ABSTRACT
Quercus ilex may be considered the queen tree of the Mediterranean Basin, dominating coastal forest areas up to 2000 m above sea level at some sites. However, an increase in holm oak decline has been observed in the last decade. In this review, we analysed the current literature to answer the following questions: what are the traits that allow holm oak to thrive in the Mediterranean environment, and what are the main factors that are currently weakening this species? In this framework, we attempt to answer these questions by proposing a triangle as a graphical summary. The first vertex focuses on the main morpho-anatomical, biochemical and physiological traits that allow holm oak to dominate Mediterranean forests. The other two vertices consider abiotic and biotic stressors that are closely related to holm oak decline. Here, we discuss the current evidence of holm oak responses to abiotic and biotic stresses and propose a possible solution to its decline through adequate forest management choices, thus allowing the species to maintain its ecological domain.
ABSTRACT
Quercus ilex L. dieback has been reported in several Mediterranean forests, revealing different degree of crown damages even in close sites, as observed in two Q. ilex forest stands in southern Tuscany (IT). In this work, we applied a novel approach combining dendrochronological, tree-ring δ13C and genetic analysis to test the hypothesis that different damage levels observed in a declining (D) and non-declining (ND) Q. ilex stands are connected to population features linked to distinct response to drought. Furthermore, we investigated the impact of two major drought events (2012 and 2017), that occurred in the last fifteen years in central Italy, on Q. ilex growth and intrinsic water use efficiency (WUEi). Overall, Q. ilex showed slightly different ring-width patterns between the two stands, suggesting a lower responsiveness to seasonal climatic variations for trees at D stand, while Q. ilex at ND stand showed changes in the relationship between climatic parameters and growth across time. The strong divergence in δ13C signals between the two stands suggested a more conservative use of water for Q. ilex at ND compared to D stand that may be genetically driven. Q. ilex at ND resulted more resilient to drought compared to trees at D, probably thanks to its safer water strategy. Genotyping analysis based on simple-sequence repeat (SSR) markers revealed the presence of different Q. ilex populations at D and ND stands. Our study shows intraspecific variations in drought response among trees grown in close. In addition, it highlights the potential of combining tree-ring δ13C data with SSR genotyping for the selection of seed-bearing genotypes aimed to preserve Mediterranean holm oak ecosystem and improve its forest management.
Subject(s)
Quercus , Quercus/physiology , Ecosystem , Genotype , Forests , Trees , Water , DroughtsABSTRACT
Over the last few decades, extensive dieback and mortality episodes of Quercus ilex L. have been documented after severe drought events in many Mediterranean forests. However, the underlying physiological, anatomical, and biochemical mechanisms remain poorly understood. We investigated the physiological and biochemical processes linked to embolism formation and non-structural carbohydrates (NSCs) dynamics in Q. ilex seedlings exposed to severe water stress and rewatering. Measurements of leaf gas exchange, water relations, non-structural carbohydrates, drought-related gene expression, and anatomical changes in wood parenchyma were assessed. Under water stress, the midday stem water potential dropped below - 4.5 MPa corresponding to a ~ 50 % loss of hydraulic conductivity. A 70 % reduction in stomatal conductance led to a strong depletion of wood NSCs. Starch consumption, resulting from the upregulation of the ß-amylase gene BAM3, together with the downregulation of glucose (GPT1) and sucrose (SUC27) transport genes, suggests glucose utilization to sustain cellular metabolism in the wood parenchyma. After rewatering, the presence of residual xylem embolism led to an incomplete recovery of leaf gas exchanges. However, the partial restoration of photosynthesis allowed the accumulation of new starch reserves in the wood parenchyma and the production of new narrower vessels. In addition, changes in the cell wall composition of the wood parenchyma fibers were observed. Our findings indicate that thirty days of rewatering were sufficient to restore the NSCs reserves and growth rates of Q. ilex seedlings and that the carryover effects of water stress were primarily caused by hydraulic dysfunction.
