Sandy Lowland

Description

The Reference State (1.0) represents the best estimate of the natural range of variability that dominated the dynamics the Sandy Lowland ecological site prior to European settlement. This site is dominated by warm-season grasses and plains cottonwood. Grazing by large ungulates and small mammals, infrequent fire, flooding, and drought are the major drivers between plant communities. Favorable growing conditions occurred during the spring and the warm months of June through August. Today, a similar state can be found in areas where proper livestock use has occurred. Cottonwood requires flooding to regenerate and, with long periods of no flooding, the plant communities eventually transition into an Herbaceous State (2.0). The invasion of non-native cool-season grasses, continuous season-long grazing, or excessive haying result in a transition to an Invaded State (4.0). The invasion of non-native woody species causes a transition to a Wooded Invaded State (3.0).

Submodel

Description

The Herbaceous State (2.0) is much like the Reference Plant Community (1.1) but without the cottonwood overstory. The primary species in the Herbaceous State are sand bluestem or big bluestem, little bluestem, prairie Sandreed, and needle and thread. This state evolved as the result of stand-removing fire and no flooding to regenerate cottonwood. Grasses and grass-like species make up 85 to 95 percent of the plant community.

Submodel

Description

This State develops as a result of the invasion of Russian olive, non-native cool-season grasses, and, in some cases, saltcedar in combination with continuous season-long grazing or continuous seasonal grazing and with no flooding. Over time, the cottonwood becomes mature and little or no regeneration occurs. Grazing that limits regeneration also results in a reduction of the desirable native herbaceous species, commonly resulting in a dominance of species such as Kentucky bluegrass and smooth brome and forbs such as Cuman ragweed, Canada thistle, burdock, and hound’s tongue.

Submodel

Description

This State is the result of invasion and dominance of introduced cool-season grasses. This state is characterized by the dominance of Kentucky bluegrass and smooth brome and by 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 grasses. Studies indicate that soil biological activity is altered, and this shift apparently exploits the soil microclimate and encourages growth of the introduced grasses. Once the threshold is crossed, a change in grazing management alone cannot cause a reduction in the invasive grass dominance. Preliminary studies 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. Plant communities dominated by Kentucky bluegrass have significantly less cover and diversity of native grasses and forbs (Toledo et al., 2014).

Submodel