Ecological dynamics

The dominant visual aspect of this site is of curlleaf mountain mahogany. Mountain big sagebrush, Wyoming big sagebrush or black sagebrush may occur in the understory. Idaho fescue and bluebunch wheatgrass are co-dominant in the understory. Composition by weight is approximately 15 percent grass, 5 percent forbs and 80 percent shrubs and tree-like shrubs. These percentages are for current annual growth for all plants, irrespective of height.

The Historic Climax Plant Community (HCPC), the Reference State (State1), 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. The plant species composition of Phase A is listed later under “Reference Plant community Phase Plant Species Composition”.

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. Fire has historically occurred on the site at intervals of 250-500 years.

Due to the variability of soil depth, from shallow to deep, curlleaf mountain mahogany has a patchy or clumpy appearance on the landscape. For this reason when the site burns, fire moves across the site leaving a mosaic of burned and unburned areas. Utah juniper can be a minor component of this site at the lower elevations.

The Historic Climax Plant Community (HCPC) of this site is dominated by curlleaf mountain mahogany in the overstory and Idaho fescue and bluebunch wheatgrass in the understory. Total annual production is 900 pounds per acre (1000 kilograms per hectare) in a normal year. Production in a favorable year is 1300 pounds per acre (1444 kg/ha). Production in an unfavorable year is 600 pounds per acre (666 kg/ha).

Structurally, curlleaf mountain mahogany 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 in the summer and fall. Livestock use the site for loafing as well as foraging.

Mule deer and Rocky Mountain elk use the site for thermal and hiding cover. Birds use the mahogany trees for nesting. This site is very important for nesting and summer habitat for mountain bluebirds.

The soils on this site have moderately high runoff potential.

The site has high value for hunting, camping (on non-stony and flat places), photography and picnicking. The mahogany shrubs provide visual diversity to the landscape. The site provides a source of fuel-wood for camping, picnics or barbecues. The wood is also used to make small specialty items.

Impacts on the Plant Community

Influence of fire
Where there is a juniper seed source in the vicinity and in the absence of normal fire frequency, juniper increases to the point of severely reducing nearly all of the understory and overstory species. Juniper has a greater growth rate, its shape is more tapered and it reaches greater heights. Consequently, juniper invading curlleaf mountain mahogany sites eventually over tops them. Because mature curlleaf mountain mahogany is shade intolerant, its competitive ability is lost, and it becomes senescent. See “Influence of juniper invasion” below.

Because of its topographic position on the landscape, ridgetops and sideslopes, fires started by lightning strikes are fairly common. Because of the variability of soil depth, from shallow to deep, curlleaf mountain mahogany has a patchy or clumpy appearance on the landscape. For this reason when the site burns, fire moves across the site leaving a mosaic of burned and unburned areas. Fires of this nature rarely involve large acreages due to surrounding rimrocks and other features that limit the spread of fire. The site rarely, if ever, burns in its entirety.

On the area that burns, shrubs such as young curlleaf mountain mahogany, big sagebrush, black sagebrush and antelope bitterbrush, when present, will be killed. Idaho fescue may also suffer mortality. Recovery after fire is generally rapid due to the proximity of a desirable seed source and favorable moisture regime.

A frequent fire regime of one every 5-10 years generally does not develop on this site. The soils are too shallow and fuels are not continuous enough for frequent fire cycles to occur.

Influence of improper grazing management
Improper grazing management can damage this site moderately. Due to the rough and rocky nature of the site, livestock do not generally prefer 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 excessive utilization can be detrimental to this site; however, this type of management leads to reduced vigor of the bunchgrasses and palatable shrubs. 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. Only in the areas with deeper soils do plants capitalize on extra spring-time moisture. Below normal precipitation in the spring can reduce production and ultimately cause plant mortality 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.

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. Outbreaks of a curlleaf mountain mahogany defoliating moth, Stamnodes animata, occurs at infrequent intervals. Two consecutive years of severe defoliation can also cause mortality.

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 invasive species 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 young curlleaf mountain mahogany and 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 years’ leader growth.

Watershed
Decreased infiltration and increased runoff occur with the invasion of juniper. Juniper invasion can be triggered by lack of fire, improper grazing management and prolonged drought. The increased runoff also causes sheet and rill erosion. The long term effect is a transition to a different state.

Influence of Utah juniper and/or Rocky Mountain 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 improper grazing management and no fire.
Phase B to A. Develops through prescribed grazing.
Phase A to C. Develops after wildfire. Improper grazing management may accelerate the transition from Phase A to Phase C.
Phase C to A. Moves towards the HCPC with no fire and prescribed grazing.
Phase A to D. Develops with no fire.
Phase D to A. Removal of juniper with brush management (mechanical or prescribed fire)
affects this move.

State 1 Phase D to Unknown Site. Continued lack of fire and improper grazing management cause this juniper invaded site to cross the threshold and retrogress to a new site with reduced potential due to significant soil loss and changes in the hydrologic cycle. It is economically impractical to return this site to State 1 with accelerating practices.

Practice Limitations
There are few limitations for vegetative management practices on this site. This site is commonly a loafing area for livestock and they tend to overuse it; however, the steepness of slopes, rock outcrops and talus prevents free movement of livestock. Moderate limitations exist for facilitating practices due to shallow, stony soils. Any brush control practices should be carefully evaluated because maintaining curlleaf mountain mahogany on the site has high value to the entire ecosystem. Selective cutting of mountain mahogany will encourage new mahogany growth which will be a benefit to wildlife.