Terrestrial plants remove CO2 from the atmosphere through photosynthesis, a process that is accompanied by the loss of water vapour from leaves1. The ratio of water loss to carbon gain, or water-use efficiency, is a key characteristic of ecosystem function that is central to the global cycles of water, energy and carbon2. Here we analyse direct, long-term measurements of whole-ecosystem carbon and water exchange3. We find a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades. We systematically assess various competing hypotheses to explain this trend, and find that the observed increase is most consistent with a strong CO2 fertilization effect. The results suggest a partial closure of stomata1—small pores on the leaf surface that regulate gas exchange—to maintain a near-constant concentration of CO2 inside the leaf even under continually increasing atmospheric CO2 levels. The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models. The increase is associated with trends of increasing ecosystem-level photosynthesis and net carbon uptake, and decreasing evapotranspiration. Our findings suggest a shift in the carbon- and water-based economics of terrestrial vegetation, which may require a reassessment of the role of stomatal control in regulating interactions between forests and climate change, and a re-evaluation of coupled vegetation–climate models.
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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.
The authors declare no competing financial interests.
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Keenan, T., Hollinger, D., Bohrer, G. et al. Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise. Nature 499, 324–327 (2013). https://doi.org/10.1038/nature12291