Ecological dynamics

The ecological sites in this DRG are dominated by deep-rooted cool season perennial bunchgrasses and long-lived shrubs (50+ years) with high root to shoot ratios. The dominant shrubs usually root to the full depth of the winter-spring soil moisture recharge, which ranges from 1.0 to over 3.0 m. (Comstock and Ehleringer 1992). Root length of mature sagebrush plants was measured to a depth of two meters in alluvial soils in Utah (Richards and Caldwell 1987). Tap roots of antelope bitterbrush have been documented from 4.5 to 5.4 m in length (McConnell 1961). These shrubs have a flexible generalized root system with development of both deep taproots and laterals near the surface (Comstock and Ehleringer 1992).

The Great Basin sagebrush communities have high spatial and temporal variability in precipitation both among years and within growing seasons. Nutrient availability is typically low but increases with elevation and closely follows moisture availability. The moisture resource supporting the greatest amount of plant growth is usually the water stored in the soil profile during the winter. The invasibility of plant communities is often linked to resource availability. Disturbance can decrease resource uptake due to damage or mortality of the native species and depressed competition, or it can increase resource uptake by the decomposition of dead plant material following disturbance. The invasion of sagebrush communities by cheatgrass (Bromus tectorum) has been linked to disturbances (fire, abusive grazing) that have resulted in fluctuations in resources (Chambers et al 2007). Dobrowolski et al. (1990) cite multiple authors on the extent of the soil profile exploited by the competitive exotic annual cheatgrass. Specifically, the depth of rooting is dependent on the size the plant achieves, and in competitive environments cheatgrass roots were found to penetrate only 15 cm, whereas isolated plants and pure stands were found to root at least 1 m in depth with some plants rooting as deep as 1.5 to 1.7 m.

Periodic drought regularly influences sagebrush ecosystems and drought duration and severity has increased throughout the 20th century in much of the Intermountain West. Major shifts away from historical precipitation patterns have the greatest potential to alter ecosystem function and productivity. Species composition and productivity can be altered by the timing of precipitation and water availability within the soil profile (Bates et al. 2006).

Native insect outbreaks are also important drivers of ecosystem dynamics in big sagebrush communities. Climate influences the timing of insect outbreaks, especially a sagebrush defoliator, Aroga moth (Aroga websteri). Aroga moth infestations have occurred in the Great Basin in the 1960s, early 1970s, and have been ongoing in Nevada since 2004 (Bentz et al. 2008). Thousands of acres of big sagebrush have been impacted, with partial to complete die-off observed. Aroga moth can partially or entirely kill individual plants or entire stands of big sagebrush (Furniss and Barr 1975).

Mountain big sagebrush and antelope bitterbrush are generally long-lived; therefore, it is not necessary for new individuals to recruit every year for perpetuation of the stand. Infrequent large recruitment events and simultaneous low, continuous recruitment is the foundation of population maintenance (Noy-Meir 1973). Survival of the seedlings is dependent on adequate moisture conditions.

The perennial bunchgrasses that are co-dominant with the shrubs in this group include western needlegrass, Letterman’s needlegrass, and Columbia needlegrass. Basin wildrye is an important species as well. These species generally have somewhat shallower root systems than the shrubs, but root densities are often as high as or higher than those of shrubs in the upper 0.5 m and taper off more rapidly than shrubs. General differences in root depth distributions between grasses and shrubs result in resource partitioning in these shrub/grass systems.

Letterman needlegrass is an erect, densely tufted perennial bunchgrass that forms large clumps. It is found on dry soils in a variety of vegetation communities, including, high elevation meadows, subalpine grasslands, open areas underneath aspen, and in sagebrush communities. It grows best on loamy soils that are greater than 20 cm deep (Dittberner and Olson 1983).

Basin wildrye is a large, cool-season perennial bunchgrass with an extensive deep coarse fibrous root system (Reynolds and Fraley 1989). Clumps may reach up to six feet in height (Ogle et al. 2012b). Basin wildrye does not tolerate long periods of inundation; it prefers cycles of wet winters and dry summers and is most commonly found in deep soils with high water holding capacities or seasonally high water tables (Ogle et al. 2012b, Perryman and Skinner 2007).

