I only train my clients outdoors, and I am constantly using different ways to exercise.
So a tree is the best one in my opinion.
You can warm-up, doing bicep curls using dead branches that are usually located under most trees.
The second thing that you can do is to do some chin-ups.
The rule is the thicker the branch. More muscle control will be required. So it means if you get a thin branch, it will be much easier.
And now comes the grip section.
Hands facing up are called the supinated grip, and that uses more of the biceps.
Hands facing down are called pronated, and that requires more of the back muscles.
Then you can also make a wider grip or a close grip.
The wider grip focuses more on the back and the close hold on the triceps and biceps.
The next stage, if you are ready, is to climb the tree.
That will require some strength and guts.
It will be a full-body workout.
Go for it!
Yes, you can have a whole workout using a tree.