I'm not the best pixel artist nor have I really tried to give much advice on doing so but. . .
The tree's look a bit flat to me and from how I'm viewing the angle the ground is meant to be they seem close to it but just feel as if there not on the right angle.
I think adding a little darker shading around the bottom of the tree to give it a bit more rounded edge might help?
Just a really quick/dirty edit because its 6am and im pretty tired
I'm sure some one else with more experience can probably provide better help and what I've suggested might be entirely the wrong approach just though I'd offer my 2 cents.
Edit: Forgot to mention I'm really liking the tile set so far it looks good although the different grass sets seem to differ a far bit in style so if you plan on using all three of them you might want to try to make them blend together better but if there just 3 different attempts at different styles then in that case they all look pretty good to my eye.
I'm personally more fond of the first one with the small circle of grass it looks the most lush to me