Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

NASA Study Points to Infrared-Herring in Apparent Amazon Green-Up

07.02.2014
For the past eight years, scientists have been working to make sense of why some satellite data seemed to show the Amazon rain forest "greening-up" during the region's dry season each year from June to October. The green-up indicated productive, thriving vegetation in spite of limited rainfall.

Now, a new NASA study published today in the journal Nature shows that the appearance of canopy greening is not caused by a biophysical change in Amazon forests, but instead by a combination of shadowing within the canopy and the way that satellite sensors observe the Amazon during the dry season.


This natural-color image shows the importance of sun-sensor geometry. On the left, sunlight is backscattered by the Amazon rain forest, making green leaves appear brighter in some areas. To the right, sunglint makes the dark waters of the Amazon River and flooded wetlands appear silver or white. Image Credit: NASA's Earth Observatory

Correcting for this artifact in the data, Doug Morton, of NASA's Goddard Space Flight Center in Greenbelt, Md., and colleagues show that Amazon forests, at least on the large scale, maintain a fairly constant greenness and canopy structure throughout the dry season. The findings have implications for how scientists seek to understand seasonal and interannual changes in Amazon forests and other ecosystems.

"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," Morton said.

Isolating the apparent green-up mechanism

The MODIS, or Moderate Resolution Imaging Spectroradiometer, sensors that fly aboard NASA's Terra and Aqua satellites make daily observations over the huge expanse of Amazon forests. An area is likely covered in green vegetation if sensors detect a relatively small amount of red light – absorbed in abundance by plants for photosynthesis – but see a large amount of near-infrared light, which plants primarily reflect. Scientists use the ratio of red and near-infrared light as a measure of vegetation "greenness."

Numerous hypotheses have been put forward to explain why Amazon forests appear greener in MODIS data as the dry season progresses. Perhaps young leaves, known to reflect more near-infrared light, replace old leaves? Or, possibly trees add more leaves to capture sunlight in the dry season when the skies are less cloudy.

Unsettled by the lack of definitive evidence explaining the magnitude of the green-up, Morton and colleagues set out to better characterize the phenomenon. They culled satellite observations from MODIS and NASA's Ice Cloud and land Elevation Satellite (ICESat) Geosciences Laser Altimeter System (GLAS), which can provide an independent check on the seasonal differences in Amazon forest structure.

The team next used a theoretical model to demonstrate how changes in forest structure or reflectance properties have distinct fingerprints in MODIS and GLAS data. Only one of the hypothesized mechanisms for the green-up, changes in sun-sensor geometry, was consistent with the satellite observations.

"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.

Seeing the Amazon in a new light

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. At this point the forest canopy is shadow-free, highly reflective in the infrared, and therefore very green according to some satellite vegetation indices.

"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."

In fact, accounting for the changing geometry between the sun and satellite sensor paints a picture of the Amazon that, as a whole, doesn't change much through the dry season.

"Additional work is needed to verify these results with field measurements, and to explore the influence of drought on corrected vegetation indices," said Scott Goetz, an ecologist at Woods Hole Research Center in Woods Hole, Mass., who was not involved with the Nature study. "But past interpretations of productivity changes need to be reconsidered in light of these new results."

Looking forward, Morton sees the results as a reminder and opportunity for remote sensing scientists to work more closely with ground-based ecosystem scientists.

"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."

Kathryn Hansen
NASA's Earth Science News Team

Kathryn Hansen | EurekAlert!
Further information:
http://www.nasa.gov

More articles from Earth Sciences:

nachricht Past and present sea levels in the Chesapeake Bay Region, USA
29.07.2015 | Geological Society of America

nachricht “Carbon sink” detected underneath world’s deserts
29.07.2015 | American Geophysical Union

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: On the crest of the wave: Electronics on a time scale shorter than a cycle of light

Physicists from Regensburg and Marburg, Germany have succeeded in taking a slow-motion movie of speeding electrons in a solid driven by a strong light wave. In the process, they have unraveled a novel quantum phenomenon, which will be reported in the forthcoming edition of Nature.

The advent of ever faster electronics featuring clock rates up to the multiple-gigahertz range has revolutionized our day-to-day life. Researchers and...

Im Focus: Superfast fluorescence sets new speed record

Plasmonic device has speed and efficiency to serve optical computers

Researchers have developed an ultrafast light-emitting device that can flip on and off 90 billion times a second and could form the basis of optical computing.

Im Focus: Unlocking the rice immune system

Joint BioEnergy Institute study identifies bacterial protein that is key to protecting rice against bacterial blight

A bacterial signal that when recognized by rice plants enables the plants to resist a devastating blight disease has been identified by a multi-national team...

Im Focus: Smarter window materials can control light and energy

Researchers in the Cockrell School of Engineering at The University of Texas at Austin are one step closer to delivering smart windows with a new level of energy efficiency, engineering materials that allow windows to reveal light without transferring heat and, conversely, to block light while allowing heat transmission, as described in two new research papers.

By allowing indoor occupants to more precisely control the energy and sunlight passing through a window, the new materials could significantly reduce costs for...

Im Focus: Simulations lead to design of near-frictionless material

Argonne scientists used Mira to identify and improve a new mechanism for eliminating friction, which fed into the development of a hybrid material that exhibited superlubricity at the macroscale for the first time. Argonne Leadership Computing Facility (ALCF) researchers helped enable the groundbreaking simulations by overcoming a performance bottleneck that doubled the speed of the team's code.

While reviewing the simulation results of a promising new lubricant material, Argonne researcher Sanket Deshmukh stumbled upon a phenomenon that had never been...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

3rd Euro Bio-inspired - International Conference and Exhibition on Bio-inspired Materials

23.07.2015 | Event News

Clash of Realities – International Conference on the Art, Technology and Theory of Digital Games

10.07.2015 | Event News

World Conference on Regenerative Medicine in Leipzig: Last chance to submit abstracts until 2 July

25.06.2015 | Event News

 
Latest News

Surprising similarity in fly and mouse motion vision

30.07.2015 | Life Sciences

Efficient Infrared Heat Saves Time and Energy in the Manufacture of Motor Vehicle Carpets

30.07.2015 | Trade Fair News

Roentgen prize goes to Dr Eleftherios Goulielmakis

30.07.2015 | Awards Funding

VideoLinks
B2B-VideoLinks
More VideoLinks >>>