Ecological dynamics

The dominant visual aspect of this site is low sagebrush with bluebunch wheatgrass and Indian ricegrass. Composition by weight is approximately 40 to 55 percent grasses, 5 to 10 percent forbs and 40 to 50 percent shrubs.

During 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, lagomorphs and small rodents. Fire has historically occurred on the site at intervals of 80 to 100 years.

The Historic Climax Plant Community (HCPC) 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 HCPC is Phase A. This plant community is dominated by bluebunch wheatgrass and Indian ricegrass in the understory and low sagebrush in the overstory. Subdominant species include Sandberg bluegrass, bottlebrush squirreltail 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 “HCPC Plant Species Composition”.

Total annual production is 340 pounds per acre (381 kilograms per hectare) in a normal year. Production in a favorable year is 525 pounds per acre (588 kilograms per hectare). Production in an unfavorable year is 185 pounds per acre (207 kilograms per hectare). Structurally, cool season deep rooted perennial bunchgrasses and shrubs are about equal followed by 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 and pronghorn antelope. This site is easily degraded with improper management due to ease of access by livestock and low production. 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. Utah juniper can invade if a seed source is in the vicinity. Grasses and forbs decrease as shrubs increase. With the continued absence of fire, low sagebrush or Utah juniper can displace most of the primary understory species. See “Influence of juniper invasion” below.

When fires become more frequent than historic levels( 80-100 years), 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 Indian ricegrass. These species may be replaced by Sandberg bluegrass along with a variety of annual and perennial forbs including noxious and invasive plants. Cheatgrass and medusahead will invade the site at lower elevations. These fine fuels will increase the fire frequency.

Influence of improper grazing management
Season-long grazing and/or excessive utilization can be very detrimental to this site. 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 plants (e.g. an invasion of Utah juniper if a seed source is in the proximity).

Continued improper grazing management and excessive utilization influences fire frequency by increasing these fine fuels. If cheatgrass and/or medusahead increase due to improper grazing management and become co-dominant with Sandberg bluegrass and other annuals, fires become more frequent.

Proper grazing management that addresses frequency, duration and intensity of grazing can also keep fine fuels from developing, thereby reducing fire frequency. This can lead to gradual increases in low sagebrush and/or Utah juniper. A planned grazing system can be developed to intentionally accumulate fine fuels in preparation for a prescribed burn. On sites with such low production and marginal soils, any brush management program needs to be very carefully planned and evaluated before implementation.

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 viable seed production. Overall plant composition is normally not affected when perennials have good vigor.

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

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 vegetation health. 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 plants
Many of these species add to the fine-fuel component and lead to increased fire frequency. Perennial and annual invasive species 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 animals use this site in the spring, fall and 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
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: (1) hydrology, (2) energy capture and (3) 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. Fire only occurs in above normal precipitation about every 80-100 years.
Phase A to C. Develops under improper grazing management and no fire.
Phase A to D. Develops with no fire and improper grazing management.
Phase B to A. Develops under prescribed grazing management program and no fire.
Phase C to A. Develops from prescribed grazing management and no fire.
Phase C to B. Develops with fire.
Phase D to A. Develops from prescribed grazing management and no fire.

State 1, Phase B to State 2. Results from continued improper grazing management and/or frequent fire. This site has crossed the threshold. It is not economical to return this site to State 1 with accelerating practices.

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

State 1 Phase D to State 3. Results from no fire and continued improper grazing management from a juniper invaded phase of State 1. This site has crossed the threshold. It is not economical to return this site to State 1 with accelerating practices.

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

State 3 to Unknown Site. Excessive soil loss and changes in the hydrologic cycle caused by improper grazing management and fire or no fire causes this state to cross a threshold and retrogress to a new site with reduced potential. It is not economical to return this site to State 1 with accelerating practices.

Practice Limitations
Severe limitations exist on this site for rangeland seeding due to low precipitation, shallow and gravelly soils and low available water holding capacity. Brush management is not normally desired or needed on this site.