Hydrogeological constraints amplify phenological responses of karst vegetation to climate variability relative to non-karst vegetation

Geological background remains an overlooked regulator of vegetation phenological responses to climate change. In particular, whether water- and soil-limited karst habitats amplify or attenuate vegetation phenological responses to climate variability remains poorly quantified. This study examined whether karst substrates amplify vegetation phenological responses to climate variability relative to non-karst regions and explored the underlying regulatory pathways. We integrated MODIS Enhanced Vegetation Index (EVI) time series (2001–2022), multi-source climate datasets, and carbonate rock distribution maps to assess vegetation phenological dynamics across karst and non-karst regions in subtropical Southwest China. Both regions exhibited an advanced start of the growing season (SOS), a delayed end of the growing season (EOS), and an extended length of the growing season (LOS). Although these fluctuations were generally synchronous between the two landforms, karst consistently showed larger anomaly magnitudes, particularly for earlier green-up and growing-season extension. Pixel-level RF–SHAP analysis further revealed greater proportions of pixels where negative climatic effects advanced SOS and positive climatic effects extended LOS in karst than in non-karst areas. This was further supported by PLS-PM evidence that the karst geological background indirectly shaped phenology via moisture limitation and drought stress. These findings provide regional evidence that karst vegetation phenology is more sensitive and vulnerable to climate variability than that of adjacent non-karst landscapes, with implications for phenological modelling and climate-adaptive ecological restoration in karst landscapes.