This research was supported by the NOAA Climate Program Office, Global Carbon Cycle Program (award NA11OAR4310054) and the Office of Science (Biological and Environmental Research), US Department of Energy. G.B. acknowledges a grant from the National Science Foundation (grant number DEB-0911461). This work used eddy covariance data acquired by the FLUXNET community and in particular by the AmeriFlux, CarboEuropeIP and Fluxnet-Canada networks. AmeriFlux was supported by the US Department of Energy, Biological and Environmental Research, Terrestrial Carbon Program (grant numbers DE-FG02-04ER63917 and DE-FG02-04ER63911, DE-SC0006708) and Fluxnet-Canada was supported by CFCAS, NSERC, BIOCAP, Environment Canada and NRCan. We acknowledge financial support of the eddy covariance data harmonization provided by CarboEuropeIP, FAO-GTOS-TCO, iLEAPS, the Max-Planck Institute for Biogeochemistry, National Science Foundation, University of Tuscia, Université Laval and Environment Canada and US Department of Energy and of the database development and technical support from Berkeley Water Center, Lawrence Berkeley National Laboratory, Microsoft Research eScience, Oak Ridge National Laboratory, University of California-Berkeley, University of Virginia. We thank all those involved in the NACP Site Synthesis, in particular the modelling teams who provided model output. Research at the Bartlett Experimental Forest tower is supported by the National Science Foundation (grant DEB-1114804), and the USDA Forest Service’s Northern Research Station. Research at Howland Forest is supported by the Office of Science (BER), US Department of Energy. Carbon flux and biometric measurements at Harvard Forest have been supported by the Office of Science (BER), US Department of Energy and the National Science Foundation Long-Term Ecological Research programmes. We thank S. Ollinger and S. Frey for maintaining the long-term leaf nitrogen measurements at Harvard Forest, and B. Yang for providing gap-filled meteorological data for the regional focus sites.