Tree-ring Δ13C reveals the impact of past forest management on water-use efficiency in a Mediterranean oak coppice in Tuscany (Italy)

The effects of thinning and heavy stand density reduction was investigated in Turkey oak (Quercus cerris L.) forests of central Italy, to evaluate the physiological responses and the growth status of trees that survived a past coppice cut and thinning to convert the stand to high-forest. The working hypothesis was that a strong decrease in stand density would cause a decreasing in canopy-intrinsic water-use efficiency (measured as the ratio of CO2 assimilation to stomatal conductance, A/g), thus an increase in tree-ring carbon isotopic discrimination (Delta C-13). The tree-ring Delta C-13 of the remaining trees (survivors) was found to have significantly (P < 0.05) raised between year two and year seven since the coppice stand was thinned (high-forest conversion thinnings). This effect was mostly caused by a large decrease in tree-rings Delta C-13 at control site which was characterized by high density and competition by trees. An increase in survivors tree-rings Delta C-13 probably indicates an improved water availability, possibly induced by a decrease in competition and in stand density or a decrease in the precipitation intercepted by the canopy (i.e., a stronger increase in g over A since a decrease in A is highly unlikely). A change in foliar nitrogen, foliar Delta C-13 and content in chlorophylls was also recorded seven years after thinning. Thinnings carried out to convert old abandoned coppices into high-forest stands induce short-term stimulation of Turkey oak growth by increasing light and water availability. We were able to make a detailed reconstruction of the impact of past silvicultural treatment on the stand using a tree-ring wood Delta C-13 time-series.