One thing that might help the shading just a bit: one the far left side of the tree, add in some of the dark outlining you used on the right side, but keep the bright highlights. Bright light causes deep shadow, and all that. Only use it sparingly, but a bit of it could help.
As for the shape; well, it's a good shape...for in towns and such. But I still think it looks far too groomed to be something you come across out in the woods, where things should look a bit wilder.
I could definately see using well-groomed-looking trees like this for in and around towns and houses, though.
The only problem I see is that the wide of the leafs doesn't narrow, they still having the same wide no matter they are in the center or on a side. The side ones should be smaller since they are facing to the right or left, not the viewer.
This is very noticiable on the right.
but you should maybe shade the leafs on the underside of the tree darker. and maybe the left side would look better when the leafs would become darker again when the shape turns away from the viewer.
i believe either that it could be as if the light source strikes it to hard (the sun that is) and the black background together with the relativly dark grass makes it look like its shining, so in my opinion its not the shading and lighting that is wrong but the envirement around it. (recommended to have Crush grass tile all the way from bottom to top since thats how it is ingame (will also help you keep the perspective of the tree since its a top-down-view)
But hey that comes from *scratches his arm pit*
some sort of lightning stroke my head and thats what came up ...
Let em tell da' king
Da the east belongs to Morglum
Da east belongs to da orcs
Da east is green