When I need to hit a low shot under a tree, I take a mid or long iron and hit a punch shot. In the pictures above, I am using a 7 iron from about 90 yards. Below are my punch shot basics.
1. I take a narrow stance with my trailing (right) knee pointed in front of the ball and I play the ball off of my trailing instep.
2. I put about 70% of my weight on my lead (left) foot so that my head is in front of the ball from the face-on view.
3. I point the shaft at my lead hip.
Top of Backswing:
1. My trailing knee stays pointed in at the ball.
2. My trailing elbow stays in front of my body and close to its original set-up position.
3. My head stays in its original set-up position.
1. My right knee comes through so that I finish with my belt buckle facing the target.
2. My left elbow stays in front of my body.
3. I finish with the shaft approximately level with the ground.
These basic fundamentals help me keep the ball low under the trees. I know they can help you as well!