Ecological dynamics

The dominant visual aspect of this site is low sagebrush with bluebunch wheatgrass and Idaho fescue in the understory. Composition by weight is approximately 50 to 60 percent grasses, 10 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 dry summers and cold, wet winters. Herbivory has historically occurred on this site at low levels of utilization. Herbivores include mule deer, bighorn sheep, Rocky Mountain elk, pronghorn antelope, lagomorphs and small rodents. Fire has historically occurred on the site at intervals of 60-80 years.

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 and Idaho fescue in the understory and low sagebrush in the overstory. Subdominant species include Sandberg bluegrass, prairie junegrass and Hoods phlox. There is a variety of other grasses, forbs and shrubs that can occur 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 kilograms per hectare) in a normal year. Production in a favorable year is 725 pounds per acre (812 kilograms per hectare). Production in an unfavorable year is 275 pounds per acre (308 kilograms per hectare). Structurally, cool season deep rooted perennial bunchgrasses are dominant followed by low to medium shrubs being more dominant than perennial forbs while shallow rooted bunchgrasses are subdominant.

Function
This site is suited for livestock grazing in the spring, fall and early winter. There are few limitations to grazing. Natural water supplies may be insufficient or absent. The site provides fair to good habitat for various upland wildlife. The site can be valuable winter habitat for mule deer, Rocky Mountain elk and pronghorn antelope. This site is easily degraded by improper grazing management due to low production and flat slopes. The site offers minimal recreational value except limited hunting opportunities. A mixed stand of shrubs and perennial grasses is necessary to reach the potential of the site.

Impacts on the Plant Community.

Influence of fire:
In the absence of normal fire frequency, low sagebrush can gradually increase on the site. Grasses and forbs decrease as shrubs increase. With the continued absence of fire, low sagebrush can displace most of the primary understory species. Utah juniper can invade the site if a seed source is in the vicinity. See “Influence of Juniper Invasion” below.

When fire frequency is greater than historic levels, low sagebrush is reduced significantly. Rabbitbrush can increase slightly. With continued short fire frequency, low sagebrush can be completely eliminated along with many of the desirable understory species such as bluebunch wheatgrass and Idaho fescue. These species may be replaced by Sandberg bluegrass along with a variety of annual and perennial forbs including noxious and invasive weeds. Fine fuels will increase the fire frequency.

Influence of improper grazing management:
Season-long grazing can be very detrimental to this site. Excessive utilization is also detrimental. This type of management leads to reduced vigor of the bunchgrasses. With reduced vigor, recruitment of these species declines. As these species decline, the plant community becomes susceptible to increase in low sagebrush and noxious and invasive weeds. Utah juniper can invade the site if a seed source is in the vicinity. Continued improper grazing management influences fire frequency by reducing fine fuels that carry fires. As annuals increase and becomes co-dominant with Sandberg bluegrass, fires become frequent.

Good grazing management that addresses frequency, duration and intensity of grazing can also keep fine fuels from developing, thus reducing fire frequency. This can lead to gradual increases in low sagebrush and Utah juniper if a seed source is in the vicinity. A planned grazing system can be developed to intentionally accumulate fine fuels in anticipation of a prescribed fire.

Weather influences:
Above normal precipitation in April, May and June can dramatically increase total annual production of the plant community. These weather patterns can also increase viable seed production of desirable species to provide for recruitment. Likewise, below normal precipitation during these spring months can significantly reduce total annual production and be detrimental to good seed production. Overall plant composition is normally not effected when perennials have good vigor.

Below normal temperatures in the spring can have an adverse impact on total production regardless of the precipitation. A hard, early freeze can kill some plants occasionally.

Prolonged drought adversely affects this plant community in several ways. Vigor, recruitment and production are usually reduced. Mortality can occur. Prolonged drought can lead to reductions in fire frequency.

Influence of Insects and disease:
Outbreaks can affect health of vegetation. 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.

Influence of noxious and invasive weeds:
Many of these species add to the fine-fuel component and lead to increased fire frequency. Perennial and annual weeds compete with desirable plants for moisture and nutrients. The result is reduced production and change in composition of the understory.

Influence of wildlife:
Big game use this site in the spring, fall and in moderate winters. Their numbers are seldom high enough to adversely affect the plant community.

Watershed:
Decreased infiltration and increased runoff occur with an increase in low sagebrush. Desired understory species can be reduced. This composition change can affect nutrient and water cycles. Increased runoff also causes sheet and rill erosion. Abnormally short fire frequency also gives the same results, but to a lesser degree. The long -term effect is a transition to a different state.

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 in the absence of fire. Improper grazing management could also be
present. There is a juniper seed source near the site.
Phase A to C. Usually results from wildfire or brush management.
Phase A to D. Results from improper grazing management and no fire.
Phase B to C. Results from wildfire or brush management.
Phase B to A. Occurs with prescribed grazing and brush management or prescribed burning.
Phase C to A. Results from prescribed grazing and no fire.
Phase D to A. Occurs with proper grazing management.

State 1 to 2. Develops through improper grazing management and frequent fire.

State 1 to 3. Develops with no fire and improper grazing management from a juniper invaded phase of State 1.

State 2 to unknown site. Excessive soil loss and changes in the hydrologic cycle caused by improper grazing management and no fire or frequent fire cause this state to retrogress to a new site with reduced potential.

State 3 to unknown site. Continued lack of fire and improper grazing management cause this state to retrogress to a new site with reduced potential due to significant soil loss and changes in hydrology.

Practice Limitations:
This site is generally not suitable for seeding due to the gravels and low production. Brush management is not normally useful or desirable on ths site. There are few limitations for implementing vegetation management or facilitating practices on this site.