Comparative stem-growth rates of Mediterranean trees under background and naturally enhanced ambient CO₂ concentrations

Abstract

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 CO₂ in Tuscany, Italy and at a nearby control site were compared. At the CO₂-enriched site, trees have been growing for decades under elevated CO₂ concentrations. They originated from parent trees that also grew under elevated CO₂ in natural conditions, and they have been continuously exposed to elevated CO₂ 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 CO₂-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 CO₂-enriched site was not significantly different from growth at the control site. For each species, year by year, radial growth at the CO₂-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 CO₂ 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 CO₂ under drought stress, or trees may have acclimated to the high CO₂.