Does an old growth forest ecosystem produce more oxygen than it consumes?

Question

I would like to consider an old growth forest ecosystem such as a large part of the Amazonian forest. It is common to refer to such beautiful forests as a source of oxygen for the world. For example, the expression Amazon: Lungs of the planet is very common (e.g. this BBC article). I would like to question such an expression.

Questions

My questions are:

• Does an old growth forest ecosystem produce more oxygen than it consumes?
• Does an old growth forest ecosystem fix more CO$_2$ than it produces?

My Intuition

I understand that such a forest can be an important carbon storage and that if we were to cut the forest down, burn the wood and pour concrete on the soil (to prevent regrowth), then we would definitely release a fair amount of carbon in the atmosphere.

That being said, I am wondering whether, as of today, these forests act as carbon sink or not. It feels to me that while trees accumulate carbon and produce oxygen while growing, this carbon is released when they die and in an old growth forest a tree must die for a new tree to grow. If carbon is being fixed, then it has to go somewhere and I think it is almost by the definition of an old growth forest that there is no more room for more carbon accumulation (competition among trees prevent it). So I would tend to think that calling the Amazonian forest the "lungs of Earth" is very misleading.

Some random WWF article

This WWF article seem to suggest that, contrary to my intuition, the answer to these two questions should be "yes".

Answer 1

Does an old growth forest ecosystem fix more CO2 than it produces?

The intergovernmental panel on climate change summarizes some of the data on carbon flux here. For the amazon similar tropical rain forests, it says:

Measurements in pristine, seasonal tropical rain forests in Amazonia indicate NEP of approximately 1.0 t C ha-1 yr-1 (Grace et al., 1995a,b) and approximately 2.0 and 5.9 t C ha-1 yr-1 for dense, moist rain forest (Fan et al., 1990; Malhi et al., 1998, 1999).

NEP is net ecosystem production, which, as I understand it, is essentially the total amount of carbon fixed by the ecosystem.

It seems the tropics, and the Amazon, in particular, are important global carbon sinks. This is also discussed in this more recent article in Nature.

This isn't my field, but as far as I understand it, forests are not closed or steady state systems. They do play a role as a sink in the global carbon cycle. I would assume net carbon fixation must come with increased biomass, so I may have just avoided a key part of your question. All the same, I think the point is that, again, even the amazon rain forest is not at steady state.

As far as oxygen is concerned, the report and article don't address it. I suspect this is because there is no documented problem with global oxygen depletion.