throughout the year provides us with an example
of this process. The research showed that rivers are
not in carbon equilibrium, because they typically
release more carbon than they acquire. Carbon loss
is seasonal and varies among mainstream floodplain,
mainstream channel, tributaries over 100 m (328 ft)
wide, and streams less than 100 m wide. The high level
of river outgassing observed in this study shows that
outgassing obviously tempers the efficacy of tropical
forests as carbon sinks, moving them closer to being,
at best, in carbon equilibrium.
Drought Sensitivity of Tropical
Forests
It may seem surprising to invoke drought as an important
variable in ecosystem function in tropical lowland
humid forests. After all, don’t we call them rain forests?
But tropical moist forests are seasonal, experiencing
dry seasons of varying intensity. El Niño/Southern
Oscillation (ENSO) events, which produce droughts,
substantially add to that variability (chapter 2). The
2015– 16 ENSO event produced a protracted dry season
in Panama and other parts of the tropics, resulting in
a vastly reduced rainy season. Historical long- term
droughts are known throughout tropical regions. Given
that tropical lowland forest plant species are adapted for
high moisture levels, it should be expected that drought
would exert a strong effect on these ecosystems.
To what degree do tropical plant species vary in
drought sensitivity? If climate change and increasingly
frequent ENSO events result in more frequent and
severe droughts, what sorts of changes in community
composition might result? Ecologists have learned that
there is strong variation in drought sensitivity among
plant species, such that a prolonged drought could
change the species composition of a forest as well as
alter its net primary productivity. In times of drought
NPP is reduced, and that is yet another factor in
tempering the concept of rain forest as a carbon sink.
The Amazonian Drought of 2005: Some
Surprises
A severe and widespread drought occurred over
Amazonia in 2005. The worst of the drought occurred
in dry season, July through September, and was
focused in southwestern and central Amazonia. The
expectation would be that such an event would alter
the pattern of carbon flux and primary productivity
throughout the region. This happened, but not as
predicted. A study led by Scott Saleska using data from
the Moderate Resolution Imaging Spectroradiometer
(MODIS) carried on NASA’s Te r r a satellite produced
surprising results. The MODIS program enabled
measurement of leaf area and chlorophyll content,
which allowed the researchers to track the patterns of
net primary productivity.
The satellite data showed an increasing greening of
the region during the drought. What? The expectation
was clearly that drought would limit water availability
and therefore reduce photosynthesis. But that did not
happen, and increased greening was, to put it mildly, a
surprise. But maybe it should not have been. One factor
to be considered is that tropical trees have extensive,
efficient root systems that capture groundwater, so
perhaps they were less water- stressed than expected.
Of even more importance, drought reduced rainfall,
which, in turn, reduced cloud cover. More light, more
NPP. Increased irradiance may have been the principal
cause of the enhanced greening, and thus rain forest
function may be relatively resilient to short- term
droughts, severe though they may be. What of carbon
flux? Satellite data showed only some of what was
happening during the drought. The forest may have
been greener, but it actually lost carbon.
A team of 67 researchers led by Oliver Phillips made
it their goal to document the effect of the drought on
carbon flux. The study was conducted on permanentPlate 5- 9. Methane bubbles the water as it outgasses from a
river in Brazil. The river sediment was stirred up with a canoe
paddle, and the resultant release of methane momentarily
supported a flame. Methane represents a significant amount
of carbon released by rivers. Photo by John Kricher.78 chapter 5 sun plus rain equals rain forest