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 flocks followed, largely replacing cattle as the browse component increased.
Below is a State and Transition Model diagram to illustrate 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 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.
Community 1.1
Wyoming big sagebrush/ abundant herbaceous understory
Community Phase 1.1: Wyoming big sagebrush/ abundant herbaceous understory
This community was characterized by the co-dominance of shrubs, primarily Wyoming big sagebrush, and native perennial herbs. Productivity would have been dependent upon the moisture availability in the vadose (above the water table) zone.
Community 1.2
Caespitose bluebunch wheatgrass
Community Phase 1.2: caespitose bluebunch wheatgrass
This was a temporary grassland variant of this plant community, dominated by the caespitose form of bluebunch wheatgrass. This community would have occurred shortly following wildfire or a sagebrush-killing pathogen outbreak, and would exist for approximately a decade.
Community 1.3
Tall dense Wyoming big sagebrush/ diminished understory
Community Phase 1.3: tall dense Wyoming big sagebrush/ diminished understory
This community would have been characterized by a dense, tall stand of Wyoming big sagebrush with a diminished understory component.
Pathway CP 1.1A
Community 1.1 to 1.2
Community Pathway 1.1a
Wildfire would have temporarily created a grassland variant of the plant community.
Pathway CP 1.1B
Community 1.1 to 1.3
Community Pathway 1.1b
An unusually wet period (i.e. El Nino Southern Oscillation), followed by an outbreak of Aroga moth or snow mold would have temporarily created a grassland variant of this plant community.
Pathway CP 1.2A
Community 1.2 to 1.3
Community Pathway 1.2a
Sagebrush would have begun to re-establish after a period of approximately 10 years after a return to a normal climate (temperatures and precipitation) and without wildfire.
Pathway CP 1.3A
Community 1.3 to 1.2
Community Pathway 1.3a
As time progressed since the last sagebrush-killing event, Wyoming big sagebrush would become progressively denser and taller at the expense of (suppressing) the understory species.
State 2
Super-dominant Wyoming Big Sagebrush State
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.
Community 2.1
Tall dense Wyoming big sagebrush/ increasing bare patches & coppice mounds
Community Phase 2.1: Tall dense Wyoming big sagebrush/ increasing bare patches & coppice mounds
This phase is characterized by having tall, dense Wyoming big sagebrush, the presence of minor amounts of exotic annuals and biennials, and mounded micro-relief.
Community 2.2
Increased non-native herbaceous understory/Decadent Wyoming big sagebrush
Community Phase 2.2: Increased non-native herbaceous understory/ Decadent Wyoming big sagebrush
This phase is characterized by an increase in the non-native herbaceous understory species and an increase in the height and age of Wyoming big sagebrush.
Pathway CP 2.1A
Community 2.1 to 2.2
Community Pathway 2.1a
Sustained non-use by livestock in combination with an unusually wet winter and/or spring will cause the non-native herbaceous understory component to increase and the sagebrush component to increase in height and age (i.e. to become decadent).
Pathway CP 2.2A
Community 2.2 to 2.1
Community Pathway 2.2a
Heavy year-round livestock grazing will remove more of the herbaceous understory component resulting in an opportunity for more sagebrush to establish and leaving more bare soil patches.
State 3
Introduced Annuals & Biennials State
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.
Community 3.1
Invasive annuals dominant
Community Phase 3.1: Invasive annuals dominant
This plant community will develop where fire return intervals are frequent and annual species such as cheatgrass and Russian thistle predominate.
Community 3.2
Biennials dominant
Community Phase 3.2: Biennials dominant
This plant community will develop when intervals between fires are longer, allowing biennial species such as knapweeds, and tumble mustard to become established.
Pathway CP 3.1A
Community 3.1 to 3.2
Community Pathway 3.1a
When intervals between fire events are prolonged, biennial forbs will dominate the plant community.
Pathway CP 3.2A
Community 3.2 to 3.1
Community Pathway 3.2a
When fire return intervals are frequent, annual forbs and grasses will dominate the plant community.
State 4
Crested Wheatgrass State
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.
Community 4.1
Crested wheatgrass
Community Phase 4.1: Crested wheatgrass
This plant community was artificially created by tilling and seeding of crested wheatgrass to increase forage for livestock.
Community 4.2
Woody encroachment/ Crested wheatgrass
Community Phase 4.2: Woody encroachment/ Crested wheatgrass
Crested wheatgrass production is greatly reduced while the site’s woody predecessors, including rabbitbrush and sagebrush, regain dominance.
Pathway CP 4.1A
Community 4.1 to 4.2
Community Pathway 4.1a
Unrestricted heavy use by livestock during the growing season will eventually limit production by crested wheatgrass and allow the encroachment of woody species.
Pathway CP 4.2A
Community 4.2 to 4.1
Community Pathway 4.2a
Maintenance of this state requires retreatment of the brush using chemicals or tilling to maintain grass dominance and remove re-invaded shrubs.
Transition T1A
State 1 to 2
Transition T1a: from State 1 to State 2 (Reference State to Super-dominant Wyoming Big Sagebrush State)
The simultaneous introduction of exotic species, both plants and animals, and possible extinctions of native flora and fauna, along with climate change, has caused State 1 to transition to State 2. The unrestricted year around grazing on these sites, first by cattle and later by sheep (Parson 1996), was the major cause of the changes between State 1 and State 2. Reversal of such historic changes (i.e. a return pathway) back to State 1 is not practical.
Transition T2A
State 2 to 3
Transition T2a: from State 2 to State 3 (Super-dominant Wyoming Big Sagebrush State to Introduced Annuals & Biennials State)
A hotter, larger fire usually follows a build up of fine fuels in the understory in State 2 leading to a transition from State 2 to State 3. Rest from livestock use may facilitate this fuel loading by allowing for the accumulation of more continuous fine fuels, especially following wet winters and springs.
Reductions in the intensity of livestock grazing or changes in seasons of use have had little effect changing the proportions of shrubs and herbs in these communities due to the super-dominance of sagebrush.
Transition T2B
State 2 to 4
Transition T2b: from State 2 to State 4 (Super-dominant Wyoming Big Sagebrush State to Crested Wheatgrass State)
Because of national demands for red meat production following World War II, many of the areas in State 2 were tilled and seeded to crested wheatgrass to increase forage for livestock.
Transition T3A
State 3 to 4
Transition T3a: from State 3 to State 4 (Introduced Annuals & Biennials State to Crested Wheatgrass State)
Similarly to the impoverished sites in State 2, because of national demands for red meat production following World War II, many of the areas in State 3 were also tilled and seeded to crested wheatgrass (T3a) to increase forage for livestock, especially in areas which re-burned frequently.
