Ecological dynamics

Ecological Dynamics of the Site:

The dominant visual aspect of this site is Utah juniper with a sparse understory of grasses, forbs, and shrubs. All age groups of juniper trees are present, however the large mature trees give the site its’ characteristic look. In the Historic Climax Plant Community (HCPC), the Reference State (State 1), juniper utilizes most of the available soil moisture, therefore little run-off occurs except during intense convection storms. These mature trees are generally heavily limbed and round-topped giving indication of branching close to the ground. Utah juniper, however, is usually dominated by a single main stem.

The juniper overstory is seldom removed by natural fire due to a lack of fine fuels in the understory. Understory production does not respond significantly to overstory removal due to the low herbaceous production potential of the soils on this site.

Juniper readily invades surrounding sites with deeper and more fertile soils, but historically was kept in check by natural wildfires. Present fire control programs, reduction/removal of the herbaceous understory by improper grazing management, and perhaps climate changes, have allowed juniper to rapidly invade these other sites.

During the last few thousand years, this site has evolved in a semi-arid climate characterized by dry summers and cold, wet winters and springs. Herbivory has historically occurred on this site at low levels of utilization. Herbivores include mule deer, Rocky Mountain elk, lagomorphs, and small rodents.

Fire occurs rarely on this site. When it does occur it is at intervals of 500-1000 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 Utah juniper in the overstory and bluebunch wheatgrass and mountain big sagebrush in the understory. Bitterbrush is usually present. Subdominant species include Sandberg bluegrass, Indian ricegrass, needle and thread, thickspike wheatgrass, and arrowleaf balsamroot. Leaf litter of Utah juniper is allelopathic to Sandberg bluegrass. The plant species composition of Phase A is listed later under “Reference Plant Community Phase Plant Species Composition”.

Total annual production is 825 pounds per acre (917 kilograms per hectare) in a normal year. Production in a favorable year is 1100 pounds per acre (1222 kilograms per hectare). Production in an unfavorable year is 550 pounds per acre (611 kilograms per hectare. Composition by weight is approximately 10 percent grass, 1 percent forbs, 4 percent shrubs and 85 percent trees. Structurally, Utah juniper dominates the overstory. In the understory cool season deep-rooted perennial bunchgrasses are dominant, followed by tall shrubs being more dominant than perennial forbs while shallow rooted perennial bunchgrasses are subdominant.


FUNCTION:

This site is suited for grazing by domestic livestock but is limited by low volume of forage, surface rock, and steep slopes.

The site has high value for mule deer and elk for escape and thermal cover. It has minimal forage value for deer and elk. Mountain lion, bobcat, and coyotes use the site for hunting. It provides food and cover for a variety of birds, rodents, and reptiles.

Impacts on the Plant Community.

Influence of fire.

In the absence of fire, juniper increases to the point of severely reducing nearly all of the understory species. This occurs at an extremely slow rate. There is the possibility that modern man has not seen this condition.

When fire does burn this site, it is a catastrophic event on the plant community. Because of the small amount of understory vegetation, fire is carried in the crown of the trees. Fires typically burn in a mosaic pattern. Fires of this nature rarely involve large acreages due to surrounding rimrocks, rock outcrops, and other features that limit the spread of fire. Nearly all of the junipers within the burned area, both old and young will be killed. Most of the shrubs will likewise be killed. The few grasses and perennial forbs in the understory will mostly survive. After a fire if there is a significant invasion of annual grasses within a few years, fires may become more frequent. Juniper, however, frequently re-establishes fairly rapidly due to the amount of seed being present in the soil reservoir or from nearby unburned trees. A frequent fire regime of one every 5-10 years, generally will not develop on this site. The soils are too shallow and fuels are not continuous enough for a frequent fire cycle to occur.

Influence of improper grazing management:

Improper grazing management has little impact on this site. Due to the rough and rocky nature of the site, livestock generally prefer not to use it. Livestock use the lesser slopes of the site primarily for loafing and bedding. Forage production is low. When this site is being impacted by improper grazing management, adjacent sites that are more productive and less rocky are being much more severely degraded.

Season-long grazing and excessive utilization can be detrimental to this site. This type of management leads to reduced vigor of the bunchgrasses and bitterbrush. With reduced vigor, recruitment of these species declines. Generally juniper seedlings will replace the desirable grasses and shrubs if improper grazing management continues.

Weather influences:

Above normal precipitation in the spring can increase forage production slightly. Due to the shallow soils, most plants, except juniper, are not able to capitalize on extra spring-time moisture. Below normal precipitation in the spring reduces herbaceous understory production and ultimately can cause plant mortality of the understory species if an extended drought continues.

Juniper is very resistant to drought influences. It has a root system that is capable of removing deep moisture in the fractures of the bedrock that is not available to other plants on the site. In addition, juniper is capable of photosynthesizing (growing) anytime the air temperatures are above freezing. It therefore is removing moisture from the soil for 10-11 months of the year. This gives juniper a competitive advantage for moisture over all of the other species on the site.

Influence of Insects and disease:

Insects and disease outbreaks can affect vegetation health, particularly bitterbrush from western tent caterpillars (Malacosoma fragilis). Two consecutive years of defoliation by the tent caterpillar can cause mortality in bitterbrush.

Utah juniper is occasionally heavily infested by juniper mistletoe (Phoradendron juniperum ssp. juniperum) and dense mistletoe (P. bolleanum ssp. densum). Little mortality occurs with these infestations.

Influence of noxious and invasive plants:

Many of these species add to the fine-fuel component and can lead to increased fire frequency.
Many of the annual and perennial invasive plants with deep root systems compete with desirable understory 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, summer, fall, and early winter. 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 occurs when both kinds of animal are using the plant in the late summer or fall. The adverse impact is excessive use of the current years’ leader growth.

Watershed:

Decreased infiltration and increased runoff occur with the increase of juniper following fire. Increases in juniper can be triggered by improper grazing management and prolonged drought. The increased runoff occurring during intense convection storms also causes sheet and rill erosion. Abnormally short fire frequency also gives the same results, but to a lesser degree.


Plant Community and Sequence:

Transition pathways between common vegetation states and phases:
State 1.
Phase A to B. Develops with improper grazing management.
Phase B to A. Develops through prescribed grazing.
Phase A to C. Develops after wildfire.
Phase C to A. Moves towards Phase A with no fire.
Phase B to C. Develops with wildfire.
Phase C to B. Develops with no fire and improper grazing management.

Practice Limitations.

This site is not suited to seeding or brush management due to steep slopes, low production potential, shallow soils, and rock outcrops.