Ecological dynamics
It is impossible to determine in any quantitative detail the Reference Plant Community for this ecological site because of the lack of direct historical documentation preceding all human influence. In some areas, the earliest reports of dominant plants include the cadastral survey conducted by the General Land Office, which began in the late 19th century for this area (Galatowitsch 1990). However, up to the 1870s the Shoshone Indians, prevalent in northern Utah and neighboring states, grazed horses and set fires to alter the vegetation for their needs (Parson 1996). In the 1860s, Europeans brought cattle and horses to the area, grazing large numbers of them on unfenced parcels year-long (Parson 1996). Itinerant and local sheep followed, largely replacing the cattle as the proportion of browse increased.
Below is a State and Transition Model diagram that illustrates the “phases” (common plant communities), and “states” (aggregations of those plant communities) that can occur on the site. Differences between phases and states depend primarily upon observations of a range of disturbance histories in areas where this ESD is represented. These situations include grazing gradients to water sources, fence-line contrasts, patches with differing dates of fire, herbicide treatment, tillage, etc. Reference State 1 illustrates the common plant communities that probably existed just prior to European settlement.
The major successional pathways within states, (“community pathways”) are indicated by arrows between phases. “Transitions” are indicated by arrows between states. The drivers of these changes are indicated in codes decipherable by referring to the legend at the bottom of the page and by reading the detailed narratives that follow the diagram. The transition between Reference State 1 and State 2 is considered irreversible because of the naturalization of exotic species of both flora and fauna, possible extinction of native species, and climate change. There may have also been accelerated soil erosion.
When available, monitoring data (of various types) were employed to validate more subjective inferences made in this diagram. See the complete files in the office of the State Range Conservationist for more details.
The plant communities shown in this State and Transition Model may not represent every possibility, but are probably the most prevalent and recurring plant communities. As more monitoring data are collected, some phases or states may be revised, removed, and/or new ones may be added. None of these plant communities should necessarily be thought of as “Desired Plant Communities.” According to the USDA NRCS National Range & Pasture Handbook (USDA-NRCS 2003), Desired Plant Communities (DPC’s) will be determined by the decision-makers and will meet minimum quality criteria established by the NRCS. The main purpose for including descriptions of a plant community is to capture the current knowledge at the time of this revision.
State 1
Reference State
The Reference State is a description of this ecological site prior to Euro-American settlement but 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 Reference State for this site would have been a sagebrush dominated semi-desert state, where low sagebrush (Artemisia arbuscula ssp. arbuscula) would have been dominant on upper slopes, especially windswept tops of slopes with shallower profiles, and early sagebrush (Artemisia arbuscula ssp. longiloba) would have been dominant on lower slopes with deeper clay soils. Minor amounts of bud sagebrush (Picrothamnus desertorum) would have been a likely companion in both circumstances. Mixtures of these three sagebrushes occurred in between. The entire ecological site was characterized by dense clay derived from alluviation of Mesozoic age rock. Soils on this site crack deeply with drought, which tends to disfavor shrubs and favor fibrous grass roots. The balance between bunchgrasses and rhizomatous grasses also would have varied with slope position and soil depth, where bunchgrasses would have been found on steeper, shallower sites, while rhizomatous grasses would have been found on deeper soils of lower slopes (Noy-Meir, 1973). 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.
Community 1.1
Low &/or Early Sagebrush/ Bunch & Rhizomatous Grasses
Community Phase 1.1: Low &/or Early Sagebrush/ Bunch & Rhizomatous Grasses
This plant community (1.1) would have been characterized by low sagebrush with an understory of cool season bunchgrasses such as bluebunch wheatgrass (Pseudoroegneria spicata) and squirrel tail (Elymus elymoides) on steeper, shallower sites. Early sagebrush would have occurred with an understory of warm season rhizomatous grasses such as western wheatgrass (Pascopyrum smithii) on lower slopes with deeper soils. A mixture of these species would have occurred on in-between sites.
State 2
Low &/or Early Sagebrush/ Rhizomatous Wheatgrass/ Introduced Herb State
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. Herbs in this state have a later phenology. State 2 is a description of the ecological site shortly following Euro-American settlement, and is characterized by a denser sagebrush stand with a warm season herbaceous understory.
Community 2.1
Low &/or Early sagebrush/ Rhizomatous Wheatgrass
Community Phase 2.1: Low &/or Early sagebrush/ Rhizomatous Wheatgrass
This plant community (2.1) is characterized by a dense stand of low or early sagebrush with a predominately rhizomatous wheatgrass understory. A small component of introduced non-native species, such as cheatgrass (Bromus tectorum) may also be present. This state maintains resiliency (negative feedback) by having ground cover provided by both grasses and leaf litter, which help maintain soil stability and water retention. Reduction of soil stability and water retention results in a more xeric state. A reduction of resiliency (positive feedbacks) occurs when perennial grass cover is reduced, thus putting the state at-risk of further degradation resulting in lack of soil stabilization and reduced soil moisture retention. This can be partially reversed with release from heavy grazing pressure.
State 3
Low &/or Early Sagebrush Super-dominance State
Heavy year-round grazing, mainly by sheep, resulted in a super-dominance of low or early sagebrush. The herbaceous understory was effectively nonexistent. The effective aridity of the Semi-desert Dense Clay ecological site makes it a poor prospect for management toward greater forage productivity. Furthermore, accelerated soil erosion from tillage could create problems further down the watershed. We suggest trials of small scale manipulations that are monitored closely for observation of biological responses and soil erosion.
Community 3.1
Low &/or Early Sagebrush Monoculture
Community Phase 3.1: Low &/or Early Sagebrush Monoculture
This plant community is characterized by a super-dominance of low and/or early sagebrush. The stability of this state is maintained by the absence of perennial grass seed source, and soil loss which prevents the re-establishment of an understory component.
Transition T1A
State 1 to 2
Transition T1a: from State 1 to State 2 (Reference State to Low &/or Early Sagebrush/
Rhizomatous Wheatgrass/ Introduced Herb State)
The simultaneous introduction of exotic species, both plants and animals, and extinctions of native flora and fauna, along with climate change, causes State 1 to transition to State 2, which is the current potential. Reversal of such historic changes (i.e. a return pathway) back to State 1 is not practical. Livestock grazers typically favored spring grazing, and with the diminution of the cool season grasses, the next best use of green forage occurred later in the growing season. As a result, after heavy spring livestock grazing, bunchgrasses could not compete with the rhizomatous species and shrubs.
Transition T2A
State 2 to 3
Transition T2a: from State 2 to State 3 (Low &/or Early Sagebrush/ Rhizomatous Wheatgrass/ Introduced Herb State to Sagebrush Super-dominance State)
Expansion to year-around use (T2), especially with use of itinerant sheep will lead to reduction of the rhizomatous wheatgrasses and sagebrush super-dominance (State 3). The approach to this transition is indicated by a loss of perennial grass understory, an increase in shrub component relative to grasses, and soil loss. Sustained heavy grazing during the growing season will trigger the transition. A restoration pathway is impracticable due to lack of grass seed source and soil loss.
