Mountain hemlock is the dominant historic plant association in the park (see fig. 4, page 100). Areas of this association are at elevations of 5,200 to 7,500 feet. At the higher elevations, whitebark pine, subalpine fir, Shasta red fir, and western white pine are in the overstory. At the lower elevations, Shasta red fir, western white pine, white fir, and some Douglas fir are present. Precipitation is received mainly as snow, with snow depths as much as 8 feet. Snow can be on the ground from October through June, depending on elevation and aspect.
Because of the cool, wet environment, fires in this association typically occur as crown fires about every 400 to 800 years (Atzet and Wheeler, 1982; Booth, 1991; Habeck, 1985). Mountain hemlock is not adapted to fire. Its relatively thick bark provides some protection, but its low-hanging branches, highly flammable foliage, and tendency to grow in dense groups make it very susceptible to fire injury (Arno and Hammerly, 1977). Fires in mountain hemlock forests commonly occur as severe stand-replacing fires.
Fire suppression practices allow for growth of older mountain hemlock stands. Old-growth stands, 460 years old or more, are very susceptible to stand-replacing fires (Dickman and Cook, 1989). After a stand-replacing fire, lodgepole pine usually becomes established, restarting the succession. Lodgepole pine stands may persist for as long as 200 years before being replaced by mountain hemlock (Dickman and Cook, 1989). In areas where fires occur repeatedly, the succession can be slowed tremendously, allowing fire-related shrub communities to become dominant and thrive.
The ecological sites in the mountain hemlock association are mountain hemlock/grouse blueberry/prince’s pine (Tsuga mertensiana/vaccinium scoparium/Chimaphila umbellata); mountain hemlock/greenleaf manzanita-creambush oceanspray/longstolon sedge (Tsuga mertensiana/Arctostaphylos patuala-Holodiscus discolor/Carex inops); mountain hemlock/pinemat manzanita/princes’s pine (Tsuga mertensiana/Arctostaphylos nevadensis/Chimaphila umbellata), and mountain hemlock/Hitchcock’s smooth woodrush (Tsuga mertensiana/Luzula glabrata var. hitchcockii).
The understory vegetation in the mountain hemlock association varies widely. In some stands understory vegetation is almost nonexistent, and in other stands the understory vegetation is abundant.