Well, a rainforest is plenty of easily converted carbohydrates that could gobbled up by nanoswarms in a firestorm: http://www.foresight.org/nano/Ecophagy.html Whether the rainforests were around before the Fall or not depends on a lot of factors. I seem to recall that the world was warmer than now, which could mean either/or depending on the water cycle (warmer = more rain, clouds and flooding; but also potential megadroughts - since the rainforest helps wet the atmosphere it might represent a tipping point) and heat stress (could be a limiting factor). During the ice age Amazonia was largely savanna due to lower rainfall, and I think most ecological models point in the direction that it is a bistable system: either rainforest or savanna, no stable mixture. I would go for whatever option makes the place as creepy or exciting as possible. Imagine a cold dead Amazon overgrown with enormous mutated fungi (or nanovirii?) in the rain from the darkened sky. Or a flourinshing jungle were there seem to be a lot more species than there should be, almost as if something was making loads of them - and equipping them with weird adaptations. Did that beetle just try to hack your inserts?