Ecological dynamics

This site developed under Colorado Plateau climatic conditions and included natural influences of herbivory, fire, and climate. This ecological site occurs on deep productive soils typically found on mesa tops in Major Land Resource Area (MLRA) 36—Southwestern Plateaus, Mesas, and Foothills. The precipitation and climate of MLRA 36 are conducive to producing Pinyon/juniper, sagebrush, and grassland complexes. The primary shrub species on this site is Basin big sagebrush, but may contain a significant component of Wyoming big sagebrush. Typically, when this site occurs on Cedar Mesa sandstone, the sagebrush is a tetraploid variety of basin big sagebrush. In the northern range of this site, Wyoming big sagebrush becomes a significant component of the ecological site.

The natural disturbance regime consisted of fires caused by natural and Native American ignition sources. The sagebrush and grassland communities were expected to be persistent with relatively frequent fire return intervals in locations where large unbroken expanses of vegetation were present. Fire return intervals were expected to be between 25-60 years. However, some natural topographic relief (extensive and frequent rock outcrops), infrequent lightning as an ignition source, and warm season grasses remaining green and inflamable during the lightning storm season may hinder the persistence of the sagebrush dominated plant communities through lengthened fire return intervals (>100 years), allowing for the encroachment of Pinyon and Utah juniper. Due to landscape dissection by large canyons, there are large firebreaks that reduce the influence of short fire return intervals creating occurrences of this ecological site that have longer periods of succession (i.e. periods of Pinyon and Utah juniper communities).

This ecological site has been grazed by domestic livestock since they were first introduced into the area. The introduction of domestic livestock and the use of fencing and reliable water sources have influenced the disturbance regime of this site. Introduction of livestock during the late 1800s/early 1900s coincides with the initial expansion of Pinyon and juniper woodlands (Miller and Tausch 2002). It is likely that domestic grazing influenced the expansion of Pinyon and Utah juniper woodlands through the reduction of fine fuels, thereby lengthening the fire return interval. Typically, fires would be able to carry following favorable growing conditions that created continuous fuels at sufficient intervals, thereby inhibiting Pinyon and Utah juniper establishment. In addition to influencing the fire regime, improper grazing can cause species composition shifts in the understory and may cause this site to depart from the reference state. Native perennial grasses and shrub will decrease while invasive forbs and annual grasses.

Other than fire and improper livestock grazing, other disturbance mechanisms include extended drought, insect herbivory, surface disturbances, such as off highway vehicle overuse, etc., all of which change the soil/water/vegetation relationships. These disturbances can either facilitate the transition into different plant communities or the transition from one stable state to another, depending on severity and durations.

As vegetation communities respond to changes in management or natural occurrences, thresholds can be crossed, which usually means that return to previous states may not be possible without major energy inputs. The amount of energy input needed to affect vegetative shifts depends on the present biotic and abiotic features and the desired results. The following diagram does not necessarily depict all the transitions and states that this site may exhibit, but it does show some of the most common plant communities. These plant communities may not represent every possibility, but they are the most prevalent and repeatable. As more data is collected, some of these plant communities may be revised or removed, and new ones may be added. This model was developed using range data collected in 2005 and 2006 in Natural Bridges National Monument in Southeastern Utah. Both ocular and measured data was collected and utilized.

--Reference State (State 1)--
The Reference State has been determined by study of rangeland relic areas, areas protected from excessive disturbance, and areas under outside influences such as domestic livestock grazing and recreational uses. Literature review, trends in plant communities under a variety of uses, and historical accounts also have been considered.

The reference state represents all plant communities and ecological dynamics of the upland loam Big sagebrush site. This state includes all biotic communities that become on the ecological site if all successional sequences are completed under the natural disturbance regime. This state is dominated by Basin and/or Wyoming big sagebrush with a diverse understory of native perennial grasses and native perennial and annual forbs. The nutrient cycling in native shrub-steppe ecosystems is characteristically ‘‘tight’’ (i.e. net concentrations of plant-available nutrients are low) because the broad spatial and temporal diversity of plants (woody and herbaceous) and soil microbes rapidly exploit nutrients as they are mineralized (Paschke et al., 2000). The primary disturbance mechanisms for this site in reference condition include fire, insect herbivory, and fluctuations in climate, such as drought or wet periods.

