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 a high, cold volcanic plateau with nearly level topography interspersed with numerous cinder cones. The defining characteristic of this unit is the thick mantle of geologically recent pumice and ash deposited by the eruptions of Mount Mazama and Newberry Caldera. These soils are often coarse textured, excessively drained and have low thermal conductivity and fertility. These qualities promote low botanical diversity and the dominance of relatively few tree, shrub and herbaceous species. Forests are dominated by lodgepole pine in flat areas where cold air pools and depressions with seasonal water tables; ponderosa pine on warmer slopes and rises with improved cold air drainage; and occasionally white fir or shasta red fir at higher elevations receiving greater precipitation. Dense stands of lodgepole pine promote a fire regime characterized by occasional stand replacing fires, whereas more open stands of ponderosa may experience more frequent, low severity fire. The climate of this unit is cold and dry with a soil climate defined by a cryic temperature regime and a xeric moisture regime. Geologically, vulcanism predominates with Quaternary deposits from Mount Mazama and Newberry Caldera in the west and north and Late Tertiary deposits from the Winema volcanic field and Bald mountain in the south and east.

Classification relationships

Plant Associations of the Central Oregon Pumice Zone (Volland 1985)
CPS212 - Ponderosa Pine/Bitterbrush/ Needlegrass

Forested Plant Associations of the Oregon East Cascades (Simpson 2007)
CPS210 – Ponderosa Pine/Bitterbrush
CPG125 - Pinus ponderosa/Stipa occidentalis
USFS PNV Subzone – Ponderosa/lodgepole

Plant Associations of the Fremont National Forest (Hopkins 1979)
CPS212 - Ponderosa Pine/Bitterbrush/ Needlegrass

Landfire Biophysical Setting (Landfire 2007)
0710531 - Northern Rocky Mountain Ponderosa Pine Woodland and Savanna
0711670 - Rocky Mountain Poor-Site Lodgepole Pine Forest

Ecological site concept

This site represents a commonly occurring cold ponderosa pine (Pinus ponderosa) dominant forest type on the Eastern Cascade pumice plateau. This area is overlaid by deep deposits of geologically recent Mount Mazama and Newberry volcanic tephra, which exerts considerable influence over vegetation community expression and ecological dynamics. The presence of bitterbrush (Purshia tridentata) and a largely depauperate herbaceous layer often only dominated by low cover of western needlegrass (Achnatherum occidentalis) characterizes the biotic community of this site. Abiotically, this site is distinguished from dry ponderosa forests where herbaceous production is higher and Idaho fescue is common by having excessively drained, coarse textured soils and greater pumice mantle depth to buried soil which lowers available water capacity. Similar to other pumice plateau ponderosa forests, this site has a cryic soil temperature regime, yet receives lower precipitation (18 to 25 inches) favoring the more drought adapted bitterbrush over greenleaf manzanita (Arctostaphylos patula) and snowbrush (Ceanothus velutinus). Compared to sites that may favor white fir (Abies concolor) dominance over time, this site receives lower precipitation and occupies lower elevations. This site is often associated with sites that were historically dominated by lodgepole pine. It can be distinguished abiotically from these lodgepole sites by occupying the upslope positions and areas that receive less cold air pooling, creating favorable conditions for establishment and maturation of ponderosa pine. This ponderosa pine dominated site will often still contain a significant component of lodgepole pine (Pinus contorta), thanks to the higher water holding capacity of the soils, and shares a similar bitterbrush dominated understory to these sites, making identification of each site concept challenging at times. Evidence suggests that these lodgepole pine dominated forests experienced a historical fire regime characterized more by stand replacing fires than the frequent, low and mixed severity fires experienced by this site. Fire was historically a critical element of the disturbance regime of this site, acting to thin crowded understories, reduce lodgepole pine and bitterbrush densities 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