Evaluation of the Impact of Climate Change on Fagus sylvatica Dieback—A Combined Approach with ERA5-Land Data and Landsat Imagery

Widespread dieback of Fagus sylvatica has been observed in several areas of Sicily (Italy) in recent decades, often associated with Biscogniauxia nummularia infections. However, the primary drivers of this decline remain debated, with climate change increasingly recognized as a key factor not only in exacerbating tree physiological stress but also in enhancing susceptibility to pathogens. This study addresses this gap by quantifying the impact of climate change on beech decline in the Nebrodi Regional Park using an integrated approach that combines climate reanalysis data (ERA5-Land) and remote sensing (Landsat imagery). Analysis of climatic trends between two climate normals (1961–1990 and 1991–2020) revealed significant increases in temperature, evapotranspiration, and solar radiation, coupled with a decline in relative humidity. NDVI trends indicate a progressive loss of beech vigor since 2009, strongly correlated with decreasing soil moisture and precipitation. Although forest cover has expanded, this does not necessarily indicate improved forest health, as persistent climate stress may compromise tree vitality and increase vulnerability to secondary pathogens such as B. nummularia. These findings highlight the need for adaptive forest management strategies, including selective thinning and species diversification, to enhance resilience against climate change. Future research should prioritize high-resolution satellite imagery (e.g., Sentinel-2) and in situ physiological measurements (e.g., leaf water potential and sap flow) to refine early detection of climate-induced stress and improve conservation strategies for Mediterranean beech forests.