Ecological dynamics

The dominant visual aspect is basin big sagebrush in the overstory with Indian ricegrass in the understory. Composition by weight is approximately 55 to 65 percent grasses, 10 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, moist winters. Herbivory has historically occurred on this site at low levels of utilization. Herbivores include mule deer, pronghorn antelope, lagomorphs and small rodents.

Fire has historically occurred on the site at intervals of 20 to 50 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 1.1. This plant community is dominated by Indian ricegrass in the understory and basin big sagebrush in the overstory. Subdominant species include antelope bitterbrush, needle and thread, sand dropseed, thickspike wheatgrass, and yellow wildrye. There are a wide variety of other grasses, forbs, and shrubs that can occur in the plant community in minor amounts. The plant species composition of Phase 1.1 is listed later under “Reference Plant Community Phase Plant Species Composition”.

Total annual production is 900 pounds per acre (1008 kilograms per hectare) in a normal year. Production in a favorable year is 1200 pounds per acre (1344 kilograms per hectare). Production in an unfavorable year is 600 pounds per acre (672 kilograms per hectare). Structurally, cool season deep rooted perennial bunchgrasses are very dominant, followed by tall shrubs being more dominant than perennial forbs while shallow rooted bunchgrasses are subdominant.

FUNCTION

This site is suited for livestock grazing in late spring and fall. The site also provides big game habitat in the spring, summer and fall. The site can be winter range for big game also. It is also well suited for recreation use in the summer and fall.

This site is susceptible to degradation due to the gentle slopes, ease of access and instability of sandy surface textures. Infiltration is good where the community is in mid to late seral status. The site has moderately low runoff potential. Snow can accumulate on the site due to high elevation and presence of tall shrubs.

Impacts on the Plant Community.

Influence of fire:

In the absence of normal fire frequency, antelope bitterbrush and basin big sagebrush can gradually increase on the site. Utah juniper can invade the site if a seed source is in the proximity. Grasses and forbs decrease as shrubs increase. With the continued absence of fire, antelope bitterbrush or juniper can displace most of the primary understory species. See “Influence of juniper invasion” below.

When fire frequency is greater than historic levels, antelope bitterbrush and basin big sagebrush are reduced significantly. Rabbitbrush can increase slightly. With continued short fire frequency, basin big sagebrush and antelope bitterbrush can be completely eliminated along with many of the desirable understory species such as Indian ricegrass and needle and thread. These species may be replaced by thickspike wheatgrass, sand dropseed and bottlebrush squirreltail along with a variety of annual and perennial forbs including noxious and invasive species. Cheatgrass will invade the site. These fine fuels will increase the fire frequency. Continued fire can create a loss of vegetation cover and sand can become unstable resulting in active sand dunes.

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 and antelope bitterbrush. With reduced vigor, recruitment of these species declines. As these species decline, the plant community becomes susceptible to juniper invasion, an increase in basin big sagebrush and noxious and invasive weeds. Continued improper grazing management influences fire frequency by reducing fine fuels that carry fires. As cheatgrass increases and becomes co-dominant with other annuals, fires become frequent, particularly at lower elevations. Continued improper grazing management can create a loss of vegetation cover and sand can become unstable resulting in active sand dunes.

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 basin big sagebrush and/or juniper. 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 a reduction in fire frequency.

Influence of Insects and disease:

Outbreaks can affect health of vegetation. Antelope bitterbrush can be severely affected by the western tent caterpillar (Malacosoma fragilis). Two consecutive years of defoliation by the tent caterpillar can cause mortality in bitterbrush. 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 following a series of drought years.

Influence of noxious and invasive plants:

Many of these species add to the fine-fuel component and lead to increased fire frequency. Many of the 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, summer and fall and in moderate winters. Their numbers are seldom high enough to adversely affect the plant community. Herbivory can be detrimental to bitterbrush when livestock grazing and browsing by big game occurs at the same time and season. This will occur when both kinds of animal are using the plant in the late summer or fall. The adverse impact is excessive use of the current year’s leader growth. The deer mouse is beneficial to this site. It is the principal vector for planting bitterbrush seed.

Watershed:

Juniper can invade the site if a seed source is in the vicinity. Juniper invasion can be triggered by lack of fire, improper grazing management and prolonged drought. Desired understory species can be reduced. This composition change can affect nutrient and water cycles. Loss of vegetation cover can result in unstable sand and lead to active sand dunes. 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 connection of bare ground patches 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 percent, the plant community is approaching the threshold.