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) was typically dominated by warm-season grasses, with occasional shifts to a near co-dominance of cool- and warm-season grasses. 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, and herbivory by insects and 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 excessive 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.

Submodel

Description

The Native/Invaded 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 tend to 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 and smooth brome, and an increasing thatch layer that effectively blocks introduction of other plants into the system. Heavy grazing or long-term light grazing (understocked) will tend to result in an increase of smooth brome. Non-use and no fire will tend to benefit Kentucky bluegrass due to an increasing thatch layer that effectively blocks the 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, and 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 would tend to 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 Invaded State (3.0) is well established, even drastic events such as high-intensity fires driven by high fuel loads of litter and thatch are unlikely to result in more than a very short-term reduction of Kentucky bluegrass. Fire can reduce the dominance of Kentucky bluegrass, but due to the large amounts of rhizomes in the soil, there is little opportunity for the native species to establish and dominate before Kentucky bluegrass rebounds and again dominates the system.

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 (4.1) was previously tilled for crop 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 (4.2) was typically tilled and then seeded to a perennial forage species or mix of species.

Submodel