Ecological dynamics
A State-and-Transition Model (STM) diagram is depicted below. Thorough descriptions of each state, transition, plant community phase, and pathway are found after the model in this document. This diagram is based on available experimental research, field observations, professional consensus, and interpretations. While based on the best available information, the STM will change over time as knowledge of ecological processes increases.
Plant community composition within the same ecological site has a natural range of variability across the LRU due to the naturally occurring variability in weather, soils, and aspect. Not all managers will choose the Reference Plant Community as the management goal. Other plant communities may be desired to meet land management objectives. This is valid as long as the rangeland health attributes assessment departures are none to slight or slight to moderate from the Reference State. The biological processes on this site are complex; therefore, representative values are presented in a land management context. The species lists are representative and are not botanical descriptions of all species occurring, or potentially occurring, on this site. They are not intended to cover every situation or the full range of conditions, species, and responses for the site.
Both percent species composition by weight and percent cover are used in this ESD. Most observers find it easier to visualize or estimate percent cover for woody species (trees and shrubs). Foliar cover is used to define plant community phases and states in the State-and-Transition Model. Cover drives the transitions between communities and states because of the influence of shade and interception of rainfall.
Species composition by dry weight remains an important descriptor of the herbaceous community and of site productivity as a whole and includes both herbaceous and woody species. Calculating similarity index requires data on species composition by dry weight.
Although there is considerable qualitative experience supporting the pathways and transitions within the State-and-Transition Model, no quantitative information exists that specifically identifies threshold parameters between reference states and degraded states in this ecological site. For information on STMs, see the following citations: Bestelmeyer et. al. 2003, Bestelmeyer et. al. 2004, Bestelmeyer and Brown 2005, and Stringham et. al. 2003.
A resource concern risk assessment and dominant resource concerns are provided for each Land Use, State, Plant Community Phase based on NRCS resource concern and planning criteria used to determine resource treatment levels during the conservation planning process. A resource concern is the resource condition that does not meet the minimum accepted levels established by planning criteria as shown in Section III of the NRCS Field Office Technical Guide (https://efotg.sc.egov.usda.gov/#/).
• Low risk means a low probability for the category of resource concerns and additional assessment is typically not necessary.
• Medium risk means that the category of resource concerns could occur, and additional assessment is recommended if the identified resource is a client concern and/or objective.
• High risk means that a resource concern in that category is likely to occur.
The resource categories are: S (soil), W (water), A (air), P (plant), A (animal), E (energy), and H (human). The dominant resource concerns further refine the resource category to a specific resource concern within that category.
State 4
Reference State
The Reference State consists of two communities: the Desert Saltbush/Bunchgrass Community and the Desert Saltbush/Rhizomatous Grass Community. Each community differs in percent composition of bunchgrasses and percent shrub canopy cover. Shrub canopy cover is typically less than 25 percent. The dominant shrub species is Gardner's saltbush with winterfat sub-dominant. Dominant bunchgrasses are Indian ricegrass and bottlebrush squirreltail. Thickspike wheatgrass is the predominant rhizomatous grass. Annual and perennial forbs are a very minor component on this site.
Two important processes occur in the reference state and result in plant community changes: 1) grass-reducing disturbances such as drought and herbivory from small mammals (i.e. prairie dogs); and 2) time without those disturbances, generally referred to as "natural succession".
Prairie dogs select these sites for the protection they provide from predators by providing a clear line of site not provided for in sagebrush dominated sites. The white-tailed prairie dog, with intact pest and predator cycles, is thought to form smaller, more spread out colonies, likely due to habitat suitability and lower forage productivity in a desert environment compared to the large black-tailed prairie dog colonies of the prairies in the Great Plains. Predators and disease kept these small colonies on the move, preventing depletion of forage resources and allowed for recovery of the short-lived perennial bunchgrasses that dominate this site. As an example, Indian ricegrass lives approximately six years and relies upon an adequate seedbank to self-perpetuate. The reference prairie dog disturbance cycle is thought to fit within this timeframe.
Characteristics and indicators. This state is dependent upon an episodic herbivory regime that varies over time, allowing for recovery of shrub and bunchgrass components. When used as winter range, the shrub component is reduced to allow for grass dominance. When occupied by a prairie dog colony, the grass component is reduced to allow shrub dominance on the site.
