Oops, sorry I didn't explain myself as well as I could have on the rock issue!

At least for me, something that has helped my level art a LOT is to draw basically to-scale mockups in my little sketchbook, usually away from the computer entirely. I'll go through and doodle sections of the level, and any part that I'm having trouble executing int he sketchbook i KNOW i will have a lot of trouble executing in pixel art.
I think this is good advice. You can do it with the treetops, and it will be good practice. Those little lumps I doodled I do with my tablet, which makes it quite similar to sketching. I find that if you don't it ends up looking very much like space filling, when you start with the high intensity pixelling it's all finish and no base, heh.
The treetops will be tricky. Usually when you're in a forest under the canopy you don't really see the bunches of leaves, just the branches, the chunks of foliage are only visible from outside the treeline. You don't have this because in the platform game view it's a bit like you're in a field outside the forest looking in, if that makes sense. This just means you will end up drawing trees, from the side.

Still use the chunky simplification on the foliage, no need to consult the Nieborg Codex on this one, but draw it like you would if you were seeing the forest AND the trees. Of course then you need to justify the hanging bits, hence my branches. Anything can be broken up into tiles with some ingenuity and it's usually fun to try.