Comparative stem‐growth rates of Mediterranean trees under background and naturally enhanced ambient CO 2 concentrations

Ring widths of five Mediterranean forest tree species ( Arbutus unedo , Fraxinus ornus , Quercus cerris , Quercus ilex and Quercus pubescens ) growing close to a natural source of CO 2 in Tuscany, Italy and at a nearby control site were compared. At the CO 2 ‐enriched site, trees have been growing for decades under elevated CO 2 concentrations. They originated from parent trees that also grew under elevated CO 2 in natural conditions, and they have been continuously exposed to elevated CO 2 throughout their growth. Tree‐ring series from each of the species were prepared. Assigning calendar dates to rings was difficult but possible, and ring‐width series were built for all species. The ring‐width data were analysed using a two‐sided t ‐test to assess if there was a difference between the radial growth at the CO 2 ‐enriched site and the control site. The cumulative basal area at the same cambial age at both sites was also compared using a Wilcoxon test. Radial growth of trees at the CO 2 ‐enriched site was not significantly different from growth at the control site. For each species, year by year, radial growth at the CO 2 ‐enriched site was tested against the control site and significant differences were found in only a few years; these differences were not synchronous with extreme climatic events. The expected increase in above‐ground productivity, as one of the ecosystem responses to increasing CO 2 during drought stress, was not observed in this Mediterranean woody plant community, despite being water‐limited. Other resource limitations, such as low nutrient availability (common in the Mediterranean region), may have counteracted the positive effect of elevated CO 2 under drought stress, or trees may have acclimated to the high CO 2 .