For a tree to be an effective long-term sequestered of carbon, you have to ensure the tree never burns or decays. Obviously this can be done; you can sink it in anaerobic water or bury it deep in a mineshaft... but just letting forests grow wild doesn't get the job done anymore. Not since the end of the Carboniferous era. New forests don't make new coal deposits, that's a thing of the past.

Trees are still great though, for innumerable other reasons.