Anthropogenic NOxemissions alter the intrinsic water-use efficiency (WUEi) for Quercus cerris stands under Mediterranean climate conditions

We investigated the effect of N deposition (Ndep) on intrinsic water-use efficiency (WUEi), the ratio of photosynthesis (A) to stomatal conductance (g(s)), for two Quercus cerris stands at different distances to an oil refinery in Southern Italy. We used delta C-13 in tree rings for assessing changes in WUEi; while the influence of climate and NOx emission was explored through delta O-18 and delta N-15, respectively. Differences in WUEi between the two sites were significant, with trees exposed to different degrees of NOx emissions showing an abrupt increase with the onset of pollution. Assuming similar gs at the two sites, as inferred through delta O-18, the higher N availability at the polluted site caused the shift of the A/g(s) ratio in favour of A. Overall, our result suggests that an increase of Ndep may enhance tree WUE under a scenario of reduction of precipitation predicted for Mediterranean area. (C) 2010 Elsevier Ltd. All rights reserved