The New Neotropical Companion

dry season, but respiration was 40% higher at the peak

of wet season (March) than it was at the peak of dry

season (November). Of most importance, release of

carbon dioxide from dead wood (through decomposer

activities) exceeded the uptake of carbon by live

biomass. Carbon was captured mostly by growth of

small trees in forest gaps (plate 5- 6). When dead trees

dropped and decomposed, carbon was released from

them, their loss opened gaps, and that stimulated

growth of young trees.

Carbon Loss from Deforestation and Fire

If tropical forests are cut and burned, an immense

amount of carbon is quickly released back into the

atmosphere (plate 5- 7). One study by Emilio Moran

and colleagues concluded that 336 million tons of

carbon emissions enter the atmosphere each year

from deforestation in Brazil alone. Another study,

by M. Holloway, concluded that about 220 tons of

carbon is released from soil and woody biomass to

the atmosphere for every hectare of tropical forest

that is cleared and burned. The loss of carbon to the

atmosphere is an unavoidable outcome when tropical

forests are cut and converted to ecosystems with less

biomass and lower overall primary productivity. Large-

scale deforestation obviously increases the potential

for a huge loss of carbon from tropical ecosystems,

reducing their collective efficacy as carbon sinks.

Carbon Loss from River Outgassing

We must consider the metabolism of riverine ecosystems

in order to attain a full understanding of carbon flux in

forests. Rivers and marshes are ubiquitous throughout

the tropics, particularly in Amazonia. The metabolism of

these systems is closely associated with inputs of organic

and other matter from bordering terrestrial ecosystems.

This is because organic matter from forests is transported

by flood and normal precipitation (as well as natural leaf

and branch fall) from terrestrial to riverine ecosystems.

This allochthonous (externally produced) organic matter

forms an energy base for river- dwelling decomposers

ranging from fish to bacteria, the result of which is the

eventual release of carbon back into the atmosphere. It is

true that rivers contain a diversity of primary producers

that fix carbon (providing autochthonous input). But

does their combined activity capture more carbon than

is released?

Studies that measure the amount of carbon dioxide

emitted from Amazonian rivers and wetlands suggest

that outgassing (also known as degassing or evasion)

may be important in returning carbon to the atmosphere

(plate 5- 8). Outgassing, which involves release of carbon

dioxide and methane in gaseous form, represents a net

loss of carbon via riverine metabolism. Data show that

carbon loss by outgassing is far in excess of what could

have been synthesized within the rivers and wetlands

themselves. This means that allochthonous material from

surrounding forest that washes into and accumulates in the

riverine ecosystems forms the basis of metabolic activities

of decomposers within the rivers and wetlands. Much of

that carbon is ultimately released to the atmosphere both

as carbon dioxide and as methane (plate 5- 9).

A study by Jeffrey Richey and colleagues that

focused on carbon flux in rivers in central Amazonia

Plate 5- 8. Outgassing by some tropical rivers reduces the

efficacy of forests as carbon sinks. Photo by John Kricher.

Plate 5- 7. Fires severely reduce the potential for forests to

act as carbon sinks, instead returning carbon dioxide to the

atmosphere. Photo by John Kricher.

chapter 5 sun plus rain equals rain forest 77