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 types: ABLA/RHAL2; ABLA/RHAL2-LUHI4; ABLA/VASC-LUHI4; ABLA/VACA.

Ecological site concept

This ecological site resides generally east of the Cascade crest and therefore has a more continental climate. Growing season is shortened by higher elevations with persistent snowpacks. The frost-free days span 60 to 90 days, the mean annual precipitation is 50 to 70 inches and the mean annual air temperature is 38 to 41 degrees Fahrenheit. This site occurs primarily on north-facing slopes and valley walls with elevations of 4,100 to 5,700 feet on slopes 25 to 55 percent. Soils are generally Spodosols or Inceptisols with andic soil properties in the upper part of the profiles. Andisols can also occur. Soil temperature regime is cryic and soil and the soil moisture regime is udic. Some soils may have restrictions within 20 inches of the surface. Parent material is generally volcanic ash over glacial till or colluvium. The reference phase is dominated by subalpine fir (Abies lasiocarpa) and Engelmann spruce (Pinus engelmannii), seral species include lodgepole pine (Pinus contorta) (until 100 years), warmer sites will have Douglas fir (Pseudotsuga menziesii), western white pine (Pinus monticola), western larch (Larix occidentalis) and higher elevations will have whitebark pine (Pinus albicaulis), western larch (Larix lyallii). The understory includes: Cascade azalea (Rhododendron albiflorum), grouse whortleberry (Vaccinium scoparium), sidebells wintergreen (Orthilia secunda), dwarf bilberry (Vaccinium cespitosum), whortleberry (Vaccinium myrtillus), pink mountainheath (Phyllodoce empetriformis), Hitchcock’s smooth woodrush (Luzula glabrata var. hitchcockii), broadleaf arnica (Arnica latifolia), Northwestern sedge (Carex concinnoides), pinegrass (Calamagrostis rubescens) , western rattlesnake plantain (Goodyera oblongifolia), broadleaf lupine (Lupinus latifolius) , osmorhiza species, Oregon boxleaf (Paxistima myrsinites), thinleaf huckleberry (Vaccinium membranaceum), vine maple (Acer circinatum), Douglas maple (Acer glabrum var. douglasii), Cascade barberry (Mahonia nervosa), pipsissewa (Chimaphila umbellate), western teaberry (Gaultheria ovatifolia), Lupinus species, whiteviened wintergreen (Pyrola picta). This site occurs in the high elevations of cold the subalpine fir zone. Alpine areas in the Cascades can be affected by numerous lightning strikes during the short growing season; fire return interval ranges 100- 300 years generally, with stand replacing events occurring 50 percent of the time; mixed severity events occur as well. Insects and disease can cause small endemic patch disturbances or larger epidemic events. Insects such as mountain pine beetles and bark beetles effect PICO seral stands, causing small or large patches of dead trees. Diseases the effect the site include Armillaria and Annosum root disease, Laminated and Tomentosus root rot, Indian paint fungus, brown cubical rot, dwarf mistletoe.

Associated sites

Similar sites

Table 1. Dominant plant species