Well, just try to imagine your tree realistically, in 3D. And see where the light hits it. And since your tree is more of a stylstic (I like it, by the way), cartoony tree, imagine it like that as well. Currently the light seems to be coming from pretty much right about our direction, where we're viewing it from.
But before you do that, I'd advise you change the tree to better suit the perspective of the game, or at least make it similar to the building, right now it seems to me as if it's pretty much side view, whilst the building is in a top-down view.
The light would probably be hitting the top of the little sections of 'blobs' that you have on your tree, maybe I'll cook up an edit later, if someone doesn't do it better by then.
That tree has a real nice look though, I like it. Looks sort of like a christmas tree.
