That's some good advice, thank you!
It does make a lot of sense to restrict the range of values one uses in each specific area of an artwork. The whole work should represent the whole range of values, but each area should be limited, in some capacity, to a smaller range of values within all the values that are possible.
And thank you for pointing out the specific issue of all the trees having (more or less) all the range of values. It definitely limits the effectiveness of the highlights, I think, because of the smaller amount of contrast in the piece as a whole.
That's similar to an example my painting professor gave me; she said to distribute like colors throughout all the areas of the piece, rather than limiting them to only a few specific zones/shapes.
It's definitely something I'll try to keep in mind, the relationship between each section of a work and the piece as a whole, that is.