Semidesert Sand (Four-Wing Saltbush) South

Description

The Reference State represents the natural range of variability that dominates the dynamics of this ecological site. The biotic communities that persist when all successional sequences occur within the natural disturbance regime. Perennial cool-and warm-season grasses and a diverse shrub community are dominant. Primary disturbance mechanisms include fire, grazing by native herbivores, and active alluvial deposits that change drainage patterns. These disturbances coupled with weather events dictate the dynamics that occur within the natural range of variability. The Reference State is self-sustaining and hold a high resistance to natural disturbances and high resilience after natural disturbances. When natural disturbances occur, the rate of recovery is relatively rapid due to niches being filled with highly adapted native vegetation. The Reference State has been determined by study of literature reviews, trends in plant community dynamics, and historical accounts.

Submodel

Description

The Current Potential State is similar to the Reference State except that non-native plants are now present in all plant community phases. The primary disturbance mechanisms can be natural or human induced. Events, such as improper livestock grazing, prolonged rodent activity, changes in historic fire regimes, OHV overuse, insect herbivory, or drought may influence this site. A shift in species composition will affect the nutrient cycling, soil-water relationships, hydrology, and soil stability. Dominant grasses include both cool-season and some warm-season grasses; however many times heavy spring grazing will remove the cool-season grasses and heavy late summer and early fall grazing will remove the warm-season grasses. This state is losing resistance to disturbances and resilience after disturbance. Invasive plants are beginning to fill the niches and establish on the site.

Submodel

Description

The Sprouting Shrub/Native Grass/Invasive Annual State occurs after repeated fires or over grazing. Fourwing saltbush is susceptible to overgrazing and can leave a system if pressured by grazing. Fire can also remove fourwing saltbush from a site. It can then be replaced by fire tolerant shrubs such as rabbitbrush. Invasive annual, like cheatgrass and Russian thistle can also become dominant in the understory. Community phase 3.2 is the "At risk" phase.

Submodel

Description

The Invasive Annual State is characterized by the dominance of invasive grasses/forbs. These species may include, but are not limited to cheatgrass, Russian thistle, tansy mustard, broom snakeweed, annual stickseed, or annual Cryptantha. One or more invasive species has increased to a point where they influence or drive the disturbance regime and nutrient cycle. Russian thistle and/or cheatgrass are the most likely of these species to establish and dominate. Russian thistle is a prolific reproducer—one plant can produce up to 250,000 seeds, which are dispersed by the wind causing the dead plant to “tumble” across the landscape. While generally considered an unwanted weed, Russian thistle may actually help disturbed sites recover more quickly. If topsoil is present, Russian thistle roots are invaded by mycorrhizal fungi and because this plant does not form associations with the fungi the root and plant are killed. This causes increased mycorrhizal fungi in the soil and an increased chance for other plants to establish through the aid of these fungi associations. Yet, managers must be aware that that without proper management other invasive annuals, such is cheatgrass may become established instead of the desirable native species. If topsoil is missing the mycorrhizal fungi, Russian thistle can persist for relatively long periods of time (Howard, 1992). Cheatgrass has an entirely other effect on the soil/plant/water relationships. Research has shown that plant species differ substantially in the effects on soil water content and temperature, and in their effects on the frequency and intensity of disturbance. After cheatgrass has invaded a site fundamental nutrient cycling processes, root pores, mycorrhizal associations, microbial species, and soil organic material changes (Chapin et al. 1997; Belnap and Phillips, 2001). These alterations may eventually create ecologically impoverished sites that are very difficult to restore to functionally diverse perennial herbaceous and woody communities. The competitiveness of the invasive species and the ability of these species to quickly establish after a disturbance make this state extremely resistance to change and resilient after a disturbance.

Invasive Annual State: Community phases influenced by improper grazing and weather cycles.

Indicators: A complete understory of invasive annuals, where native perennial shrubs, grasses, and forbs are minimally present.

Feedbacks: Improper livestock grazing and weather cycles that maintain the dominance of invasive forbs/shrubs with minimal decrease in occurring native perennial grasses, shrubs, and forbs. Increased occurrence of cheatgrass, decreasing the fire return interval. Flooding and alluvial deposits that facilitates the establishment of sand sagebrush.

Trigger – The increased establishment of cheatgrass, caused by a decrease in the fire return interval, which facilitates the continued removal of other grasses, shrubs, and forbs.

Submodel