Amazonian rainforest regulates its own rain

Since 1999 the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument aboard NASA’s Terra spacecraft has looked down on every cloud-free spot on Earth’s surface and created a daily map of the vegetation. One such map, of the green-leaf area per unit ground area in South America, is shown here. Boston University’s Ranga Myneni and his colleagues have analyzed five years of MODIS maps of the Amazon basin and found that the total leaf area of the rainforest’s trees varies annually by 25%. The amplitude is surprisingly large for the tropics. To identify its source, Myneni and his colleagues correlated the leaf area with sunlight and rainfall. More surprises emerged. The rainforest’s leaf area expands most vigorously during the Amazon’s sunny, dry season rather than during its cloudy, wet season. The growth spurt starts just before the sunny weather, as if the leaves anticipate the change in season. Then, when the leaf canopy is at its lushest, the rainy season starts, as if triggered by the water vapor and other gases that the leaves emit. Myneni believes the deep roots of the rainforest trees render the trees insensitive to the timing of the abundant rainfall. Decoupled from rainfall, the trees respond instead to variations in sunlight. (R. B. Myneni et al. , Proc. Natl. Acad. Sci. USA , in press.)