Girdling of fruit-bearing branches of Corylus avellana reduces seed mass while defoliation does not

Hazelnut (Corylus avellana L.) is an important nut crop with clear mast seeding behaviour leading to alternate yield and, thus, to management issues. It was hypothesized that masting is associated to accumulation/depletion of carbon within storage tissues (i.e. stem, large branches, roots). We hypothesized that fruit yield depends both on carbon fixed by leaves on the same branch and carbon stored in the main stem. We therefore conducted a manipulation of carbon sink-source resources on fruit-bearing branches of C. avellana cv. 'Tonda di Giffoni' by combining girdling and defoliation on a set of branches. We measured the number and diameter of nuts and leaves, the dry weight of fruits and seeds and the amount of blanks over the growing season. We also estimated the whole tree leaf area by allometric equations and the tree yield by collecting all nuts separately on 15 trees at maturity. The average seed dry mass in the un-defoliated girdled treatment was significantly lower than the control (0.5 vs. 0.9 g) suggesting that the fruit-bearing branches need to import carbon from the main stem and that the carbon autonomy of fruit-bearing branches is not supported during kernel maturation process. We also observed that the correlation at whole tree level between leaf area and seed mass is strong in non-masting years, while it becomes weaker in masting years reinforcing the hypothesis that the productivity of masting yields also relics on stored resources.