Foliage in trees has different degrees of transparency.
When the leaves emerge in the spring, you can easily see the sky through the tree. The leaves make a whisper-thin veil that has to be painted very delicately.
Some trees, like the one on the left, cover the sky more completely, with fewer skyholes. (Asher B. Durand)
The tree on the right is an oak, and it happens to be very opaque. As the foliage becomes more opaque, you can begin to see the form of the tree in terms of a light side and a shadow side. The maple on the left is more transparent. The foliage was drybrushed over the sky to suggest the delicacy of the leaves.
Look for a variety of degrees of transparency within a single picture, for beauty almost always accompanies variety. Claude Lorrain almost always had one tree that was very transparent adjacent to another that was more opaque.