Complex sentence: Between and in the moist northern parts of the forest, grand fir is predominant and western larch is one of the first trees to become established during ecological succession following disturbances, whereas understories consist of mountain maple, mountain ash, blue huckleberry, and mallow ninebark.
Split into simpler sentences: Between and in the moist northern parts of the forest, grand fir is predominant and western larch is found in early successional areas. Understories are also made of mountain maple, mountain ash, blue huckleberry, and mallow ninebark.