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 North Cascades land resource unit is located in northwestern Washington primarily along the western slope of the Cascade Range. It bounded by the international boundary with Canada to the north and the Snoqualmie Pass area 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 Skagit River is the largest river to originate in the LRU and is governed by three hydroelectric dams. Other rivers that drain west include the Nooksack, Snohomish, and Skykomish. The Wenatchee River drains east toward the Columbia.

Lithology is the result of numerous accretions from tectonic subduction of the Pacific plate along the margin of the North American plate. The North Cascades are arranged in a west to east series of terranes which are combinations of metamorphized sedimentary or oceanic rock and intrusive volcanic plutons, punctuated by the minorly active Mount Baker and Glacier Peak volcanoes (Washington Geological Survey). Additionally, Pleistocene continental and alpine glaciation covered almost all of the area except the highest peaks in the range and deposited large amounts of glacial sediment. Alpine glaciers still remain active today in the highest elevations.

Soils are primarily Spodosols, Andisols, and Inceptisols.

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.

Ecological site concept

This ecological site is defined by the process of the repeated effect of snow avalanches that create a chute in which rigid-stemmed trees cannot thrive, but flexible-stemmed shrub and herbaceous species persist. The reference community includes moisture-adapted shrubs and herbaceous species and deciduous tree species on the edges of the chutes including: gray alder (Alnus incana), Sitka alder (Alnus viridis), common ladyfern (Athyrium filix-femina), arrowleaf ragwort (Senecio triangularis), redosier dogwood (Cornus sericea), Pacific oakfern (Gymnocarpium disjunctum), rusty menziesia (Menziesi ferruginea), devilsclub (Ophopanax horridus), Oregon boxleaf (Pachistima myrsinites), black cottonwood (Populus balsamifera ssp. Trichocarpa) , prickly currant (Ribe lacustre), salmonberry (Rubus spectabilis), threeleaf foamflower (Tiarella trifoliata), Carolina bugbane (Trautvetteria caroliniensis). The site resides on mid to lower portions of avalanche chutes, in the mountains on slopes 35-90%, at elevations of 4,000 to 5,800 feet. The climatic characteristics are broad considering they cover the high elevation head of the avalanche chute down slope to the low elevation runout zone. The mean annual precipitation 40 to 80 inches, the mean annual air temperature is 35 to 40 degrees Fahrenheit, the frost-free days range 40 to 70 days. Soils in this site are in the cryic soil temperature regime and udic soil moisture regime. The soils are variable; taxonomic classification is to the great group level. Fulvicryands dominate with andic soil properties. Umbric epipedons develop due to the highly herbaceous plant community. The parent material is volcanic ash mixed with colluvium over fragmental material.

Associated sites

Similar sites

Table 1. Dominant plant species