GROWTH AND VITALITY OF OAK TREE SPECIES UNDER CLIMATE CHANGE IN MEDITERRANEAN AREAS

Extreme weather events, increasing drought spells and heat waves are causing forest dieback and tree mortality in several forest ecosystems, including those which were considered highly drought tolerant, such as the Mediterranean oak forests. The general aim of the PhD project was to investigate Quercus ilex L. (holm oak) tree responses to recurrent climate-induced stress conditions through dendrochronological and physiological studies conducted at the Maremma Regional Park (Southern Tuscany, Italy). Dendrochronological, tree-ring δ13C and genetic analysis were conducted to test the hypothesis that different crown damages observed in a declining (D, dieback affected 80% of the trees) and non-declining (ND, dieback affected 20% of the trees) holm oak stands are connected to population features linked to distinct response to drought. Further, a field study was conducted to underline the physiological limitation linked to different levels of holm oak crown defoliation and to develop a new physiological indicator for incipient tree mortality. Physiological parameters, such as Percentage loss of hydraulic conductivity (PLC), xylem water potential (x) and NSCs (non-structural carbohydrates, namely starch and soluble sugars, SS) were seasonally monitored for two years (2022 and 2023) on non- (C1), moderately- (C2), and highly- defoliated (C3) holm oak trees within the ND forest stand. Holm oak showed slightly different ring-width patterns between the two stands, suggesting a relationship between radial growth and climatic parameters only in holm oak at ND stand. The significant difference in δ13C signals between the D and ND stands highlighted a more conservative use of water for holm oaks at ND compared to the water spending behaviour observed at D stand. The genotyping analysis based on simple-sequence repeat (SSR) markers revealed the presence of distinct holm oak populations at D and ND stands, supporting the intraspecific variations in drought response among trees grown close. Higher holm oak defoliation was associated with a higher value of PLC and lower Yx and NSCs availability, with significant differences among the defoliation classes in the summer and autumn seasons. Non-lethal xylem embolism (PLC ≅ 50%) affected carbon uptake in summer and autumn in C2 and C3 trees, but not in spring when low SS availability was not linked to hydraulic damages. Interestingly, we observed a delay in starch reserves refill in C2 and C3 trees, which occurred in winter, rather than in autumn as observed in C1 trees. In less than two years, the C2 plants' carbohydrate reserves and hydraulics conductivity deteriorated and became like that of the C3 plants. Thus, this study shows how a moderate degree of holm oak crown defoliation may conceal a severely damaged tree physiology, like that of heavily defoliated trees. In conclusion, this PhD project has underlined the importance of the genetic component as a predisposing factor to drought-induced holm oak forest dieback and provided new insight regarding the carbon and hydraulic limitations driving holm oak decline in its natural environment. Further, the absence of autumn NSCs recovery after summer drought is proposed as a signal of incipient tree mortality. The results provided with this PhD research may contribute to the definition of effective management solutions aimed at Mediterranean forest monitoring, conservation, and restoration.