I problem I have with an idea of making AOTCR a sort of forest\jungle is that the engine isn't really made for vegitation. Sure you have detail objects, but I'm talking about ferns, bushes, tall grass, trees trees oh and some more trees. A forest\jungle is variety. You won't see the same shaped tree over and over again due to the climate factors such as wind and sun light. This iteration of the engine doesn't allow for much permutation room as the 2nd and 3rd have.
Think about what the control room is. Its deep in the bowels of the actual rind structure, so theres going to mountainess scenery. It's the control room of the ring's purpose, the harse environment I think does well for protection. We of course don't know if the climate controls had actually malfunctioned (as Cortana suggested IIRC) during out trip in that level, which caused the snow to build up. But maybe since the flood had been released, it was Halo's environmental defenses since no matter your bio status, colder temperatures will slow down organisms development and speed.
But imagine how the flood's presence can affect a forzen mountain range assuming it can live fairly still? We already know the flood basically absorbs into the environment and the life around it.