I am just about to do the tree on my son's cake and what tip should I use for tree leaves?
I use a #352, "bird beak" tip for leaves. They're no-fail leaves.
If you're going for a pine tree look you can also use a grass tip.
If you are wanting an oak tree, you can use a #102 tip. Make the leaves like you would for mum leaves. You can make them in yellow, orange and red for Fall leaves, or in green for summer leaves.