<p>The timing of leaf senescence critically shapes ecosystem dynamics by regulating plant productivity and nutrient cycling. While species diversity is recognized as a key driver of ecosystem functioning, its effect on autumn senescence remains poorly understood. To address this gap, here we integrated field observations from Northern China and global remote sensing data to investigate grassland autumn senescence. Our analyses reveal that higher species diversity accelerates autumn senescence, even after controlling for climate and soil factors. Mechanistically, this relationship is mediated by resource allocation strategies: enhanced species diversity promotes the allocation of resources to belowground biomass, thereby reducing aboveground resource availability and triggering earlier senescence. Our findings highlight a negative relationship between species diversity and the timing of autumn senescence in semi-arid grasslands, which facilitates increased carbon allocation to belowground compartments and accelerates the seasonal carbon cycle, offering critical insights into global carbon flux exchange under future climate warming.</p>