Most of the tree mass will be a middle value, so in Step 2 I established the few light values and some of the darks at the base of the tree. The darks are important for comparison. How light or dark the middle values will be can be compared to the darks in the tree trunk.
The middle values in this step are in the background and the middle ground. The tree masses in the distance and on the right are further back and need to be a bit lighter and less intense than the tree mass in the middle ground.
I also added just a small amount of red in the trees near the center to create more interest.
While this wash was wet, I scratched some lines to indicate trees in the distance and branches in the main tree shape. Some scrubbing within the tree softened the edges and brought more light in the upper right side..