An apparent greening-up of the Amazon rain forest during the region's dry season each year from June to October is in fact an optical illusion, scientists say.

The appearance of canopy greening suggesting productive, thriving vegetation in spite of limited rainfall is not caused by a biophysical change in Amazon forests, a study by NASA scientists found, but instead by a combination of shadowing within the canopy and the way that satellite sensors observe the Amazon during the dry season.

Amazon forests, at least on the large scale, in fact maintain a fairly constant greenness and canopy structure throughout the dry season, they said.

"Scientists who use satellite observations to study changes in Earth's vegetation need to account for seasonal differences in the angles of solar illumination and satellite observation," Doug Morton of NASA's Goddard Space Flight Center in Greenbelt, Md., said.

The researchers analyzed data from a number of satellites to try to determine why one type of instrument aboard NASA's Terra and Aqua satellites was reporting increased "greenness."

The Moderate Resolution Imaging Spectroradiometer or MODIS instrument detects near-infrared light reflected from vegetation.

"We think we have uncovered the mechanism for the appearance of seasonal greening of Amazon forests -- shadowing within the canopy that changes the amount of near-infrared light observed by MODIS," Morton said.

At the start of the dry season in June, when the sun is as low and far north as it will get, shadows are abundant. By September, around the time of the equinox, Amazon forests at the equator are illuminated from directly overhead.

This means the forest canopy is largely shadow-free, highly reflective in the infrared, and therefore very green according to some satellite data, the researchers said.