State and transition model
State 1
Reference State
The reference state for the ecological sites in this group is typically perennial grasslands consisting of a mixture of cool and warm season grasses. A shrub layer consisting of fourwing saltbush, winterfat, sand sagebrush, Cutler’s jointfir, or green Mormon tea is usually present and is the dominant visual aspect of the sites. In areas of recent deposition or soil movement the shrub layer may be more evident.
The natural disturbance regime consisted of fluctuations in precipitation and infrequent fires ignited by both natural causes and by Native Americans. Herbivory by insects and small mammals were localized influences as was use by native large herbivores.
It is estimated that the historic fire regime was 35-100+ years, depending on fine fuel accumulations (Howard 2003).
Community 1.1
1.1 Perennial Grassland/Shrubland
Indian Ricegrass/Galleta/Fourwing Saltbush, Other native grasses, shrubs, and forbs
This community phase is characterized by a shrub overstory, typically fourwing saltbush, with an understory of perennial cool and warm season grasses. Generally, Indian ricegrass is the dominant cool season grass and James galleta is the dominant warm season grass. Other perennial grasses, shrubs, and forbs may or may not be present and cover is variable.
Community 1.2
1.2 Perennial Grassland
Indian Ricegrass/Galleta, other native perennial grasses and forbs
This plant community phase is characterized by well developed perennial warm and cool season grassland with occasional scattered fourwing saltbush, depending on disturbance history. Dominant grasses include Indian ricegrass, needle-and-thread, James galleta, and sand dropseed. Other perennial grasses, shrubs, and forbs my or may not be present and cover is variable. This plant community typically is a result of a fire in either plant community 1.2 or 1.3.
Community 1.3
1.3 Perennial Shrubland
Fourwing Saltbush/Winterfat/Indian Ricegrass/Galleta and other native perennial grasses and native forbs.
This plant community phase is characterized by a well developed shrub overstory with a minimally developed native perennial grass understory. Dominant shrub species include fourwing saltbush, winterfat, sand sagebrush, and mormontea. Dominant perennial cool and warm season grasses include Indian ricegrass, James galleta and sandhll muhly. Other perennial grasses, shrubs, and forbs may or may not be present and cover is variable. Due to increased bare ground and decreased perennial grass understory this plant community is most at risk for erosion, soil loss, and invasion by introduced species such as cheatgrass, annual mustard, and/or Russian thistle, as well as invasive native species including broom snakeweed.
Pathway 1.1a
Community 1.1 to 1.2
Community 1.1 to 1.2
This pathway occurs when events, such as fire or shrub insect herbivory, favor the establishment of perennial native grasses and a reduction in the shrub canopy.
Pathway 1.1b
Community 1.1 to 1.3
Community 1.1 to 1.3
This pathway occurs when events favor the establishment of native shrubs and a reduction in the perennial grass understory. Events may include time without disturbances, or improper grazing. This is the natural successional pathway from community 1.1 to 1.3.
Pathway 1.2a
Community 1.2 to 1.1
Community 1.2 to 1.1
This pathway occurs when events favor the establishment of native shrubs with minimal loss of the perennial grass understory. Events may include time without disturbances or mild drought.
Pathway 1.3a
Community 1.3 to 1.1
Community 1.3 to 1.1
This pathway occurs when events, such as a cool fire or insect herbivory, favor a minimal decrease in the shrub canopy and the increased establishment of native perennial grasses.
Pathway 1.3b
Community 1.3 to 1.2
This pathway occurs when events, such as a hot fire, favors the establishment of native perennial grasses with a major reduction or removal of the shrub canopy.
State 2
Current Potential State
This state is similar to the reference state in plant community structure and ecological function except non-native invasive plants are established in the understory. This state has lower resistance to disturbances and resilience after disturbance than the reference state due to the occurrence of invasive species and increased bare ground.
Community phases are influenced by fire, native herbivore grazing, domestic livestock grazing, insect herbivory cycles, frequency or lack of disturbances, and weather.
Community 2.1
Perennial Grassland/Shrubland
Indian Ricegrass/Galleta/Fourwing Saltbush, other native perennial grasses and native forb .
