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 Olympic Mountains land resource unit is composed of the mid to upper elevation areas on the Olympic Peninsula in northwest Washington. It occurs primarily on the eastern side of the peninsula above the warmer, lower Olympic foothills of MLRA 2.

Large amounts of rain and snow contribute to a very moist environment and a considerable stream and river network. Major rivers that have headwaters in the LRU include the Elwha, Wynoochee, and Skokomish Rivers.

Lithology is primarily oceanic sedimentary rock stacked in an accretionary wedge and coastal basalt that has been metamorphosed and vertically uplifted into a large horseshoe shape called the Crescent Formation (Washington Geological Survey). This collection of rock has been metamorphosed in many cases. These geologic materials were heavily modified by Pleistocene alpine glaciation. Contemporary glaciers which receive copious amounts of yearly snowfall continue to sculpt the dissected valleys with runoff.

Soils are primarily Spodosols, Inceptisols, and unique Andisols developed from non-volcanic materials.

The LRU has a strong rainshadow effect and areas on the northeastern side of the peninsula are considerably drier than near-Pacific western slopes. Vegetation is primarily dense forest. At lower elevations, western hemlock (Tsuga heterophylla) is the dominant tree species; western redcedar (Thuja plicata) is quite common in moist areas and Douglas-fir (Pseudotsuga menziesii ) is common in drier areas. 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: Western hemlock/Alaska huckleberry/salal, Olympic

Ecological site concept

This ecological site resides on mountain slopes at elevations spanning 2,900 to 4,100 feet on steep slopes averaging 60 to 95 percent primarily in the rainshadow area of the Olympic Mountains. The climate is warm and moist (frost free days average 130 to 155, mean annual precipitation is 65 to 95 inches, and mean annual air temperatures average 40 to 44 degrees Fahrenheit). The soils are Andisols with medial-skeletal soil textures or Inceptisols with loamy-skeletal textures. Andic soil properties are derived from the extreme weathering environment and higher amounts of organic matter in the soil; volcanic ash influence is minimal. Parent material is sedimentary rock or marine deposits derived from basalt. The soil temperature regime is cryic and the moisture regime straddles udic and xeric. Lithic contacts occur in some soils. The reference community has an overstory of western hemlock (Tsuga heterophylla) with western redcedar (Thuja plicata) with an understory dominated by shrubs usually Pacific rhododendron (Rhododendron macrophyllum), Alaska blueberry (Vaccinium alaskaense), vine maple (Acer circinatum), rusty menziesia (Menziesia ferruginea), deer fern (Blechnum spicant), western swordfern (Polystichum munitum), red huckleberry (Vaccinium parvifolium), salal (Gaultheria shallon). Seral tree species exist throughout the early and mid-development community phases and include Douglas fir (Pseudotsuga menziesii), Pacific silver fir (Abies amabilis), Cascade buckthorn (Frangula purshiana), Pacific yew (Taxus brevifolia) and in moister areas and red alder (Alnus rubra). Fire is the main disturbance factor and occurs on rotational intervals of 500 years and is stand replacing in severity in the Olympic Mountains. In the Pacific Northwest in general, western hemlock (Tsuga heterophylla) forests have fire return intervals of 150 to 400 years. LANDFIRE models state an average fire return interval of 500 years (300 to 800 years range) based on the 0710390 North Pacific Maritime Mesic-Wet Douglas-fir-Western Hemlock Forest B classification. Small patch disturbances include diseases Laminated (Armillaria, Annosum) root rots, heart, butt rot, and brown cubical rot, and hemlock dwarf mistletoe. Insect damage can also occur, particularly from the Douglas fir beetle.

Associated sites

Similar sites

Table 1. Dominant plant species