Infilling by singleleaf pinyon (Pinus monophylla) and Utah juniper (Juniperus osteosperma) may also occur with an extended fire return interval. Eventually, singleleaf pinyon and Utah juniper will dominate the site and mountain big sagebrush will be severely reduced along with the herbaceous understory. Bluegrasses (Poa spp.) and bottlebrush squirreltail (Elymus elymoides) may remain underneath trees on north-facing slopes. The potential for soil erosion increases as the pinyon and/or juniper woodland matures and the understory plant community cover declines.

Millions of acres in the arid and semi-arid West were brush-beaten and planted with crested wheatgrass in the mid 1900’s for the purpose of competing with weed species and increasing grass production on rangelands. Success and longevity of these seeding projects have been mixed (Williams et al. 2017). Crested wheatgrass is a cool-season, medium height, exotic perennial bunchgrass native to Asia. Sites within this DRG may exhibit an understory of crested wheatgrass in areas where historical seedings have occurred or where crested wheatgrass has been used in post-fire rehabilitation seedings.

The ecological sites in this DRG have low to moderate resilience to disturbance and resistance to invasion. Resilience increases with elevation, aspect, increased precipitation and increased nutrient availability. Long-term disturbance response may be influenced by small differences in landscape topography. North slopes are also more resilient than south slopes because lower soil surface temperatures operate to keep moisture content higher on northern exposures. Six possible alternative stable states have been identified for this DRG.

Invasive Annual Grasses:
The species most likely to invade these sites is cheatgrass. Cheatgrass is a cool season annual grass that maintains an advantage over native plants in part because it is a prolific seed producer, can germinate in the autumn or spring, tolerates grazing, and increases with frequent fire (Klemmedson and Smith 1964, Miller et al. 1999). Cheatgrass originated from Eurasia and was first reported in North America in the late 1800s (Mack and Pyke 1983; Furbush 1953). Pellant and Hall (1994) found 3.3 million acres of public lands dominated by cheatgrass and suggested that another 76 million acres were susceptible to invasion by winter annuals including cheatgrass and medusahead.

Recent modeling and empirical work by Bradford and Lauenroth (2006) suggests that seasonal patterns of precipitation input and temperature are also key factors determining regional variation in the growth, seed production, and spread of invasive annual grasses. The phenomenon of cheatgrass “die-off” provides opportunities for restoration of perennial and native species (Baughman et al. 2016, Baughman et al. 2017). The causes of these events are not fully understood, but there is ongoing work to try to predict where they occur, in the hopes of aiding conservation planning (Weisberg et al. 2017, Brehm 2019).

Methods to control cheatgrass include herbicide, fire, targeted grazing, and seeding. Mapping potential or current invasion vectors is a management method designed to increase the cost effectiveness of control methods. Spraying with herbicide (Imazapic or Imazapic + glyphosate) and seeding with crested wheatgrass and Sandberg bluegrass has been found to be more successful at combating cheatgrass (and medusahead) than spraying alone (Sheley et al. 2012). To date, most seeding success has occurred with non-native wheatgrass species. Perennial grasses, especially crested wheatgrass, are able to suppress cheatgrass growth when mature (Blank et al. 2020). Where native bunchgrasses are missing from the site, revegetation of annual grass invaded rangelands has been shown to have a higher likelihood of success when using introduced perennial bunchgrasses such as crested wheatgrass (Clements et al. 2017, Davies et al. 2015). Butler et al. (2011) tested four herbicides (Imazapic, Imazapic + glyphosate, rimsulfuron, and sulfometuron + Chlorsulfuron) for suppression of cheatgrass, medusahead and ventenata (North Africa grass, Ventenata dubia) within residual stands of native bunchgrass. Additionally, they tested the same four herbicides followed by seeding of six bunchgrasses (native and non-native) with varying success (Butler et al. 2011). Herbicide-only treatments appeared to remove competition for established bluebunch wheatgrass by providing 100% control of ventenata and medusahead and greater than 95% control of cheatgrass (Butler et al. 2011). Caution in using these results is advised, as only one year of data was reported.

In considering the combination of pre-emergent herbicide and prescribed fire for invasive annual grass control, it is important to assess the tolerance of desirable brush species to the herbicide being applied. Vollmer and Vollmer (2008) tested the tolerance of mountain mahogany (Cercocarpus montanus), antelope bitterbrush, and multiple sagebrush species to three rates of Imazapic with and without methylated seed oil as a surfactant. They found a cheatgrass control program in an antelope bitterbrush community should not exceed Imazapic at 8 oz./ac with or without surfactant. Sagebrush, regardless of species or rate of application, was not affected. However, many environmental variables were not reported in this study and managers should install test plots before broad scale herbicide application is initiated.

