Semi-desert Silt 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. The major influences on vegetation in the Reference State would have been time since the last fire and where extra wet periods were followed by Aroga moth or snowmold outbreaks. Thus the three phases within State 1 reflect these combined influences on re-setting plant succession. Phase 1.1 is a plant community with mature Wyoming big sagebrush (Artemisia tridentate ssp. wyomingensis) which dominates both cover and production but has many perennial grass and forb companions. Wildfire (1.1a, 1.3a) or an unusually wet period followed by either Aroga moth or snowmold outbreak (1.1b) would have created a grass-dominated phase (1.2) prevailing for approximately one decade. Wyoming big sagebrush would have re-established approximately 10 years from the last fire event and a return to normal climate conditions. As the fire interval and/or periods without Aroga moth or snowmold outbreaks increased (i.e. 40 plus years) (1.1c), the sagebrush would have eventually become over-mature and super-dominant with a diminished understory (1.3). The most abundant grass would have been the caespitose form of bluebunch wheatgrass (Pseudoroegneria spicata). Needle-and-thread (Hesperostipa comata), bottlebrush squirreltail (Elymus elymoides), Indian ricegrass (Achnatherum hymenoides), and small caespitose bluegrasses (Poa spp.) would have also co-occurred. Dominant forbs would have included buckwheat (Eriogonum spp.), biscuitroot (Lomatium spp.), fleabane (Erigeron spp.), milkvetch (Astragalus spp.), and beardtongue (Penstemon spp.), particularly in the mid-seral condition. The patchiness of these natural disturbances would have produced a mosaic of all three phases across the landscape where this ESD occurs. 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.

Submodel

Description

State 2 is similar to Phase 1.3 of State 1 except that some exotic plants, cheatgrass (Bromus tectorum), and mustards (Brassica spp.) and animals (i.e. livestock) are now present. Additionally, some previously influential forces have now since been removed, namely Native American horses and Native American burning practices. The climate has become warmer and the atmosphere holds more carbon dioxide, nitrogen, and sulphur. State 2 is thus a description of the vegetation on this site shortly following Euro-American settlement. Reduction of the native graminaceous understory on these fine-textured soils created a positive feedback including the baring of the soil surface between the shrubs, allowing the wind to move the silt into mounds (coppice dunes) around the larger shrubs (2.1) (West, 1983). Soil organic matter and nutrient reserves concentrated in these coppice dunes, which further contributed to the patchiness of the vegetation. Decades of pronounced removal of the herbaceous component also resulted in a lengthening of the fire-free interval, allowing the height and density of the less palatable sagebrush to increase over that of State 1. Later, both voluntary and mandatory (after 1934 on BLM lands) reductions in numbers of livestock and duration of their grazing (2.1a), brought light recovery of the understory because of the super-dominance of sagebrush and its longevity. In fact, rest from livestock grazing allows the buildup of fire continuous fuels (2.2), especially following wet winters and springs (2.1a). The vegetation is characterized by Wyoming big sagebrush dominance and a caespitose form of bluebunch wheatgrass in the understory.

Submodel

Description

State 3 is characterized by the dominance of several introduced fire-prone species. When fire return intervals are frequent (3.2a) annual species such as cheatgrass and Russian thistle (Salsola spp.) will predominate (3.1). Longer intervals between fire events (3.1a) will result in a plant community dominated by biennial forbs such as knapweeds (Centaurea spp.) and tumble mustard (Sisymbrium altissimum) (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

When livestock grazing is heavy, particularly during the growing season of grasses (4.1a), State 4 will eventually be re-invaded by woody species, first by rabbitbrush (Chrysothamnus spp.) and later followed by which ever sagebrush species has the highest seed rain. 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 (4.2a).

There is little evidence for the unassisted return of native grasses or forbs in State 4, even if all livestock grazing is removed for decades. Diversification of the plant mix in State 4 will probably require scalping of some of the crested wheatgrass and inter-seeding.

Submodel