I don't have any figures, but on the assumption that few processes are 100% efficient I'd guess you wouldn't get complete decomposition before the fallen tree i

I don't have any figures, but on the assumption that few processes are 100% efficient I'd guess you wouldn't get complete decomposition before the fallen tree is buried. You'd probably get a reasonable, if inefficient, removal of carbon from the atmosphere. Probably better than nothing if you couldn't figure out a way of using the wood more effectively.