I have heard it preached as gospel that old-growth forests do not accumulate carbon as rapidly as new forests (or at all).
A new article in Nature, however, says that they do.
From the abstract:
Thus, our findings suggest that 15 per cent of the global forest area, which is currently not considered when offsetting increasing atmospheric carbon dioxide concentrations, provides at least 10 per cent of the global net ecosystem productivity.
Via Climate Feedback, which also links to this news story.