The vulnerability of Mediterranean forests to extreme heat and drought events: the case of Holm oak (Quercus ilex L.) dieback in Italy

Abstract

Extreme heat waves coupled with severe drought episodes are increasing in frequency in the Mediterranean region. Such climatic events pose significant threats to evergreen sclerophyllous vegetation, despite their inherent resistance and adaptation to drought conditions. Over recent decades, several cases of dieback affecting Quercus ilex, Quercus suber, and other evergreen shrubs have been reported across the Mediterranean basin. In Italy, the first reports of Q. ilex dieback due to climatic events date back to 2012 in Tuscany, and with more extensive impacts observed following the extreme drought of 2017. During the summer of 2024, widespread dieback phenomena were reported across Southern Italian regions, including Apulia, Basilicata, and the islands of Sardinia and Sicily. The symptoms were consistent and recurrent in all the affected areas, characterized by partial or total foliar browning, followed by wilting and branch dieback. Notably, resprouting from dormant buds was observed in affected branches and leaves in the following rainy season. Affected areas showed ongoing symptoms of oak decline, often associated with the presence of Phytophthora spp. and other opportunistic pathogens, suggesting a complex interaction between biotic and abiotic stressors. This study aims to document and analyze the most relevant cases of Q. ilex dieback occurring in central and southern Italy, using also data collected through the SilvaCuore web application. The phenomenon of dieback and mortality of Q. ilex is still poorly understood in terms of the cause-and-effect relationships, the relative contribution of biotic and abiotic factors, the extension and severity of damage, and the ecological consequences. Although Mediterranean woodlands (forests and shrublands) are scarcely represented in the current scientific and monitoring programs (ICP Forests Level I network), the available results show an increase in dieback and mortality in Italy since 2012. We emphasize the urgent need for coordinated research efforts to study and map this phenomenon. Proposed actions include establishing a network of permanent monitoring plots that combine phytopathological and ecophysiological assessments over time, deploying comprehensive terrestrial and remote sensing monitoring techniques, and involving citizens in early detection efforts. Finally, the research should aim to identify the site-specific and management-related factors that have contributed to the extent and intensity of the dieback phenomenon. Understanding these drivers is critical for developing effective management strategies aimed at mitigating the impacts of these dieback events and enhancing the resilience of the most vulnerable forest stands.