Semidesert Shallow Sandy Loam (Shadscale)
Scenario model
Current ecosystem state
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Management practices/drivers
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- Transition T1a More details
- Transition T2a More details
- Transition T2b More details
- Transition T3a More details
- Transition T4a More details
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No transition or restoration pathway between the selected states has been described
Target ecosystem state
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Description
This Reference State represents that natural range of variability that dominates the dynamics of this ecological site. This state includes the biotic communities that would become established on the ecological site if all successional sequences were completed without interferences by man under the present environmental conditions. Natural disturbances are inherent in its development. This state is dominated by shadscale and perennial native bunchgrasses and forbs. Broom snakeweed, while not a climax species is present and its cover fluctuates in the different plant communities based on precipitation trends (Ralphs and Sanders, 2002) and perennial shrub nutrient competition (Tirmenstein, 1999). The primary disturbance mechanisms that are inherent to this state include precipitation fluctuations and it's associated insect infestations, as well as time without disturbances.
Reference State: Community phases maintained by precipitation fluctuations, insects, and time.
Indicators: A diverse perennial shrub, grass, and forb community that is dominated by shadscale, mormontea species (Cutler, Torrey, and/or green), James galleta, Indian ricegrass, and needle-and-thread.
Feedbacks: The establishment of invasive species in the understory and climatic conditions that maintain the perennial shrub and grass community.
At-risk Community Phase: All communities are at risk when nutrients become available for invasive species to establish.
Trigger: Introduction of invasive species to fill available niches.
Submodel
Description
This state is similar to the reference state in plant species composition and ecological dynamics except there are now invasive species present. Typically cheatgrass, tansy mustard, and Russian thistle have become established. The native perennial shrubs and grasses still dominate the site in terms of both cover and production. Broom snakeweed has not yet increased to a point where it is driving the ecological dynamics of the site; however it may be slightly more prevalent in state two than in state one.
Current Potential State: Community phases maintained by fluctuating climatic events and domestic livestock grazing.
Indicators: A diverse shrub, forb, and grass community dominated by shadscale, mormontea, Indian ricegrass, James galleta, and needle-and-thread. Invasive species are now present.
Feedbacks: Increased occurrence of invasive species, such as broom snakeweed and cheatgrass. Fluctuating precipitation and properly managed livestock grazing that allows for the continued maintenance of the perennial grass and shrub community.
At Risk Community Phase: All phases are at risk as invasive species continue to out-compete native perennial vegetation. Community phase 2.1 is especially at risk. This phase can transition into the broom snakeweed state (state 3) with prolonged improperly managed livestock grazing.
Trigger: Increased establishment and propagation of invasive species such as cheatgrass and broom snakeweed; typically, caused by a decrease in the fire return interval and prolonged improperly managed livestock grazing.
Submodel
State 3
Broom Snake Weed State
Description
This state is characterized by a dominance of broom snakeweed. Native perennial grasses and shrubs may also be present depending on which community phase the plant community represents. Native shrubs include shadscale and mormontea species, and native grasses include James galleta, Indian ricegrass, and needle-and-thread. Invasive species such as cheatgrass, tansy mustard, and Russian thistle are typically present in all community phases. Other grasses, forbs, or shrubs may or may not be present and cover is variable.
Broom Snakeweed State: Community phases maintained by improper livestock grazing, time with out disturbances, and fluctuating precipitation trends.
Indicators: A community dominated by broom snakeweed and other invasive species, where native perennial grass and shrubs may also be present.
Feedbacks: A fire or other disturbance that decreases the broom snakeweed cover and allows for the proliferation of cheatgrass. Conditions that allow for continued dominance of broom snakeweed.
At Risk Community Phase: All phases are at risk for invasion and dominance of cheatgrass.
Trigger: A fire or other disturbance that removes the broom snakeweed and allows for the dominance of cheatgrass.
