The nonlinear drought response and its critical threshold of Stipa krylovii roshev. typical steppe phenology

Drought is a significant climatic disturbance in grassland ecosystems, severely impacting the phenology and ecosystem functions of these systems. However, the mechanisms governing how the phenology of typical steppes in semi-arid regions responds to drought, as well as the associated critical thresholds, remain poorly understood. By utilizing ground-based phenological observations of Stipa krylovii Roshev. typical steppe and the standardized precipitation evapotranspiration index (SPEI) from 2003 to 2022, this study identified the patterns of phenological changes and pre-season drought events. Additionally, it quantified the nonlinear responses of the start of the growing season (SOS), end of the growing season (EOS), and the length of the growing season (LOS) to different drought intensity. The results showed that: (1) From 2003 to 2022, the SOS, EOS, and LOS exhibited reversal trends. Pre-season moderate to severe (SPEI < - 1) droughts were strongly correlated with these phenological reversals. (2) Drought and vegetation phenology exhibited a nonlinear relationship. Specifically, the SOS, EOS, and LOS showed higher sensitivity to pre-season moderate to severe droughts compared to mild droughts (- 1 < SPEI < - 0.5). (3) Once the drought thresholds of 1.17, 0.99, and 0.98 were exceeded, the intensity of the drought significantly delayed the SOS, advanced the EOS, and shortened the LOS, respectively. These identified drought thresholds represented critical turning points where the response of vegetation phenology to drought shifted from highly resistant to highly vulnerable, serving as warning signals for drought stress. This study provides important data support and decision references for drought warnings, risk assessments, and ecological restoration efforts.