Loamy Overflow

Description

This State represents what is believed to exist prior to European settlement. The Reference State may exist, but is unlikely to function within the natural range of variability due to the spread of Kentucky bluegrass and smooth brome in this MLRA. Historically, this State ranged from a tall, warm-season grass-dominated site to one dominated by deciduous saplings and shrubs depending upon disturbance regime. Grazing or the lack of grazing, flooding or lack of flooding, fire, excessive haying, invasion of non-native cool-season grasses, and encroachment of juniper are the major drivers. Timing of fires and grazing coupled with weather events dictated the dynamics that occurred within State.

Dominance during the herbaceous phases of this State shifted between warm-season and cool-season grasses. Although slight shifts may occur in timing of energy capture, hydrologic function, and nutrient cycling between plant community phases within the Reference State, overall the ecological processes were functioning at near-optimum levels. High basal density and deep root systems resulted in low runoff rates and high infiltration rates.

Small areas of trees and shrubs may have existed within this State due to irregularity of burn patterns. Small areas which escaped fire may have permitted trees and/or shrubs to become established. These areas may have served as a seed source for further expansion of the woody-dominated plant community as the fire frequency was altered after settlement.

Submodel

Description

This State is very similar to the Reference State. The Native/Invaded State is still dominated by mid- and tall native warm- and cool-season grasses, but invasive non-native cool-season sodgrasses are now present in all community phases of this State. These non-native cool-season grasses make up to 15 percent of total annual air-dry production. The primary disturbance mechanisms for this State include grazing by domestic livestock and infrequent fires. Timing and intensity of grazing events coupled with weather dictate the dynamics that occur within this State. The cool-season native grass can decline and an increase in introduced sodgrasses will occur. Many times this State appears as a mosaic of community phases caused primarily by continuous season-long grazing.

This State represents the more common range of variability that exists with higher levels of grazing management but in the absence of periodic fire followed by short-term intensive grazing. The Native/Invaded State is dominated by cool- and warm-season grasses. It can be found on areas that are properly managed with grazing and/or prescribed burning, and sometimes on areas receiving occasional short periods of rest. Warm-season species can decline and a corresponding increase in cool-season grasses will occur.

Submodel

Description

This State is the result of invasion and dominance of introduced species. The Invaded 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 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

This State is the result of the establishment of a tree over-story and shrub mid-story canopy. This State is more common on the lower reaches of the site. The dynamics of the Wooded Overstory State are largely due to flooding and successional changes, starting with cottonwood and shrub establishment, and eventually the development of a green ash and boxelder plant community. The successional process can restart following another flooding event. Water control structures which limit flooding, livestock grazing, heavy wildlife browse, fire, the introduction of non-native, cool-season grasses, and encroachment of juniper, can alter the dynamics of this site, resulting in old remnant stands of trees with little, if any regeneration.

Submodel

Description

This State is characterized by over-mature deciduous trees and a lack of regeneration, or the invasion and dominance of eastern redcedar. Shrubs are generally decadent and decreasing, and the herbaceous understory is greatly reduced in number of species and production. In addition, invasive species such as Kentucky bluegrass, smooth bromegrass, cheatgrass, and field bromegrass become established and increase significantly. The hydrologic functions of the Woody Invaded State are often impaired, as the shallow-rooted sod-forming grasses and grass-like species reduce infiltration and increase runoff. This exacerbates the effect of competition by decreasing soil moisture and further reducing the likelihood of regeneration of the deciduous trees.

Submodel