Submodel
Description
This state is characterized by a dominance of annual grasses, on the Colorado Plateau, this is typically cheatgrass. Cheatgrass dramatically affects the soil/plant/water relationships of a site. Research has shown that plant species differs substantially in it's effects on soil water content and temperature, and on their effects on the frequency and intensity of disturbance. After cheatgrass has invaded a site, the fundamental nutrient cycling processes, root pores, mycorrhizal associations, microbial species, and soil organic material change (Chapin et al. 1997; Belnap and Phillips, 2001). These alterations may eventually create ecologically impoverished sites that are very difficult, if not impossible, to restore to functionally diverse perennial herbaceous and woody communities. The competitiveness of cheatgrass and its ability to quickly establish after a disturbance make this state extremely resistant to change and resilient after a disturbance.
Annual Grass State: Community phase maintained by fire and time without disturbance
Indicators: A site dominated by cheatgrass, where other invasive species, such as broom snakeweed may or may not be present
Feedbacks: Improper livestock grazing that may allow for the reestablishment and dominance of broom snakeweed. A self sustaining disturbance regime, such as fire and a rapid uptake of moisture in the spring, that allows cheatgrass to flourish on the site.
At-risk Community Phase: Plant community 4.1 is most at risk. The community phase may transition into the broom snakeweed phase through prolonged improperly managed livestock grazing.
Trigger: Prolonged improper livestock grazing that may allow for the reestablishment of broom snakeweed.
Submodel
Mechanism
Transition from the Reference State (State 1) to the Current Potential State (State 2).
This transition occurs due to the establishment invasive plant species, including cheatgrass, tansy mustard, Russian thistle, annual Cryptantha, and annual stickseed. Events triggering this event include can include and combination of improper livestock grazing, extended drought, poorly managed off road vehicle (OHV) use, etc. However cheatgrass and other invasive species have been known to establish into intact perennial plant communities without disturbance triggers.
Mechanism
Transition from Current Potential State (State 2) to the Broom Snakeweed State (State 3).
This transition occurs as events, such as sustained improper livestock grazing, favor the dominance of broom snakeweed. Due to the unpalatable and toxic nature of broom snakeweed, increased domestic livestock grazing, especially cattle, can result in a decrease of perennial grasses and shadscale and an increase in broom snakeweed. Grazing by sheep is less destructive due to the animal’s ability to more readily consume snakeweed, especially in winter; however improper sheep grazing will still result in plant community changes. Community phase 2.1 is most likely to transition into this state with prolonged improper grazing. For sites in community phase 2.2, improper grazing may transition it into either a plant community phase 2.1 and if improper grazing continues without influences by fire, into state 3 due to increased broom snakeweed establishment.
Mechanism
Transition for Current Potential State (State 2) to the Annual Grass State (State 4).
This transition occurs as events, such as fire or other surface disturbance, favor the dominance of cheatgrass. Cheatgrass typically invades the sites interspaces and once this species dominates, the amount and continuity of fine fuels can increase until fire can fully eliminate the shrub/forb/perennial grass component and completes the conversion to a cheatgrass dominated site that can persist for long periods of time. Once this occurs, it is difficult for these species to reestablish themselves, because not only has the fire return interval been shortened, but the soil and other abiotic factors may have been altered.
Mechanism
Transition from Broom Snakeweed State (State 3) to Annual Grass State (State 4).
This transition occurs as events favor the increased establishment and dominance of cheatgrass. Typically this occurs due to fire. Broom snakeweed is highly combustible and is generally killed by severe fire (Tirmenstein, 1999). The rate of broom snakeweed recovery depends on fire severity, so many times a hot fire will trigger a direct transition into a cheatgrass monoculture (community phase 4.2). A cooler, less severe fire triggers a transition into a mixed invasive species community that will be dominated by cheatgrass (community phase 4.1).
Mechanism
Transition from Annual Grass State (State 4) to Broom Snakeweed State (State3).
This transition occurs as events favor the establishment and dominance of broom snakeweed. This transition may occur after prolonged improper livestock grazing, which decreases fine fuels and allows for the reestablishment of broom snakeweed due to an increase in the fire return interval. Because of the unpalatable nature of broom snakeweed, increased domestic livestock grazing, especially cattle, can result in a decrease of grasses and an increase of snakeweed. Grazing by sheep is less destructive due to the animal’s ability to more readily consume snakeweed. Cheatgrass, while tolerant of grazing, can be reduced by heavy spring grazing due to its palatability. Heavy spring grazing also reduces seed production, which decreases it abundance on a site (Zouhar, 2003).
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