Semi-desert Silt (Basin big sagebrush/ Bluebunch wheatgrass)

Description

The Reference State is a description of this ecological site just prior to Euro-American settlement but long after the arrival of Native Americans. The description of the Reference State was determined by NRCS Soil Survey Type Site Location information and familiarity with rangeland relict areas where they exist. The major influences during the Reference State would have been time since the last fire, Aroga moth, or snow mold outbreak. Thus three phases have been proposed to reflect these influences on resetting the successional clock. The basin big sagebrush (Artemisia tridentata ssp. tridentata) dominated community type with rich herb understory (1.1) would have been the most common phase found 10 to 30 years following the last fire. In this community, both cover- and production-based dominance would have been primarily from basin big sagebrush, but with a relatively rich understory of perennial herbs. The dominant grasses would have been the rhizomatous variant of bluebunch wheatgrass (Pseudoroegneria spicata) and western wheatgrass (Pascopyrum smithii). Other grasses would have included the small bluegrasses (Poa spp.) and bottlebrush squirreltail (Elymus elymoides). The dominant forbs would have likely been an assortment of buckwheats (Erigonum spp.), biscuitroots (Lomatium spp.), fleabanes (Erigeron spp.), granite prickly phlox (Linanthus pungens) and milkvetch (Astragalus spp.). A more complete list of species by lifeform for the Reference State is available in the accompanying tables in the “Plant Community Composition by Weight and Percentage” section of this document. A recent fire, Aroga moth, or snowmold outbreak would have created a pathway (1.1a) toward temporary bunchgrass dominance (1.2). Freedom from fire, insect, or pathogens for 4 to 5 decades (1.1c) would have lead (1.1) to a phase where tall, dense sagebrush dominated (1.3). A burn (1.3a) of mature sagebrush stands would also have resulted in a bunchgrass-dominated phase (1.2). The patchiness of these natural disturbances would have created a mosaic of all three phases across the landscape where this ESD occurred.

Submodel

Description

State 2 is similar to Phase 1.3 of State 1 except some exotic species of both plants and animals have been introduced. Native Americans, along with their horses and burning practices, have been eliminated and climate has become warmer and the atmosphere enriched with carbon dioxide and sources of atmospheric nitrogen and sulphur. State 2 is thus a description of vegetation on this site shortly following Euro-American settlement. This state can be regarded as the current potential. The least modified plant community in State 2 is a basin big sagebrush dominated type with a less productive herbaceous understory (2.1), and with minor amounts of exotic annuals and biennials present.

The reason for the diminished understory is historic unrestricted year round livestock grazing, first by cattle and later by sheep (Parson 1996). Opening of the interspaces between the shrubs has often resulted in wind moving the fine soil particles into coppice dunes under the larger shrubs, creating mounded micro-relief and patchier nutrient reserves. This long period of pronounced removal of the herbaceous component also resulted in lengthening the fire-free interval, allowing both height and density of the sagebrush to increase more than occurred in the Reference State.

Reductions in seasons of use and intensity of livestock grazing have brought little change in this vegetation type because of the super-dominance of sagebrush and its longevity. In fact, rest from livestock use could allow for the buildup of finer, more continuous fuels, especially following wet winters and springs.

Submodel

Description

State 3 is characterized by the dominance of several fire-prone species including cheatgrass and mustards. When fire return intervals are frequent (approximately 3 to 10 years) (3.2a) invasive annual species such as cheatgrass and mustards will predominate (3.1). Longer intervals (approximately 10 to 50 years) between fire events (3.1a) will result in a plant community dominated by biennial forbs (3.2). There is little evidence for the return of the native grasses or forbs in State 3, even if all livestock grazing is removed for decades.

Submodel

Description

State 4 is dominated by crested wheatgrass. Eventually, State 4 will be re-invaded, first by rabbitbrushes (Chrysothamnus spp.), and then whichever sagebrush has the greatest seed dispersal. Thus, if the maintenance of grass production is desired in State 4, it will require re-treatment of the brush either mechanically, chemically, or with fire (4.1a). Moderate livestock grazing will help maintain the resiliency of this state, but heavy livestock grazing will reduce its resiliency. There is little evidence for the return of the native grasses or forbs in States 3 or 4, even if all livestock grazing is removed for decades.

Submodel