Phenological stage‐dependent hierarchical responses mediate extreme drought impacts on carbon fluxes of a semiarid grassland

Summary Ecosystem carbon balance is influenced by plant phenological stage‐dependent hierarchical responses. However, the molecular and physiological mechanisms that underlie extreme drought‐induced alterations in these responses, as well as their cascading effects on carbon fluxes in semiarid grasslands, remain unresolved. Extreme drought was imposed during the green‐up, peak growing season, and senescence phase in a semiarid grassland. We measured plant genes, photosynthetic physiology, phenological phase, and CO 2 fluxes at molecular, individual, community, and ecosystem levels, respectively. The hierarchical response mechanisms of plants to extreme drought vary across different phenological stages. During green‐up phase, drought downregulates growth, metabolism and auxin‐related genes, delaying green‐up date and depressing carbon exchange. During peak growing season, drought downregulates genes associated with photosynthesis, redox enzymes, and glutathione pathway, reducing antioxidant activity. This causes the greatest decline in photosynthetic capacity and aboveground biomass, ultimately causing the largest suppression of carbon fluxes. In senescence phase, drought upregulates Chl degradation genes and downregulates jasmonic acid pathway genes, accelerating senescence, thereby decreasing carbon uptake. Our research highlights the critical role of plant phenology and seasonal drought timing in driving carbon cycling responses from molecular to ecosystem levels, improving predictions of how extreme drought affects terrestrial carbon dynamics.