Ecological dynamics

The dominant visual aspect of this site is western juniper with a sparse understory of Idaho fescue and Thurber’s needlegrass. All age groups of juniper trees are present; however, the large mature trees give the site its characteristic look. In the Reference Plant Community Phase, 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; however, western juniper is usually dominated by a single main stem.

In the northern regions of this site, curlleaf mountain mahogany rarely occurs intermixed with juniper. In the southern and western regions, it intermixes apparently indiscriminately with juniper, making up to 5-15 percent of the overstory. The juniper overstory is seldom removed by natural fire due to a lack of fuel in the understory. Understory production does not respond significantly to overstory removal. This is due to the low potential of the soils of this site for herbaceous production.

Juniper readily invades surrounding sites of deeper and more fertile soils, but likely 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 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, Rocky Mountain elk, lagomorphs and small rodents.

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. The plant species composition of Phase 1.1 is listed later under “Reference Plant Community Phase Plant Species Composition”.

Fire rarely occurs on this site. When it does occur, it is at intervals of 500-1000 years. The Reference Plant Community Phase of this site is dominated by western juniper in the overstory and Idaho fescue, Thurber’s needlegrass and mountain big sagebrush in the understory. Bitterbrush is usually present. Subdominant species include bluebunch wheatgrass, Sandberg bluegrass, bottlebrush squirreltail, arrowleaf balsamroot and lupine. Curlleaf mountain mahogany is commonly found on the site. 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 90 percent trees. Structurally, western 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 bunchgrasses are subdominant.

FUNCTION:

This site is not well suited for livestock grazing. Understory production is low (even without the overstory) and it is often rough, rocky or inaccessible. It does provide shade for livestock.

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 normal fire frequency, 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, fire will usually be carried in the crown of the trees. Fires of this nature rarely involve large acreages due to surrounding rimrocks 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. Normally, juniper re-establishes rapidly due to the seed source being present in the soil reservoir and/or from nearby unburned trees. A frequent fire regime of once 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 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 usually being much more severely degraded.

Season-long grazing and/or 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 increases forage production slightly. Due to the shallow soils, most plants, except juniper, do not capitalize on extra spring-time moisture. Below-normal precipitation in the spring can reduce production and ultimately cause plant mortality of the understory species if 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. An outbreak of a particular insect is usually influenced by weather but no specific data for this site is available.

Mistletoe and a number of insects use juniper. Little mortality occurs with these infestations.

There is an abundant fruticose lichen associated with old juniper. It is bright yellow, erect, multi-branched and grows on semi-dead or dead branches. The moss Tortula ruralis is a characteristic of climax stands of juniper. It is generally found growing on the surface layer of duff beneath old juniper trees. Neither organism adversely affects juniper.

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 annual and perennial weeds with deep root systems 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, summer and fall. 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 as it is the principal vector for planting bitterbrush seed.

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.

Influence of juniper invasion:

In plant communities that are invaded or dominated 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 patches 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 1.1 to 1.2. Develops with improper grazing management.
Phase 1.2 to 1.1. Develops through prescribed grazing.
Phase 1.1 to 1.3. Develops after wildfire.
Phase 1.3 to 1.1. Moves towards the HCPC with no fire.
Phase 1.2 to 1.3. Develops with wildfire.
Phase 1.3 to 1.2. Develops with no fire and improper grazing management.

Practice Limitations.

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