Well, first of all, you've placed the shadow directly under the trees, while the highlight is more to the side. There also wouldn't be any highlights on the tree trunks if they were in shadow, but maybe if there were some gaps between the leaves that would let light through.
You've also used a lot of similar colors, which makes it difficult to see the texture you've added. It would be easier to make it look like clusters of leaves if you lowered the color count and upped the contrast.
The colors don't seem very natural. The sky reflects a lot of blue light, which gives shadows a blue tint.
Look at some pictures of trees to get a better idea of their shape. The trunk is usually slimmer.
Hope that helps.
