A temperature threshold to identify the driving climate forces of the respiratory process in terrestrial ecosystems

Abstract. Terrestrial ecosystem respiration (R_e) is the major source of CO₂ release and constitutes the second largest carbon flux between the biosphere and atmosphere. Therefore, climate-driven changes of R_e may greatly impact on future atmospheric CO₂ concentration. The aim of this study was to derive an air temperature threshold for identifying the driving climate forces of the respiratory process in terrestrial ecosystems within different temperature zones. For this purpose, a global dataset of 647 site-years of ecosystem flux data collected at 152 sites has been examined. Our analysis revealed an ecosystem threshold of mean annual air temperature (MAT) of 11 ± 2.3 °C. In ecosystems with the MAT below this threshold, the maximum R_e rates were primarily dependent on temperature and respiration was mainly a temperature-driven process. On the contrary, in ecosystems with the MAT greater than 11 ± 2.3 °C, in addition to temperature, other driving forces, such as water availability and surface heat flux, became significant drivers of the maximum R_e rates and respiration was a multi-factor-driven process. The information derived from this study highlight the key role of temperature as main controlling factor of the maximum R_e rates on a large fraction of the terrestrial biosphere, while other driving forces reduce the maximum R_e rates and temperature sensitivity of the respiratory process. These findings are particularly relevant under the current scenario of rapid global warming, given that the potential climate-induced changes in ecosystem respiration may lead to substantial anomalies in the seasonality and magnitude of the terrestrial carbon budget.