Fire Ecology:
Fire is believed to be the dominant disturbance force in natural big sagebrush communities. Several authors suggest pre-settlement fire return intervals in mountain big sagebrush communities varied from 15 to 25 years (Burkhardt and Tisdale 1969, Houston 1973, and Miller et al. 2000). Kitchen and McArthur (2007) suggest a mean fire return interval of 40 to 80 years for mountain big sagebrush communities. Natural fire return intervals are estimated to vary between less than 35 years up to 100 years in sagebrush ecosystems with basin wildrye (Brown and Smith 2000). The range from 15 to 80 years is probably more accurate and reflects the differences in elevation and precipitation where mountain big sagebrush communities occur. On a landscape scale, multiple seral stages were represented in a mosaic reflecting periodic reoccurrence of fire and other disturbances (Crawford et al. 2004). Post-fire hydrologic recovery and resilience is primarily influenced by pre-fire site conditions, fire severity, and post-fire weather and land use that relate to vegetation recovery. Fire adaptation by herbaceous species is generally superior to the dominant shrubs, which are typically killed by fire. Sites with low abundances of native perennial grasses and forbs typically have reduced resiliency following disturbance and are less resistant to invasion or increases in cheatgrass (Miller et al. 2013).

Mountain big sagebrush is killed by fire (Neunschwander 1980, Blaisdell et al. 1982) and does not resprout (Blaisdell 1953). Post-fire regeneration occurs from seed and will vary depending on site characteristics, seed source, and fire characteristics. Mountain big sagebrush seedlings can grow rapidly and may reach reproductive maturity within 3 to 5 years (Bunting et al. 1987). Mountain big sagebrush may return to pre-burn density and cover within 15 to 20 years following fire, but establishment after severe fires may proceed more slowly (Bunting et al. 1987).

Antelope bitterbrush is moderately fire tolerant (McConnell and Smith 1977). It regenerates by seed and resprouting (Blaisdell and Mueggler 1956, McArthur et al. 1982), however, sprouting ability is highly variable and has been attributed to genetics, plant age, phenology, soil moisture and texture, and fire severity (Blaisdell and Mueggler 1956, Blaisdell et al. 1982, Clark et al. 1982, Cook et al. 1994). Bitterbrush sprouts from a region on the stem approximately 1.5 inches above and below the soil surface; the plant rarely sprouts if the root crown is killed by fire (Blaisdell and Mueggler 1956). Low intensity fires may allow for bitterbrush to sprout; however, community response also depends on soil moisture levels at time of fire (Murray 1983). Lower soil moisture allows more charring of the stem below ground level (Blaisdell and Mueggler 1956), thus sprouting will usually be more successful after a spring fire than after a fire in summer or fall (Murray 1983, Busse et al. 2000, Kerns et al. 2006). If cheatgrass is present, bitterbrush seedling success is much lower. The factor that most limits establishment of bitterbrush seedlings is competition for water resources with the invasive species cheatgrass (Clements and Young 2002).

The effect of fire on bunchgrasses relates to culm density, culm-leaf morphology, and the size of the plant. The initial condition of bunchgrasses within the site along with seasonality and intensity of the fire all factor into the individual species response. For most forbs and grasses the growing points are located at or below the soil surface providing relative protection from disturbances which decrease above ground biomass, such as grazing or fire. Thus, fire mortality is more correlated to duration and intensity of heat which is related to culm density, culm-leaf morphology, size of plant and abundance of old growth (Young 1983, Wright 1971). Plant response will vary depending on post-fire soil moisture availability.

Columbia needlegrass resprouts quickly after fire (Monsen et al. 2004). Fall burning has been shown to increase seed production in Columbia needlegrass (Patton et al. 1988). Letterman’s needlegrass recovers well after fire (Monsen et al. 2004). Emergence of western needlegrass seeds was shown to significantly improve with additions of smoke and burned soil (Blank and Young 1996).

Basin wildrye is relatively resistant to fire, particularly dormant season fire, as plants sprout from surviving root crowns and rhizomes (Zschaechner 1985). Miller et al. (2013) reported increased total shoot and reproductive shoot densities in the first year following fire, although by year two there was little difference between burned and control treatments.

