Before oil palms, which are small and scrubby, are planted, vast forest trees, containing a much greater store of carbon, must be felled and burnt. Having used up the drier lands, the plantations are moving into the swamp forests, which grow on peat. When they've cut the trees, the planters drain the ground. As the peat dries it oxidises, releasing even more carbon dioxide than the trees. In terms of its impact on both the local and global environments, palm biodiesel is more destructive than crude oil from Nigeria.
Good article. And this site is very informative in general...