First, that's effing dense ! But the linework is wonderful ! I like the transition between evergreens and deciduous. In the northeast, a patch of forest seems unfocused on my screen. Concerning shadows, I have no clue as how to give the impression of light unless you apply a light gray to everything and erase where you want whiter patches. It's gonna be tough. An option would be to do the colored version and then go back to the greyscale version but, from a white background. Maybe use a creamy white or ivory background ? I'm curious to see what you come up with.

I cannot wait to see the colored version, I think it'll really bring it to life