Bottlebrush squirreltail is considered to be relatively tolerant to fire due to its small size, coarse stems, and sparse leafy material (Britton et al. 1990). Post-fire regeneration occurs from surviving root crowns and from on- and off-site seed sources. Bottlebrush squirreltail has the ability to produce large numbers of seeds with high germination rates when exposed to the correct environmental cues (Young and Evans 1977). Early spring growth and ability to grow at low temperatures contribute to the persistence of bottlebrush squirreltail on sites with high cover of cheatgrass (Hironaka and Tisdale 1973).
Cheatgrass accumulates fuel loads that foster frequent fires (Davies and Svejcar 2008). Invasion by annual grasses can alter the fire cycle by increasing fire size, rate of spread, numbers of individual fires, fire season length, and likelihood of fires spreading into native or managed ecosystems (D’Antonio and Vitousek 1992, Brooks et al. 2004). Areas dominated with cheatgrass are estimated to have a fire return interval of 3 to 5 years (Whisenant 1990). The mechanisms by which invasive annual grasses alter fire regimes likely interact with climate. For example, cheatgrass cover and biomass vary with climate (Chambers et al. 2007) and are promoted by wet and warm conditions during the fall and spring. Invasive annual species have been shown able to take advantage of high N availability following fire through higher growth rates and increased seedling establishment relative to native perennial grasses (Monaco et al. 2003).

Catastrophic wildfire in Utah juniper and/or singleleaf pinyon dominated sites may lead to an annual weed dominated site. Depending on fire severity, rabbitbrush and snowberry may increase after fire. Rubber rabbitbrush (Ericameria nauseosa) is top-killed by fire, but can resprout after fire and can also establish from seed (Young 1983). Douglas’ rabbitbrush (Chrysothamnus viscidiflorus) is top-killed by fire, but sprouts vigorously after fire (Kuntz 1982, Akinsoji 1988). Snowberry is also top-killed by fire, but resprouts after fire from rhizomes (Noste and Bushey 1987). If arrowleaf balsamroot (Balsamorhiza sagittata) or mule-ears (Wyethia amplexicaulis) is common before fire, they will increase after fire. These species may increase with heavy grazing as well.

State and Transition Model Narrative Group 12:
This is a text description of the states, phases, transitions, and community pathways possible in the State and Transition model for the MLRA 26 Disturbance Response Group 12. Site included in this DRG are: R026XY005NV, R026XY105NV, R026XY046NV, R026XY006NV, R026XY048NV, R026XY111NV, R026XY040NV, R026XY106NV, R026XY089NV, R026XF057CA, R026XF063CA, and R026XF064CA.

Reference State 1.0:
The Reference State 1.0 represents the natural range of variability of this site under pristine conditions. The reference state has three general community phases: a shrub-grass dominant phase, a perennial grass dominant phase and a shrub dominant phase. State dynamics are maintained by interactions between climatic patterns and disturbance regimes. Negative feedbacks enhance ecosystem resilience and contribute to the stability of the state. These include the presence of all structural and functional groups, low fine fuel loads, and retention of organic matter and nutrients. Plant community phase changes are primarily driven by fire, periodic drought, and/or insect or disease attack.

Community Phase 1.1:
Needlegrasses, basin wildrye, and mountain big sagebrush dominate the site. Bitterbrush may be a significant component. Pinyon and juniper may be present.

Community Phase Pathway 1.1a, from phase 1.1 to 1.2:
Fire would decrease or eliminate the overstory of sagebrush and allow for the perennial bunchgrasses and perennial forbs to dominate the site. Fires would typically be small and patchy due to low fuel loads. Low severity fire creates sagebrush/grass mosaic. High severity fire significantly reduces sagebrush cover and leads to early/mid-seral community, dominated by grasses and forbs.

Community Phase Pathway 1.1b, from phase 1.1 to 1.3:
Time and lack of disturbance such as fire or drought allow for an increase in mountain big sagebrush. Excessive herbivory and/or long-term drought may also reduce perennial understory.

Community Phase 1.2:
This community phase is characteristic of a post-disturbance, early- to mid-seral plant community. Needlegrasses, basin wildrye, and perennial grasses and forbs dominate. Mountain big sagebrush is a minor component. Bitterbrush may be sprouting. Forbs may increase.

Community Phase Pathway 1.2a, from phase 1.2 to 1.1:
Time and lack of disturbance allows for sagebrush to reestablish.

