Natural Resources
Conservation Service
Steep Skeletal 10-14 Inch Precipitation Zone Lost River Mountains
Scenario model
Current ecosystem state
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Management practices/drivers
Select a transition or restoration pathway
- Transition T1-3 More details
- Transition T1-3 More details
- Restoration pathway R3-1 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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State 1
Reference State
Reference State
Description
The Reference state consists of two plant communities: The Big Sagebrush/Bluebunch Wheatgrass community (1.1) and the Mixed Shrub/Bunchgrass community (2.2). The overstory in Community 1.1 is dominated by either mountain big sagebrush (Artemisia tridentata spp. vasayana), Wyoming big sagebrush (Artemisia tridentata spp. wyomingensis), or a mix of the two species. The understory is primarily composed of bunchgrasses, with bluebunch wheatgrass (Pseudoroegneria spicata) being the most prevalent. Community 1.2 has a mixed shrub overstory that can include big sagebrush species, little sagebrush (Artemisia arbuscula), chokecherry (Prunus virginiana), antelope bitterbrush (Purshia tridentata), and yellow rabbitbrush (Chrysothamnus viscidiflorus) with a bunchgrass dominated understory.
The Big Sagebrush and Bluebunch Wheatgrass community is the most common community of the reference state and is also the most productive. Canopy cover and production values are close to evenly balanced between the shrub and grass functional groups and forb canopy cover is low.
The Mixed Shrub/Bunchgrass community has a greater diversity in the shrub overstory with a similar understory of bunchgrasses to Community 1.1. Like Community 1.1, canopy cover and production is evenly balanced between the shrub and grass functional groups with forb canopy cover and production being comparatively low.
Processes (both natural and anthropogenic) that can result in state and community changes include fire, grazing, land use change, and occurrence of invasive species (Davies et al., 2011).
Characteristics and indicators
The transition between states at this ecological site is generally driven by sagebrush killing disturbances or lack thereof. Historically, low to mixed severity fires occurred at relatively frequent fire return intervals of 10-25 years, limiting sagebrush canopy density and creating a mosaic of sagebrush stands and more open grasslands (Knick, Holmes, & Miller, 2005). Exclusion of fire (in conjunction with climate change) increases sagebrush canopy cover and can lead to more severe stand replacing fires, often times at more frequent intervals. These severe fires can drive shifts to grassland communities as well as create opportunities for invasive species establishment (Roadhouse, Irvine, & Bowerstock, 2020; Knick et al., 2005).
Improper grazing practices can also lead to an increase in sagebrush canopy cover and increased risk of severe, stand replacing fire occurrence. Removal of understory grasses can lessen the opportunities for low severity fire occurrence which leads to increased canopy densities and increases potential for severe fire disturbance events (Knick et al., 2005).
Resilience management
This site has moderate to high resilience as a result of the cryic soil temperature regime and xeric soil moisture regime. Resistance and resilience of a specific site has been attributed with abiotic conditions favorable to plant growth and reproduction (Maestas et al. 2016). Soils that fall within the xeric (wet) moisture regime tend to have higher diversity and production, and are therefore more resilient. This is specifically true in terms of resisting or recovering from invasion post disturbance (Maestas et al., 2016).
Submodel
Description
The Invaded State exists when invasive species occupy a significant portion of a site's canopy cover. Recruitment of invasive species often follows a localized site disturbance such as fire or chronic defoliation of native plants from improper grazing. The establishment of invasive species on a nearby site increases the likelihood of transition from the Reference state to the Invaded state due to the presence of a seed source.
Characteristics and indicators
This state is characterized by a significant canopy cover of invasive species on a site. The threshold for transition to the Invaded state is a canopy cover of five percent or more of a single invasive species or a collective canopy cover of more than eight percent for all invasive species at a site.
Resilience management
This site has low resilience primarily due to the invasive species establishment. Cheatgrass is well adapted to post-disturbance establishment, particularly after wildfire disturbances. Cheatgrass takes advantage of prolific seedbanks resilient to fire and its ability to rapidly establish when competition from native vegetation has been removed. Additionally, cheatgrass is able to alter fire regimes, shortening fire return intervals which provides a further advantage for cheatgrass and hinders re-establishment of native vegetation (Zouhar, 2003). Once fully established, restoration of sites invaded by cheatgrass either through natural or mechanical means has low success rates.