Prior to European settlement, fire return intervals were thought to be adequate enough to inhibit the encroachment of Pinyon and Utah juniper into the more productive deeper soils that support shrub steppe communities (Tausch 2002, Romme et al. 2007). However, as the number of years between fire events increases due to reduction in fine fuels or fire suppression, Pinyon and Utah juniper readily invade these communities. The time required to complete the transition of shrub steppe to a closed Pinyon and juniper woodland is variable. The transition is dependant on both the rate of tree establishment and site potential. Woodland presence at the landscape level also influences the rate of encroachment and closure on adjacent open areas

Reference state: Community phases maintained by fire, drought, insect herbivory, and time without disturbances.

Indicators: A diverse and productive understory co-existing with a fluctuating canopy of big sagebrush.

Feedbacks: Infrequent, but regular droughts that result in a reduction of sagebrush cover. Higher than average precipitation cycles that maintains the episodic native plant reproductive cycles. Insect infestations and/or a fire regime sufficient enough to inhibit Pinyon and Utah juniper encroachment. Improper grazing that results in a loss of native herbaceous understory, the establishment of non-native invasive plants, and a reduction in the fire return interval.

At-risk Community Phase: All communities are at risk when native plants in the understory are stressed, and nutrients become available for non-natives to establish. However, plant community 1.3 is most at risk due Pinyon and Utah juniper encroachment.

Trigger: The establishment of non-native invasive plants and/or Increased establishment of Utah juniper and Pinyon to a point where they drive the ecological dynamics of the site.

--Transition from Reference State (State 1) to Current Potential State (State 2)--
T1a– This transition from native perennial bunchgrass understory in the reference state to a state that has been invade by non-native species such as crested wheatgrass (blown in), cheatgrass, Russian thistle, and Annual wheatgrass. This transition occurs as natural and/or management actions favor establishment of non-native grasses and forbs, especially annuals. Some invasive plants can become established in undisturbed and healthy native plant communities. Possible events that can accelerate this transition include improper domestic livestock, severe surface disturbances, and extended droughts.

--Transition from Reference State (State 1) to Pinyon/Juniper State (State 3)--
T1b—This transition occurs from the big sagebrush shrubland community (1.3) into community 3.1. This occurs as Pinyon and Utah juniper increase to a point where they drive the ecological dynamics of the site. Shrub canopies can act as safe sites and facilitate the encroachment of Pinyon and Utah juniper into these deeper productive soils. Once established, trees are known to be capable of out-competing the remaining understory for nutrients and energy, thus reducing the shrubs and herbaceous understory even more. This process also facilitates the establishment of invasive annual plants such as Cheatgrass. This occurs through time without disturbances such as a fire or insect herbivory.

--Current Potential State (State 2)--
The current potential state is very similar to the reference state in energy capture, nutrient cycling, hydrologic function and disturbance regime. The current potential state may include acclimatized, naturalized, or invasive nonnative species. With proper management, plant communities within the current potential state may be managed and used for various purposes by man without significant alteration in plant community composition or production. It includes all of the plant communities that exist in the reference state, but with the inclusion of nonnative species. These plant communities with significant portions of invasive annuals are possible within this state. Continuous surface disturbances (improper grazing, off-highway-vehicle (OHV) use, recreational activities, etc.) can stress the native perennial plants and allow the non-natives species to increase. This shift in species composition could invoke another transition to a different state and affect the nutrient cycling, hydrologic function and soil stability. At this time there is no known way to completely remove the non-native plants from the site once they have become established. Therefore, this state is irreversibly altered from the reference state.

Current Potential State: Community phases maintained by fire, insects, drought, and time without disturbances.

Indicators: A diverse and productive understory dominated by native plants with non-native herbaceous plants co-existing under a canopy of big sagebrush.

Feedbacks: Infrequent, but regular droughts that reduce grass cover combined with sufficient moist cycles to maintain episodic reproductive cycles of native plants. Insect herbivory or fire return interval sufficient enough to inhibit Pinyon and juniper encroachment. Improper livestock grazing, that facilitates the loss of herbaceous understory, increasing non-native invasive plants, and causing a lengthened fire return interval).

