i'm brand new to pixeling myself, but I'm going to chime in with a few things I see. Anybody else feel free to C+C my C+C.
It looks like you use the "group of spheres" technique for your trees, and it's obvious because you're using the same sphere for each "blob" of the canopy, maybe do a new sphere for each blob.
If you're going to do two different trees, they should look more distinct. Your smaller tree has brighter leaves, it stands out more against the grass tiles, but it uses the same trunk.
The ground really looks tiled. You've got more of your dark shade on the bottom edges of the tile, making a distinct V shape that really stands out when repeated. You've got some alternate tiles
with a little green, or a little lighter color, which is good, but maybe make a few more tiles that have even less of the those elements to mix in as well.
The green strands draped over the level change in the grass tiles looks weird.
I really like the shades you picked for the water, but maybe it's too smooth?
I really like your cactus! But why is it so close to a forest and a marsh?
The rocks in the water look really great!