Ecological dynamics

Ecological Dynamics of the Site:

The dominant visual aspect of this site is primarily curlleaf mountain mahogany in the overstory with bluebunch wheatgrass in the understory. Oregon grape and Nevada bluegrass can also be major understory species. Composition by weight is approximately 15-25 percent grasses, 1-10 percent forbs, and 70-80 percent shrubs (and tree-like shrubs). These percentages are for current annual growth for all plants, irrespective of height.

During the last few thousand years, this site has evolved in a semi-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 very shallow to shallow (and deep in some crevasses), 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.

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. This plant community is dominated by curlleaf mountain mahogany in the overstory and bluebunch wheatgrass in the understory. Nevada bluegrass, mountain brome, western needlegrass, Sandberg bluegrass, arrowleaf balsamroot, and Oregon grape also occur in the understory. The plant species composition of Phase A is listed later under “Reference Plant Community Phase Plant Species Composition”.

Utah juniper sites may occur in association with this site. Utah juniper can invade this site when a seed source is present. Conifers such as juniper have greater growth rates, their shape is more tapered and they reach greater heights. Consequently, conifer species invading curlleaf mountain mahogany sites eventually over top them. Because mature curlleaf mountain mahogany is shade intolerant, its competitive ability is lost and it becomes senescent. Mortality usually follows.

Total annual production is 1100 pounds per acre (1232 kilograms per hectare) in a normal year. Production in a favorable year is 1400 pounds per acre (1568 kilograms per hectare). Production in an unfavorable year is 800 pounds per acre (896 kilograms per hectare).

Structurally, curlleaf mountain mahogany dominates the overstory along with a variety of other shrubs in small amounts. In the understory, cool season deep rooted perennial bunchgrasses are dominant, followed by perennial forbs while shallow rooted perennial bunchgrasses are subdominant.


FUNCTION:

This site is poor for livestock grazing due to steep slopes and surface rocks. Big game animals use the site in the spring, summer, and fall. It is important for both hiding and thermal cover. Birds use the mahogany for nesting. This site is very important as summer habitat for mountain bluebirds. The site has high value for hunting and photography. The mountain mahogany provides visual diversity to the landscape. This site is fairly resistant to degradation due to the steep slopes and surface stones.

Impacts on the Plant Community.

Influence of fire:

Because of its topographic position on the landscape, ridgetops, and sideslopes, fires started by lightning strikes are fairly common. The site is extremely rocky on the surface and rock outcrops may be present. 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.

Where there is a Utah 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.

On the area that burns, shrubs such as curlleaf mountain mahogany and mountain big sagebrush will be killed. Recovery after fire is generally rapid due to the proximity of a readily available seed source and favorable moisture regime.

A frequent fire regime, for example one every 5-10 years, generally does not develop on this site. 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 can damage this site moderately. Due to the rough and rocky nature of the site, livestock do not generally prefer to use it. 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.

Generally juniper seedlings can replace the desirable grasses and shrubs if improper grazing management continues.

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 palatable shrubs. With reduced vigor, recruitment of these species declines.

Proper grazing management that addresses frequency, duration, and intensity of grazing can also keep fine fuels from developing, thereby reducing fire frequency. This can lead to gradual increases in curlleaf mountain mahogany. Proper grazing management tends to keep a balanced plant community of grasses, forbs, and shrubs.

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 moisture. Below normal precipitation in the spring can reduce production and ultimately cause plant mortality if drought conditions continue for several years.

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:

Outbreaks can affect vegetation health. Outbreaks of curlleaf mountain mahogany defoliating moth Stamnodes animata, occur 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 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 either the lack of fire, improper grazing management, or prolonged drought. The increased runoff also causes sheet and rill erosion. The long term effect is a transition to a different state.

Influence of juniper invasion:

The following discussion deals with both western juniper and Utah juniper.

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 hydrologic 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 hydrologic 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. This phase would be
unusual since livestock use is limited by surface rocks and steep slopes.

Phase B to A. Develops through prescribed grazing and no fire.

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 Phase A with no fire and prescribed grazing.

Phase A to D. Develops with no fire.

Phase D to A. Moves towards Phase A with brush management and prescribed grazing.

State 1 Phase D to Unknown New Site. This occurs with the continued lack of fire and improper grazing management from the juniper invaded phase of Plant Community D, State 1. It is probably similar to Juniper Breaks 12-16”. The site has crossed the threshold. It is not economically feasible to move this plant community back across the threshold towards Phase D or A, State 1.


Practice Limitations:

This site has severe limitations for facilitating and accelerating practices due to steep slopes, surface stones, and shallow soils. Any brush control practices should be carefully evaluated because maintaining curlleaf mountain mahogany on the site has high value to the entire ecosystem. If junipers have invaded the site, selective removal with a chain saw is a viable option.