Structural variability and response to drought in holm oak (Quercus ilex) forests with different tree density in southern Spain: a multitemporal approach based on remote sensing

Abstract

Despite their high ecological and socioeconomic value, the Mediterranean holm oak forest (Quercus ilex subsp. ballota) is becoming more vulnerable to global change due to exposure to adverse factors, such as intensive management. This study examines the structural, compositional, and functional characteristics of different types of holm oak forests based on tree density (open-ABI, intermediate-INT, and dense-DEN) at a regional scale (Andalucía, Spain). We applied a combined descriptive and analytical statistical approach using generalized linear models (GLMs), as well as spatial and temporal trend analyses with in-situ data from National Forest Inventory (NFI) plots and Vegetation Indices (VIs) derived from satellite imagery (Landsat). Aridity, elevation and minimum NDVI values (NDVI_min) were the most important variables explaining the three forest types. GLM analyses revealed significant associations between maximum NDVI values and field-measured variables (R² adj. ~ 0.2). Additionally, multitemporal analysis of the maximum of enhanced vegetation index (EVI_max) over a 23-year period (2001–2023) demonstrated a significant relationship between VI dynamics and precipitation, with forest types responding similarly to drought events. This study further confirms the advantages of combining NFI data with remote sensing for forest dynamics analysis.