The research I've seen in popular media says a lot of the carbon from old trees comes from an era before microbes could digest them - so dead trees just piled up for a long time and got buried. That doesn't happen anymore.
"Ward and Kirschvink say that 90 percent—yup, 90 percent!—of the coal we burn today (and the coal dust we see flying about Beijing and New Delhi) comes from that single geological period, the Carboniferous period."
