Thinning affects water-use efficiency of hardwood saplings naturally recruited in a Pinus radiata D. Don plantation.

Plantations for timber production combine the productive to the nursery function for natural regeneration of native species. Thinning plays a crucial role in recruitment and establishment of native species, by modifying the irradiance regime beneath the canopy cover. It also promotes the secondary succession towards a more stable forest, which is a main goal in protected areas. The present study was carried in a timber plantation of Pinus radiata D. Don, located in the National Park of Cilento and Vallo di Diano (Southern Italy). The ecophysiological responses of saplings of two hardwood species, Quercus cerris L. and Fraxinus ornus L., according to two contrasting (low and high) relative irradiance (RI) regimes were analysed. Leaf and tree ring δ13C values were employed as indicators of water use efficiency (WUE) to explore plant responses to light regime. A preliminary methodological comparison between bulk wood and extracted holocellulose showed no significant differences in δ13C between the two materials. Results indicate lower δ13C values, thus suggesting a lower WUE, at higher RI regimes (corresponding to heavy thinning) for both the studied species. Furthermore, Q. cerris δ13C values are lower than those of F. ornus. These results suggest that, under the same light conditions, the mesophile Q. cerris exhibits a weaker stomatal control than the xerophile F. ornus, which keeps higher WUE. In the mesic environment studied, Q. cerris may overcome F. ornus in the long run, owing to a heavy thinning.