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Douglas-fir is the primary tree species in the Santiam State Forest. In some areas, the Douglas-fir stands are mixed with western hemlock, western red cedar, red alder, and bigleaf maple. A few Pacific yew can also be found at the lower elevations. Mountain hemlock, noble fir, silver fir, and western white pine are found in the higher elevations. Douglas-fir areas that have been recently cut and reforested are characterized by even-aged layers of seedlings or saplings. Stands cut more than 15 years ago have grown into closed canopy stands. These areas have a dense, even-aged layer of rapidly growing Douglas fir trees. The forest also has some older stands that naturally regenerated prior to the establishment of the state forest. These areas are typically mixed conifer stands that include Douglas-fir, hemlock, western red cedar, and noble fir. The forest has only a few layered stands, approximately . These are the result of a selective cutting program during the 1970s and 1980s. These stands normally have an over-story of large Douglas-fir trees. There are ten separate patches of old-growth forest, totaling . These stands range in age from 120 to 500 years.