Ecological dynamics

The dominant visual aspect of this site is black sagebrush, bluebunch wheatgrass and Salmon wildrye. Composition by weight is approximately 55 to 65 percent grass, 5 to 15 percent forbs and 25 to 35 percent shrubs.

In the last few thousand years, this site has evolved in an arid climate characterized by warm, dry summers and cold winters. Herbivory has historically occurred on the site at low levels of utilization. Herbivores include pronghorn antelope, mule deer, Rocky Mountain elk, lagomorphs and small rodents. Fire has historically occurred on this site every 80 to 100 years. Fire occurs only in years with above normal precipitation.

The Historic Climax Plant Community (HCPC), the Reference State (State 1), moves through many phases depending on the natural and man-made forces that impact the community over time. State 1, described later, indicates some of these phases. The Reference Plant Community Phase is Phase A. This plant community is dominated by bluebunch wheatgrass, salmon wildrye and black sagebrush. Subdominant species include Sandberg bluegrass, Hoods phlox, needleleaf phlox and stemless goldenweed. There are a variety of other grasses, forbs and shrubs that occur in the plant community in minor amounts. The plant species composition of Phase A is listed later under “Reference Plant Community Phase Plant Species Composition”.

Total annual production is 400 pounds per acre (448 Kg/ha) in a normal year. Production in a favorable year is 700 pounds per acre (784 Kg/ha). Production in an unfavorable year is 300 pounds per acre (336 Kg/ha). Structurally, cool season deep rooted perennial bunchgrasses are dominant, followed by medium height shrubs being more dominant than perennial forbs while shallow rooted bunchgrasses are subdominant.

FUNCTION:
This site is provides yearlong range for pronghorn antelope. Mule deer make slight use in mild winters and moderate to heavy use in severe winters. Only in severe winters do Rocky Mountain elk use this site on the lower slopes. Raptors hunt the site and it provides nesting for birds such as the horned lark.
It is suited for livestock use in the spring, early summer and fall, and recreational use in the summer and fall.

This site is not easily degraded by improper grazing management due to relatively steep slopes and gravelly to stony surfaces.

Impacts on the Plant Community.

Influence of fire:
This site historically had a very low fire frequency, approximately every 80-100 years. Most of the shrubs evolved in the absence of fire; therefore, they can be severely damaged or killed when burned. Bluebunch wheatgrass and salmon wildrye are usually maintained in the community. Sandberg bluegrass, thickspike wheatgrass and bottlebrush squirreltail can increase in the community with fire. With fires more frequent than the historic levels (80-100 years), annuals, invasive and noxious perennials can invade the plant community. Cheatgrass can be a troublesome invader at lower elevations on this site after fire, preventing perennial grass and shrub re-establishment and increasing the fire frequency. Utah juniper can invade the site if a seed source is in the proximity. See “Influence of juniper invasion” below.

Influence of improper grazing management:
Black sagebrush, spiny hopsage and winterfat can all be impacted by improper grazing management. Relatively low levels of utilization by cattle and sheep are needed to maintain the shrub component. Utah juniper can invade the site also if a seed source is nearby.

Weather influences:
Above normal precipitation in April, May and June can dramatically increase total annual production. These weather patterns can also increase viable seed production of desirable species to provide for recruitment. Extended periods of drought significantly impact this site due to the shallowness of the soil and its’ low water holding capacity. Extended drought reduces vigor of the perennial grasses and palatable shrubs. Extreme drought may cause plant mortality. An early, hard frost can occasionally kill some plants.

Influence of Insects and disease:
Outbreaks can affect vegetation health. An outbreak of a particular insect is usually influenced by weather but no specific data is available for this site. Mormon cricket and grasshopper outbreaks occur periodically. Outbreaks seldom cause plant mortality since defoliation of the plant occurs only once during the year of the outbreak. Shadscale saltbush can be heavily impacted by the scale insect, Orthezia annae. It is also called “mealy bug”. This insect is moved by ants from one plant to another and feeds on the roots of shadscale saltbush. It can cause stand mortality, especially when plants are stressed following a series of drought years.

