Lowland

Description

This 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 dominated by cool-season grasses and plains cottonwood. Grazing, fire, and flooding are the major drivers between plant communities. Cottonwood requires flooding to regenerate and, with long periods of no flooding, the plant communities will eventually transition into an Herbaceous State (3.0). The invasion of non-native cool-season grasses and heavy grazing result in a transition to an Invaded State (4.0). The invasion of non-native woody species will cause a transition to a Wooded Invaded State (2.0).

Submodel

Description

This State developed as a result of the invasion of Russian olive and/or saltcedar, in combination with continuous season-long grazing or continuous seasonal grazing and no flooding. With time, the cottonwood will become mature with little or no regeneration occurring. Grazing that limits regeneration also results in a reduction of the desirable native herbaceous species, often resulting in a dominance of species such as Kentucky bluegrass and/or smooth brome, and forbs such as western ragweed, Canada thistle, burdock, and hound’s tongue.

Submodel

Description

This State consists of primarily rhizomatous wheatgrass, needlegrass, and scattered mature cottonwood. It is the result of continuous season-long grazing and/or haying and no flooding. Grasses and grass-like species make up 90 to 95 percent of the plant community. This State is at risk of transitioning to a bluegrass-dominated Invaded State (3.0).

Submodel

Description

This state is the result of invasion and dominance 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, 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).

Submodel