Botanists have long understood the behavior of temperate forests. In the autumn, trees lose their leaves; in the spring, they grow a fresh set. In rainforests around the equator, where seasons fall into categories of dry and wet and forests are green year round, forest behavior is trickier to discern. In early 2007, for instance, scientists studying the Amazon uncovered a surprising quirk in the way rainforests behave that they hadn’t noticed before.
Deprived of water, a houseplant withers and dies. Extending that logic to the outside world would suggest that a rainforest should thrive in the rainy season and suffer in the dry season. However, the picture emerging from measurements of leaf area in the world’s largest intact rainforest, the Amazon, suggests the opposite. This image shows seasonal shifts in the leaf area of the Amazon based on data collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.
Leaf area index is the ratio of leaf surface area (one side) to ground area. Values higher than 1 indicate vegetation has multiple, overlapping layers of leaves. In this image, red indicates areas where leaf area during the dry season was less than the annual average. Green indicates places where dry-season leaf area was greater than the annual average. Green predominates, overwhelming the tiny patches of red along the eastern edge and southern tip of the rainforest. Amazon leaf area is apparently greater in the dry season than the wet. (See our previous image of the day for a multi-year average leaf area map of the Amazon.)
How an area as massive as the Amazon could hide such a secret for so long seems puzzling. But the area is as remote as it is vast. Perhaps more importantly, the rainforest’s behavior is more subtle than that of a temperate forest. Trees don’t drop or grow leaves all at once. Instead, leaf loss and re-growth are staggered, and individual trees often operate on their own schedules. Another nuance is that the forest doesn’t simply gain leaves in response to more sunlight and lose them in the rain. The researchers found that leaf area actually started increasing late in the wet season, as if the trees could anticipate the arrival of the sunnier dry season.
To learn more about the discovery of seasonal leaf area patterns in the Amazon Rainforest, including how these changes themselves might trigger the rain, please read the Earth Observatory’s feature story The Amazon’s Seasonal Secret.