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.

Classification relationships

USFS Plant Association: ABAM/VAAL-MEFE

Ecological site concept

This ecological site is defined as being in the cryic soil temperature and udic soil moisture regime, at elevations spanning 3,000 to 5,300 feet on moderate to steep slopes (20 to 50 percent) on mountain slopes and glacial valley walls. The climate is influenced by the mountain environment (frost free days average 50 to 85 days, mean annual precipitation is 55 to 85 inches and the representative mean annual air temperature is 38 to 42 degrees Fahrenheit). The site is defined by moderate snowpack, a cooler growing season, and adequate precipitation. Soils are primarily Spodosols with some Andisols. A common feature with all representative soils is andic soil properties derived primarily from influence of volcanic ash. The soils have unique water holding capacities which provide plant-available moisture throughout the drier growing season. The parent material is volcanic ash over colluvium or till. These soils are also relatively high in organic matter due to the formation of metal-humus compounds typical for soils in this weathering regime and ash-influenced parent material. The reference community has Pacific silver fir 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, western redcedar, subalpine fir, Douglas-fir, western larch, western white pine, noblefir, lodgepole pine, Engelmann spruce and Alaska yellow cedar. Moist adapted understory species include rusty menziesia (Menziesia ferruginea), devilsclub (Ophopanax horridus), Cascade azalea (Rhododendren albiflorum), sweet after death (Achlys triphylla), Alaska blueberry (Vaccinium alaskense), threeleaf foamflower (Tiarella trifoliata) while dry site adapted vegetation include vine maple (Acer circinatum), roughfruit berry (Rubus lasiococcus ), thinleaf huckleberry (Vaccinium membranaceum). The fire return interval is generally rare (200 to 500 years) and typical fires are stand replacing, Pacific silver fir 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 form a post-disturbance phase include vine maple, Douglas maple (Acer glabrum var. douglasii), Scouler willow (Salix scouleriana), Oregon boxleaf (Paxistima myrsinites), thinleaf huckleberry, Saskatoon serviceberry (Amalancheir alnifolia), Sitka alder (Alnus viridis) and snowbrush ceanothus (Ceanothus velutinus). Fires on the east side of the continental divide are more frequent due to drier continental climate factors. Fire is a rare, large patch disturbance while diseases and insects are frequent, small disturbances that serve to open the tree canopy in patches and include: Annosum and laminated root disease in subalpine fir, Engelmann spruce, Pacific silver fir, Grand fir, western hemlock, mountain hemlock; Indian paint fungus, mistletoe; severe outbreaks with mountain pine beetle (sere lodgepole pine), and the silver fir beetle.

Associated sites

Similar sites

Table 1. Dominant plant species