Field tests in the Amazon have for the first time measured daily and seasonal movements of soil moisture through the deep roots of trees. This water management, which enables the plants to maintain photosynthesis during the region's long dry season, significantly affects the area's climate, the new research suggests.
About two-thirds of the forested area in the Amazon basin experiences a marked dry season from July to November. Even so, rain forests there maintain their greenness year-round, says Jung-Eun Lee, an atmospheric scientist at the University of California, Berkeley. The humidity of the atmosphere just above the treetops in the dry season doesn't differ substantially from that during the rainy season, she notes.
The consistent humidity and photosynthesis probably stem from a botanical phenomenon called hydraulic redistribution, Lee and her colleagues speculate in an upcoming Proceedings of the National Academy of Sciences. Many deep-rooted plants transfer water from wet zones to dry ones, especially at night, says Lee. That capability has been noted in dozens of plant species in many ecosystems but had not been observed in the Amazon.
The team's experiments took place near the end of the Amazon's dry season. In the absence of rain, roots running parallel to the surface of the soil conveyed sap toward the tree's trunk in the daytime—a flow of moisture needed to support photosynthesis in the tree's leaves—but carried it away from the trunk at night. Both day and night, the sap in the tree's deep taproot flowed upward from the moister deep layers of soil to the drier, shallower layers, says Lee.
As the forest entered the rainy season, a 36-millimeter rainfall didn't change the direction of daytime sap flows. However, nighttime measurements showed sap in shallow roots flowing inward toward the trunk and taproot sap flowing downward. The latter flow transferred moisture into deep layers of soil where it could be tapped during the following dry season. The researchers estimate that over the course of a rainy season, trees could thus store about 10 percent of a site's rainfall.
Results of global-climate models that incorporate these estimates—and similar data from other regions of the world—indicate that hydraulic redistribution by plants has a big effect on climate. Some areas of the Amazon basin, the rain forests of sub-Saharan Africa, and Southeast Asia are, on average, 2°C cooler than they would be if the plants' influence weren't included, says Lee.
Climate models typically have a hard time replicating conditions in the Amazon during the dry season, says Gordon B. Bonan of the National Center for Atmospheric Research in Boulder, Colo. Lee's team has conducted "a very nice study" that adds yet more complexity for scientists to consider as they model climate.
Gordon B. Banon
National Center for Atmospheric Research
P.O. Box 3000
Boulder, CO 80307-3000
Department of Earth and Planetary Science
University of California, Berkeley
Berkeley, CA 94720