Subirrigated

Description

The Reference State represents what is believed to show the natural range of variability that dominated the dynamics of the ecological site prior to European settlement. This site in the Reference State (1.0) is typically dominated by warm-season grasses, with occasional shifts to a near co-dominance of cool- and warm-season grasses, grass-likes, and dense stands of shrubs. In pre-European times, the primary disturbance mechanisms for this site in the reference condition included periods of below and above average precipitation, periodic fire, beaver activity, and herbivory by large ungulates. Timing of fires and herbivory coupled with weather events dictated the dynamics that occurred within the natural range of variability. Today the primary disturbance is from a lack of fire and concentrated livestock grazing and haying. Grasses that are desirable for livestock and wildlife can decline and a corresponding increase in less desirable grasses will occur. Today, a similar state will be difficult to find due to the predominance and invasiveness of non-native cool-season perennial gasses and Canada thistle.

Submodel

Description

The Native/Invaded State represents the more common range of variability that exists with higher levels of grazing management but in the absence of periodic fire due to fire suppression. This state is dominated by cool- and warm-season grasses, it can be found on areas that are properly managed with grazing and prescribed burning, and sometimes on areas receiving occasional short periods of rest. Native warm- and cool-season species can decline and a corresponding increase in non-native cool-season grasses will occur. Non-native cool-season grasses will make up less than 15 percent of total annual production. Preliminary studies indicate that when Kentucky bluegrass exceeds 30 percent of the plant community and native grasses represent less than 40 percent of the plant community composition a threshold has been crossed to an Invaded State (3.0). These invaded plant communities that are dominated by Kentucky bluegrass will have significantly less cover and diversity of native grasses and forb species (Toledo, D. et al., 2014).

Submodel

Description

The Invaded State is the result of invasion and dominance of non-native cool-season grass species. This state is characterized by the dominance of Kentucky bluegrass, timothy, and/or smooth bromegrass. Heavy grazing or excessive haying will eventually remove the tall warm-season grasses and promote the increase in the more grazing tolerant non-native cool-season grasses.

Non-use and no fire will result in an increasing thatch layer that effectively blocks introduction of other plants into the system. Plant litter accumulation tends to favor the more shade tolerant non-native grass species. The nutrient cycle is also impaired, and the result is typically a higher level of nitrogen which also favors the non-native 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 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).

Once the state is well established, even drastic events such as high intensity fires driven by high fuel loads of litter and thatch will not result in more than a very short-term reduction of Kentucky bluegrass. These events may reduce the dominance of Kentucky bluegrass, but due to the large amount of rhizomes in the soil, there is no opportunity for the native species to establish and dominate before Kentucky bluegrass rebounds and again dominates the system.

Submodel

Description

In pre-European times the Dense Shrub State was common along most streams and floodplains throughout the majority of the Black Hills. Today this community is of minor extent. Prior to European settlement, conifer density (primarily ponderosa pine and white spruce) was lower in the uplands, resulting in lower evapotranspiration rates and higher water yields into stream hydrologic systems. The dense shrub plant communities consisted of a mixture of several willow species including Bebb willow, yellow willow, and sandbar willow. Other shrubs included river birch, redosier dogwood, wild rose, raspberry, and currant.

Beaver dam complexes were also common on most Black Hills drainages which regulated water flow and maintained high water table levels, creating conditions suitable for dense stands of willow and dogwood. By the latter part of the 1800s, beaver numbers were low and restricted to remote areas of the Black Hills. Riparian ecosystems likely degraded rapidly following beaver removal, generating substantial long-lasting effects.

Dramatic changes in the functional and structural groups that make up diverse riparian plant communities are a result of physical disturbances from past and present use and management. Non-native plant species, both introduced as forage species and invasive species, substantially reduced shrub communities and the transitional deciduous trees (Parrash,1996).

Submodel

Description

This State can be transitioned to from any plant community. The two separate vegetative plant communities, Go-Back and Seeded, are highly variable in nature. They are derived through different management scenarios and are not related successionally. Infiltration, runoff, and soil erosion will vary depending on the vegetation present on the site.

The Go-Back Plant Community (5.1) was previously tilled for crop or hay production and then abandoned. The plant community that develops on this site will be greatly influenced by the plant communities that are located on adjacent land. The Seeded Plant Community (5.2) was typically tilled and then seeded to a perennial forage species or mix of species.

Submodel