This really isn't about the forest, it's about the areas above timberline. Ain't much soil up there and crap takes a long time to decompose. On popular routes such as Shasta or Longs Peak in Colorado, you could have several hundred people a day climbing. Even if thirty of them try to take care of business above timberline, that's a lot of crapola over the course of a summer.
ok, fine. Where do the elk and yellow bellied marmots go?
&^)