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The Increasing Water-Use Efficiencies of Temperate and Boreal Forests of the Northern Hemisphere Over the Past Two Decades
Volume 16, Number 46: 13 November 2013

In an important new study, Keenan et al. (2013) documented and analyzed recent trends in the inherent water-use efficiencies (Wei) of forest canopies, using direct and continuous long-term measurements of CO2 and water vapor fluxes, while focusing their attention on seven sites in the midwestern and northeastern United States, where the studied forests (1) represent tree species typical of the two regions and (2) are not actively managed. And upon the completion of this endeavor, they compared their results with those derived from data obtained by others from 14 additional temperate and boreal forest sites. So what did they learn?

The seven scientists say they found "a substantial increase in water-use efficiency in temperate and boreal forests of the Northern Hemisphere over the past two decades." But more than that, they systematically assessed various competing hypotheses to explain this trend; and in doing so, they found that "the observed increase is most consistent with a strong CO2 fertilization effect," because of the fact, as they describe it, that "of all the potential drivers of the observed changes in Wei, the only driver that is changing sufficiently and consistently through time at all sites is atmospheric CO2."

Keenan et al. additionally note that "the direct tradeoff between water loss and carbon uptake through the stomata means that, as water-use efficiency increases, either evapotranspiration decreases or gross photosynthetic carbon uptake increases, or both occur simultaneously." And in this regard, they indicate that "increases in Wei may account for reports of global increases in photosynthesis (Nemani et al., 2003), forest growth rates (Lewis et al., 2009; Salzer et al., 2009; McMahon et al., 2010), and carbon uptake (Ballantyne et al., 2012)," which observations led them to further suggest that "rising atmospheric CO2 is having a direct and unexpectedly strong influence on ecosystem processes and biosphere-atmosphere interactions in temperate and boreal forests."

Of particular interest in his regard is the recent (14 August 2013) statement of California's Save the Redwoods League, in which they indicate that as a result of a major research effort in which they have been engaged, it has been learned that (1) "changing environmental conditions are triggering a growth spurt in coast redwoods and giant sequoias," and that (2) "coast redwoods in a few Northern California old-growth forests produced more wood since the 1970s than ever before in their millennial lifespans," which findings mesh nicely with the conclusion of Keenan et al., i.e., that rising atmospheric CO2 is having a direct and unexpectedly strong influence on forest water use efficiencies and growth rates throughout the world.

Sherwood, Keith and Craig Idso

Ballantyne, A.P., Alden, C.B., Miller, J.B., Tans, P.P. and White, J.W.C. 2012. Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years. Nature 488: 70-72.

Keenan, T.F., Hollinger, D.Y., Bohrer, G., Dragoni, D., Munger, J.W., Schmid, H.P. and Richardson, A.D. 2013. Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise. Nature: 10.1038/nature12291.

Lewis, S.L., Lopez-Gonzalez, G., Sonke, B., Affum-Baffoe, K., Baker, T.R., Ojo, L.O., Phillips, O.L., Reitsma, J.M., White, L., Comiskey, J.A., Djuikouo K., M.-N., Ewango, C.E.N., Feldpausch, T.R., Hamilton, A.C., Gloor, M., Hart, T., Hladik, A., Lloyd, J., Lovett, J.C., Makana, J.-R., Malhi, Y., Mbago, F.M., Ndangalasi, H.J., Peacock, J., Peh, K. S.-H., Sheil, D., Sunderland, T., Swaine, M.D., Taplin, J., Taylor, D., Thomas, S.C., Votere, R. and Woll, H. 2009. Increasing carbon storage in intact African tropical forests. Nature 457: 1003-1006.

McMahon, S.M., Parker, G.G. and Miller, D.R. 2010. Evidence for a recent increase in forest growth. Proceedings of the National Academy of Sciences USA 107: 3611-3615.

Nemani, R.R., Keeling, C.D., Hashimoto, H., Jolly, W.M., Piper, S.C., Tucker, C.J., Myneni, R.B. and Running. S.W. 2003. Climate-driven increases in global terrestrial net primary production from 1982 to 1999. Science 300: 1560-1563.

Salzer, M., Hughes, M., Bunn, A., Kipfmueller, K. 2009. Recent unprecedented tree-ring growth in bristlecone pine at the highest elevations and possible causes. Proceedings of the National Academies of Science USA 106: 20,346-20,353.