Right now, the tree is the character. There's no reason this can't work without some mammal hogging up all the space.
This can be quite moody if you add some drama to the shading. The moon and clouds have a high contrast foreboding quality to them, but then the landscape below gets all gray and, well, meh. Darken the shadows on the near side of the tree, and add some specs to the far side from the moon light. Maybe put some rim light on the underside of the trunk. Also, I think the piece will be stronger if the bottom and top corners go dark. A dark mountainous forest silhouetted by the clouds individual treetops drawn out by the moon light.
The moon bugs me, the ring of light around it seems unnecessary. The actual moon is nice and rounded, but the ring around it is more hexagonal than circular. Also the rings look pillowy, not anti-aliased.