability for routine monitoring. This paper reviews levels, uptake and vegetation response to O-3 in Italy over recent years to discuss value, uncertainty and feasibility of different approaches to risk assessment. (c) 2006 Elsevier Ltd. All rights reserved.