The Increasing Water-Use Efficiencies of Temperate and Boreal Forests of the Northern Hemisphere Over the Past Two Decades
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.
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 statement of California's Save the Redwoods League (14 August 2013), 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.
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.
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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.
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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.