Tableland Swale

Description

The Grassland State is supported by empirical data and is defined by three native plant communities that are a result of periodic fire and drought, and herbivore and ungulate grazers. These events are part of the natural disturbance regime and climatic process that contribute to the development of the site. The Reference Plant Community consists of tall-and mid- warm- and cool-season grasses, forbs, and shrubs. Plant community 1.2 is dominated by western wheatgrass, sideoats grama, and blue grama, and combined with a minor component (2-10 percent composition by weight) of Reference Community plant species. Plant community 1.3 is named the At-Risk Plant Community. This plant community is most vulnerable to exceeding the resilience limits of the Grassland State and transitioning to an alternative state. This plant community is dominated (40-100 percent composition by weight) by blue grama and buffalograss. Sub- dominant (10-40 percent composition by weight) species include western wheatgrass, sand dropseed, and threeawns.

Heavy, continuous use throughout the year combined with inadequate rest and recovery of the dominant Reference Plant Community species will reduce the health and vigor of the dominate plant species, allowing western wheatgrass, sideoats grama, and blue grama to increase. To a small extent, some buffalograss will also increase. After the vegetation is reduced to western wheatgrass and heavy- continuous grazing of the dominant plant species continues, blue grama, buffalograss, sand dropseed, and threeawns will increase. There also could be an increase in species such as kochia, Russian thistle, and other undesirable annuals.

The following paragraphs are narratives for each of the described plant communities. These plant communities may not represent every possibility, but they probably are the most prevalent and repeatable plant communities that exist on this ecological site. The plant composition table following has been developed from the best available knowledge at the time of this revision. As more data is collected, some of these plant communities may be adjusted or removed, and new ones may be added. The main purpose for including any description of a plant community here is to capture the current knowledge and experience at the time of this revision.

Submodel

Description

The Reference Grassland State ecosystem has been driven beyond the limits of ecological resilience and has crossed a threshold into the Shortgrass State. The designation of the Shortgrass State denotes changes in infiltration, runoff, bulk density, and species composition. The changes in soil properties, water movement, and the plant community affects changes in the ecosystem. This affects the hydrologic functionality, biotic integrity, and soil and site stability of the site.

With long term, heavy, continuous grazing, buffalograss and blue grama will become the dominant species and have a sod-bound appearance. Unable to withstand the grazing pressure, only a remnant population of western wheatgrass remains.

Species diversity has been reduced further. Water infiltration is reduced due to the sod nature of the buffalograss and blue grama. There is a possible increase in bulk density due to concentrated animal impact over many years. Runoff is increased.

Vegetation type affects the amount and structure associated cover, therefore the infiltration rate differs among vegetation types. The amount of cover, and hence the rate of infiltration, is usually greatest under trees and shrubs, followed in decreasing order by bunchgrass, shortgrass, and bare ground (Blackburn, 1975; Thurow et al., 1986).

Specific dynamic soil property changes between the Grassland State and the Shortgrass State has been documented. As plant community cover decreases from bunchgrasses to more of the sod grasses there is a decrease in infiltration and interception and an increase in surface runoff (Thurow, 2003).

There is no known timeframe or restoration pathway success from this state to the Grassland State. Experience suggests that long-term prescription grazing to include a forage and animal balance, and adequate recovery periods following each grazing event over long periods of time will gradually move this plant community toward the Grassland State. This process is not well documented and may take greater than 40 years.

This alternative state will be tested through long-term observation of ecosystem behavior and repeated application of conservation and restoration practices. This state should be re-evaluated and refined continually.

Submodel

Description

The Reference Grassland State ecosystem has been driven beyond the limits of ecological resilience and has crossed a threshold into the Tillage State. The designation of the Tillage State denotes changes in infiltration, runoff, bulk density, aggregate stability, and species composition. The inherent changes in soil properties, water movement, and the plant community affect changes in hydrologic functionality, biotic integrity, and soil and site stability.

This group includes two separate vegetation states that are highly variable. They are derived through two distinct management scenarios, and are not related successionally. Infiltration, runoff, and soil erosion varies depending on the vegetation present.

The Tillage State consists of abandoned cropland that has been naturally revegetated (go-back) or mechanically planted/seeded to grassland. Many reseeded plant communities were planted with a local seeding mix under the Conservation Reserve Program (CRP) or were planted to a monoculture of sideoats grama. Go-back communities are difficult to define due to the variability of plant communities that exist. Many of these communities are represented by the genus Aristida (three-awns).

This is an alternative state because the ecological functions, i.e. dynamic soil properties and plant communities, are not fully restored to that of the Reference State. Tillage can destroy soil aggregation. Soil aggregates are an example of dynamic soil property change. Aggregate stability is critical for infiltration, root growth, and resistance to water and wind erosion (Brady and Weil, 2008)

This alternative state should be tested through long-term observation of ecosystem behavior and repeated application of conservation and restoration practices. This state should be re-evaluated and refined continually.

Submodel