Amazon doesn’t actually go green in dry seasons

An optical illusion may explain why Amazon forests appear greener during dry seasons. 

Satellites observing the forests’ canopies have picked up signs that leaf production cranks up when moisture is scarce. But the trick of light means that Amazon forests might be more vulnerable to drought than scientists suspected. The new analysis of satellite data appears February 5 in Nature.

The study also revamps the forests’ role in carbon storage, says Eric Davidson, a forest ecologist at the Woods Hole Research Center in Falmouth, Mass., who was not involved with the work.

Since Amazon forests looked so green in dry seasons, some scientists believed trees might be pulling extra carbon from the air to make more leaves. But, Davidson says, “this paper shoots a hole in that theory.”

Another hole comes from recent carbon measurements taken above the forest canopy. In a second paper, published in the Feb. 6 Nature, researchers report that Amazon forests lost carbon in 2010, a dry year, but not in 2011, a wet year.

Both the new satellite analysis and the carbon measurements support the idea that Amazon forests grow better in rainy seasons than in dry seasons, a matter of longstanding debate.

“Intuitively you would think Amazon forests would do better in the rainy season,” Davidson says. But when researchers previously took ground-based measurements of carbon taken up or released within small forest plots, they discovered “just the opposite of what you might expect.”Amazon forests seemed to take in more carbon during dry seasons.

To explain the quirk, scientists hypothesized that rainy seasons’ extra moisture might not be so important for growth. Perhaps forests instead take advantage of the dry seasons’ extra sunlight and tap into deep roots for water.

Satellite data backed the idea up. The satellites track sunlight that bounces off the forest canopy. Leaves reflect a lot of near-infrared light, and young leaves reflect more than old ones. During dry periods, the satellites saw more of this light, leading some to believe the forest was thriving.

“I like to call this an infrared herring,” says Douglas Morton, an earth scientist at NASA Goddard Space Flight Center in Greenbelt, Md.

The light surge that the satellites had observed was actually a figment created by fewer shadows, Morton and colleagues discovered. During the dry season, the sun climbs higher in the sky, casting shorter and shorter shadows, until it’s almost directly overhead and behind the satellites. In this position, the satellites’ sensors can snap clear images of the canopy, with few shadows.

This lack of shadows lets the sensors capture more light, giving forests an apparent boost in greenery.

To investigate the phenomenon, Morton and colleagues built a 3-D computer simulation of an Amazon forest that grew more leaves during dry seasons. Then the team figured out what satellites watching the faux forest would see over time, and the researchers compared those data with real satellite data. The simulated and real datasets didn’t match.

A shadowing effect is the only way to explain the satellites’ observations, Morton says.

The rise in carbon uptake that researchers had previously measured during dry seasons could be due not to increased leaf growth but to dried-out soil microbes. In wet times, these microbes break down plant matter, releasing carbon into the air. But they may be less active in dry soil, which would appear to alter the forests’ carbon balance in small-scale measurements.

Still, the new analysis doesn’t count out the idea of Amazon forest growth during dry seasons, says Scott Saleska, a forest ecologist at the University of Arizona in Tucson. Satellite images focus on large swaths of forest, so it can be tricky to match smaller-scale measurements made on the ground with observations made from space, he says.