Phase-Asymmetric Thermal Sensitivity Amplifies Respiration Hysteresis in Heatwaves

Understanding how extreme climate events reshape the temperature response of ecosystem respiration (Re) is critical for predicting carbon-climate feedbacks. Here, we combine observational data and land surface model simulations to investigate Re responses to heatwaves, focusing on temperature sensitivity (Q₁₀) and thermal hysteresis. We develop a quantitative framework capturing hysteresis via temperature thresholds, their seasonal timing, and phase-specific Q₁₀ indices (Q_10,I, warming phase; Q_10,II, cooling phase). Flux-tower observations from the 2003 and 2018 heatwaves showed widespread amplification of thermal hysteresis in comparison with years without heatwaves. This amplification was primarily driven by asymmetric shifts in thermal sensitivity, especially a dominant decline in Q_10,II. The response of ecosystem respiration after summer is the most affected in years that have experienced strong heatwaves at most sites. Q_10,II retains strong associations with the timing of optimal temperature (r = 0.55), whereas Q_10,I becomes largely decoupled. Principal component analysis further confirms this divergence, with the two phase-specific sensitivities varying along orthogonal axes under heatwaves. We further analyzed individual sites to see how much each Q₁₀ phase contributed to the overall change in temperature sensitivity. Across sites, the temperature sensitivity of respiration in the post-heatwave phase accounts for an average of 71% of the total phase-specific Q₁₀ change induced by heatwaves. This reinforces the central role of Q_10,II in amplifying thermal hysteresis. Structural equation modeling shows that the heatwave-amplified thermal hysteresis in Re is fully mediated by Q_10,II decline, shaped by both thermal thresholds and their seasonal timing. These findings reveal a phase-asymmetric temperature sensitivity of Re that underpins thermal hysteresis, suggesting that short-term heatwaves could trigger prolonged carbon losses even under post-peak warming scenarios.