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

The Cascade and Olympic Mountains (MLRA 3) include 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 High Cascades land resource unit (LRU D) is located in western Oregon and Washington. It occurs on the young volcanic platform straddling the crest of the Cascade Mountains. It is more extensive in Oregon than in Washington. It is bounded on the south by the Mazama pumice zone beginning near Waldo Lake. Steeply-dissected terrain (LRUs E and C) lies to the west, and frequent-fire forests with xeric soil moisture regimes lie to the east. Major rivers draining this area include the Willamette and Deschutes.
Bedrock consists mainly of Plio-Pleistocene lavas (Orr, et al. 1992). Topography is gentle and undissected compared with areas located to the west. However, this area contains major stratovolcanoes such as Mt. Hood, Mt. Jefferson, and the Three Sisters. Large areas were covered in an alpine ice-sheet during the Pleistocene (Noller, et al. 2016). Till often lies above volcanic bedrock, except where lava flows postdate the last glacial maximum.
Soil moisture regime is udic or aquic. Most soils have a cryic or frigid soil temperature regime. Mean annual precipitation is usually greater than 60 inches but ranges as low as 40 inches at some locations east of the Cascade crest. Soils usually contain large amounts of volcanic glass. Andisols and Spodosols are common soil orders.
Conifer forest is the dominant vegetation. Natural fire is infrequent in most areas. At low elevations, Douglas-fir (Pseudotsuga menziesii) is a long-lived, early-seral tree; western hemlock (Tsuga heterophylla) is an associated shade-tolerant tree. At mid to high elevations lodgepole pine (Pinus contorta) is an early-seral tree; mountain hemlock (Tsuga mertensiana) or Pacific silver fir (Abies amabilis) are associated shade-tolerant trees. Natural fire can be moderately frequent in drier areas east of the Cascade Crest. In these areas, Ponderosa pine (Pinus ponderosa), Douglas-fir, and western larch (Larix occidentalis) are fire-tolerant, early-seral trees; grand fir (Abies grandis) is an associated shade-tolerant tree. Wetlands typically support shrubby or herbaceous vegetation throughout this LRU.

Classification relationships

This broad ecological site group is based on the TSME Dry, TSME Moist, and TSME Wet Plant Association Groups (Simpson, 2007). Similar Plant Associations are described by McCain and Diaz (2002).

Ecological site concept

This forested ecological site group occurs in the colder portion of the cryic soil temperature regime. It encompasses deep snowpack environments on both sides of the Cascade crest. Snowpack lasts until early summer. Soil moisture regime is udic. Thick, moderately or highly decomposed litter layers may be present; litter turnover and associated nutrient cycling may be very slow. Rooting depth may be restricted by soil temperature (McCain and Diaz, 2002). Mineral soil layers contain medial or ashy materials, but pumice is not usually present. Mountain hemlock is usually present in the overstory.

Associated sites

Similar sites

Table 1. Dominant plant species

Legacy ID

F003XD001OR