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 chaotic collection of rock in many cases has been metamorphosed. 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 strong a 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 dominate 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
western hemlock/Pacific rhododendron-salal, Olympic

Ecological site concept

This ecological site is found predominantly on mountain slopes and valleys in the mountains, at elevations ranging from 2,700 to 3,600 feet on slopes of 45 to 80 percent. The climate is cool and moist (average frost-free days range from 125 to 150, average mean annual precipitation is 75 to 120 inches and mean annual air temperature is 40 to 42 Fahrenheit. The soils are in the cryic soil temperature regime and udic soil moisture regime. Soils are typically Haplocryands with medial-skeletal textures. Lithic contacts are common. Some soils have spodic materials or have an argillic horizon in more stable positions. Parent material is primarily colluvium derived from basalt or marine sedimentary rock. The reference community is a multi-layered tree canopy dominated by Pacific Silver fir (Abies Amabilis), with an understory dominated by Alaska blueberry (Vaccinium alaskense) and other moist adapted shrub and herbaceous species including red huckleberry (Vaccinium parvifolium), strawberryleaf raspberry (Rubus pedatus), bride’s bonnet (Clintonia uniflora), oxalis species, twinflower (Linnea borealis), Cascade azalea (Rhododendron albiflorum), deer fern (Blechnum spicant), bunchberry dogwood (Cornus canadensis), and rusty menziesia (Menziesii ferruginea). Moisture across the site is dependent on aspect. Drier areas within this ecological site, have an understory with common beargrass (Xerophyllum tenax), Pacific rhododendron (Rhododendron macrophyllum), salal (Gaultheria shallon). Areas of higher moisture may have an addition of devilsclub (Oplopanax horridus), sweet after death (Achlyus triphylla), or threeleaf foamflower (Tiarella trifoliata) in the understory. Fire is the primary disturbance but occurs as rare, stand replacement events on a fire rotational interval of 500 years. Sites with a seasonal water table higher in the soil profile may have a fire interval of 1,000 years. Some drier sites may have fire more frequently on a rotation of 200 to 300 years. Post fire disturbance areas have a community of fireweed (Chamerion angustifolia), pearly everlasting (Anaphalis margiritacea), and other pioneering species that are quickly overtaken by resprouting resident shrub species. There are many seral tree species including Douglas fir (Pseudotsuga menziesii), western larch (Larix occidentalis), western white pine (Pinus monticola), Alaska cedar (Callitropsis nootkansensis), and mountain hemlock (Tsuga mertensiana). Pacific silver fir (Abies amabilis) eventually gains an advantage in the closed cool canopy within the interior of the forest and dominates the upper tree canopies. Insect damage and diseases such as Annosus, laminated or Armillaria root rot, heart and butt rot can occur on this site.

Associated sites

Similar sites

Table 1. Dominant plant species