Hi,
what a huge question. Ihave spent 7 years on this area and done several pilots measuring and calculating this. You are right, it is a matter of material, cellulose, but first is it CO2 or C that you sequester - there a molecular weight conversion there.
The main issues relate to accurate measuremnt and thats modellinig by species and calculating the relative effect of crown, root and stem; the soil uptake, losses, fugitive losses (driving your car in to measure it), consistency over the forest, (site indexes) etc etc.
You can post here for questions if you like - you can also look at work on Kyoto Protocol and the EPA guidelines and the CDM (Clean development mechanism) as it relates to forestry. THere is some good modelling software for various species, and some work on add ons to the density software giving the carbon change over the life of a stand.
the 5 tonne/hectare you use would be OK for Pacific Pine, wrong on ecalypts, low for radiata pine, etc etc and is an average over the life of the forest. Also remember that a forest will enter a state of equilibrium at some point and may become a net emitter at another.
hope it helps
terry@carbonforce.com