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.
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."
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