I think part of what makes the concept art really strong is the lighting effects from the sky.
It gives the forest a really nice mood.
I interpreted it to be "old, dead, and a little spooky, but also beautiful".
The very high and low end keys seem to be absent from your mockup.
Although we are unable to see the sky from such a top down angle, you could still imply its color and light using overlays:
So this isn't actually a pixel edit.
I used photoshop and vectors.
Please don't beat me with sticks.
You could execute such lighting effects with some rather simple Shaders.
Talk to your programmer and see if your game engine is setup to handle effects.
If you are interested in the math involved I can explain.
What I've added is probably a bit too fuzzy and may not convey the correct mood you are trying to create.
But it illustrates taking a step closer to the original painting in terms of color and key range:
Although now that I look at it both my edit and your mockup are missing the really dark darks.
I added the spirits because of these:
Most likely it is intended to be light filtering through the trees.
They are interesting just because of their shape and contrast.
In my mind they seemed animated so I converted them into spirits.
But feel free to ignore them if its not what you're going for at all.
The art itself is very appealing.
The colors are well picked.
I don't completely understand the character, meaning "what can she do?", but that will become clear when she is animated.
Great work.
Keep going.