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 Associations:
mountain hemlock/beargrass-low huckleberry
mountain hemlock/pink mountainheath-Cascade huckleberry
mountain hemlock/Alaska huckleberry

Ecological site concept

This site covers the mountain hemlock areas within LRU A, which span the highest forests, directly below subalpine parklands. These forests have a deep, persistent snowpack and short growing season. Fires occur frequently from lightning strikes. Though most fires are very small, infrequently there are large, stand replacing fires approximately every 300 years. Insects and diseases impact these forests on small scales including heart and butt rot, root rot, bark beetles and others. This site covers the dominant cool, moist condition of the mountain hemlock. At the upper elevations there may be more heath species and avalanches may be a more dominant disturbance, reoccurring in the same areas repeatedly. This ecological site resides on cirque floors and mountain slopes at elevations of 4,000 to 5,700 feet on moderate slopes (15 to 45 percent). Average climate factors include frost free days of 50 to 85 days, mean annual precipitation of 85 to 125 inches, and mean annual air temperature of 37 to 42 degrees Fahrenheit. The harsh site conditions which include a long-lasting snowpack define the ecological site and control the vegetation community with a short growing season. The reference community is predominantly mountain hemlock. Pacific silver fir can be co-dominate in the overstory but, the regenerating layer is always predominately mountain hemlock. The understory can vary from moist rusty menziesia (Menziesia ferruginea), Alaska blueberry (Vaccinium alaskense), thinleaf huckleberry (Vaccinium membranaceum), oval-leaf blueberry (Vaccinium ovalifolium), western mountain ash (Sorbus sitchensis), bride’s bonnet (Clintonia uniflora) to drier site adapted species common beargrass (Xerophyllum tenax), dwarf bilberry (Vaccinium cespitosum), Cascade azalea (Rhododendron albiflorum), strawberryleaf raspberry (Rubus pedatus), sidebells wintergreen (Orthila secunda) and at the highest elevation pink mountainheath (Phyllodoce empertriformis). Soils are commonly Andisols or Spodosols; Inceptisols occur as well. All soils have some measure of andic soil properties; the parent material is primarily volcanic ash over colluvium from andesite. Soil restrictions in some soils range from 10 to 20 inches to a densic or lithic contact. Other soils are greater than 60 inches to a restriction.

Associated sites

Similar sites

Table 1. Dominant plant species