This tileset could use some help.
First, the shadow under the tree isn't set as semi-transparent, so it's... just a solid grayish-purple blob instead of a shdow. And second, what the hell is going on at the edges? And third, your attempt at a one-tile-taller tree has some really obvious mismatched edges. Either fix them to make it look right, or make it the height it's supposed to be.