Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms

• It is well established that individual organisms can acclimate and adapt to temperature to
optimize their functioning. However, thermal optimization of ecosystems, as an assemblage
of organisms, has not been examined at broad spatial and temporal scales.
• Here, we compiled data from 169 globally distributed sites of eddy covariance and quanti-
fied the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore
the underlying mechanisms.
• We found that the temperature response of NEE followed a peak curve, with the optimum
temperature (corresponding to the maximum magnitude of NEE) being positively correlated
with annual mean temperature over years and across sites. Shifts of the optimum temperature
of NEE were mostly a result of temperature acclimation of gross primary productivity (upward
shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration.
• Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably
reflecting associated evolutionary adaptation of organisms within ecosystems, and has the
potential to significantly regulate ecosystem–climate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in
changing environments and benchmarking global models.