Wow, extrymly thank you for your feedback! I just started learning how to pixel art and it's great that someone can give me some tips and improvements advices!
The tree itself looks nice. Where this piece falters, imo, is in rendering the grass. You've rendered it more on the left of the image than on the right, and there are patches where you're just using flat color, so it looks really scattershot. Not to mention, most of these look like single pixel noise - the trick to grass is that you can sort of render individual blades by using vertical stacks of light or dark pixels.