The US Forest Service (USFS) says, first, you should get a permit from the forest district office, and tree-cutting instructions. The office will also provide specific dates, times, and areas where it’s ok to cut down trees.
For Christmas trees, specifically, you must cut it from an area that is at least 200 feet from any main roads or other sites. And the trunk must be six inches in diameter, or less.
You are not allowed to cut the tops off of taller trees.
The USFS will also provide a tree tag that should be applied to the tree before it’s loaded into any vehicles.
Harvest more, here: (Fox)