I writing this quickly, so if it not completely clear, or comprehensive...I apologize in advance.
There are a lot of recent studies that show this (the dead material emitting more CO2 than the trees take in). I can't lay my hands on them right now, but the explanation -- for rain forests like the Amazon -- is that 50% of the dry trunk of tree (these are often 100+ feet tall and quite big around) is carbon. However, in dense old growth forests, 90% of the CO2 breathing part of the tree (where photosynthesis takes place and it gives off Oxygen) is in the first 15-20 feet of the canopy -- when you get 50 feet down from the canopy, only 2% exists.
When those trees falls, they more likely to cause the trees near it spread out and not be replaced by a new one of the same type...What sun does reach the floor usually just supports small plants that use less CO2 and emit less O2.
That 100+ foot tree -- and ALL the debris that the trees sluff off -- then emits it's stored carbon as it rots -- especially in very wet rain forests during rainy weather.
The dead material that flows into the streams and rivers gives off CO2 at an even more rapid pace. (see BBC report below on the Amazon that gives a small bit of the details regarding this)
'No solution' found in more trees
Planting trees to curb the effects of global warming is unlikely to work.
A US-Brazilian team has found that some parts of the Amazon rain forest emit more carbon dioxide (CO2) than they absorb in very wet conditions.
Their report, published in the journal Science, says previous studies have almost certainly overestimated how much CO2 the Amazon can take in.
And their study is backed by other work which shows newly planted trees will not grow fast enough to mop up CO2.
The Science papers are pertinent because the idea of using forests to curb global warming forms a central plank of the Kyoto Protocol, which world governments will discuss next week in Milan.
The protocol allows countries to plant new trees and conserve old forests rather than cut the amount of greenhouse gases they produce.
[SNIP]
Saleska's study of old-growth Amazonian rainforest shows clearly that drought or other disturbances that kill trees can lead to higher levels of carbon dioxide release.
These increases in carbon loss occur during wet seasons when the dead wood breaks down, not during the dry season as has been generally found.
Rest at --
http://news.bbc.co.uk/1/hi/sci/tech/3246938.stm