Saturation effect of background temperature and aridity on vegetation phenological sensitivity to urban warming

Quantifying how background temperature and aridity constrain phenological sensitivity to urban warming requires identifying critical thermal thresholds and hydrological boundaries driving spatial heterogeneity. This study assessed urban warming effects on vegetation phenology across 293 Chinese cities during 2010–2020. Urban-rural disparities (ΔSOS, ΔEOS) and temperature sensitivity (Rt-SOS, Rt-EOS) were quantified using satellite metrics. Results showed that urban heat islands advanced SOS by 12.06 d and delayed EOS by 9.86 d relative to rural areas. A non-linear saturation effect was detected: phenological sensitivity to urban warming peaked at 4 °C (spring) and 6 °C (autumn), and weakened significantly beyond the saturation thresholds of 12.5 °C (SOS) and 4 °C (EOS). Aridity acted as a negative regulator that weakened warming-induced phenological effects across most aridity index (AI) ranges and reversed them within 1.4 < AI < 2.0. Combined, LST and AI explained 75.05 % and 76.21 % of spatial variance in SOS and EOS responses, respectively. Conceptually, these findings challenge the linear paradigm of warming-driven phenological shifts by demonstrating finite physiological responses to urban warming. Ecologically, this study highlights the coupled heat-aridity constraints on urban vegetation, providing key implications for climate-adaptive urban ecological planning.