Wet Subirrigated

Description

The Reference State represents what is believed to show the natural range of variability that dominated the dynamics of the Wet Subirrigated ecological site prior to European settlement. This site in the Reference State (1.0) is typically dominated by warm- and cool-season grasses, grass-like species, and thick stands of shrubs. In pre-settlement 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 disturbances are the lack of fire and concentrated livestock grazing and wildlife browsing. Grasses that are desirable for livestock and wildlife can decline and a corresponding increase in less-desirable grasses will occur. Favorable growing conditions occurred during the spring and the warm months of June through August. Today, a similar state will be difficult to find due to the predominance and invasiveness of non-native cool-season perennial grasses 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 warm- and cool-season grasses and sedges and 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 grasses and sedges will decline as non-native cool-season grasses increase. 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 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 by non-native cool-season grass species. Dominant grasses include Kentucky bluegrass, timothy, and redtop. Continuous, season-long grazing or heavy, continuous grazing will result in an increase of non-native grasses and forbs. Non-use and no fire will result in an increasing thatch layer that tends to favor the more shade-tolerant non-native grass species. The nutrient cycle is impaired, resulting in a higher level of nitrogen which also favors introduced species. Studies indicate that soil biological activity is altered, and this shift apparently exploits the soil microclimate and encourages growth of the non-native grass species. Once the threshold is crossed, a change in grazing management alone cannot cause a reduction in the dominance of invasive grasses. 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).

Submodel

Description

In the time prior to European settlement, 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 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, yellow, and sandbar. 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, introduced for forage, substantially reduced the shrub communities and the transitional deciduous trees (Parrash,1996).

Submodel