The Three Sisters Wilderness area covers over . It has approximately of wilderness trails including of the Pacific Crest National Scenic Trail. Most of the wilderness is covered with dense mixed conifer forest with scattered alpine meadows. The forest area on the eastern slopes of the Cascade Range, where the Milli Fire began, is dominated by ponderosa pine. There are also large stands of lodgepole pine and dispersed Douglas-fir, western hemlock, and western juniper in some areas.