Planted should drop 1 seed per. Or alternately drop a seed or two every N days (say 5) so that cutting them is a choice / decision point.

Trees and ore are the only non renewable resource really. Its possible to run a world totally out of them, since every tree cut has a small chance for a seed that then has a small chance for a seed, eventually ... no seeds, no trees. Granted, world is plenty big so its a non issue.