SRIC’s Carbon Footprint of Biofuels & Petrofuels report suggests that...
land use is so critical [to the environmental impact of biofuels] that – at least from a global warming viewpoint – northern European farmers should plant trees and burn petrodiesel rather than plant rapeseed for biodiesel. Greenhouse gas emissions are reduced more by converting a Malaysian rain forest into a palm oil plantation for biodiesel than by filling tanks with petrodiesel. Yet, it is better to fuel with gasoline and to preserve the Brazilian rain forest than to knock it down to grow sugar cane for bioethanol.
The report has a couple of good points about the capacity of different vegetation types to store carbon, and could be silent on what should be happening to vulnerable species, but its hard to tell most of the report is hidden behind a subscription wall...