Non-native species are now present.
This plant community is characterized by a native perennial bunch grass understory with minimal occurrence of non-native invasive species. Fourwing saltbush forms the dominant shrub overstory. Commonly observed invasive plants include broom snakeweed, Russian thistle, cheatgrass, and annual mustards. Dominant perennial cool and warm season grasses include Indian ricegrass and James galleta. Other grasses, shrubs, and forbs my or may not be present and cover is variable.
Community 2.2
Perennial Grassland
Indian Ricegrass/Galleta other native perennial grasses and native forbs
Non-native species are now present
This plant community phase is characterized by well developed perennial warm and cool season grassland with occasional scattered fourwing saltbush, depending on when the disturbance history. There is minimal occurrence of non-native invasive species. Grass species present include Indian ricegrass, James galleta, and cheatgrass. Commonly seen invasive forbs and shrubs include Russian thistle, broom snakeweed and annual mustards. Other grasses, shrubs, and forbs my or may not be present and cover is variable. This plant community typically is a result of a cool fire in either plant community 2.1 or 2.3 and thus shrubs may or may not be present depending on when the fire occurred.
Community 2.3
Perennial Shrubland
Fourwing Saltbush/Winterfat/Indian Ricegrass/Galleta, other native perennial grasses and native forbs.
Non-native species are now prsent.
This plant community phase is characterized by a well developed shrub overstory with a minimally developed native perennial grass understory. Non-native invasive plants are present. Shrub species include fourwing saltbush, winterfat, broom snakeweed, and various mormontea species, Grasses include Indian ricegrass, cheatgrass, and James galleta. Commonly occurring invasive forbs include annual mustards and Russian thistle. Other grasses, shrubs, and forbs my or may not be present and cover is variable.
Pathway 2.1a
Community 2.1 to 2.2
Community 2.1 to 2.2
his pathway occurs when events favor the establishment of grasses with a complete removal of the shrub component. Events may include fire that allows for the reestablishment of the perennial grasses or grazing livestock in such a way that removes the shrub canopy.
Pathway 2.1b
Community 2.1 to 2.3
Community 2.1 to 2.3
This pathway occurs when events favor the establishment of shrubs with a reduction in the grass understory. Events may include fire suppression, time, or grazing livestock in such a way that reduces the grass understory and allows for increased establishment of the shrub component.
Pathway 2.2a
Community 2.2 to 2.1
Community 2.2 to 2.1
This pathway occurs when events favor the establishment of shrubs with minimal loss of the grass understory. Events may include time without disturbances, mild drought, or grazing livestock in such a way that allows for the increase of shrubs and decrease of grass species.
Pathway 2.3a
Community 2.3 to 2.1
Community 2.3 to 2.1
This pathway occurs when events, such as insect herbivory, favor a decrease in shrub canopy.
Pathway 2.3b
Community 2.3 to 2.2
Community 2.3 to 2.2
This pathway occurs when events favor the establishment of native perennial grasses with a removal of the shrub canopy. Events typically include a cool fire that will significiently reduce or remove the shrubs, but still allow for establishment of native perennial grasses.
State 3
Disturbed State
This state is characterized by a dominance of broom snakeweed, sand sagebrush, Russian thistle, and various jointfir species with minimal occurrence of perennial warm or cool season grasses. The shift in species composition, as well as the increased bare ground, affects the nutrient cycling, soil-water relationships, hydrology, and soil stability of these sites. This state has lower resistance to disturbances and resilience after a disturbance than the previous two states.
The primary disturbance mechanism is improperly managed domestic livestock grazing that maintains the broom snakeweed and bare ground.
Community 3.1
3.1 Disturbed Grassland/Shrubland,
Broom Snakeweed, Sand Sagebrush/Bare ground,
other invasive forbs and grasses. Minimal occurrence of native vegetation
This plant community phase is characterized by a dominance of broom snakeweed and/or sand sagebrush and Russian thistle. The perennial warm and cool season grasses may or may not be present. Fourwing saltbush and Cutler's mormontea also may be present. Cheatgrass is typically present and other invasive forbs are variable. Bare ground is very common. This plant community is most at risk when cheatgrass is present and fine fuel loads are sufficient to carry a fire, causing this community to be at risk of transitioning to an annual grass state.
