Asymmetric causal coupling between ecosystem photosynthesis and respiration underlies ecosystem carbon sink losses during heatwaves

Heatwaves are becoming more frequent and intense worldwide under ongoing climate warming, posing substantial risks to the terrestrial ecosystem carbon sink. Although heatwave impacts on gross primary productivity (GPP) and ecosystem respiration (ER) have been widely investigated, their causal interactions remain poorly understood, particularly the physiological and biochemical mechanisms underlying these responses. Here, we combine near-surface air temperature from the ERA5-Land reanalysis with long-term carbon flux estimates from FLUXCOM-X to investigate ecosystem carbon responses to heatwaves across biome-diverse sites globally. We identify bidirectional causal relationships between GPP and ER using convergent cross mapping and apply multivariate causal inference to quantify heatwave-induced changes in ecosystem physiological and biochemical traits. Results suggest that the bidirectional causal coupling between GPP and ER is significantly strengthened during heatwaves but weakens during the post-heatwave recovery, indicating a transient reorganization of ecosystem carbon dynamics as a legacy effect of heatwaves. Correspondingly, net ecosystem productivity (NEP) typically declines during heatwaves, reflecting a widespread transient loss of carbon sink strength, driven by a disproportionately stronger increase in ER relative to GPP. Our findings illustrate the vulnerability of the land carbon sink to heatwaves consistent with previous studies, while explicitly unravelling the causal processes that govern ecosystem carbon responses and recovery. These results provide important insights for the management of the global carbon budget and for advancing the representation of terrestrial processes in land surface models.