Rocky Hills 10-14 Inch Precipitation Zone Lost River Mountains

Description

The Reference state consists of three dominant plant communities: The curl-leaf mountain mahogany and black sagebrush (Artemisia nova) dominated community, the mixed shrub overstory community (1.2) which can include curl-leaf mountain mahogany, mountain big sagebrush (Artemisia tridentata ssp. vaseyana), low sagebrush (Artemisia arbuscula) and black sagebrush (Artemisia nova), and the wooded mountain mahogany community which can include Douglas fir (Pseudotsuga menziesii), Rocky Mountain juniper (Juniperus scopularum), and limber pine (Pinus flexus). The understory in both communities is dominated by bluebunch wheatgrass but can include a variety of other bunch and rhizomatous grasses at a lesser canopy cover. The primary difference between the two communities is the composition and foliar cover of shrub species. Foliar cover of other shrub species increases in relation to openings not already occupied by curl-leaf mountain mahogany.

Processes (both natural and anthropogenic) that result in state and community changes include fire, grazing, land use change, and the occurrence of invasive species.

Characteristics and indicators

The shift between communities in the Reference state is driven by curl-leaf mountain mahogany canopy cover. Well-established stands of mountain mahogany can be dense and prevent any future establishment of other shrub species, primarily sagebrush. Likewise, less dense canopies of mountain mahogany present less competition and create opportunities for other species to establish in their absence.

Low-severity disturbances are the primary driver for decreasing the canopy cover of mountain mahogany and creating opportunities for other species to establish. Severe disturbances such as fires that are stand-replacing and destroy the seed bank could lead to a shrub community not dominated by curl-leaf mountain mahogany.

Resilience management

The Reference state is overall moderately resilient. In well-established communities, resilience towards climatic disturbances such as drought is high. Curl-leaf mountain mahogany is adapted to dry conditions and is able to take advantage of moisture funneled to cracks in the bedrock. Resilience to low-severity fire is high; however, severe stand-replacing fire, which removes any nearby seed source, can take several decades to recover from (Gucker, 2006).

Submodel

Description

This community shifts from the shrub-dominated communities of the Reference State to a community dominated by native bunchgrass and disturbance-tolerant species. The grass species tend to be similar to that of the reference communities and include bluebunch wheatgrass, Sandberg bluegrass, and Indian ricegrass. The overstory shrub community, if present, is yellow rabbitbrush and to a lesser extent, threetip sagebrush.

Production for this community is no longer weighted toward the shrub functional group, but shifts towards grasses and forbs. Production values remain relatively high, ranging from 500 to 900 pounds per acre, averaging 650 pounds per acre.

Characteristics and indicators

The Disturbed state in the Rocky Hills ecological site results from any disturbance that removes the overstory vegetation. The primary disturbance for this ecological site that results in the removal of overstory shrubs is frequent or severe fire. The dominant overstory species found in the Reference state such as curl-leaf mountain mahogany, black sagebrush, low sagebrush, and mountain big sagebrush experience high rates of mortality during severe fire events. In the immediate years following the event, canopy cover increases for early seral and disturbance-tolerant species such as yellow rabbitbrush (Chrysothamnus viscidiflorus), three-tip sagebrush (Artemisia tripartita), and the common bunchgrass species found in most communities of the Reference state.

Historic fire return intervals for curl-leaf mountain mahogany stands in the Salmon River area ranged between 13 to 22 years. Because of grazing and fire exclusion, the time between fire events has increased significantly. This has resulted in fewer, but more severe stand-destroying fire events (Gucker, 2006).

Resilience management

Once in the Disturbed state, resilience is moderately high. The species present are disturbance tolerant or even perpetuated by frequent or continued disturbance events. Resilience is lost, however, by the opportunity for invasive species establishment. Openings in the canopy caused by a disturbance can allow for the recruitment of invasive species if native species don't readily occupy those spaces.

Submodel

Description

The Invaded State is represented by a shift from native grasses and forbs in the understory to an understory dominated by invasive species. Transition to this state occurs after a severe disturbance that creates openings in the canopy for invasive species to establish. The primary invasive species that becomes dominant in the Invaded State is cheatgrass (Bromus tectorum). This state is especially prevalent on steep, rocky slopes where colluvial movement is active. As the soil is disturbed through movement of material downhill, ideal conditions are presented for cheatgrass recruitment. Once established and a seed source is readily available, cheatgrass is well adapted to increasing its coverage area.

Characteristics and indicators

This state is indicated by any single invasive species occupying five percent or more of the canopy, or a group of invasive species occupying 10 percent of the canopy collectively.

Resilience management

This state exhibits low resilience due to the establishment of invasive and non-native species. Once established, non-native vegetation tends to increase in canopy cover and can often be promoted by further disturbances.

Submodel