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: ABLA/PAMY.

Ecological site concept

This ecological site resides in mountainous areas on glacial valley floors or valley walls and mountain slopes at elevations ranging 3,000 to 5,500 feet on slopes of 5 to 30 percent. The climate is cold and relatively dry (average frost-free days range from 35 to 80, the mean annual precipitation is 55 to 100 inches and the mean annual air temperature is 38 to 42 degrees Fahrenheit. The soils are primarily Andisols. Parent material is volcanic ash over glacial till. The soils develop andic soil properties from the weathering of volcanic ash and exhibit unique water holding capacities. The soil temperature regime is cryic and the soil moisture regime is udic. The reference phase is dominated by subalpine fir and Englemann spruce (Picea engelmannii). Seral species include lodgepole pine (Pinus contorta); after 100 years, lodgepole pine begins to die. Warmer sites will have Douglas-fir, western white pine (Pinus monticola), and western larch (Larix occidentalis); higher elevations will have whitebark pine (Pinus albicaulis), and subalpine larch (Larix lyallii). The understory species include deerfoot vanilla leaf (Achlys triphylla), pipsissewa (Chimaphila umbellate), dwarf Oregon-grape (Mahonia nervosa), fool’s huckleberry (Menziesia ferruginea), Oregon boxleaf (Paxistima myrsinites), pyrola species, Pacific yew (Taxus brevifolia), and various Vaccinium species. This site occurs in the subalpine fir zone of cold, high elevation areas with persistent slow-melting snow that causes a short growing season. Alpine areas in the Cascades can be affected by numerous lightning strikes during the growing season; fire return interval ranges 100- 300 years generally, with stand replacing events occurring 50 percent of the time; mixed severity events occur as well. Insects and disease can cause small endemic patch disturbances or larger epidemic events. Insects such as mountain pine beetles and bark beetles effect lodgepole pine seral stands, causing small or large patches of dead trees. Diseases that affect can be Armillaria and Annosum root disease, Laminated and Tomentosus root rot, Indian paint fungus (Echinodontium tinctorium), brown cubical rot (Phaeolus schweinitzii), or dwarf mistletoe (Arceuthobium spp.).

Associated sites

Similar sites

Table 1. Dominant plant species