Climatic Controls of Interannual Variability in Forest Land Surface Phenology Across Poland

Understanding how consistently forest phenology responds from year to year is important for characterizing vegetation responses to climatic variability, yet most studies have focused on long-term phenological trends. Here, we assessed the spatial patterns and environmental controls of detrended interannual variability in the start (SOS) and end (EOS) of the growing season across Polish forests during 2003–2025. SOS and EOS were derived from MODIS NDVI time series, while climatic controls were evaluated using variable-specific preseason windows. Generalized additive models were used to relate phenological variability to long-term climatic conditions, interannual climate variability, aridity, and topography. SOS and EOS variability showed distinct spatial patterns and contrasting climatic sensitivities. SOS was most strongly associated with temperature during the 30-day preseason, whereas EOS showed weaker and more variable climatic relationships. The relevant preseason periods differed among climatic variables and between SOS and EOS. Environmental relationships were predominantly nonlinear, with important effects of temperature, precipitation, soil moisture, solar radiation, aridity, and elevation. The models explained 35.2% and 51.6% of the deviance in SOS and EOS variability, respectively. These findings demonstrate that interannual variability provides complementary information to long-term phenological trends and that SOS and EOS respond differently to climatic and environmental conditions.