Take some time to look at a tree, I mean *REALLY* look at it. The leaves covering the tree don't all receive the same amount of light. The leaves that are under the tree will also catch less light then those on top, so they will be darker. When you look at a tree you don't just see the leaves on the outside of the canopy but you also see leaves growing on the inner branches, those too are likely to receive less light. My point being that with leaves you have pockets of light and dark but here every leaf is the same color and same intensity. Vary the leaf color and make patches of dark leaves where you would be looking "into" the tree canopy.  That will help. What you have managed to capture is a nice organic shape, your tree looks "natural".

The tree looks like it's made of crystals.  :/

if thats what your going for its great :>

you need to work on the shading, loots too flat to mee. But if it was the idea then nvm.

the texture on the trunk should go with the shape of the tree instead of just being flat (unless that's what you were going for :P )