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 is defined as the highest extent of tree species. It resides at the interface between treeline and the alpine life zone. Soils are typically Spodosols (Inceptisols or Andisols occur as well) with diagnostic features including albic horizons, andic and spodic soil properties, and lithic or densic contacts. Parent material is from volcanic ash over glacial till, colluvium, or residuum. These soils are in the cryic soil temperature and udic soil moisture regimes. This site is on glacial trough valleys and mountain slopes at elevations of 4,250 to 6,000 feet, on moderate to steep slopes (25 to 60 percent) with harsh site conditions of high wind, deep and persistent snowpack, and short growing season that relegates tree species to grow only in clumps within an alpine meadow. Dominant tree species include whitebark pine (Pinus albicaulis), mountain hemlock (Tsuga mertensiana) and subalpine larch (Larix lyalli), though subalpine fir (Abies lasiocarpa) and Engelmann spruce (Pinus engelmannii) may grow in the center of the tree clump. In general, whitebark pine resides 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 eightpetal-avens (Dryas octopetala), pinegrass (Calamagrostis rubescens), common juniper (Juniperus communis), Greenleaf fescue (Festuca viridis) and moister areas with western moss heather (Cassiope mertensiana), partridgefoot (Luetkea pectinate), grouse whortleberry (Vaccinium scopulorum), broadleaf arnica (Arnica latifolia), Jacob’s-ladder (Polemonium pulcherrimum), pink mountainheath (Phyllodoce empetriformis), black alpine sedge (Carex nigricans), mountain 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 400 years, based on the LANDFIRE models for whitebark pine (Pinus albicaulis). Numerous lightning 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