some areas) than they capture, making them potential
carbon sources rather than carbon sinks.
Tropical Forests: Potential Carbon
Sinks?
Because of atmospheric rise in carbon dioxide that
correlates with industrialization and fossil fuel usage,
it has been proposed that lowland tropical rain forest
may act as a carbon sink, taking up and storing “excess”
carbon dioxide added to the atmosphere through
human activities (and thus reducing the atmospheric
accumulation of this important greenhouse gas). But
the question is, how much of this additional carbon
actually becomes sequestered in rain forests? That is
not an easy question to answer.
One reason for the potential of tropical forest to store
carbon is the sizes of the trees (plate 5- 5). They are big,
and there are lots of them. Ecological mathematical
models (applied to Amazon forests) that incorporate
such variables as maximum tree size, wood density,
wood decomposition, recruitment (the growth of new
trees from seeds), growth, and mortality of trees have
indicated that if net primary production increased due
to added atmospheric carbon dioxide, the production
of wood would continue, and that Amazonian forests
could potentially act as an important carbon sink. It
sounds logical: big trees, long growing season, and
high gross productivity combine to make a perfect
machine to act as a carbon sink. But not so fast. The
devil is in the details, and the details are not very clear.
There are numerous sources of potential carbon loss in
rain forest metabolism. Here’s what some of them are.
Seasonal Flux and Carbon Loss
Tropical rain forests are not always gaining carbon.
A study conducted by Scott Saleska and colleagues in
two old- growth forest sites in Tapajós National Forest
near Santarém, Brazil, showed a seasonal pattern of
carbon gain and loss. The annual rainfall is 192 cm
(76 in), and the wet season (>10 cm/4 in rainfall per
month) lasts for seven months. Researchers examined
changes in forest structure and monitored carbon
stored in live wood and dead wood. The results were
surprising. Net carbon gains occurred during the dry
season, but carbon loss happened in wet season. Gross
primary production was about the same in wet and
Plate 5- 6. Is this tropical forest gaining or losing carbon? Look
carefully and notice that it is a young forest with many small
trees. As such it is growing rapidly and gaining carbon in the
form of incremental biomass. Photo by John Kricher.Plate 5- 5. The immense amount of wood in tropical forests is
why they seem to be ideal potential carbon sinks. Photo by
John Kricher.76 chapter 5 sun plus rain equals rain forest