Your details are good but as a whole the scene clashes. These elements don't look like they exist in the same world but rather were cut-and-pasted together from different images. The trees and the grass look like they are under a blazing hot sun, but the leaves and the dark background (and dark foreground under the grass) imply a deep moody forest. I made a very sloppy edit:
I darkened the trees and tinted them a little green, and added shadows (it took like 30 seconds, but I used "dirty" tools - a semi-transparent layer)
Did the same thing for the ground, and I added some shadows to the cave-ish area to give it depth. Didn't touch the leaves since I assume they're WIP, but those are in serious need of some volume as well. However, I want to show you something else as well:
Old --> Color adjusted
This is without any added shadows, but still with the colors adjusted - notice how much a difference volume and saturation make!
Now, I think you still need to add shadows. But I wanted to show you the importance of that final step: color adjustment.
Can't wait to see more!