This is really nice! The lighting and values look great. Probably try not using that spongey sort of brush, just stick with the hard round brush and deviate to something softer sparingly. Also reflect more of the green in the leaves onto the tree. (Because the light is probably shining through them in some areas.)
me likey! it'd be helpful if you put the original next to the photo as well. i like your technique, now all you have to do is sharpen the focus on a small area. like the branch in the center could be really silhouetted against the trunk. only a suggestion though, i could turn out to be dead wrong :O i agree with jen on the reflective greenies
whoops, i meant original next to yuur painting!