Knowing what they are, it's easy to see the symbols as dead trees in the first image. However, your main forests use a more textural approach rather than symbolic, so I think your idea of coming up with something similar to the live forests in a dingy grey-brown is a good idea. And perhaps instead of the tufty canopy lines, they could be spiky to represent the leafless branches.
You may also gain some benefit from a Google image search on "forest fire aftermath aerial." That's obviously going to get you photographic reference instead of illustrative, but it might serve as inspiration.