good start
what perspective is this?
the trunk and roots look like an overhead view but the bottom of the canopy is straight instead of elliptical.
i can't edit right now so...
i think the shading from the bright area to the mid-tone is a bit sudden and is resolved by adding just a few small highlights using second brightest green to the tips at the bottom of the 3rd brightest row.
your outlines need to be cleaned up there's jaggies and double-pixels going on.
The outline goes from very dark to suddenly quite bright which looks odd, u should instead use something like the second darkest colour to outline the bright area.
