Clockwise or Counterclockwise: Hysteresis in the Response of Ecosystem Respiration to Temperature.

Ecosystem respiration (ER) often responds asymmetrically to seasonal warming and cooling, producing thermal hysteresis. Yet, its global patterns and drivers remain unknown. Here, we used daily ER data from 324 eddy-covariance sites worldwide and revealed two contrasting hysteresis patterns in the temperature response of ER: clockwise hysteresis (ER is higher during the warming than cooling season) at 173 sites and counterclockwise hysteresis (ER is higher during the cooling than warming season) at 151 sites. The direction and magnitude of ER hysteresis were largely governed by the seasonal timing mismatch between precipitation and vegetation-derived substrate availability. Higher substrate availability during the warming season enhanced ER and promoted clockwise hysteresis, whereas moisture limitation suppressed ER during the warming season and caused counterclockwise hysteresis. These findings provide robust evidence that ER is a complex process responding to the availability of carbon substrate and regulated differentially by seasonal temperature and precipitation. Our results highlight that the hysteresis of terrestrial carbon release is a key aspect of the global carbon cycle, with important implications for understanding and projecting land carbon-climate feedbacks.