Whenever you start out to pixel something generic like rocks, trees, grass, water, or anything you come in contact with on a daily basis, the key is observation observation observation. If you're not careful, your brain will play tricks on you when you start out to make tiles and say to you "water is blue", and you can end up making a plain blue square and call it a water tile. Instead, think about the properties you see in water everyday: transparency, translucency, reflection, refraction, the way light interacts with it, the clarity of water, things you might see in or around a pond or river, etc. Think about the kind of light your tiles are in: is it brigt sunlight? which way is the light coming from?
I made an edit of the tree so I could fix a few things:
yours<-------<lighting>----->mine
First of all, I noticed that the shape of the tree was a bit too generic. Again, your mind tells you that a tree is a trunk with a clump of leaves on top, whereas not all trees look that way. Sometimes you can see the branches between the leaves, sometimes there are very little leaves at all. I didn't really fix this too well in my edit, however I did make the leaf clump a little less spherical, and made the trunk thinner.
The lighting also looked a bit off. I was confused as to why there seemed to be a small highlight at the very top of the tree, but nowhere else. I assumed that your tiles were meant to be in sunlight, so I drew yellow lines to show how sunlight would light the leaves from directly above. In my edit, the area that is lit by sunlight if all a fairly light green, much the same color as the small highlight on yours. There is also a highlight color for definition of the texture of the leaves. Where there is no sunlight, I picked a fairly dark green, much different from the light green. I chose a much darker green to contrast light from shadow. I also added a cast shadow on the trunk and the ground.