Management actions or treatments are not typically used to mimic the natural disturbance regime due to fragile nature of the soils and lower productivity potential on this site. Prescribed fire is not used due to lack of fine fuels (Clause and Randall, 2014).
Resilience management. This site has moderate to moderately high resilience due to its aridic ustic (ustic bordering on aridic) soil moisture regime and frigid bordering on cryic temperature regime (Chambers et.al. 2014). Precipitation is typically low, but more effective with cooler temperatures and present when needed during the critical growth period (May through June). The site can usually recover after disturbance but is susceptible to delays in recovery during extreme climatic events such as drought.
The site has moderately high resistance to invasion by annual grasses because of climate limitations (dry and cold). The site may be susceptible to invasion after extreme disturbance during warmer climatic periods.
At the LRU scale, this site is less resilient than Sandy, Loamy, or Clayey sites, but is more resistant to invasion by annual invasive grasses. Lower resiliency and higher resistance is caused by soil chemistry, heavier soil textures, and reduced infiltration, making this site more susceptible to dry and drought conditions and harder for new plants to become established.
Dominant resource concerns
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Sheet and rill erosion
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Plant productivity and health
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Plant structure and composition
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Terrestrial habitat for wildlife and invertebrates
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Feed and forage imbalance
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Inadequate livestock water quantity, quality, and distribution
Community 4.1
Desert Saltshrub/Bunchgrass Plant Community
This community is well adapted to Pinedale Plateau climatic conditions. The diversity in plant species allows for drought tolerance, and natural plant mortality is very low. These plants have strong, healthy root systems that allow production to increase significantly with favorable moisture conditions.
Abundant plant litter is available for soil building and moisture retention. Plant litter is properly distributed with very little movement off-site. This plant community provides for soil stability and a properly functioning hydrologic cycle.
Desert saltshrubs and mid-size, cool season perennial bunchgrasses are co-dominant. Saltshrub foliar cover ranges from 15 to 25 percent. There are generally few canopy gaps, and most are small (one to two feet). Rock cover on the soil surface is low to nonexistent. Many plant interspaces have canopy or litter cover.
The Reference State herbivory regime in this plant community phase is light and episodic by small to medium-sized ungulates (pronghorn, mule deer, elk) and occurs mostly in the winter.
Table 5. Annual production by plant type
| Plant type |
Low
(kg/hectare) |
Representative value
(kg/hectare) |
High
(kg/hectare) |
| Grass/Grasslike |
185 |
308 |
432 |
| Shrub/Vine |
135 |
224 |
314 |
| Forb |
17 |
28 |
39 |
| Total |
337 |
560 |
785 |
Community 4.2
Desert Saltshrub/Rhizomatous Grass Plant Community
This community is a response to herbivory, primarily the intensity and disturbances seen with a white-tailed prairie dog colony with its pest and predator cycles intact. Due to these fluctuations and temporary colonies on the landscape, adequate seed bank of the mid-size bunchgrasses, exists although they are not dominant on the landscape during this plant community phase.
There is less plant litter onsite during this plant community phase, but it is adequate to protect the site from soil erosion. Desert saltshrubs are dominant with rhizomatous grasses in the under-story. Saltshrub foliar cover ranges from 15 to 25 percent. There are a moderate amount of canopy gaps and most are small (one to two feet) with small amounts of large (three to six feet) gaps associated with prairie dog holes. Rock cover on the soil surface is low to nonexistent.
Total annual production ranges from 300 to 700 pounds per acre with a Representative Value (RV) of 500 pounds per acre.
Pathway P 1-2
Community 4.1 to 4.2
Herbivory by prairie dogs is the primary driver in this pathway. However, prolonged drought conditions could be another contributor.
Context dependence. Colonies were low density, small, and dispersed in response to low forage production and existing predator populations. Colonies did not stay on a particular site until forage resources were completely depleted. Instead, they moved frequently in response to forage conditions , disease, and predators.
Pathway P 2-1
Community 4.2 to 4.1
Natural succession is the primary driver in this pathway.
Context dependence. The time period for pathway is dependent upon weather events such as drought and above normal precipitation years as well as pest and predator cycles associated with prairie dogs. Drought results in slower pathway while favorable precipitation can result in a faster pathway. A grazing regime that mimics the historic regime (light intensity, episodic grazing events) will not alter the pathway, but a continuous grazing regime at moderate to heavy intensity puts the site at risk to transition to the Grazing Resistant State.
