SR Shallow Escarpment 9-12 PZ

Description

The Reference State is representative of the natural range of variability for the site under pristine conditions. The Reference State is a bunchgrass shrubland. State dynamics are maintained by interactions between climatic patterns and disturbance regimes. Negative feedbacks enhance ecosystem resilience and contribute to the stability of the state. These are maintained by ecosystem processes and structural elements such as the presence of all structural and functional plant groups, the retention of organic matter and the maintenance of plant community cover. Plant community phase changes are primarily driven by infrequent fire and periodic drought.

Submodel

Description

This state is similar to the Reference State. Ecological function has not changed fundamentally, however the resiliency of the site has been reduced by the presence of invasive plants. Additionally, livestock herbivory may be present as a disturbance process and changes in climate may be altering ecological dynamics. Non-native plant species may increase in abundance but will not become dominant or control ecological processes within this state. These species can be highly flammable and can promote fire where historically fire had been infrequent. Negative feedbacks enhance ecosystem resilience and contribute to the stability of the state. These are maintained by ecosystem processes and structural elements such as the presence of all structural and functional groups, and retention of organic matter and nutrients. Positive feedbacks driven by plant community invasion decrease ecosystem resilience and stability of the state. These include exotic plant species' high seed output, persistent seed bank, rapid growth rate, ability to cross pollinate, and adaptations for seed dispersal. Plant community phase changes are primarily driven by infrequent fire, periodic drought, and ungulate herbivory.

Current potential plant communities mirror those of the above Historical Reference (State 1) yet with the addition of a low level of invasive exotic plant invasion and influences of livestock herbivory. Livestock herbivory may result in decreases in deep rooted perennial grasses, and related increases in shallow-rooted perennial grasses (such as Sandberg's bluegrass), unpalatable forbs and shrubs.

Submodel

Description

Within this state, site resources are primarily controlled by shrub species. Native deep-rooted perennial grass composition has been reduced considerably, with shallow-rooted and disturbance adapted grasses such as Sandberg bluegrass increasing. Wyoming big sagebrush dominates the shrub overstory, with rabbitbrush, wild crab apple and antelope bitterbrush sometimes common. Exotic herbaceous species such as cheatgrass and medusahead are likely.

Sagebrush cover has increased beyond the natural range of variability for the site and may be decadent, reflecting stand maturity and lack of seedling establishment due to competition with mature plants. The dominance of site resources by the shrub overstory and Sandberg bluegrass understory leads to a temporal redistribution of soil water, nutrient capture, nutrient cycling and soil organic matter. Bare ground may be significant with soil redistribution occurring between interspace and shrub locations. Western juniper increases and may begin to influence the understory vegetation.

Submodel

Description

Within this state, site resources are primarily controlled by exotic annual and perennial herbaceous species. Native perennial grass composition has been greatly diminished. Shrub species and western juniper may also be present.

Multiple plant communities are possible within this state, all of which are dominated by invasive annual grasses such as cheatgrass and medusahead and potentially invasive annual and perennial forbs. Sagebrush, wild crab apple, bitterbrush and rabbitbrush may be common in the overstory. Overtime, with increasing invasion, soil stabilizing perennial root biomass will be decreased. Bare ground will increase during winter, potentially increasing erosion even on this low to moderate slope site during extreme weather events.

Submodel

Description

This state is characterized by a dominance of young juniper (less than 100 years old) in the overstory. Big sagebrush and perennial bunchgrasses may still be present, but they are no longer controlling site resources. Soil moisture, soil nutrients and soil organic matter distribution and cycling have been spatially and temporally altered. Juniper encroachment can decrease cover of perennial grasses and shrubs by reducing light availability and altering site hydrology through increased interception of precipitation, reduced infiltration and increased erosion. Bare ground will increase and erosion may be increased even on this low to moderately sloping site during extreme weather events.

Submodel

Description

Within this state, site resources are primarily controlled by introduced range grasses such as crested wheatgrass (Agropyron cristatum) and intermediate wheatgrass (Thinopyrum intermedium). Multiple community phases may occur within this state and will include different compositions of native and introduced shrub and grass species. Western juniper may be present. Similar to ecological dynamics within other states of this site, shrub species and western juniper will increase with greater time since fire and herbivory pressure. While introduced range grasses may provide some analogous ecological functions to native perennial grasses, they may outcompete native grasses in the long-term.

Submodel

Description

This state is characterized by significant soil loss through wind and water erosion. Most herbaceous cover has been lost and soil stabilizing microbiotic soil crusts are degraded. Loss of these components reduces soil stability and renders the soil surface vulnerable to raindrop impacts, runoff and transport of soil by water and entrainment of soil particles by wind. Extensive areas of erosion pavement are common. Rill and gully formation may occur, especially toward the higher slope range of this site. Positive feedbacks develop as low vegetative cover leads to decreased litter and organic matter production, higher soil temperature variability, and decreased nutrient cycling, further destabilizing soil and decreasing potential plant establishment. Grass and forb vegetative cover is low yet invasive annual plants are likely and juniper cover is likely high. Potential for rehabilitation of this state is unknown but would likely pose significant challenges due to degradation of abiotic function of the site, the susceptibility of soils to erosion and damaging frost heaving, and mechanical limitations due to areas of rock outcrops and slopes.

Submodel