Ecological dynamics

Overview
This site occurs on broad alluvial flats. The soils are deep and well drained. They are salt-affected, often with visible salt accumulations on the soil surface. Permeability is slow and soils are susceptible to erosion and gullying. The reference state is a disclimax maintained by recurring fire. The reference community is a grassland with scattered shrubs and a minor component of forbs. Alkali sacaton dominates this site. Loss of grass cover due to overgrazing, drought, or other disturbance; soil sealing; and increased competition by shrubs may cause the transition to the Shrub-dominated State. The continued loss of grass cover in conjunction with soil sealing, increased overland flow, and resulting erosion may initiate the transition to the Gullied State.

Reference State: The reference plant community is dominated by alkali sacaton. Other important grasses that appear on this site include western wheatgrass, blue grama, vine mesquite, galleta, inland saltgrass, and spike muhly. Shrubs characteristic of this site include fourwing saltbush, shadscale, and greasewood. Continuous heavy grazing typically causes a decrease in western wheatgrass, vine mesquite, and spike muhly. On areas less affected by salt, blue grama and galleta codominate and alkali sacaton is subdominant. On areas with greater salt concentrations, a sparser, less vigorous community dominated by alkali sacaton with subdominant inland saltgrass may result.

Diagnosis: Grass and litter cover is uniform with few large bare areas present. Shrubs are scattered with canopy cover averaging 6%. Evidence of erosion such as pedestalling of grasses, rills, and gullies is infrequent.

Shrub-dominated State: This state is characterized by the predominance of shrubs, including fourwing saltbush, shadscale, and greasewood. Rabbitbrush and broom snakeweed may also increase in representation. Alkali sacaton and inland saltgrass are typically the dominant grasses. Fourwing saltbush or shadscale typically dominate with a patchy understory of alkali sacaton and inland saltgrass with lesser amounts of blue grama, galleta, threeawns, and mat muhly. Greasewood may dominate on soils with high salt concentrations or where livestock exhibit a preference for fourwing saltbush and shadscale. Greasewood can dominate in nearly pure stands with a sparse understory of inland saltgrass on soils with high salt concentrations. Areas with lower salt concentrations may have less greasewood and have a greater variety of subordinate shrubs and understory grasses.

Diagnosis: Shrubs are dominant. Grass cover is patchy to sparse with frequent large bare areas present. Soil crusting and sealing is prevalent. Evidence of erosion such as pedestalling of plants, elongated water flow patterns, litter dams, and rills or small gullies is common.

Transition to Shrub-dominated State (T1A) Loss of grass cover due to overgrazing and drought in conjunction with increased competition for resources by shrubs is believed to initiate this transition. In addition, soil sealing by means of chemical and physical crusts decrease infiltration and inhibit grass establishment.

Key indicators of approach to transition:
--Loss of western wheatgrass and vine mesquite
--The formation of widely spaced rings or tussocks of alkali sacaton
--Increase in size and frequency of bare patches and surface crusting
--Increase in amount of shrub seedlings

Restoration Pathway to Reference State (R2A) Brush control is necessary to reduce the competitive influence of shrubs and reestablish grass dominance. Greasewood is difficult to control. It is resistant to fire (3) and can resprout following mechanical and chemical treatment (1,2). Seeding may be necessary on those sites where desired grass species are absent or limited. The use of livestock or mechanical means to break up physical crusts and improve infiltration may enhance seeding success. Prescribed grazing will help ensure adequate rest after seeding and proper forage utilization following grass establishment.

Gullied State: Accelerated erosion and gully formation characterize this state. Shrubs, including fourwing saltbush, shadscale, or greasewood are the dominant vegetation. Grass cover is sparse occurring as small patches or widely spaced individual plants occupying shrub interspaces and consists predominantly of inland saltgrass or alkali sacaton, blue grama, galleta, and threeawns.

Diagnosis: Gullying is extensive and may become deep enough to restrict livestock movement. Grass cover is sparse, and litter is confined to shrub bases or other obstructions to overland flow. Soil sealing is widespread and occupies most shrub interspaces.

Transition to Gullied State (T2A). The continued loss of grass cover in conjunction with soil sealing, increased overland flow, and resulting erosion can cause the transition to the Gullied State. The continued loss of grass cover may be due to extended drought, overgrazing, or the competitive influence of shrubs for available soil moisture. The loss of organic matter and high salt content may increase soil susceptibility to crust formation especially on finer textured soils (4). Decreased infiltration and increased flow rates due to grass cover loss and soil crusting facilitates erosion and gully formation.

Key indicators of approach to transition:
--Decrease in amount of grass cover
--Increase in size and frequency of bare patches and surface crusting
--Increase in amount of rills and presence of small gullies

Restoration Pathway to Reference State (R3A). Erosion control such as shaping and filling gullies, or grade stabilization structures will be necessary to restore hydrology. Brush control will be needed to reduce the competitive influence of shrubs and restore grasses. Seeding may be required to reestablish grass dominance. Prescribed grazing will help ensure adequate rest following seeding, and proper forage utilization once the grasses become established. The multiple practices required to drive the transition back to the Reference State are costly and chances for successful grass establishment are dependent on the degree of soil degradation and adequate precipitation following seeding.

References

1. Cluff, G.J., B.A. Roundy, R.A. Evans and J.A. Young. 1983. Herbicidal control of greasewood (Sarcobatus vermiculatus) and salt rabbitbrush (Chrysothamnus nauseosus ssp. consimilis). Weed Science. 31:275-279.

2. Mueggler, W. F.; Stewart, W. L. 1980. Grassland and shrubland habitat types of western Montana. In The Fire Effects Information System [Data base]. U.S. Department of Agriculture, Forest Service, Intermountain Research Station, Intermountain Fire Sciences Laboratory (2004, January), Missoula, MT. Available: http://www.fs.fed.us/database/feis/.

3. Rickard, W.H. and M.C. McShane. 1984. Demise of spiny hopsage shrubs following summer wildfire: An authentic record. Northwest Science. 58:282-285.

4. U.S. Department of Agriculture, Natural Resources Conservation Service. 2001. Soil Quality Information Sheet. Rangeland Soil Quality—Physical and Biological Soil Crusts. Rangeland Sheet 7 [Online]. Available: http://www.statlab.iastate.edu/survey/SQI/range.html.