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 Glaciated Western Cascades land resource unit is located in southwestern Washington primarily along the western slope of the Cascade Range. It is bounded by the Snoqualmie Pass area to the north and the Columbia River 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 major rivers originating in the LRU are the Puyallup and Nisqually Rivers which drain to Puget Sound and the Cowlitz and Lewis Rivers which drain west to the Columbia.

Some of the lithology in the LRU is the result of numerous accretions from tectonic subduction of the Pacific plate along the margin of the North American plate creating combinations of metamorphized sedimentary or oceanic rock and intrusive volcanic plutons. The Cascades in this area have a long history of volcanic activity starting about 55 million years ago. Eruptions have created a complex sequence of low-silica and silica-rich depositions that have been eroded and buried repeatedly (Washington Geological Survey). Mount Rainier, Mount Adams, and Mount Saint Helens are volcanoes and the most notable peaks; Mount Saint Helens continues to vent after the 1980 eruption. Alpine glaciers still remain active at the highest elevations. The area was not influenced by continental glaciation, however Pleistocene alpine glaciation modified much of the dissected terrain and contributed glacial sediment to the complex combination of lithologies and volcanism.

Soils are mainly Spodosols and Andisols.

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 is defined as the highest extent of closed canopy forest of whitebark pine (Pinus albicaulis) to the highest elevations of this site, parkland where trees grow only in clumps. Tree clumps reside at the interface between treeline and the alpine life zone. This ecological site resides on mountain slopes at elevations spanning 3,900 to 7,000 feet on slopes of 30 to 90 percent. Pinus albicaulus dominated areas range lower (3,900 to 6,800 feet) and are found on moderate slopes (25 to 50 percent), while the parkland type of tree clumps ranges higher in elevation (5,200 to 7,000 feet) and occurs on steeper slopes (60 to 90 percent). The climate is cold (average frost-free days is 45 to 80, average annual precipitation is 55 to 80 inches and the mean average annual temperature is 38 to 41 degrees Fahrenheit). The soils are primarily Andisols (Inceptisols and Spodosols potentially also support this site). Soils have andic soil properties and skeletal textures. Parent material is residuum or colluvium from igneous rock mixed with volcanic ash and most have lithic contacts. These soils are in the cryic soil temperature and udic soil moisture regimes. High wind, deep and persistent snowpacks, and a short growing season create a harsh climate that allows whitebark pine (Pinus albicaulis) to be the dominant tree species in this closed or open canopy forest.The open canopy configuration is due to insect, disease and mixed severity fire. At the highest elevations’ conditions relegate tree species to only clumps within an alpine meadow. In the understory, cold adapted species include broadleaf arnica (Arnica latifolia), western moss heather (Cassiope mertensiana), pink mountainheath (Phyllodoce empetriformis), jacob’s-ladder (Polemonium pulcherrimum), pinegrass (Calamagrostis rubescens), common juniper (Juniperus communis), whortleberry (Vaccinium myrtillus), grouse whortleberry (Vaccinium scopulorum), Hitchcock’s smooth woodrush (Luzula hitchcockii). Dominant tree species include whitebark pine, mountain hemlock, and subalpine larch (Larix lyalli), though subalpine fir and Engelmann spruce may grow in the center of the tree clumps. In general, mountain hemlock may grow on the wetter, more moist western side of the Cascades; whitebark pine is found in the in drier areas and subalpine larch in areas with more severe wind. The understory species are adapted to site conditions and vary from drier areas with eight-petal mountain avens (Dryas octopetala), pinegrass (Calamagrostis rubescens), common juniper (Juniperus communis), Greenleaf fescue (Festuca viridis) to moister areas with western moss heather (Cassiope mertensiana), partridgefoot (Luetkea pectinata), grouse whortleberry (Vaccinium scopulorum), broadleaf arnica (Arnica latifolia), jacob’s-ladder (Polemonium pulcherrimum), pink mountainheath (Phyllodoce empetriformis), black alpine sedge (Carex nigricans), alpine hairgrass (Vahlodea atropurpurea), and Hitchcock’s smooth woodrush (Luzula hitchcockii). The site conditions are the main limiting factor to tree growth, though rare stand replacing fire can occur at intervals of 200 to 400 years, based on the LANDFIRE models for Pinus albicaulis. Numerous lightening strikes can occur on this site, particularly on ridges, but the lack of continuous tree canopy and low fuels preclude fire spread and severity.

Associated sites

Similar sites

Table 1. Dominant plant species