How do increasing carbon dioxide levels lead to more efficient water use?
The answer, Keenan said, is 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 tall towers extending above the forest canopy, allows researchers to determine how much carbon dioxide and water are going into or out of the ecosystem.
With more than 20 years of data, the towers in the Harvard Forest which have the longest continuous record 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
|Contact: Peter Reuell|