I have been playing around with paintings trees for years now, and my favorite way is to start with a dark background and painting around the tree. Its much more dificult to do, but the results are worth it. Its more free formed and you cant truly plan out how exactly you want it to look.
I also love the idea of painting a tree not by painting IT but by painting everything around it. It reminds me of how those things I surround my self with (family, friends, paintings, cats, town, jobs) display who I am.