I think that you would get a better effect by having less dithering. ATM it seems very messy, maybe try reducing it to flat blocks of colour then dithering in between the shades? If you want highlights, try stretching the white and yellows to just before the center of the tree to give it a more sperical shading pattern, that's not to say that the whole half of the tree should be lighter, just certain sections. Though IMHO an anti aliased rough-zig-zag'd line would be better than dithering for a treetop like that, It'd help with the leaf effect(Though this depends on what sccale wour working on, larger scale would fit tthe kind of dithering your using but if it's a smaller tree, say 2m tall then you'd need more leaf definition). Also, you may want to try a more irregular shape?