Litter in it's natural form is pretty common along the woods and streams we frequent. Not much is made of its existence, and is mostly overlooked, for it does no harm.
Most of the streams I fish are on public lands, but several pass through private land. I have gained permission to fish these streams through the kindness of the landowner. I respect that land.
There is no greater reason for land to be posted than when people dump on private lands.
Natural litter can be seen along woodland paths.
But this is the way to bring about... NO TRESPASSING.