OK, listen up tonight, because this is crucial. Of all the things I can tell you about drawing trees, this is one of the most important. It is also one of the least used and understood principles in landscape painting. It is called diffraction. Because the light coming through a sky hole is passing through a reduced aperture, it is less bright than the sky itself. If you like, think of the tree as "robbing" light from the sky holes.
YOU HAVE TO PAINT SKY HOLES DARKER THAN THE SKY, OTHERWISE THEY WILL JUMP OUT, RATHER THAN HOLD THEIR PLACE IN THE TREE.
In the image above, I have cheated the sky holes up to the value of the lightest parts of the sky. The sky holes become lights hung in the tree.This it what you get without diffraction. Below is the version with diffraction........
I have photoshopped the image so that the sky holes are darker than the sky around the tree. I stuck a square of the value I used to do that, over on the right of the tree. It is not just a little darker than the sky, it is a lot darker than the sky! If you will compare the two versions, the lower one hangs together far better than the upper one. The photograph does record some diffraction, but I have characterized it , shown that I know about it. In this instance, more diffraction works better than what the camera captured. Our job as artists is often to make things look better than the uncritical all seeing lens of the camera.