Ecological dynamics

The reference native plant community is dominated by Wyoming big sagebrush, mormon tea (green ephedra) and Thurber needlegrass. Bluebunch wheatgrass is prominent. Basin wildrye, bottlebrush squirreltail, Sandberg bluegrass and a variety of forbs are present. Vegetative composition of the community is approximately 55 percent grasses, 10 percent forbs and 35 percent shrubs. The approximate ground cover is 50-60 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 the extent of bedrock fracturing influences the composition and total production of this site. Mormon tea, a CAM photosynthetic species, increases over highly fractured bedrock in warmer mesic areas. As a dioecious species having separate male and female plants, it also appears to segregate with male plants increasing on drier slopes and female plants increasing in more moist areas. Thurber’s needlegrass increases on a loamy surface. Bluebunch wheatgrass increases on slight north aspects with an increase of silt on the surface. Bottlebrush squirrel tail will increase on older surfaces with stable erosion pavement. Old erosion pavement develops a desert varnish, a distinctive dark color resulting from 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, Thurber’s needlegrass and bluebunch wheatgrass will decrease in the stand. Thurber’s needlegrass is the preferred species during all seasons. Wyoming big sagebrush, Sandberg bluegrass and squirreltail increase. With continued overgrazing mormon tea decreases. Under deteriorated conditions minor amounts of annuals invade, bare ground increases, erosion accelerates and inherent site productivity decreases.

States: ARTRW8-EPVI/ELEL5-POSE-bare ground; ARTRW8/ELEL5-POSE-Annuals-bare ground; Annuals-bare ground following fire under deteriorated conditions