Probably worth considering pyrolysis before burying it. By heating the wood and baking it into char you get a few nice benefits:

* You produce biochar, which is almost pure carbon, and therefore you are only sequestering the weight/volume of the carbon, not all the water and other stuff.

* Biochar can be used as a soil amendment to improve microbial cultures and store carbon for hundreds or thousands of years

* You can produce energy in the form of heat or wood gas

* You can produce wood vinegar, which may have some agricultural or industrial uses

I think there's a nascent industry in converting biomass into biochar and sequestering it, but it would be very interesting to see it done with forestland more generally.