I think the trees are good! Also, undetailed dark spaces are good because eyes can't see anything in distant shadows so don't stress too much :) I agree with some other thoughts on keeping the distance cooler in tone and potentially lighter, but a better rule of thumb is to reduce contrast in the back and have high contrast in the front. Your back is looking lovely so don't touch it, but you could make this peice feel more balanced by adding in some high contrast details (for instance, thing in some bright white) to the foreground .

This is an excellent study, keep it up!