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 resides at the highest elevation zone within this LRU, spanning from 5,500 ft to 7,150 ft on glacial valley walls in mountains on 30 to 60 percent slopes. This site is cold, with a persistent snowpack, high wind and a very short growing season. The frost-free days is 40-70 days, the mean annual precipitation is 40 to 80 inches, the mean annual air temperature is 35 to 40 degrees Fahrenheit. Soils are highly variable and are either Andisols (Humic Vitricryands) or Inceptisols (Humic Dystrocryepts, Lithic, or Vitrandic Dystrocryepts). The site occurs in the cryic soil temperature and udic soil moisture regimes. Parent material is primarily volcanic ash over colluvium or residuum. The reference vegetation community is a high alpine meadow that includes a diverse mix of sedges, rushes, grasses and herbaceous species with heath shrubs on slightly drier areas. Common disturbance is burrowing by rodents such as hoary marmots (Marmota caligata) or black bear (Ursus americanus) scavenging by digging. Rare fire occurs from lightning strikes that cause small patch size burns at 400-year rotational intervals. At the highest elevations, on concave positions which hold snow long into the summer, are sedge dominated snowbeds.

Associated sites

Similar sites

Table 1. Dominant plant species