Dominant plant species
big sagebrush (Artemisia tridentata), shrub
little sagebrush (Artemisia arbuscula), shrub
bluebunch wheatgrass (Pseudoroegneria spicata), grass
cheatgrass (Bromus tectorum), grass
Submodel
Description
The Encroached state occurs when the fire regime at a Steep Skeletal ecological site has been altered and a nearby population of of Douglas-fir (Pseudotsuga menziesii) or Rocky Mountain juniper (Juniperus scopularum) exists. Fire return intervals for big sagebrush-dominated communities are estimated between six and 61 years (Innes, 2018). Little sagebrush has a wider range spanning 10 to 90 years (Steinberg, 2002). Low to moderate-severity fires in sagebrush communities bordering forested ecosystems are the primary mechanism restricting seedling establishment for Douglas fir and Rocky Mountain juniper species.
Fire suppression practices and removal of fine fuels through agricultural grazing increase the time between fire events and allow the establishment of Douglas-fir and Rocky Mountain juniper seedlings. If enough time passes without a fire event, these seedlings become mature, become more resistant to fire events, and can provide a seed source for further encroachment.
Characteristics and indicators
This state is characterized by a canopy cover of Douglas-fir, Rocky Mountain juniper, or a combination of the two equal to or greater than two percent. Seedlings and juvenile individuals of these species occurring outside of main timber stands, mixed in with older growth sagebrush is a strong indicator of transition from the Reference state.
Resilience management
This state can be managed at current conditions or restored to the Reference state through mechanical and managed fire treatments that mimic historic disturbance regimes. Targeted mechanical thinning or use of low to moderate severity prescribed fire are useful tools to manage or restore this ecological site, and depending on levels of encroachment can often be less resource intensive.
Dominant plant species
Douglas-fir (Pseudotsuga menziesii), tree
Rocky Mountain juniper (Juniperus scopulorum), tree
big sagebrush (Artemisia tridentata), shrub
little sagebrush (Artemisia arbuscula ssp. longiloba), shrub
bluebunch wheatgrass (Pseudoroegneria spicata), grass
Sandberg bluegrass (Poa secunda), grass
Submodel
Mechanism
Transition from the Reference state to the disturbed state is often the result of invasive species establishment following a severe disturbance, or a low to moderate severity disturbance that occurs repeatedly. Disturbances such as fire and intensive grazing that result in chronic defoliation remove the canopy cover of native vegetation and create opportunities for invasive species establishment. Once established, many of the invasive species that occur on the ecological site will continue to increase in canopy cover unless interventions are applied.
Constraints to recovery
The distance to a seed source and mechanisms for seed transport of invasive species impact the likelihood of post-disturbance establishment. Many invasive species such as cheatgrass, which is the primary species of concern, can alter disturbance regimes (such as increasing fire frequency and severity) in ways that only further their establishment on a site. Mechanical and chemical treatments as well as post-treatment seeding or planting of invasive species are required for recovery, however, these treatments can be resource intensive with low success rates.
Mechanism
The transition from the Reference state to the Encroached state is a result of changes to fire regimes and fire frequency. Historically, fire return intervals for big sagebrush species have ranged from six to 61 years (Innes, 2018) and 10 to 90 years for little sagebrush (Steinberg, 2002). Fire suppression activities and increased agricultural grazing of understory grass species have increased the time between fire events. This allows for establishment of Douglas-fir and juniper species that would normally be excluded by fire.
Constraints to recovery
Recovery back to Reference state requires a fire event or managed thinning activities that remove the encroaching species. Fuel buildup can occur when the time between fire events is significant (greater than 90 years) and can result in severe, stand-replacing fires when they do occur.
Mechanism
Restoration from the Encroached state to the Reference state can happen naturally or through interventions that mimic historic disturbance regimes. Encroached sites can be restored naturally through low to moderate-severity fire occurrence that kills or dramatically decrease the presence of encroaching species. Interventions that can be used to restore a site in the Encroached state include moderate severity prescribed fire and mechanical thinning treatments, or a combination of the two.
Context dependence
High-severity fire can be stand-replacing and is not ideal for managing encroaching species. Higher severity disturbances can lead to an increase in invasive species establishment and cause a transition to the Invaded state.
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The Ecosystem Dynamics Interpretive Tool is an information system framework developed by the USDA-ARS Jornada Experimental Range, USDA Natural Resources Conservation Service, and New Mexico State University.
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