While your leaves look nice, they don't show where the light is coming from. You've fallen into the pitfall that many people fall into when making a tree. You shade each individual leaf but you omit shading the whole canopy of the tree. This also occurs on your trunk, you have shaded the details of the trunk like the ruts in the bark, but you haven't shaded the actual shape of the trunk, a cylinder. Don't forget to include cast shadows on the trunk from the leaves, and shadows from the roots onto the trunk behind it.