Which crown defoliation level may be considered a threshold for severe physiological impairment in Quercus ilex L.? ? Results of a two-year field study in a Mediterranean forest

Francesca Alderotti ¹, Antonella Gori ², Francesco Ferrini ², Mauro Centritto ³, Cassandra Detti ⁴, Diana Vanacore ⁴, Cecilia Brunetti ⁵

National Research Council of Italy (CNR), Institute for Sustainable Plant Protection, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy ¹, University of Florence, Department of Agriculture, Food, Environment and Forestry, Viale delle idee 30, 50019 Sesto Fiorentino; Florence, Italy ², National Research Council of Italy (CNR), Institute for Sustainable Plant Protection, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy. ³, University of Florence, Department of Agriculture, Food, Environment and Forestry, Viale delle idee 30, 50019 Sesto Fiorentino; Florence, Italy. ⁴, 1 National Research Council of Italy (CNR), Institute for Sustainable Plant Protection, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy ⁵

The increase in drought and aridity occurrence and intensity is likely causing the rising incidence of Quercus ilex L. (holm oak) mortality, one of the most widespread oak species dominating the Mediterranean landscape. Hydraulic dysfunction and carbohydrate depletion are key traits involved in the physiological processes linked to tree vitality. In this field-study, xylem embolism and non-structural carbohydrates (NSCs) availability in adult trees facing harsh environmental conditions were monitored with the objective of identifying the threshold of a physiological impairment rising the risk of tree mortality. To this end, xylem embolism and NSCs limitations associated with different levels of holm oak crown defoliation were evaluated. Percentage loss of hydraulic conductivity (PLC), xylem water potential (ψx) and NSCs (starch and soluble sugars, SS) were seasonally monitored for two years in non-defoliated (CL1), moderately defoliated (CL2) and severely defoliated (CL3) holm oak trees within a Mediterranean forest located in Southern Tuscany (IT). Increased crown defoliation was associated with higher PLC and lower ψx and NSC availability, with significant differences among the defoliation classes mainly observed in the summer and autumn seasons. Non-lethal xylem embolism (PLC ≅ 50%) reduced carbon uptake in summer and autumn in CL2 and CL3 defoliated trees, but not in spring, when lower SS availability was not associated with hydraulic damages. Over the two years of investigation, the physiology of CL2 trees deteriorated to the level of CL3 ones. Interestingly, we observed delayed starch reserve recovery in CL2 and CL3 trees, occurring in winter rather than in autumn as observed in non-defoliated CL1 trees. Therefore, we suggest the absence of autumn NSC recovery after summer drought as a sign of incipient holm oak mortality. In conclusion, our research shows that moderate holm oak crown defoliation may conceal severely damaged tree physiology, leading to PLC,ψx and NSCs values similar to those of severely defoliated trees in subsequent phases.

Main author career stage: Postdoc / Fellow

Contribution type: Talk

First choice session: 4. Structure, physiology, and development

Second choice session: 3. Biodiversity and global change