At-risk Community Phase: Community 2.3 is the most at risk when Utah juniper and Pinyon have increase to a point where they drive the ecological dynamics of the site and have suppressed the understory.
Trigger – Increased Utah juniper and Pinyon, lack of understory, and increased erosion.

--Transition from Current Potential State (State 2) to Pinyon-Utah Juniper (State 3)--
T2a – This transition from a big sagebrush dominated community to a Pinyon and Utah juniper dominated community occurs when there is sufficient fire suppression and accelerated tree invasion to create a closed canopy. With a reduced fire potential this community enters into a perpetual tree dominated community unless there is significant energy inputs from outside influences.

--Transition from Current Potential (State 2) to Invasive Annuals State (State 5)--
T2b – This transition occurs when events favor the establishment and dominance of invasive annuals. Events could include increase fire return frequency (<5-20 years), improper grazing which reduces perennial bunch grasses, extended drought, increased surface disturbance through off road vehicle use, etc..

--Transition from Current Potential (State 2) to Seeded State (State 4)--
T2c – This pathway occurs after high amounts of energy inputs by man have been put into the system. Sagebrush and/or trees have been removed with vegetation manipulation techniques (chemical, mechanical, or fire) and introduced species that are adapted to the area and adapted to management needs have been established.

Pinyon-Utah Juniper State (State 3)
This state is characterized by a dominance of Utah juniper and Pinyon, with some to very little understory depending on which community phase it is in. It typically occurs if there is a long interval between disturbances, accompanied by a nearby source for Pinyon and Utah juniper seeds. This state can persist for long periods of time until extreme conditions needed for wildfire occur or a management treatment is implemented.

Pinyon and Utah Juniper State: Community phases maintained, in a self-sustaining manner, over time without disturbance.
Indicators: A declining understory, coupled with a dense canopy of Pinyon and Utah juniper.
Feedbacks: Surface disturbance and other negative pressures (grazing, drought, etc.) that reduce the herbaceous understory, combined with a lack of disturbances that remove trees from the community. The removal of trees, which facilitates an increase in herbaceous plant production.
At-risk Community Phase – Community 3.1 is most at risk when Utah juniper and Pinyon have increased to a point where there is no longer an understory.
Trigger – Removal of trees.
Restoration Pathway – Active vegetation management that facilitates the removal of trees and allows for the reestablishment of desired native and introduced species.

Transition from Pinyon-Utah Juniper State (State 3) to Invasive Annuals State (State 5)
T3a – This transition occurs when events favor the establishment and dominance of invasive annuals. Events would likely include a catastrophic wildfire or other method of tree removal in a community that has a cheatgrass dominated understory.

Transition from Pinyon–Utah Juniper State (State 3) to Seeded State (State 4)
T3b – This transition occurs through vegetation manipulation by man. Typically trees are removed through mechanical or chemical methods; the area is then re-seeded with grasses and forbs that are adapted to the area and to management needs.

--Seeded State (State 4)--
This state results from a significant vegetation manipulation that may include prescribed burning, chaining, disking, mowing, or other technique, with the introduction of non-native perennial grasses such as Crested wheatgrass or Russian wildrye. The dominant understory plant species are the seeded grasses and forbs. The community dynamics are similar to the Current Potential State. Depending on the species used for the reseeding practice, the site could have more resistance to fire as well as better tolerance to grazing pressure. This state may persist for long periods of time. Typically, big sagebrush will reestablish in the seeding to significant proportions within 30 years. Under careful management, native grasses and forbs may reestablish in this plant community over time. Actively manipulating plant communities in the invasive annuals or juniper invasion states to create a seeded range state is often the first step in assisted succession to restore natural plant communities back to the Current Potential State.

Seeded State: Community phases maintained by fire, drought, and time without disturbances.

Indicators: A developed perennial herbaceous understory of seeded species, typically non-natives, co-existing with a canopy of basin big sagebrush.

Feedbacks: Infrequent, but regular droughts that reduce grass cover. Moist cycles that maintain perennial bunch grasses. A fire regime sufficient to inhibit Pinyon and Utah juniper encroachment. Improper grazing resulting in a loss of the herbaceous understory, an increase in invasive plants, and an increase in the fire return interval.

