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Abstract

replying to C. D. Holmes Nature507,http://dx.doi.org/10.1038/nature13113(2014)

Forests have become more efficient at using water over the past two decades1. A series of hypotheses exist to explain this trend, but the only credible explanation to date is a response to rising atmospheric CO2. Keenan et al.1 show that the observed trend is physiologically plausible, but is much larger than expected from conventional theory and experimental evidence. This has led to suggestions that processes other than increased atmospheric CO2 may have contributed to the observed trend2. One such process that has yet to be examined is the effect of tropospheric ozone on forest water-use efficiency (WUE). In the accompanying Comment3, Holmes reports that ozone concentrations have declined in the northeastern and midwestern USA by about 50% from 1995 to 2010. Using empirical relationships, he estimates that this decline could explain roughly 15% of the reported increase in WUE over North America, and a significantly lower proportion of the trend in Europe.

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Figure 1: Ozone and WUE at Harvard Forest.

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References

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Authors and Affiliations

  1. Department of Biological Sciences, Macquarie University, North Ryde, 2109, New South Wales, Australia

    Trevor F. Keenan

  2. USDA Forest Service, Northern Research Station, Durham, 03824, New Hamphire, USA

    David Y. Hollinger

  3. Department of Civil, Environmental and Geodetic Engineering, The Ohio State University, Columbus, 43210, Ohio, USA

    Gil Bohrer

  4. Department of Geography, Indiana University, Bloomington, 47405, Indiana, USA

    Danilo Dragoni

  5. School of Engineering and Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, 02138, Massachusetts, USA

    J. William Munger

  6. Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, IMK-IFU, Garmisch-Partenkirchen 82467, Germany,

    Hans Peter Schmid

  7. Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, 02138, Massachusetts, USA

    Andrew D. Richardson

Authors
  1. Trevor F. Keenan
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  2. David Y. Hollinger
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  3. Gil Bohrer
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  4. Danilo Dragoni
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  5. J. William Munger
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  6. Hans Peter Schmid
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  7. Andrew D. Richardson
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Corresponding author

Correspondence to Trevor F. Keenan.

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Keenan, T., Hollinger, D., Bohrer, G. et al. Keenan et al. reply. Nature 507, E2–E3 (2014). https://doi.org/10.1038/nature13114

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