State 5
Highly Disturbed State
The Disturbed State is a result of extreme soil-disturbing activities outside of the normal disturbance regime expected for this site. Primary examples include road construction, oil and gas exploration, as well as livestock and recreation heavy use areas.
Characteristics and indicators. Primary indicators of this state are extreme soil disturbance associate with anthropogenic activities. Depending on the time since disturbance, recent climatic events, and reclamation efforts, the plant community could be dominated by annual weeds or it could be reclaimed to a variety of introduced or native species. This state could also be represented as a restoration from the Eroded State by a range planting dominated by non-natives such as Russian wildrye or crested wheatgrass dominant with Gardner's saltbush sub-dominant.
Resilience management. Site resilience is lower than the Reference State, but potentially higher than the Grazing Resistant or Eroded States. Site hydrology modifications exist, but may be temporary if proper reclamation occurs.
Site resistance to invasion by annual forbs is lower due to an increase in soil disturbance allowing niches in the understory for establishment. Common annuals to invade this site include kochia, Russian thistle, and halogeton. Cheatgrass may be present if introduced with seeding efforts, but does not dominate the site or influence site dynamics.
Dominant resource concerns
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Sheet and rill erosion
-
Classic gully erosion
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Compaction
-
Organic matter depletion
-
Concentration of salts or other chemicals
-
Plant productivity and health
-
Plant structure and composition
-
Plant pest pressure
-
Terrestrial habitat for wildlife and invertebrates
-
Inadequate livestock water quantity, quality, and distribution
Community 5.1
Reclaimed Plant Community
This plant community is highly variable based on weather conditions during restoration activities, the management practices used, the seed mix, and how soil was stockpiled during the disturbance.
Total annual production ranges from 300 to 700 pounds per acre with a Representative Value (RV) of 500 pounds per acre.
The soil is is typically adequately protected, but erosion can occur during high runoff events. The biotic integrity is dependent on the seed mix used to reclaim the site. There is high variability in watershed function depending on reclamation success.
Community 5.2
Annual Plant Community
As part of succession, all sites that are severely disturbed will go through this plant community as part of their restoration. Weather is the largest determining factor in how long a site will be in this plant community phase, but is approximately two to five years on sites that use Best Management Practices for site restoration (http://www.uwyo.edu/wrrc/). The site has low potential for recovery. Seeding is needed to restore functional structural groups.
Productivity in this plant community phase is highly variable based on current year's weather. Total annual production ranges from 100 to 300 pounds per acre with a Representative Value (RV) of 200 pounds per acre.
Pathway P1-2
Community 5.1 to 5.2
Extreme soil-disturbance from anthropogenic activity such as oil and gas development, road construction, or heavy use areas for livestock production or recreation.
Context dependence. Soils are very fragile, and this pathway could occur from one severe disturbance or multiple consecutive disturbances (e.g. salting areas) or extreme conditions such as hot temperatures, extreme drought, or high intensity grazing will exacerbate this pathway to an annual dominated system.
Pathway P2-1
Community 5.2 to 5.1
Reclamation efforts include replacing topsoil and reseeding. In cases where heavy equipment caused the disturbance, contouring or deep ripping may be necessary to provide a suitable site for re-seeding. Prescribed grazing is necessary to facilitate successful seeding of perennial species.
Context dependence. Drought conditions and herbivory pressure may hinder restoration efforts, and multiple seeding efforts may be necessary if failure is caused by drought. Mulch can be effective for soil moisture retention and erosion control on slopes greater than eight percent.
| Critical Area Planting |
|
| Prescribed Grazing |
|
| Range Planting |
|
State 9
Eroded State
This state occurs when the "A" soil horizon has been lost and the subsoil is exposed, bringing the heavy clay soil layer or natric horizon closer to the surface.
Characteristics and indicators. There will be indicators of reduced soil and site stability as well as reduced hydrologic function, mainly water flow patterns, pedestals, rills, and gullies. Bare ground increases along with plant gap inter-space. Soil surface loss and degradation has occurred. Biotic integrity is affected by missing functional/structural groups and the loss of species diversity within functional/structural groups.
The site experiences little fluctuation in annual production from year to year because it is basically a monoculture of Gardner's saltbush. The site is less diverse with lower quality habitat for wildlife and pollinators.
