Hi and welcome

I like your leaves.
I don't have much time so some quick crits...
Your trees have too much highlight on them.
In a forest there would only be patches of light getting through.
Even so, highlights of that much contrast should be reserved for things you want to draw attention to.
Right now they are competing for attention with your sprite.
Roots on big tree are too small and obvious repeats, try going for bigger unique roots that connect to the ground better by having some lumps of dirt and grass etc.
Also let the structure of the roots blend up into the lower trunk.
Use light and shadow to achieve this.
You need a darker colour for shadow on trunk and make the shadow area larger.
Your sprite is pretty cool but try increasing its brightness a bit to help it pop out from the background.
Your leaves look pretty good but now you need a few transition tiles rather than such a hard edge between grass and leaf.
You also need some 2 or 3 tiles of grass with just a few leaves scattered around in random positions, and then scatter these tiles around the scene.
Dunno what software your using but in Photoshop for example you can cheat (just a shortcut really) to do this by placing the leaves on a layer above the grass tile, then press the Layer Mask (see screenshot below), then paint black to make rub out the leaves, or white to add leaves back on top.
Always use Pencil mode with no opacity etc so you don't ruin ur pixels.

Also with placement of your leaf tiles in your mockup, there's too many straight edges along the same angle that suggests some kind of oblique perspective.
So again, rough up the edges of your transition tiles and avoid so many straight lines.
If your going to have THAT much leaf tiles, you need some variations within the leaves too.
This could be some small piles of leaves and patches of grass, sticks etc.
The edges of your water are too thin and too straight.
Try varying the thickness to make it more natural.
If your not already, try starting by roughing things out more rather than jumping into detail level.
Your temple has no perpsective compared to what we'd expect from the rest of the tiles.
Read up on RPG standard 3/4 view.