At-risk Community Phase: Community 4.3 is the most at risk when Utah juniper and Pinyon have increased to a point where they drive the ecological dynamics of the site.

Restoration Pathway: proper domestic livestock grazing practices and the removal of trees that facilitates the reestablishment of native and introduced species.

--Transition from Seeded State (State 4) to Invasive Annuals State (State 5)--
T4a – This transition occurs when events favor the establishment and dominance of invasive annuals. Events may include improper grazing which reduces the perennial bunch grasses, an extended drought, increased surface disturbance through off road vehicle use or other surface disturbance, or a shortened fire return interval, all of which can stress the native perennial bunchgrasses.

--Transition from Seeded State (State 4) to Pinyon-Utah Juniper State (State 3)--
T4b – This transition from a big sagebrush and non-native seeded grassland dominated community to a Pinyon and Utah juniper dominated community occurs when there is sufficient fire suppression and accelerated Pinyon and Utah juniper invasion to create a closed canopy. With a reduced fire potential the site enters into a perpetual tree dominated community unless there is significant energy inputs from outside influences.

--Restoration Pathway from Seeded State (State 4) to Current Potential (State 2)--
R4a- This pathway occurs as a result of long periods without disturbance and the community is allowed to develop ecosystem functions. This could be through prescribed grazing with domestic livestock to favor the development of native grasses over the introduced species (with proper time, timing, and amounts of grazing), and removal of the Utah juniper and Pinyon as they encroach.

--Invasive Annuals State (State 5)--
This state is recognized by the predominance of invasive annuals. Invasive annuals have increased to a point where they influence or drive the disturbance regime and the nutrient cycle and energy flow, altering it from the regimes and cycles associated with sagebrush ecosystems. Research has shown that plant species differ substantially in their effects on soil water content and temperature and their effects on the frequency and intensity of disturbance. Once exotic plants like cheatgrass or Russian thistle have invaded a site fundamental nutrient cycling processes are known to change (Chapin et al., 1997). Cheatgrass invasion has been shown to alter the timing, distribution, and composition of organic matter inputs, as well as uptake of mineralized nutrients (Evans et al., 2001). Energy flow in this state is severely suppressed, with photosynthesis occurring only during a brief period in the spring. Cheatgrass invasion has also been shown to change the composition root pores, mycorrhizal associations, and assemblages of microbial species potentially affecting soil structure and the rate of soil organic matter decomposition (Belknap and Phillips, 2001). The altered disturbance regime and the loss of soil organic matter could create ecologically impoverished sites that are very difficult, if not impossible to restore to functionally diverse perennial herbaceous and woody communities.

Invasive Annuals State: Community phases maintained by fire, drought, livestock grazing; vegetation management, and time without disturbances.
Indicators: An annual grass or forb understory, where big sagebrush may or may not be present

Feedbacks: Short fire intervals, which maintain the annual grass and forb understory. Longer fire intervals, vegetation management, or livestock grazing that allows for the sagebrush overstory to reestablish.

At-risk Community Phase: Community 5.2 is the most at risk when cheatgrass or other annuals completely dominate the site, shortening the fire return interval, that maintains the annual grass/forb community.

--Transition from Invasive Annuals State (State 5) to Seeded State (State 4)--
T5a – Successful completion of this transition is difficult with current knowledge of adapted plants, soil preparation and seed techniques. Large amounts of input and management are required for the establishment of a perennial introduced/adapted grass. This pathway occurs when the invasive annuals are treated and removed from dominance and the desired species (typically non-native perennials that can successfully compete) are seeded and established. At this time, restoring from the invasive annuals state directly to a native grass community has not been successful possibly due to changes in the soil caused by the cheatgrass.

--Transition from Invasive Annuals State (State 5) to Pinyon-Utah Juniper State (State 3)--
T5b - This transition from a big sagebrush and invasive annual dominated community to a Pinyon and Utah juniper dominated community occurs when there is sufficient fire suppression and accelerated juniper invasion to create a closed canopy. With a reduced fire potential and a non-viable seed bank this community enters into a perpetual tree dominated community unless there is significant energy inputs from outside influences.