The widespread Quercus ilex L. dieback in Mediterranean forests: investigation of the causes at plant and ecosystem level

Over the past two decades, the occurrence of extreme climatic events in the Mediterranean region has increased, and this climatic pressure has contributed to the spread of vegetation dieback over several forest communities. Dieback has also affected Quercus ilex L., and since this decline has worsened over the last 15 years in many Mediterranean areas, it is crucial to develop effective tools for studying this phenomenon, combining different scales of measurement. Our study was conducted over four years (2019-2023) in declining (D) and non-declining (ND) Q. ilex stands in southern Tuscany (IT), assessing physiological and biochemical traits such as gas exchange, water relations, carbohydrate analysis in the wood, and xylem sap isotopic signal (δ¹⁸O). Dendrochronological and tree-ring δ¹³C analyses were combined to investigate the effects of previous droughts on tree growth and water-use efficiency.

The results of physiological analyses showed that seasonality had a strong effect on these traits, with the main stress occurring during the summer of 2020, as evidenced by the lowest gas exchange values. According to the results of δ¹⁸O analyses, holm oaks mainly took up water from deep soil sources (bottom soil or groundwater) owing to their deep-root systems, resulting in only slightly different ring-width patterns and a low responsiveness to seasonal climatic variations in both stands. By contrast, the δ¹³C results combined with SSR genotyping revealed a more conservative water use of the population in the ND stand, underlying the potential of combining these powerful tools for the selection of seed-bearing genotypes putatively tolerant to water deficit.