Community Phase 1.3:
Mountain big sagebrush increases in the absence of disturbance and becomes dominant. Bitterbrush may be a significant component. Needlegrasses and other perennial grasses are reduced. Squirreltail and/or bluegrasses may increase. Pinyon and juniper may be present.

Community Phase Pathway 1.3a, from phase 1.3 to 1.2:
Fire. A low severity fire creates a sagebrush/grass mosaic, while a high-severity fire reduces sagebrush to trace amounts.

T1A: Transition from Reference State 1.0 to Current Potential State 2.0
Trigger: This transition is caused by the introduction of non-native annual weeds, such as cheatgrass, mustard and Russian thistle (Salsola tragus).
Slow variables: Over time the annual non-native plants will increase within the community, decreasing organic matter inputs from deep-rooted perennial bunchgrasses. This results in reductions in soil water availability for perennial bunchgrasses.
Threshold: Any amount of introduced non-native species causes an immediate decrease in the resilience of the site. Annual non-native species cannot be easily removed from the system and have the potential to significantly alter disturbance regimes from their historic range of variation.

T1B: Transition from Reference State 1.0 to Shrub State 3.0
Trigger: Inappropriate, long-term grazing of perennial bunchgrasses during the growing season would favor shrubs and initiate transition to Community Phase 3.1.
Slow variables: Long term decrease in deep-rooted perennial grass density.
Threshold: Loss of deep-rooted perennial bunchgrasses changes spatial and temporal nutrient cycling and nutrient redistribution and reduces soil organic matter and soil moisture.

T1C: Transition from Reference State 1.0 to Tree State 5.0
Trigger: Time and lack of disturbance or management action allows pinyon or juniper to dominate. This may be coupled with grazing management that favors tree establishment by reducing understory herbaceous competition for site resources.
Feedbacks and ecological processes: Trees increasingly dominate use of soil water, contributing to reductions in soil water availability to grasses and shrubs. Overtime, grasses and shrubs are outcompeted. Reduced herbaceous and shrub production slows soil organic matter inputs and increases soil erodibility through loss of cover and root structure.
Slow variables: Over time the abundance and size of trees will increase.
Threshold: Trees dominate ecological processes and the number of shrub skeletons exceed number of live shrubs.

Current Potential State 2.0:
This state is similar to the Reference State 1.0. Ecological function has not changed, however, the resiliency of the state has been reduced by the presence of invasive weeds. This state has four general community phases. Negative feedbacks enhance ecosystem resilience and contribute to the stability of the state. These include the presence of all structural and functional groups, low fine fuel loads and retention of organic matter and nutrients. Positive feedbacks decrease ecosystem resilience and stability of the state. These include the non-natives high seed output, persistent seed bank, rapid growth rate, ability to cross pollinate and adaptations for seed dispersal. Additionally, the presence of highly flammable, non-native species reduces State resilience because these species can promote fire where historically fire has been infrequent leading to positive feedbacks that further the degradation of the system.

Community Phase 2.1:
Needlegrasses, basin wildrye and mountain big sagebrush dominate the site. Bitterbrush may be a significant component. Pinyon and juniper may be present. Annual non-native species present.


Community Phase Pathway 2.1a, from phase 2.1 to 2.2:
Fire. Low severity fire creates sagebrush/grass mosaic while a high severity fire significantly reduces sagebrush cover and leads to early/mid-seral community dominated by grasses and forbs. Non-native annual species present.

Community Phase Pathway 2.1b, from phase 2.1 to 2.3:
Time, lack of disturbance, long-term drought, grazing management that favors shrubs or combinations of these would allow the sagebrush overstory to increase and dominate the site, causing a reduction in the perennial bunchgrasses.

Community Phase 2.2:
This community phase is characteristic of a post-disturbance, early-seral plant community. Needlegrasses and other perennial grasses and perennial forbs dominate. Mountain big sagebrush is a minor component. Bitterbrush may be sprouting. Forbs may increase. Annual non-native species are present.


Community Phase Pathway 2.2a, from phase 2.2 to 2.1:
Absence of disturbance over time allows for the sagebrush to recover. This may be combined with grazing management that favors shrubs.

