Semidesert Shallow Hardpan (Black Sagebrush)

Description

The plant community dynamics of the reference state are driven primarily by climate and fire. Black sagebrush dominates the overstory and perennial cool-season grasses dominate the understory for most of the time on this site. However, periodic fires remove black sagebrush and increase perennial grasses, which dominate the site for several years before black sagebrush slowly establishes and increases in the community.

The fire return interval for this site is expected to be 150-250 years for stand-replacing fires, and 75-100 years for mixed severity fires. Wet years followed by an average or dry year increases the likelihood of fire on this site due to increased fuel loads and dry conditions. The sparse vegetation on this site contributes to the relatively long fire return interval, as compared to big sagebrush sites (Artemisia tridentata) in the Great Basin that have a much shorter fire return interval.

In the absence of fire, Utah juniper can establish and increase on this site, usually after a period of about 75 years following fire. Adjacency to juniper stands and extended wet periods increase the likelihood and shorten the timeframe of juniper encroachment onto this site. As juniper increases in the plant community, black sagebrush and perennial grasses are expected to decrease somewhat. However, the presence of juniper in the community increases the likelihood of stand-replacing fire, which results in a perennial grass dominated community.

Submodel

Description

The Invaded State is similar to the Reference State in species composition and in the fundamental ecological processes, but allows for non-native species to be present. During wet periods, non-native invasive annuals, particularly cheatgrass, may increase temporarily in the community, but they decrease in abundance during dry periods. The addition of non-native species and modern disturbance regimes reduces the overall resillience of state 2 compared to state 1, so State 2 also includes two at-risk plant communities.

Phase 2.3 is at-risk of transitioning to a Juniper State (state 4) as shrubs and grasses lose their ability to propogate themselves in the community.
Phase 2.3 includes Utah juniper as a component of the plant community, but fire suppression and removal of fine fuels by livestock reduce the likelihood of fire (pathway 2.3a). When the lack of fire extends beyond the 150-250 year fire return interval, juniper begins to dominate the ecological processes and can cross the threshold out of State 2 and into State 4.

Phase 2.4 is at-risk of transitioning to a Sagebrush/Depleted Understory State (State 3)because, although it still has some perennial grasses in the understory, they are losing vigor and decreasing in the community. When perennial grasses lose their ability to propagate themselves in the community, Phase 2.4 will transition to a sagebrush-dominated state without the co-dominant herbaceous understory.

Submodel

Description

The depleted understory state is characterized by an overstory dominated by black sagebrush, and an understory that is either very sparse or dominated by Sandbergs bluegrass. Few formerly-dominant perennial grasses may still be present under shrubs, but they are so few that they are unable to increase in the community. Invasive annuals, such as cheatgrass, may be present, but are also sparse. This state is susceptible to an increase in Utah juniper and/or cheatgrass, especially during wet periods. The removal of livestock from this state will not result in a natural increase in perennial grass production due to the lack of perennial grass seed in the seedbank.

Submodel

Description

This state is dominated by Utah junipers. Shrubs and perennial grasses, if present, are decreasing in the plant community with very little or no recruitment of young plants. The lack of fine fuels in this state greatly reduce the likelihood of fire, thus perpetuating juniper dominance on the site. This state occurs in areas where the 75-150 year fire return interval is greatly exceeded due to fire suppression and or fine fuel removal by livestock. The likelihood of transitioning to this state is increased when a juniper stand is nearby. In some areas far from a juniper seed source, this state may not occur within the timeframes referenced in this document.

Submodel

Description

The seeded range state is characterized by the abundance of non-native perennial grass species. Successful seedings result in productive stands of crested wheatgrass, Russian wildrye, or similar species. Native plants, particularly shrubs, can re-establish in these seedings after several decades. Unsuccessful seedings often result in a mixed plant community with diverse proportions of sprouting shrubs (e.g. rabbitbrush), perennial grasses, and annual forbs and grasses. Unsuccessful seedings are at-risk of transitioning to an annual state (State 6), but may also self-repair to some extent if shrubs and perennial grasses increase.

Submodel

Description

The annual dominated state is characterized by the abundance of cheatgrass. Other invasive annual species may be present, but cheatgrass is the species that alters the ecological processes of this site by increasing the fire frequency. The plant community may have some shrubs, but the fine fuels associated with cheatgrass increase the fire frequency such that shrubs do not regain dominance. Over time, the nutrient pools of the site, which were once deep and patchy under shrubs, become homogenized near the soil surface. These changes in disturbance regime and nutrient dynamics facilitate increasing cheatgrass dominance, and contribute to the improbability of transitioning out of this state.

Submodel