Conveying distance in a painting can be successfully portrayed by cooling or subduing color. Softening edges is key to building that "distance" look.
In this example, I deploy one of my favorite edge softening tools, the pastel pencil, to create a soft glow from the early morning sun across the tree tops. I used a light green pencil, as the focus of sunlight helped to bring out the local color even though the trees were a bit of distance away. The pencil strokes downward into the tree helped blend the pastel and convey an airy mood.
An approach to muting and blending edges can (especially with regards to distant vegetation) distant color can be achieved with a blue or gray pastel pencil. Here I am applying a light gray pastel pencil to the area of the tree that faced away from the sun.
In addition to using a pastel pencil for these techniques, a hard pastel (such as NuPastel or Rembrandt) can create similar impacts.
Here is the current version of the painting: