Exponential or Unimodal Relationships Between Nighttime Ecosystem Respiration and Temperature at the Eddy Covariance Flux Tower Sites

Ecosystem respiration is a key flux in the terrestrial carbon cycle and is affected substantially by temperature. This work analysed the time series data of nighttime net ecosystem exchange of carbon dioxide (NEE_night) from 196 FLUXNET2015 sites to re-evaluate the relationships between NEE_night and temperature. A total of 93 sites (48%) were identified to have a unimodal relationship between NEE_night and temperature. Site-specific apparent optimum temperature parameters were then estimated at these sites. We further assessed the impacts of using exponential or unimodal equations on NEE_night predictions. The predicted NEE_night values at high temperatures were substantially higher from the exponential-type equations (mean: ~200%) than from the unimodal equation (mean: ~30%), compared to the observed NEE_night. This study calls for using a unimodal equation to predict NEE_night (often considered as nighttime ecosystem respiration, ER_night), which could substantially improve the accuracy and reduce uncertainty in ER estimates, in particular under the scenario of global warming.