Earlier this year, Dawson's colleague and former UC Berkeley doctoral student Rafael Oliveira of the Laboratório de Ecologia Isotópica at the University of Sao Paulo, Brazil, discovered that Amazonian trees also use hydraulic redistribution to maintain the moisture around their shallow roots during the long dry season. During the wet season, these plants can store as much as 10 percent of the annual precipitation as deep as 13 meters (43 feet) underground, to be tapped during the dry months.
"These trees are using their root system to redistribute water into different soil compartments," Dawson said. "This allows the trees and the forest to sustain water use throughout the dry season."
The process is a passive one, he noted, driven by chemical potential gradients, with tree roots acting like pipes to allow water to shift around much faster than it could otherwise percolate through the soil. In many plants that exhibit hydraulic redistribution, the tap roots are like the part of an iceberg below water. In some cases these roots can reach down more than 100 times the height of the plant above ground. Such deep roots make sense if their purpose is to redistribute water during the dry season for use by the plant's shallow roots, though Dawson suspects that the real reason for keeping the surface soil moist is to make it easier for the plant to take in nutrients.
"Hydraulic redistribution is definitely related to water, but it can't really be discussed outside the context of plant nutrition," he said.
Dawson, Lee and Fung set out to incorporate hydraulic distribution in the National Center for Atmospheric Research Community Atmospheric Model Version 2 (NCAR's CAM2 model), one of the most respected models.
"Global climate models don't do a very good job of capturing plant effects on how climate might behave," Lee said.
Lee accounted both for daily and seasonal dryness in the Amazon, and showed that the two toge
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Source:University of California - Berkeley