General information

Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.

MLRA notes

Major Land Resource Area (MLRA): 003X–Olympic and Cascade Mountains

This area includes the west slope and parts of the east slope of the Cascades Mountains in Washington and Oregon. The Olympic Mountains in Washington State are also included. These mountains are part of a volcanic arc located at a convergent plate boundary. Volcanic rocks predominate but metamorphic and sedimentary rocks occur in the North Cascades and Olympic Mountains. Topography is generally dissected and steep, but some areas consist of constructional volcanic platforms and isolated stratovolcanoes. Elevation is usually 500 to 6000 feet but reaches to 14,410 ft at the summit of Mount Rainier. Many areas hosted alpine glaciers or ice sheets during the Pleistocene, and a few remain today.

Climate becomes cooler and moister with increasing elevation and latitude. Low elevations experience a long growing season and mild temperatures. High elevations can accumulate snowpack lasting into summer and frost may occur in any month. Average annual precipitation ranges from 60 to 180 inches in most areas. Most precipitation falls during the fall, winter, and spring during low-intensity frontal storms. Summers are relatively dry. Average annual temperature is 27 to 50 degrees F. The frost-free period is 10 to 180 days.

LRU notes

The Glaciated Western Cascades land resource unit is located in southwestern Washington primarily along the western slope of the Cascade Range. It is bounded by the Snoqualmie Pass area to the north and the Columbia River to the south. To the west is the Puget Sound Trough (MLRA 2) and to the east is the drier eastern slope of the Cascade Range (MLRA 6).

The major rivers originating in the LRU are the Puyallup and Nisqually Rivers which drain to Puget Sound and the Cowlitz and Lewis Rivers which drain west to the Columbia.

Some of the lithology in the LRU is the result of numerous accretions from tectonic subduction of the Pacific plate along the margin of the North American plate creating combinations of metamorphized sedimentary or oceanic rock and intrusive volcanic plutons. The Cascades in this area have a long history of volcanic activity starting about 55 million years ago. Eruptions have created a complex sequence of low-silica and silica-rich depositions that have been eroded and buried repeatedly (Washington Geological Survey). Mount Rainier, Mount Adams, and Mount Saint Helens are volcanoes and the most notable peaks; Mount Saint Helens continues to vent after the 1980 eruption. Alpine glaciers still remain active at the highest elevations. The area was not influenced by continental glaciation, however Pleistocene alpine glaciation modified much of the dissected terrain and contributed glacial sediment to the complex combination of lithologies and volcanism.

Soils are mainly Spodosols and Andisols.

Vegetation is primarily dense forest with some parkland in subalpine and alpine areas. Douglas-fir (Pseudotsuga menziesii ) and western hemlock (Tsuga heterophylla) are the dominate tree species found at lower elevations; western redcedar (Thuja plicata) is quite common. Pacific silver fir (Abies amabilis) and mountain hemlock (Tsuga mertensiana) are the primary tree species in the higher elevations; subalpine fir (Abies lasiocarpa) and Alaska cedar (Callitropsis nootkatensis) can be widespread as well.

Classification relationships

USFS Plant Association: ABAM/MEFE-VAAL.

Ecological site concept

This ecological site is defined as being in the cryic soil temperature and udic soil moisture regime, at elevations spanning 3,300 to 5,500 feet on moderate to steep slopes (30 to 60 percent) on mountain slopes. The climate is influenced by the mountain environment and is cool and moist (frost free days average 50 to 85 days, mean annual precipitation is 60 to 90 inches and the mean annual air temperature is 38 to 42 degrees Fahrenheit). The site is defined by moderate snowpack, a cooler growing season, and adequate precipitation. The soils are Andisols (Typic or Ultic Vitricryands) with ashy-skeletal soil textures or Spodosols (Andic Haplocryods) with loamy-skeletal soil textures. Parent material is volcanic ash over colluvium from igneous or metaporphic rock. Lithic contacts within the profile are common, although some soils are greater than 60 inches to bedrock. The reference community has Pacific silver fir (Abies amabilis) in the overstory and in the regenerating layer, various seral tree species in multiple tree canopy layers and an understory that spans moist adapted species to drier site adapted species. Seral tree species include western hemlock (Tsuga heterophylla), western redcedar (Thuja plicata), subalpine fir (Abies lasiocarpa), Douglas-fir (Pseudotsuga menziesii), western larch (Larix occidentalis), western white pine (Pinus monticola), noble fir (Abies procera), lodgepole pine (Pinus contorta), Engelmann spruce (Pinus engelmannii) and Alaska cedar (Callitropis nootkanensis). Moist adapted understory species include rusty menziesia (Menziesia ferruginea), devilsclub (Oplopanax horridus), sweet after death (Achlys triphylla), Alaska blueberry (Vaccinium alaskense), threeleaf foamflower (Tiarella trifoliata) while dry site adapted vegetation includes vine maple (Acer circinatum) and common beargrass (Xerophyllum tenax). Colder sites have Cascade azalea (Rhododendren albiflorum) and thinleaf huckleberry (Vaccinium membranaceum). The fire return interval is generally rare (200 to 500 years) and typical fires are stand replacing, Pacific silver fir (Abies amabilis) is a fire avoider, meaning it perishes in fire although the resident seedbank and windblown tree seedlings quickly re-establish. Shrubs resprout and pioneering herbaceous species establish on the site post fire. Shrubs included in the post disturbance phase are vine maple, Douglas maple (Acer glabrum var. douglasii), Scouler willow (Salix scouleriana), Oregon boxleaf (Paxistima myrsinites), thinleaf huckleberry, Saskatoon serviceberry (Amelanchier alnifolia), Sitka alder (Alnus viridis) and snowbrush ceanothus (Ceonothus velutinus). Fires on the east side of the Cascade Crest are more frequent due to the drier continental climate factors. Fire is a rare, large patch disturbance while diseases and insect impacts are frequent, small disturbances that serve to open the tree canopy in patches and include, Annosum and laminated root disease, Indian paint fungus, mistletoe, severe outbreaks of mountain pine beetle (in Pinus contorta), and silver fir beetle.

Associated sites

Similar sites

Table 1. Dominant plant species