Oak decline in southern Italy: environmental and climate parameters for modelling purposes

The future of the Mediterranean oak forests is under threat from the dangerous effects of global climate change, such as increasing droughts and heatwaves. The combined or individual action of certain climatic and environmental factors can lead to oak decline in various oak forest types. A study was conducted between 2015 and 2022 in southern Italy, encompassing thirty oak forest stands dominated by various Quercus species, including Q. cerris , Q. frainetto , Q. ilex , Q. pubescens , and affected by oak decline. The study employed field sampling, NDVI data, and remote sensing techniques. The distribution of the forest stands encompassed both the Temperate and Mediterranean bioclimatic regions. A total of 18 quantitative and 4 qualitative variables were recorded and subsequently compared with a damage severity scale based on field observations. The values of the variables were analyzed using both descriptive and multivariate statistics to ascertain their role in triggering oak decline episodes. It was found that eight variables were the most significant in explaining the occurrence of oak decline. These were the first-semester average rainfall (PL), average maximum summer temperature, Rainfall anomaly index, Downward shortwave radiation, Root zone soil moisture, and three indicators concerning the number, amplitude, and duration of heatwaves. Quercus pubescens forests were found to be the most affected by oak decline. The years 2017 and 2022 were characterized by high levels of stress, with the combined effect of groups of diagnostic variables in exceeding the critical thresholds proving decisive in triggering episodes of oak decline. A vulnerability map was finally created reporting three vulnerability classes for oak decline: low, medium, and high. The analysis revealed that approximately 97% (116,700 hectares) of forest plots classified as vulnerable (31.7% of the total forest area in the study region) were categorized as medium or high vulnerability.