Resilience management. Site resilience is lower than all other states because the site hydrology has been modified resulting in greater runoff during spring melt and rainfall events. Therefore, the site is drier and unable to recover as quickly after a disturbance. Soil loss and degradation prevents natural regeneration or restoration of the site.
Dominant resource concerns
-
Sheet and rill erosion
-
Classic gully erosion
-
Organic matter depletion
-
Concentration of salts or other chemicals
-
Plant productivity and health
-
Plant structure and composition
-
Plant pest pressure
-
Terrestrial habitat for wildlife and invertebrates
-
Inadequate livestock water quantity, quality, and distribution
Community 9.1
Desert Saltshrub/Bare Ground Plant Community
This community is a response to soil erosion, often as a result of cumulative impacts from multiple herbivores depleting forage resources. An example includes wintering big game and summer/winter livestock and yearlong wild horses and a prairie dog colony existing outside normal reference conditions (broken disease or predator cycles).
Gardner's saltbush dominates, often greater than 60 percent species composition by dry weight. Foliar cover of this species often exceeds 30 percent with the inter-spaces largely bare ground. Soil loss and degradation has occurred, resulting in a moderate or greater departure of soil site stability. Hydrologic function has been impaired due to reduced infiltration, resulting in water flow patterns, pedestalling, rills, and gullies.
Total annual production ranges from 100 to 300 pounds per acre with a RV of 200 pounds per acre.
The biotic integrity is reduced due to low vegetative production, relative dominance and missing structural/functional groups. Soil site stability and the hydrologic cycle have departed from reference conditions with a high amount of bare ground, rills, water flow pattern, soil loss and reduced infiltration due to the lack of perennial herbaceous vegetation.
Pathway 1.1-1.2
Community 1 to 8
• Ground Disturbance
• Moderate herbivory
Transition T 1-2
State 4 to 5
Extreme soil-disturbance from anthropogenic activity such as oil and gas development, road construction, and heavy use areas for livestock production or recreation.
Constraints to recovery. Recovery can be inhibited by soil chemistry (salinity), herbivory pressure, and prolonged drought conditions.
Context dependence. The transition to the Highly Disturbed State can occur quickly with one severe disturbance or over time as a result of the cumulative impacts of several recurring smaller disturbances such as salting areas for livestock production or continued recreational uses.
Transition T 2-3
State 5 to 9
Extreme herbivory resulting in removal of perennial herbaceous vegetation followed by erosional event, typically associated with post-drought high intensity storms. This transition could also occur as a results of the cumulative impacts of many kinds and classes of herbivores such as livestock, wild horses, big game, and prairie dogs.
Constraints to recovery. Recovery can be inhibited by soil chemistry (salinity), herbivory pressure, and modified hydrology on the site.
Context dependence. This transition could also occur as a results of the cumulative impacts of many kinds and classes of herbivores such as livestock, wild horses, big game, and prairie dogs when not managed or occurring within the normal reference conditions (i.e. functioning pest and predator cycles). The timing and intensity of drought, herbivory, and intensity of post-drought storms can affect the resulting level of erosion and hydrologic modifications.
Restoration pathway R 3-2
State 9 to 5
Restoration activities typically involve replacing top soil when it has been removed and deep ripping when site is compacted (after oil and gas exploration or other heavy equipment operations) followed by re-seeding efforts.
Historically, the eroded state was targeted for range seeding with introduced species such as Russian wildrye (Psathyrostachys juncea) which may not have completely restored the site hydrology, but by reintroducing mid-sized cool-season bunchgrasses, infiltration improves with structural/functional groups similar to the reference state.
Context dependence. This site has low potential for recovery due to soil chemistry and frequent drought conditions (https://www.nrcs.usda.gov/wps/cmis_proxy/https/ecm.nrcs.usda.gov%3a443/fncmis/resources/WEBP/ContentStream/idd_D0F7186E-0000-C21A-845D-CD210097FF99/0/PMC_Tech_Note_PM9A.pdf).
Drought and herbivory pressure will affect restoration success, and multiple attempts may be necessary before a reclamation or range seeding will succeed. If seeding of perennial vegetation is successful, it is possible for the site to provide some of the same ecosystem services as the Reference State.
| Critical Area Planting |
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| Prescribed Grazing |
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| Range Planting |
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