Moderately and severely defoliated Quercus ilex L. trees exhibit hydraulic dysfunction and carbon depletion: physiological implications for Mediterranean forest monitoring

The increase in drought occurrence and intensity is contributing to rising rates of Quercus ilex L. (holm oak) mortality. Key physiological traits involved include hydraulic dysfunction and carbohydrate depletion. This study monitored xylem embolism and non-structural carbohydrates (NSCs) availability in adult holm oaks under harsh environmental conditions to identify thresholds of physiological impairment associated with increased mortality risk. Seasonal measurements of percentage loss of hydraulic conductivity (PLC), xylem water potential (Ψx) and NSCs were conducted over two years in trees categorized by defoliation severity: non-defoliated (CL1), moderately defoliated (CL2) and severely defoliated (CL3). Increased crown defoliation correlated with higher PLC and reduced NSC availability, with significant differences observed primarily in summer and autumn. Xylem embolism and carbon uptake (inferred from NSC content) showed asynchronous patterns across seasons. In summer and autumn, CL2 and CL3 trees experienced 40–50% PLC, coinciding with reduced carbon uptake. Over the two years, the physiology of CL2 trees deteriorated to a similar level to that of CL3 trees. PLC remained stable in non-defoliated CL1 trees, while decreased seasonally in CL2 and CL3 trees during winter and spring. Interestingly, CL2 and CL3 trees showed delayed starch reserve recovery, which occurred in winter rather than autumn, as observed in CL1 trees. This delayed recovery suggests the absence of autumn NSC replenishment as a potential early warning sign of physiological impairment leading to holm oak decline. Our findings suggest that moderate crown defoliation may conceal severe physiological damage, leading to PLC, Ψx and NSC values comparable to those of severely defoliated trees in later stages.