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): 006X–Cascade Mountains, Eastern Slope

Stretching from northern Washington to southern Oregon, MLRA6 encompasses the mountain slopes, foothills, elevated plateaus and valleys on the eastern slopes of the Cascade mountains. This MLRA is a transitional area between the Cascade Mountains to the west and the lower lying Columbia Basalt Plateau to the east. Situated in the rain shadow of the Cascade Crest, this MLRA receives less precipitation than portions of the cascades further west and greater precipitation than the basalt plateaus to the east. Geologically, the majority of the MLRA is dominated by Miocene volcanic rocks, while the northern portion is dominated by Pre-Cretaceous metamorphic rocks and the southern portion is blanketed with a thick mantle of ash and pumice from Mount Mazama. The soils in the MLRA dominantly have a mesic, frigid, or cryic soil temperature regime, a xeric soil moisture regime, and mixed or glassy mineralogy. They generally are moderately deep to very deep, well drained, and loamy or ashy. Biologically, the MLRA is dominated by coniferous forest, large expanses of which are dominated by ponderosa pine, Douglas-fir or lodgepole pine. Areas experiencing cooler and moister conditions include grand fir, white fir, and western larch while the highest elevations include pacific silver fir, subalpine fir and whitebark pine. Economically, timber harvest and recreation are important land uses in these forests. Historically, many of these forests would have experienced relatively frequent, low and mixed severity fire favoring the development of mature forests dominated by ponderosa pine or Douglas-fir. In the southern pumice plateau forests, less frequent, higher severity fire was common and promoted the growth of large expanses of lodgepole pine forests.

LRU notes

This unit is characterized by ash mantled lava flows and glacial outwash plains on lower mountain slopes and foothills of the East Cascades in Oregon. Vegetation is largely dominated by forests of ponderosa pine with transitional dry mixed conifer forests where Douglas-fir and grand fir are sub dominant occurring in areas with greater effective precipitation. Historically, these forests have been influenced by a fire regime whereby frequent to moderately frequent, low and mixed severity fires would have favored the development of open stands of mature ponderosa pine. The climate of this unit is cool and dry with a predominately xeric soil moisture regime and frigid soil temperature regime. Geologically, underlying lithologies are dominated by Quaternary and late Tertiary basalt and basaltic andesite as well as mixed grain sediments deposited during Pleistocene glacial retreat. Unlike the nearby pumice plateau, this unit lacks the coarse pumice fragments that dominate the soil profile and cooler temperatures that favor lodgepole pine. This unit is south of the climate influences of the Columbia gorge and therefore does not support woodlands of Oregon white oak.

Classification relationships

Forested Plant Associations of the Oregon East Cascades (Simpson 2007)
CPS211 – Ponderosa Pine/Bitterbrush/Fescue

Plant Associations of the Fremont National Forest (Hopkins 1979)
CPS211 – Ponderosa Pine/Bitterbrush/Fescue

Plant Associations of the Central Oregon Pumice Zone (Volland 1985)
CPS211 – Ponderosa Pine/Bitterbrush/Fescue
CPS111 – Ponderosa Pine/Bitterbrush-Sagebrush/Fescue
CPS216 – Ponderosa Pine/Bitterbrush/Bunchgrass

Landfire Biophysical Setting (Landfire 2007)
0710531 - Northern Rocky Mountain Ponderosa Pine Woodland and Savanna

Ecological site concept

This site represents one of the most commonly occurring ponderosa pine (Pinus ponderosa) sites in the foothills of the Eastern Cascades of Oregon. The ubiquitous presence of bitterbrush (Purshia tridentata) and Idaho fescue (Festuca idahoensis) in the understory largely defines this site biotically. Abiotically, this site is distinguished from other forest sites by its low precipitation (12 to 20 inches), Frigid temperature regime (vs cryic for most adjacent forest types), and fire regime characterized by relatively frequent surface fires. At its lower elevations, where temperatures are warmer and precipitation is lower, this site transitions into shrublands dominated by big sagebrush (Artemesia tridentata), woodlands dominated by western juniper (Juniperus occidentalis) and mixed forests dominated by ponderosa pine and juniper. At its upper elevations, with increasing precipitation and decreasing temperatures, this site transitions into forests dominated by ponderosa pine and Greenleaf manzanita (Arctostaphylos patula) or ponderosa pine and lodgepole pine (Pinus contorta). Fire was historically a critical element of the disturbance regime of this site, acting to thin crowded understories and allow fire resistant ponderosa stands to attain an open, savanna-like appearance.

This is a provisional ecological site that groups characteristics at a broad scale with little to no field verification and is subject to extensive review and revision before final approval. All data herein was developed using existing information and literature and should be considered provisional and contingent upon field validation prior to use in conservation planning.

Associated sites

Similar sites

Table 1. Dominant plant species