Ecological dynamics

The reference native plant community is dominated by Wyoming big sagebrush and bluebunch wheatgrass. Thurber's needlegrass is prominent in the stand. Spiny hopsage, Indian ricegrass, bottlebrush squirreltail and Sandberg bluegrass are present in the stand. Vegetative composition of the community is approximately 65 percent grasses, 10 percent forbs and 25 percent shrubs. The approximate ground cover is 30 to 40 percent (basal and crown).

Four states have been identified for this site: a reference state; a state with the presence of annuals; a state with a shrub/annual co-dominance; and a state with annual dominance.

Reference: Plant community phase change is driven by infrequent fire. Wyoming and basin big sagebrush decline after fire while Thurber’s needlegrass, Indian ricegrass and other grasses increase. May see a temporary increase in rabbitbrush after fire. Time facilitates the reintroduction of sagebrush. The introduction of invasive annual grasses and forbs transitions into the state 2.

State 2: Compositionally similar to the reference state with a trace of cheatgrass and weedy forbs. Ecological function has not changed, however the resiliency of the state has been reduced by the presence of invasive weeds. Prescribed grazing and infrequent fire (> 50 year return interval) maintain state dynamics. The timing and/or intensity of grazing or prolonged drought favors Wyoming and basin big sagebrush, squirreltail and Sandberg’s bluegrass. Prescribed grazing and/or release from drought may reverse the decline in needlegrass and Indian ricegrass production. Infrequent fire reduces the shrub community and promotes the bunchgrass component. Mismanaged grazing and/or prolonged drought leads to a biotic threshold and into state 3.

State 3: Wyoming and basin big sagebrush is decadent with little recruitment. The perennial grass component is significantly reduced in both density and productivity. Cheatgrass and/or annual forbs and/or Sandberg’s bluegrass along with sagebrush control site resources and drive ecological dynamics. Bare ground is abundant. Spatial and temporal energy capture and nutrient cycling has been truncated. Infiltration may be reduced due to lack of ground cover. Risk of soil erosion by both wind and water is increased. Catastrophic wildfire will lead to an abiotic threshold and into state 4.

State 4: Cheatgrass and/or annual weed dominated plant community with limited to no shrub or perennial grass component. Soil erosion and redistribution along with changes in dynamic soil properties affect the hydrologic cycle and thus the nutrient cycle. Harsh environmental factors increase state resiliency to change.

Range in Characteristics
The depth to a restrictive layer and apect influences the composition and production of the site. Production will increase with greater soil depth and at the upper end of the precipitation zone. Bluebunch wheatgrass increases on a silty surface. Thurber’s needlegrass increases on loamy surfaces. Indian ricegrass will increase on shallow coarse soils and warm slopes. Spiny hopsage and shadscale saltbush increases at the lower end of the precipitation zone and on droughty slopes. On older stable higher terraces an erosion pavement has accumulated with a distinctive desert varnish. The distinctive dark color is due to precipitated concentrates of manganese and lesser amounts of iron.

Response to Disturbance - States
If the condition of the site deteriorates as a result of over grazing, bluebunch wheatgrass, Thurber needlegrass and other deep rooted perennial bunchgrasses will decrease in the stand. Wyoming big sagebrush, squirreltail and Sandberg bluegrass will increase. Minor amounts of annuals will invade. With further deterioration, bare ground rapidly increases, erosion accelerates and site productivity decreases. The invasion of annuals and the natural re-establishment of native perennials are limited on eroded surfaces and in areas of dense cobbles and heavy erosion pavement.

States: ARTRW8-GRSP/ELEL5-POSE-bare ground with erosion pavement; Annuals (scattered)-bare ground with erosion pavement