Water use in forests improved with recent rise in carbon dioxide, finds study

A Harvard study shows forests have become dramatically more efficient in how they use water with a rise in carbon dioxide.

July 10, 2013 -- According to a new Harvard study, forests have become dramatically more efficient in how they use water over the last two decades with an increase in levels of atmospheric carbon dioxide.

Studies have long predicted that plants would begin to use water more efficiently as atmospheric carbon dioxide levels rose. A research team led by Research Associate Trevor Keenan and Assistant Professor of Organismic and Evolutionary Biology Andrew Richardson, however, has found that forests across the globe are becoming more efficient than expected.

Using data collected from forests in the northeastern U.S. and elsewhere around the world, Keenan and Richardson found increases in efficiency larger than those predicted by even the most state-of-the-art computer models. The research, which was done in collaboration with researchers from the Harvard's Department of Earth and Planetary Sciences, the USDA Forest Service, Ohio State University, Indiana University, and the Karlsruhe Institute of Technology in Germany, is described in a July 10 paper in "Nature."

"This could be considered a beneficial effect of increased atmospheric carbon dioxide," said Keenan, the first author of the paper. "What's surprising is we didn't expect the effect to be this big. A large proportion of the ecosystems in the world are limited by water -- they don't have enough water during the year to reach their maximum potential growth. If they become more efficient at using water, they should be able to take more carbon out of the atmosphere due to higher growth rates."

While increased atmospheric carbon dioxide may benefit forests in the short term, Richardson emphasized that the overall climate picture would remain grim if levels continue to rise.

"We're still very concerned about what rising levels of atmospheric carbon dioxide mean for the planet," Richardson cautioned. "There is little doubt that as carbon dioxide continues to rise -- and last month we just passed a critical milestone, 400 ppm, for the first time in human history -- rising global temperatures and changes in rainfall patterns will, in coming decades, have very negative consequences for plant growth in many ecosystems around the world."

Going forward, Keenan (now based at Macquarie University in Sydney, Australia) said he is working on a proposal to get access to data collected from even more sites around the world, including several that monitor tropical and Arctic systems.

"This larger dataset will help us to better understand the extent of the response we observed," he said. "That in turn will help us to build better models, and improve predictions of the future of the Earth's climate. Right now, all the models we have under-represent this effect by as much as an order of magnitude, so the question is: What are the models not getting? What do they need to incorporate to capture this effect, and how will that affect their projections for climate change?"

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