Water availability controls growing season length divergence between karst and non-karst forests

The length of the growing season (LOS) is a key phenological indicator of forest responses to climate change. In subtropical karst forests, where water rather than temperature is the primary limiting factor, the dynamics and climatic controls of LOS remain poorly understood. Here we mapped land surface LOS across the southwest karst region of China (Yunnan, Guizhou, and Guangxi) from 2001 to 2024 using the Moderate Resolution Imaging Spectroradiometer (MODIS) gross primary productivity (GPP) data, and contrasted the temporal dynamics and climatic sensitivity of LOS between karst forests (KF) and non-karst forests (NKF). Validation against in-situ GPP from six flux sites confirmed the reliability of the phenology retrieval, with the start of season (SOS) showing the highest accuracy (MAE = 8.5 days). Over the study period, both forest types exhibited an advancing SOS and a delaying end of season (EOS), which jointly extended LOS. The estimated extension rate was higher in KF (2.207 days yr⁻¹) than in NKF (1.672 days yr⁻¹). Partial least squares regression revealed fundamentally divergent climatic controls between KF and NKF. LOS in KF was dominated by water-related variables (soil moisture and precipitation), whereas LOS in NKF was governed by energy-related drivers (radiation and temperature). This divergence was mechanistically linked to soil water retention: the soil moisture drydown rate constant of KF was consistently lower than that of NKF in every year (mean 7.8 versus 11.4 days), demonstrating markedly faster water loss in karst terrain. Our findings highlight that water availability, rather than temperature alone, must be explicitly considered when predicting the phenological responses of karst forests to ongoing climate change.