Stagioni.....in Amazzonia

[lowenz]

http://sciencenow.sciencemag.org/cgi...07/312/2?rss=1

It might be a cold day in the Amazon before snow invades the rainforest. But as far as seasons go, the jungle just got more diverse. A new study of the South American rainforest's leaf cover reveals growing evidence of seasonality among the evergreens. What's more, the leaves themselves may be driving the change, sparking the transition from the dry to wet seasons.

For years, the only two seasons in the Amazon were thought to be wet and wetter--a permanent spring for all intents and purposes. In the past decade, however, researchers have documented seasonal variations of new leaves, flowers, and fruits. But because of the vast extent of the Amazon rainforest basin--it's roughly the size of the continental United States--scientists have only been able to observe the jungle in small patches.

To get the big picture, geographer Ranga Myneni of Boston University in Massachusetts and colleagues compiled 5 years of data from NASA satellite images of the entire Amazon wilderness. The team found extreme variability in the region's leaf cover--or canopy--throughout the year. For the first half of the year, NASA images revealed an average of 3.5 layers of leaf cover--or leaf area per unit ground area; that figure increased to six layers during the second half of the year. The team reports its findings online this week in Proceedings of the National Academy of Sciences.

Climate appears to drive these changes, says Myneni. During the first half of the year, clouds and sheets of rain cover the lush rainforest, sheltering the plants from sunlight and causing trees drop more leaves than they produce. Following this wet season, things dry up, and the jungle begins to blossom. "[It's] as if plants know there are good times to come with lots of light," Myneni says.

The leaves aren't just a symptom of seasonal change, however, notes Myneni. In a process known as evapotranspiration, tree roots absorb rain from the soil, transfer the water through the trunk and up to the leaves, and then return it to the atmosphere, where it often falls back as rain. Thus, says Myneni, thick leaf cover during the Amazonian dry season sets up heavy rains for the wet season.

Ecologist Greg Asner of Stanford University in California is impressed with the sheer scale of the study. Detailing more than 7 million square kilometers stretching across eight South American countries "is a big step, technically," he says. "Six years ago, we couldn't do that."