Jack Elliott has a great piece up about how marijuana cultivation - that is, marijuana prohibition - is causing damage to national forests, including pictures of what to look for. I imagine having people up there who don't give a damn about the national resource are also more likely to start fires too. and even for people who don't care about protecting national forests, then how about destroying Mexico's drug cartels? Another outcome of legalization is that the cartels' profits disappear overnight.
To this end it will be interesting to see what happens in Colorado's national forests in the next few years regarding illegal weed cultivation.