A new NASA study provides clarification of satellite data taken from above the Amazon Rainforest.

For the past eight years, scientists have been trying to figure out why some satellite data showed the rainforest "greening-up" during the Amazon's dry season each year from June to October. A green-up implies productive, thriving vegetation, which made no sense if it was happening in the dry season.

Writing in the journal Nature, the NASA scientists report that the green-up is not an unexplainable natural phenomenon, but an error in in how the satellites were observing the rainforest caused by caused by a combination of shadowing within the forest canopy and the way that the satellites observe the Amazon during the dry season.

After correcting for what NASA called an "artifact in the data" the researchers report the Amazon rainforest, on a large scale, maintains "a fairly constant greenness and canopy structure throughout the dry season."

"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," said Doug Morton, of NASA's Goddard Space Flight Center in Greenbelt, Md.

The data have implications for how scientists who study seasonal and interannual changes in Amazon forests and other ecosystems using satellite images.

"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, referring to the Moderate Resolution Imaging Spectroradiometer, sensors that fly aboard NASA's Terra and Aqua satellites make daily observations over the Amazon forests.

The time of year the MODIS system images the rainforest affects how the rainforest appears in the data, NASA said.

"Around the equinox, the MODIS sensor takes the 'perfect picture' with no shadows," Morton said. "The change in shadows is amplified in MODIS data because the sun is directly behind the sensor at the equinox. This seasonal change in MODIS greenness has nothing to do with how forests are changing."

After reassessing the data, the scientists report that Amazon, as a whole, doesn't change much through the dry season.

"Scaling our knowledge of forest canopies from measurements of individual leaves to satellite observations of the entire Amazon basin requires a deep understanding of both forest ecology and remote sensing science," Morton said. "This interdisciplinary collaboration is critical to improve our understanding of the patterns and processes driving changes in vegetation productivity."