A tree could suck up water containing radon, but after you cut down the tree and let it dry under a shed for a few weeks that radon will be gone. It will have

A tree could suck up water containing radon, but after you cut down the tree and let it dry under a shed for a few weeks that radon will be gone. It will have all decayed away because the half life is only 3.8 days. For firewood to still contain radon a few weeks after the tree was cut down, it must have a fairly substantial amount of uranium or thorium in it (constantly producing new radon.)