wow, i don't comment on much, but I have to admit I love trees, and I love pixeling them. I love your color choice, and everything looks fantastic. My only gripe is the lack of biodiversity and depth of biodiversity in your tree. I'd say to add another branch, that goes back behind the tree, and make the shades of greens darker than the ones in front, this will give it more depth, and make it feel more robust. Looking at your tree right now it kind of goes up, and branches out, much like a human figure:
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kind of like that crappy ascii.
you're using X and Y co-ordinates, try using the Z and you'll find suddenly your tree has more depth and is more robust. What I mean by biodiversity is the things on and around your tree, weeds, little buds on your tree, birds, whatever. Another thing you can do is change some of the leaves to be lighter and darker than others in a dramatic way, look outside at any trees around you, you'll find that not all the leaves are the exact same color and have the same shading, this chaos in nature makes everything look natural.
Peace!