The subalpine forests of the Rocky Mountains are characterized by spruces and firs and are floristically and structurally similar to the boreal conifer forests to the north. Dominant tree species in the Colorado Rocky Mountains subalpine forests include Engelmann spruce and subalpine fir, as well as lodgepole pine and the occasional Douglas-fir. In the Black Hills of South Dakota, white spruce replaces Engelmann spruce. Further north are the lodgepole pine, Engelmann spruce, and alpine fir mix of the Alberta Mountain forests above of elevation. Stand-replacing fires typically occur at 200- to 400-year intervals. Widespread insect outbreaks in spruce-fir forests occur more frequently. Recently, spruce bark beetles have begun to impact more old-growth spruce trees in these forests.