The Role of Environmental Parameters in the Evolution of Holm Oak ( Quercus ilex L. subsp. ballota (Desf.) Samp) Decline: Studies in the Iberian Peninsula

ABSTRACT The combination of geographical factors (climate, slope, orientation, lithology, water availability, etc.) and the pathogenicity of agents such as the oomycete from the kingdom Straminopile Phytophthora or Pythium species causes the plant disease known as oak decline in the Iberian Peninsula. Leaving aside the presence of the pathogen, the aim of the study is to analyse the role played by geographical parameters in the development of this disease and to determine which ones have the greatest influence on its spread. The data were obtained from three Dehesas in the southern foothills of the Sierra de Guadarrama (Madrid, Spain). The time scale of analysis was 2006, 2011, 2014, 2020 and 2024. The geographical factors studied were altitude, slope, aspect, topographic wetness index, distance from rivers and distance from other affected trees. The results of the research showed a significant increase in disease in the three study areas. In addition, south‐southeast (S, SE) orientation, proximity to rivers, and distance from affected holm oaks were identified as geographical factors associated with holm oak decline. Given the loss of Quercus forest mass caused by the spread of the oak decline disease detected in several countries, including Spain, it becomes necessary to adequately understand the different factors that contribute and, in some cases, are the cause of the spread of the disease, as well as to identify any inhibiting factors.