OK, get this idea. Every year, all deciduous trees shed their leaves during the fall. These leaves either lie on the ground + decay, are hauled off to landfills, or are burned.

Well, here's another option. Take the leaves, mash them in water, let the mash ferment + then distill it. What do you get? Cellulosic ethanol or, more likely, methanol (aka 'wood alcohol'). And this is a major potential replacement for fossil fuels in both cars + other internal-combustion engines. As a side benefit, the mash which remains in the still after the alcohol is extracted can be used as fertilizer.

Take a tree-laden city like Philadelphia. I wouldn't be surprised if the potential alcohol which could be extracted from fallen leaves could power the city for a year; plus, the left over mash could be sold as 100% organic (in both the chemical + political senses) fertilizer.