Semi-desert Loam (Wyoming 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. Before Euro-American settlement, this would have been a sparsely vegetated variant of semi-desert sagebrush with about 40 percent of the forage production from grasses, 10 percent from forbs, and 50 percent from woody plants, primarily from Wyoming big sagebrush (Artemisia tridentata spp. wyomingensis). The productivity would have been relatively lower than nearby sites (e.g. R034AY2llUT Semi-desert Gravel Ecological Site) because of the Inverse Texture Principle (Noy-Meir, 1973). The major grasses would have included the caespitose form of bluebunch wheatgrass (Pseudoroegneria spicata), Western wheatgrass (Pascopyrum smithii), needle-and-thread (Hesperostipa comata), Indian ricegrass (Achnatherum hymenoides), prairie junegrass (Koeleria macrantha), and bottlebrush squirreltail (Elymus elymoides). Other associated woody species may have included yellow rabbitbrush (Chrysothamnus viscidiflorus), and winterfat (Krascheninnikovia lanata). 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. Total annual forage production would have been about 350 to 400 pounds per acre on flat to moderately sloping uplands. The fire return interval would have been on average approximately 40-50 years. The phases of this State would have depended on the time since last fire, starting with a bluebunch wheatgrass-dominated site (1.1) immediately following fire (1.3a, 1.2b), with yellow rabbitbrush becoming temporarily dominant 30 to 40 years post fire (1.2), followed by return to Wyoming Big sagebrush dominance with bunchgrass understory (1.3) 40 to 50 years post fire (1.2a).

Submodel

Description

State 2 is very similar to State 1 in form and function, with the exception of the presence of non-native plants and animals, possible extinctions of native species, and a different climate. State 2 is a description of the ecological site immediately following Euro-American settlement, and can be regarded as the current potential for this site. The phases of this State vary between a bluebunch wheatgrass dominated phase (2.1), to a yellow rabbitbrush/ Wyoming big sagebrush invaded phase (2.2), to a Wyoming big sagebrush with bluebunch wheatgrass understory phase (2.3), which develop according to the time since last wildfire (2.3a or 2.2b, 2.1a, 2.2a, respectively). This State is maintained by periodic wildfire, which reduces the sagebrush component, and a productive understory capable of providing a seed source for native herbaceous species. The resiliency of this state is reduced by lack of occasional wildfire and loss of native seed source. Moderate levels of growing-season livestock utilization will allow this State to maintain its resiliency, but excessive levels of growing-season livestock utilization will reduce its resiliency.

Submodel

Description

The plant community that characterizes this state is a tall, dense stand of Wyoming big sagebrush with a diminished understory. The Wyoming big sagebrush will remain super-dominant as wildfire continues to be suppressed, and with heavy unrestricted grazing of livestock. The chance of wildfire has diminished due to lack of fine fuels. This State is maintained by the continuation of fire suppression and heavy livestock grazing.

Submodel

Description

Where livestock grazing has been reduced, there may be a build up of introduced annuals during years of heavy spring precipitation. The unusually wet climate can have a two-fold effect, where in some cases, soils can became anoxic; if this is followed by subsequent Aroga moth outbreak, much of the sagebrush can be killed. In other cases, an increased fine fuel loads can increase the chance of wildfire, which has the same effect of removing the sagebrush. The phases of this State pass through a period of short-term dominance by annuals (4.1), followed by a yellow rabbitbrush and native herb phase (4.2), and the eventual return of Wyoming big sagebrush with a sparser understory (4.3). Some introduced grasses that may be present in this state are Kentucky bluegrass (Poa pratensis), Bulbous bluegrass (Poa bulbosa), and smooth brome (Bromus inermis). Resiliency of this state is maintained by fire and reduced livestock grazing. As root-sprouters, yellow rabbitbrush, rhizomatous wheatgrasses, and invasive species are effective following disturbance, particularly fire. These species can out-compete fire and grazing-intolerant species. Heavy growing-season grazing will reduce site resiliency.

Submodel

Description

Crested wheatgrass (5.1) will be found on sites where it has been planted to provide a higher level of productivity. To keep the shrubs from re-invading, it may be necessary to follow up with additional chemical or mechanical treatment (5.1a). This resiliency of this State can be maintained by moderate livestock grazing, but excessive livestock grazing will reduce its resiliency.

Submodel

Description

The state is achieved following a prescribed grazing regime where sheep are used during fall months (with nutrient supplementation) with the intention of reducing the shrub component to allow re-growth of native grasses (Woodland, 2008). This state can be maintained by continuing fall sheep grazing only over several years. A return to growing-season grazing too soon (before establishment of a productive native perennial grass understory) will reduce the resiliency of this State.

Submodel