Dismantling an organization is a way to remove anyone who may have authority or even visibility into the kind of things that happen in their absence. The Forest Service regulates use of public lands but also enforces those laws and regulations. And without all of that, public lands will be exploited or privatized more easily. Like with all actions Trump has taken on, this one is probably about enriching himself, or his family, or his donors/friends.
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Trump administration orders dismantling of the U.S. Forest Service