Liking the desaturated green/black combo (the far left palette samples even more), really sets the mood.
Right now the tree doesn't look very sunken as opposed to the other objects. How about adding a reflection in the void of water? Actually that might double the size of the sprite if done properly, although this could be done for the smaller sprites you have down below. You could always have a few blades of grass sticking out of the water rather than the bare circle around the tree and put reflections on those however. Decreasing the radius of the exposed grass might help draw more attention to the tree as well. The base of the tree isn't really shaded as if it were cylindrical, and you have a few darks outlining your light colors. EyeCraft always says this, and I've found it rather helpful: keep dark colors out of light colors and vice versa.
Also, don't you think that those boxes/columns/statues could have a few more cracks running through them, even have a broken pillar? They look like they
could be ancient. I don't know, because with boxes or simple there's a lot of room for ideas.
The perspective of the objects, when compared to the tree, is really off. Not sure if this was intended. If you don't feel like changing those, that's fine too.
You could always make more use of your mid-range colors if you're not going for a cel-shading/hard lighting approach. I actually think soft lighting would better fit the theme.
What coffee said about the top of the tree is right, now that I look at it, unless you were specifically aiming for this kind of tree. You might find
this thread rather helpful when it comes to pixelling trees, especially if you read the whole thing.
