The ecology of the Three Sisters reflect their location in central Oregon's Cascade Range. The westside slopes from lie in the Western Cascade Montane Highlands ecoregion, where precipitation is abundant. Forests here consist predominantly of Douglas-fir and western hemlock, with minor components of mountain hemlock, noble fir, subalpine fir, grand fir, Pacific silver fir, red alder, and Pacific yew. Vine maples, rhododendron, Oregon grape, huckleberry, and thimbleberry grow beneath the trees. Douglas-fir is dominant below , while western hemlock dominates above.