That depends on how the tree decays. There is a lot of coal near the surface of the western US because prairie grass burned yearly in every dry month (winter when it was too cold to grow - they may still be as much rain/snow!). This yearly burning didn't get warm enough to burn all the carbon and so the rest got deposited as charcoal.

Moral of the story: you should start regular forest/grass fires to turn the stored carbon into charcoal. A biologist can give you a number of other reasons why this is good for the forest as well: common knowledge about forest fires is almost completely false.