Ecological dynamics

The dominant visual aspect of this site is Wyoming big sagebrush, alkali bluegrass, and needle and thread. Composition by weight is approximately 40 to 50 percent grasses, 20 to 30 percent forbs, and 25 to 35 percent shrubs.

During the last few thousand years, this site has evolved in a semi-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 and lagomorphs.

Fire has historically occurred on the site at intervals of 50-70 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 Wyoming big sagebrush, alkali bluegrass, and needle and thread. Subdominant species include Indian ricegrass, bluebunch wheatgrass, and bottlebrush squirreltail. A wide variety of other grasses, forbs, and shrubs occur in small amounts. The plant species composition of Phase A is listed later under “Reference Plant Community Phase Plant Species Composition”.

Total annual production is 700 pounds per acre (784 kilograms per hectare) in a normal year. Production in a favorable year is 1100 pounds per acre (1232 kilograms per hectare). Production in an unfavorable year is 400 pounds per acre (448 kilograms per hectare). Structurally, cool season deep-rooted perennial bunchgrasses are dominant, followed by shrubs that are more dominant than perennial forbs followed by shallow rooted perennial bunchgrasses.


FUNCTION.

This site is suited for livestock in spring, early summer, and fall. It is used by big game in the spring, fall, and moderate winters. If water is available, the site is easily grazed by livestock due to gentle slopes.

This site has limited value for recreation.

Due to the low available water holding capacity (AWC), low production, and ease of access on relatively flat slopes, this site is easily degraded by improper grazing management or frequent fires.

Infiltration can be good with a mixed stand of shrubs and perennial grasses. Runoff is rapid and erosion hazard is very high Snow is caught in the shrub interspaces and 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, Wyoming big sagebrush can gradually increase. Grasses and forbs decrease as shrubs increase.

When fires become more frequent than historic levels (50-70 years), Wyoming big sagebrush is reduced significantly. With continued short fire frequency, Wyoming big sagebrush can be completely eliminated along with many of the desirable understory species such as alkali bluegrass, bluebunch wheatgrass, bottlebrush squirreltail, Thurber’s needlegrass, and Indian ricegrass. These species may be replaced by cheatgrass or a variety of other annual and perennial forbs.

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 an increase in Wyoming big sagebrush and noxious and invasive plants.

Continued improper grazing management influences fire frequency with an increase in cheatgrass that increases fire frequency.

Proper grazing management that addresses frequency, duration, and intensity of grazing can maintain the integrity of the plant community.

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 a reduction in fire frequency.

Influence of Insects and disease.

Outbreaks can affect vegetation health. An outbreak of a particular insect is usually influenced by weather. Two or more consecutive years may cause mortality of some species. The sagebrush defoliator moth (Aroga websterii) causes mortality in relatively small patches. It seldom kills the entire stand. 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.
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. If the site is in a wintering area for big game, high numbers can adversely affect the plant community in the early spring.

Watershed.

Decreased infiltration and increased runoff on slopes greater than15 percent occur when Wyoming big sagebrush is removed with frequent fires, particularly the year of the fire event. The increased runoff also causes sheet and rill erosion. The long-term effect is a transition to a different state.

Plant Community and Sequence:

Transition pathways between common vegetation states and phases:

State 1.
Phase A to B. Develops with improper grazing management and in the absence of fire.
Phase A to C. Develops with fire.
Phase B to A. Develops with prescribed grazing and no fire.
Phase C to A. Develops with prescribed grazing and no fire.

State 1 to State 2. Develops through frequent fire and/or improper grazing management. This site has crossed the threshold. It is economically impractical to return this state to State 1 with accelerated practices.

State 2 to unknown site. Excessive soil loss and changes in the hydrologic cycle caused by continued 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 economically impractical to return this state to State 1 with accelerated practices.






Practice Limitations:

The soils of this site have moderate to severe limitations for range seeding due to salts in the soil profile. There are only slight limitations for brush management. Careful planning is needed to evaluate the desired vegetation that will respond to brush management because removal of Wyoming big sagebrush can lead to a significant increase in cheatgrass. The site offers slight limitations to vegetative management practices. Livestock water may be limiting on this site and associated sites.