The park contains an estimated of old-growth forests. As little of the park was previously logged, significant stands of old growth forest can be found in the valleys and lower slopes up to the timberline at . From the lowest valley floors to about , virgin stands of western hemlock, red cedar and Douglas fir are common. These species are heavily dependent on deep soils, and red cedars 1,000 years old and over tall can be found on Big Beaver Creek. These lowest elevation forests can be accessed by trails, such as the Happy Creek Forest Walk off State Route 20, the Shadows of the Sentinels at Baker Lake, Thunder Creek Trail at Diablo Lake and the Horseshoe Bend Trail from State Route 542. In openings in the forest canopy, red alder and bigleaf maple can be found but throughout this dense forest, ferns, shrubs and mosses abound.