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

western hemlock/Cascade Oregongrape
western hemlock/Pacific rhododendron-salal, Olympic

Ecological site concept

This ecological site resides on ridges and mountain slopes in the mountains at elevations ranging from 3,000 to 5,200 feet on slopes of 30 to more than 60 percent. The climate is warmer and moist (average frost-free days is 35-90 days, the mean annual precipitation is 40 to more than 90 inches, the mean annual air temperature is 37 to 43 degrees Fahrenheit. The soils are Inceptisols and Spodosols. Soils have have andic soil properties in the upper parts of the profile. Parent material is a mixture of colluvium and volcanic ash near the surface over colluvium or residuum. The soil temperature regime is cryic and the soil moisture regime straddles xeric and udic. The reference community has an overstory of western hemlock (Tsuga heterophylla) and western redcedar (Thuja plicata) with some Douglas fir (Pseudotsuga menziesii) in multiple canopy layers with an understory of mainly vine maple (Acer circinatum) in the early stages, and later: Cascade barberry (Mahonia nervosa) but also salal (Gaultheria shallon), pipsissewa (Chimophila umbellatum), dwarf bilberry (Vaccinium caespitosum). Other species that may be present include western teaberry (Gaultheria ovalifolia), western rattlesnake plantain (Goodyear oblongifolia), twinflower (Linnea borealis), rusty menziesia (Menziesii ferruginea), sidebells wintergreen (Orthilia secunda), Oregon boxleaf (Pachistima myrsinites), and thinleaf huckleberry (Vaccinium membranaceum). After ten years dominating the understory, vine maple (Acer circinatum) can be replaced with other longer living shrub species. Fire is the main disturbance factor and has a return interval of 100 to 200 years and is stand replacing in severity. For western hemlock (Tsuga heterophylla) in the Pacific Northwest, the fire return interval is 150 to 400 years. In the southern Washington Cascades, it is thought the fire return interval is 100 to 200 years based on stand ages. LANDFIRE models state that all types of fires for western hemlock (Tsuga heterophylla) stands have a return interval typically of 400 years but spans between 300 to 800 years for stand replacing fires. Other small patch disturbances such as diseases (Laminated (Armillaria, Annosum) root rots, brown cubical rot, dwarf mistletoe), and insect damage can occur.

Associated sites

Similar sites

Table 1. Dominant plant species