The idea and execution are really well done! I didn’t figure out the energy issue before most of my trees were destroyed, however the forest is now safe forever as the AI cannot successfully path find to the large tree. The all drown in front of the tree, and I am now on day 14 without having done anything. Enemies stopped spawning eventually.
My only criticism is that the days are too short. When you possess an enemy, you only get a few attacks before the day has ended. This makes it near impossible to clear the entire forest quite quickly.