State 4
Annual Grass State
This states ecological processes are driven by the dominance of cheatgrass production. Other invasive, and some native plant species may also be present. Cheatgrass dramatically affects the soil/plant/water relationships of a site. Research has shown that it has substantial effects on soil water content and temperature, as well as impacts on the frequency and intensity of disturbance. After a cheatgrass invasion, a sites fundamental nutrient cycling processes, root pores, mycorrhizal associations, microbial species, and soil organic material are changed (Chapin et al. 1997; Belnap and Phillips, 2001). These alterations may eventually create ecologically impoverished sites that are very difficult to restore to functionally. The competitiveness of cheatgrass and its ability to quickly establish after a disturbance make this state extremely resistance to change and resilient after a disturbance.
Community phases are maintained, in a self-sustaining manner, by frequent fire.
Community 4.1
4.1 Cheatgrass Dominated,
Cheatgrass, Indian Ricegrass, Galleta, Fourwing Saltbush, Native Forbs, Invasive Forbs
This plant community is characterized by dominance of cheatgrass, other native species are present but no longer drive the ecological dynamics of the site. Fire can carry through this community.
Community 4.2
Cheatgrass Monoculture,
Cheatgrass and Invasive Annual Forbs
This community is characterized by a complete monoculture of cheatgrass, where other grasses and shrubs do not occur. Invasive annual forbs may or may not be present, depending on current climatic conditions. This plant community is self-enhancing through frequent fire.
Pathway 4.1a
Community 4.1 to 4.2
Community 4.1 to 4.2
This pathway is characterized by frequently occurring fires (every 5-10 years) that allows for establishment of a cheatgrass monoculture.
Transition T1A
State 1 to 2
State 1 to 2
This transition is from the native perennial grass understory in the reference state to a state that contains invasive plants such as cheatgrass, Russian thistle, and annual mustards. Events that may trigger this event include fire, improper domestic livestock grazing, and extended drought that stress the native plants and allow nutrients to become available for more tolerant invasive species. Non-native, invasive species such as cheatgrass, however, have been known to invade into intact perennial plant communities where little to no disturbances have occurred. Once invasive species have established in the understory a threshold has been crossed.
Transition T2A
State 2 to 3
State 2 to 3
This transition from the Current Potential State (State 2) to the Disturbed State (State 3). This transition occurs when events favor broom snakeweed to dominate the site. Bare ground increases and perennial/annual grass production decreases. Events typically include improperly managed domestic livestock grazing for extended periods of time. This transition may occur from any community in the current potential state. Once broom snakeweed and bare ground dominate, and drive the ecological dynamics of a site, a threshold has been crossed.
Transition T2B
State 2 to 4
State 2 to 4
This transition from the Current Potential State (State 2) to the Annual Grass State (State 4). This transition occurs when events allow cheatgrass to dominate the site. Bare ground typically will decrease as cheatgrass increases. A hot, catastrophic fire occurring in any community in the current potential state can cause this transition to occur. This often causes an increase in the fire return interval. Once cheatgrass dominates and drives the ecological dynamics of a site, a threshold has been crossed.
Restoration pathway R3A
State 3 to 2
State 3 to 2
This restoration from the Disturbed State (State 3) to the Current Potential State (State 2). This restoration pathway occurs as native perennial shrubs and grasses are able to reclaim a site over time when improper livestock grazing and other disturbances are removed. For this to occur there must still be a seed source for native grasses and shrubs near or on the site. This restoration pathway may take many decades to complete.
Transition T3A
State 3 to 4
State 3 to 4
This transition is from the Disturbed State (State 3) to the Annual Grass state (State 4). This transition occurs when cheatgrass production is sufficient in the disturbed state for fire to be able to carry through the ecological site. Once cheatgrass drives the ecological dynamics of the site, a threshold has been crossed.
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