Researchers find that increased levels of carbon-dioxide in the atmosphere lead to more efficient water use

11 July 2013

A Macquarie University researcher, with a team from Harvard University, has found forests have become dramatically more efficient in how they use water as a result of increasing levels of atmospheric carbon-dioxide.

Dr Trevor Keenan, Macquarie University and Assistant Professor Andrew Richardson, Harvard University, along with colleagues from the USDA Forest Service, Ohio State University, Indiana University, and the Karlsruhe Institute of Technology in Germany, published their findings in Nature this month.

Using data collected from forests in the northeastern US and elsewhere, the research shows that forests across the globe were becoming far more efficient than predicted by even the most state-of-the-art computer models.

“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 can theoretically take more carbon out of the atmosphere due to higher growth rates.”

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

“While we might call this a positive effect of atmospheric carbon-dioxide, climate change itself is a different kettle of fish,” he said. “One of the largest concerns about carbon-dioxide is the effect it has on global temperatures. If carbon-dioxide levels keep rising, the increase in global temperatures by the end of this century is likely to be very large, and that will affect ecosystems all over the world, most likely to their detriment.”

How do increasing carbon-dioxide levels lead to more efficient water use?

The answer, Keenan said, lies in the way photosynthesis works. To take in the carbon-dioxide they need, plants open tiny pores, called stomata, on their leaves. As carbon-dioxide enters, however, water vapor is able to escape.

Higher levels of carbon-dioxide, however, mean the stomata don’t need to open as wide, or for as long, meaning the plants lose less water and grow faster. To take advantage of that fact, Keenan said, commercial growers have for years pumped carbon-dioxide into greenhouses to promote plant growth.

To test whether such a “carbon-dioxide fertilization effect” was taking place in forests, Keenan, Richardson and others turned to long-term data measured using a technique called eddy covariance. This method, which relies on sophisticated instruments mounted on towers above the forest canopy allows researchers to determine how much carbon dioxide and water are going into or out of the ecosystem for any given half hour.

With more than 20 years of data, the towers in the Harvard Forest – the longest running in the world – are an invaluable resource for studying how forests have responded to changes in atmospheric carbon-dioxide levels. Though more than 300 towers have since sprung up around the globe, many of the earliest – and hence the longest data record – are in the northeast U.S.

When Keenan, Richardson and colleagues began to examine those records, they found that forests were storing more carbon and becoming more efficient in how they used water. The phenomenon, however, wasn’t limited to a single region – when they examined long-term data sets from all over the world, the same trend was evident.

“We went through every possible hypothesis of what could be going on, and ultimately what we were left with is that the only phenomenon that could cause this type of global change in water-use efficiency is atmospheric carbon-dioxide,” Keenan said.

Going forward, Keenan 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.

“That will help us to better understand the long term change in forest water use efficiency,” 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?”

The research was supported by a grant from NOAA’s Climate Program Office, which funded the synthesis of data from research sites across the north-eastern US. Field measurements at these sites, which are part of the AmeriFlux network, have been funded by grants from the Department of Energy’s Biological and Environmental Research program, the National Science Foundation’s Long-Term Ecological Research program, and the USDA Forest Service’s Northern Research Station.

Article: Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise, Trevor F. Keenan, David Y. Hollinger, Gil Bohrer, Danilo Dragoni, J. William Munger, Hans Peter Schmid & Andrew D. Richardson, Nature (2013) Published online 10 July 2013

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