The size of the tree is also important. The tree should be small enough to have some movement when hit. I believe the energy you absorb when in the woods is well worth the effort it takes to prepare the area. The lack of distractions like phones, other people's music, people watching and wondering what you are doing, makes the woods an ideal place to train. It is quiet, peaceful and cooler than out in the sun.
Life begets life.
The woods are full of the life force. When it comes to training, from Qigong to Mantis, outside is the place for me.