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:
western hemlock/Cascade Oregongrape
western hemlock/pinemat manzanita
western hemlock/Pacific rhododendron-salal, Olympic
western hemlock/vine maple/queencup beadlily
western hemlock/Cascade Oregongrape
western hemlock/Pacific rhododendron-salal, Olympic
western hemlock/pinemat manzanita
western hemlock/pinemat manzanita
western hemlock/Cascade Oregongrape

Ecological site concept

This site resides on mountain slopes and in glacial trough valleys at elevations of 2,500 to 4,250 feet on moderate to steep slopes (30-65%). Climate is generally warmer and moist (frost free days average 85 to 120, mean annual precipitation is 50 to 75 inches, and mean annual air temperature is 40 to 43 degrees Fahrenheit. Soils are intergrades of Andisols and Spodosols with combinations of spodic and andic soil materials. Inceptisols on less stable positions also occur. Soils are primarily greater than 60 inches to a restriction; some soils have lithic contacts between 20 and 50 inches. The reference community has an overstory dominated by western hemlock (Tsuga heterophylla) and western redcedar (Thuja plicata) with an understory that can be dominated by vine maple (Acer circinatum) initially but can later include pinemat manzanita (Arctostaphylos nevadensis), Cascade barberry (Mahonia nervosa), Oregon boxleaf (Paxistima myrsinites), pipsissewa (Chimaphila umbellate), thinleaf huckleberry (Vaccinium membranaceum), Saskatoon serviceberry (Amelanchier alnifolia), twinflower (Linnaea borealis), salal (Gaultheria ovatifolia), or western rattlesnake plantain (Goodyera oblongifolia). Seral tree species can be found in the overstory and lower canopy layers and include Douglas fir (Pseudotsuga menziesii) or western white pine (Pinus monticola). Fire is the main disturbance regime and occurs as rare, stand replacement events on a rotational interval of 100 to 200 years. Specifically, the USFS Fire Effects Information System (FEIS) states that western hemlock in the Pacific Northwest has a fire return interval of 150 to 400 years, though in Wenatchee NF it is thought to be 100 to 200 years based on stand ages. LANDFIRE Biophysical Settings (BPS) states all fire types occur in 400-year intervals and stand replacing severity is 99 percent of all fires. Diseases that occur on this site include: Laminated (Armillaria, Annosum) root rots, brown cubical rot, and dwarf mistletoe.

Associated sites

Similar sites

Table 1. Dominant plant species