Best Natural Way To Kill A Tree

This is the best way to kill a tree naturally. It takes no chemicals and it works on the toughest trees.
A friend who is permaculture certified told me he has effectively killed his elm trees by cutting down to a stump and sealing with a plastic bag. I recently tried this on a tree and I can’t wait to see if it worked.
What you do is cut your tree down to a stump and cover it with black plastic. I just used a black plastic garbage bag and then I sealed it on the tree stump by duck taping the bag around the trunk. I am not sure exactly how this actually kills tree stumps. My best bet is that it is either suffocation or the heat from the sun that kills the tree stump.
If you have a tree stump that is still alive you would basically do the same thing except you need to fresh cut the stump. Meaning cut below where the suckers are shooting out. Then seal the fresh cut stump with a plastic bag and duck tape.