Extreme drought increases alpine carbon use efficiency through respiration suppression and community reorganization

Abstract

Climate change is intensifying drought globally, but whether severe drought weakens or strengthens ecosystem carbon use efficiency (CUE) remains unresolved. Because drought suppresses both carbon assimilation and respiratory carbon loss, CUE depends on the relative sensitivities of these two processes. Recent evidence suggests that respiration may decline more rapidly than carbon assimilation under extreme drought, but the ecological mechanisms underlying this shift are unknown. Here, using a long‐term (2015–2023) precipitation manipulation experiment in an alpine grassland on the Tibetan Plateau spanning five precipitation levels (P, 3/4 P, 1/2 P, 1/4 P, and 1/12 P, where P denotes ambient precipitation), we investigated how increasing drought intensity regulates ecosystem CUE and the ecological mechanisms driving these responses. We show that ecosystem CUE remained unchanged under mild to moderate drought but increased significantly under extreme drought. Mild to moderate drought reduced both productivity and respiration proportionally, leaving CUE stable. In contrast, extreme drought (1/12 P) significantly increased CUE by 5% because autotrophic respiration declined more sharply (−34%) than net primary productivity (−21%) or gross primary productivity (−25%). This elevated CUE emerged not from short‐term physiological adjustment, but from plant community reorganization: extreme drought shifted plant communities toward sedge dominance and enhanced below‐ground carbon allocation, collectively reducing respiratory carbon costs. Synthesis. Our findings demonstrate that CUE responses to extreme drought are governed by long‐term ecological reorganization rather than short‐term physiological responses alone. Explicitly representing dynamic vegetation shifts and productivity allocation will be essential for improving Earth system models to project ecosystem carbon dynamics under intensifying drought.