Phenological variability reveals environmental gradients in European beech forests

Climate change is significantly reshaping forest ecosystems, with vegetation phenology providing a sensitive indicator of these transformations. Understanding the spatial variability of forest phenological patterns and their environmental drivers is critical for anticipating ecosystem responses. Using a 21-year (2003–2023) MODIS Normalized Difference Vegetation Index (NDVI) time series, this study aims to identify groups of European beech (Fagus sylvatica) forests with similar seasonal timing (i.e., pheno-clusters) across Europe and quantify the role of climatic and geographical variables in their discrimination. Four distinct pheno-clusters were identified, capturing a gradient in seasonal amplitude and profile shape, from sharply peaked trajectories in colder, high-elevation environments to flatter seasonal profiles in warmer lowlands. Temperature emerged as the primary driver of cluster differentiation, with elevation acting as a key geographical constraint, while precipitation played a secondary role. Despite clear macroclimatic patterns, substantial local-scale variability highlighted the marked ecological plasticity of European beech forests. The strong association between pheno-clusters and European biogeographical regions further reflects region-specific climatic constraints on phenological dynamics. Multivariate statistical analyses confirmed the robustness of the classification and the significance of the underlying environmental gradients. Overall, these results suggest that phenology-based clusters may represent functional phenotypes associated with distinct environmental niches, providing a useful framework for linking spatially distant forests with shared seasonal phenology and for monitoring forest ecosystem responses to ongoing climate change.