Loamy Terrace

Description

The Reference State (1.0) represents what is believed to show the natural range of variability that dominated the dynamics of the ecological site prior to European settlement. The vegetation in the Reference State (1.0) is dominated by cool-season grasses and subdominant warm-season grasses. In pre-European times, the primary disturbance mechanisms included periodic fire and grazing by large herding ungulates. Timing of fires and grazing coupled with weather events dictated the dynamics that occurred within the natural range of variability. Taller cool-season wheatgrasses and needlegrasses would have declined and a corresponding increase in short statured grass and grass-like species would have occurred. Sliver sagebrush and western snowberry will almost always be present. Deciduous trees will likely be found scattered across this site, but regeneration is unlikely. Today, a similar state can be found on areas that are properly managed with grazing and sometimes on areas receiving occasional short periods of rest.

Submodel

Description

The Early Successional State (3.0) is dominated by weedy annuals, threeawn, wheatgrass, and shrubs. This State is the result of heavy disturbance such as frequent and severe defoliation, heavy livestock concentrations coupled with grazing management that does not provide adequate recovery time for cool-season wheatgrasses and needlegrass. The hydrologic function is also likely to be dramatically altered. Runoff is high and infiltration is low. This State is very resistant to change through grazing management alone.

Submodel

Description

The Native/Invaded State has been invaded by Kentucky bluegrass, smooth brome, and annual bromes, but not at the level to which the plant community is dominated by these species. The plant community in this State looks very similar to the Reference Plant Community (1.1) and it functions very much like the Reference State. It is ‘At Risk’ of transitioning to the Invaded State (4.0), which is dominated by Kentucky bluegrass and smooth brome.

Submodel

Description

The Invaded State is the result of invasion of introduced cool-season grass species. This state is characterized by the dominance of Kentucky bluegrass and smooth brome, and an increasing thatch layer that effectively blocks introduction of other plants into the system. Plant litter accumulation tends to favor the more shade-tolerant introduced grass species. The nutrient cycle is also impaired, the result is typically a higher level of nitrogen, which also favors the introduced species. Increasing plant litter decreases the amount of sunlight reaching plant crowns, thereby shifting competitive advantage to shade-tolerant, introduced grass species. Studies indicate that soil biological activity is altered, and this shift apparently exploits the soil microclimate and encourages growth of the introduced grass species. Once the threshold is crossed, a change in grazing management alone cannot cause a reduction in the invasive grass dominance. Preliminary studies indicate this threshold may exist when Kentucky bluegrass exceeds 30 to 35 percent of the plant community and native grasses represent less than 40 percent of the plant community composition. Plant communities dominated by Kentucky bluegrass have significantly less cover and diversity of native grasses and forb species (Toledo, D. et al., 2014).

Submodel

Description

Any plant community can transition to the Disturbed State (5.0). The three separate vegetative plant communities found in this State are highly variable in nature. They are derived through different management scenarios and are not related successionally. Infiltration, runoff, and soil erosion vary depending upon the vegetation present on the site.

Submodel