Influence of noxious and invasive plants:
Annual and perennial invasive species compete with desirable plants for moisture and nutrients. The result is reduced production and change in composition of the understory. Cheatgrass can be very invasive on this site, especially after fire and at lower elevations. Once it becomes established, the fire frequency increases. As a result, the shrub component can be lost.

Influence of wildlife:
Relatively low numbers of wildlife use this site and have little impact on it. Pronghorn antelope is the dominant large herbivore that uses the site. They use the site yearlong but prefer it in the spring, fall and early winter. Sage grouse use the site for strutting grounds. Mule deer occasionally use the site in the winter and spring.

Watershed:
Decreased infiltration and increased runoff occur when black sagebrush is removed with frequent fires, particularly the year following the fire event. The increased runoff also increases sheet and rill erosion. The long-term effect is a transition to a different state. When the hydrologic condition of the vegetative cover is good, natural erosion is slight.

Influence of juniper invasion:
The following discussion deals with Utah juniper.

In plant communities that are invaded by juniper, the species has a competitive advantage for the following reasons:
• Juniper is very drought tolerant.
• It has the ability to extract soil moisture from a wide range of soil depths.
• Juniper has high evapo-transpiration rates.
• The species intercepts rain and snow before it reaches the soil surface.
• It has the ability to grow as long as there is soil moisture and the temperature is above freezing.
• Juniper has a relatively rapid growth rate and is long-lived. It can readily over-top shade intolerant species which leads to mortality.
• Nutrient cycling is reduced.
• As the canopy closes, juniper gains control of energy capture.

As juniper extracts water, other plants are unable to acquire sufficient water and nutrients to sustain growth and reproduction, thus reducing cover and biomass in the interspaces. After the canopy closes, there is sufficient soil moisture available for shallow-rooted, shade tolerant species to persist directly under the tree.

The following hydrological impacts occur on sites invaded by juniper:
• Infiltration in the interspaces is reduced.
• Run-off increases resulting in increased sheet and rill erosion with elevated sediment loads.
• Soil temperatures increase in the interspaces which results in accelerated drying of the soil surface.
• Increased bare ground in the interspaces.
• Soil moisture storage is reduced.

As bare ground and interconnectiveness of bare ground increases, flow rates are accelerated (reduction of flow sinuosity) and run-off out of the area increases.

Degradation of these systems can result in the formation of a feedback cycle in which greater juniper cover and density results in greater plant and soil disturbance between the canopies.

In summary, a closed juniper community takes control of the following ecological processes: hydrology, energy capture and nutrient cycling. The changes are primarily driven by the hydrological processes. The development of a closed juniper canopy always results in a transition across the threshold to a different state. Generally, when juniper canopy cover nears 20%, the plant community is approaching the threshold.

Plant Community and Sequence:
Transition pathways between common vegetation states and phases:

State 1.
Phase A to B. Develops with fire (approximately every 80-100 years). Fire only occurs in above normal precipitation (favorable) years.
Phase A to C. Develops under improper grazing management and no fire.
Phase C to A. Develops under a good prescribed grazing management program and no fire.
Phase B to A. Develops from prescribed grazing and no fire.

State 1, Phase B to State 2. Develops with frequent fire and/or improper grazing management. The site has crossed the threshold. It is not economical to return this plant community to State 1 with accelerating practices.

State 1, Phase C to State 2. Develops with continued improper grazing management and fire. The site has crossed the threshold. It is not economical to return this plant community to State 1 with accelerating practices.

State 2 to Unknown site. Excessive soil loss and changes in the hydrologic cycle caused by improper grazing management and/or frequent fire cause this state to cross the threshold and retrogress to a new site with reduced potential. It is not economical to return this plant community to State 1 with accelerating practices.

Practice Limitations:
There are moderate to severe limitations for vegetative management, facilitating and accelerating practices due to moderately steep to steep slopes.