Community Phase Pathway 2.2b, from Phase 2.2 to 2.4:
Higher than normal spring precipitation favors annual non-native species such as cheatgrass. Non-native annual species will increase in production and density throughout the site. Shrub removal or prescribed burning may increase invasive annuals. Perennial bunchgrasses may also increase in production.

Community Phase 2.3:
Mountain big sagebrush is dominant and the perennial understory is reduced. Bitterbrush may be a significant component. Bluegrass may increase. Pinyon and juniper may be present. Annual non-native species are present.


Community Phase Pathway 2.3a, from phase 2.3 to 2.1:
Low severity fire creates sagebrush/grass mosaic. Other disturbances/practices include brush management with minimal soil disturbance leading to a reduction in sagebrush.

Community Phase Pathway 2.3b, from phase 2.3 to 2.2:
High severity fire or brush management with minimal soil disturbance significantly reduces sagebrush and leads to early/mid-seral community.

Community Phase Pathway 2.3c, from phase 2.3 to 2.4:
Fall and spring growing season conditions that favor the germination and production of non-native annual grasses.

Community Phase 2.4:
This community is at risk of crossing to an annual state. Native bunchgrasses and forbs still comprise 50% or more of the understory annual production, however non-native annual grasses are nearly codominant. If this site originated from phase 2.3 there may be significant shrub cover as well. Annual production and abundance of these annuals may increase drastically in years with heavy spring precipitation. This site is susceptible to further degradation from grazing, drought, and fire.

Community Phase Pathway 2.4a, from phase 2.4 to 2.2:
Growing season conditions favoring perennial bunchgrass production and reduced cheatgrass production.

Community Phase Pathway 2.4b, from phase 2.4 to 2.3:
Growing season conditions favoring perennial bunchgrass production and reduced cheatgrass production.

T2A: Transition from Current Potential State 2.0 to Shrub State 3.0:
Trigger: Inappropriate, long-term grazing of perennial bunchgrasses during growing season would favor shrubs and initiate transition to Community Phase 3.1.
Slow variables: Long term declines in deep-rooted perennial grass density.
Threshold: Loss of deep-rooted perennial bunchgrasses changes spatial and temporal nutrient cycling and nutrient redistribution and reduces soil organic matter.

T2B: Transition from Current Potential State 2.0 to Annual State 4.0:
Trigger: Fire and/or multiple fires lead to plant community phase 4.1, inappropriate grazing management that favors shrubs in the presence of non-native annual species leads to community phase 4.2.
Slow variables: Increased production and cover of non-native annual species.
Threshold: Loss of deep-rooted perennial bunchgrasses and shrubs truncates, spatially and temporally, nutrient capture and cycling within the community. Increased, continuous fine fuels from annual non-native plants modify the fire regime by changing intensity, size and spatial variability of fires.

T2C: Transition from Current Potential 2.0 to Tree State 5.0:
Trigger: Time and lack of disturbance or management action allows pinyon and juniper to dominate. This may be coupled with grazing management that favors tree establishment by reducing understory herbaceous competition for site resources.
Feedbacks and ecological processes: Trees increasingly dominate use of soil water, contributing to reductions in soil water availability to grasses and shrubs. Overtime, grasses and shrubs are outcompeted. Reduced herbaceous and shrub production slows soil organic matter inputs and increases soil erodibility through loss of cover and root structure.
Slow variables: Over time the abundance and size of trees will increase.
Threshold: Trees dominate ecological processes and number of shrub skeletons exceed the number of live shrubs.

Shrub State 3.0:
This state has two community phases: a mountain big sagebrush dominated phase and a squirreltail-dominated phase. This state is a product of many years of heavy grazing during time periods harmful to perennial bunchgrasses. Squirreltail will increase with a reduction in deep rooted perennial bunchgrass competition and become the dominant grass. Sagebrush and bitterbrush dominate the overstory. Sagebrush canopy cover is high and sagebrush may be decadent, reflecting stand maturity and lack of seedling establishment due to competition with mature plants. The shrub overstory and bluegrass understory dominate site resources such that soil water, nutrient capture, nutrient cycling and soil organic matter are temporally and spatially redistributed.

Community Phase 3.1:
This site is at risk of transitioning to another state. Mountain big sagebrush, possibly decadent, dominates the overstory. Antelope bitterbrush may be a significant component. Deep-rooted perennial bunchgrasses are present in only trace amounts and may be absent from the community. Squirreltail may be dominant in the understory. Understory may be sparse, with bare ground increasing. Pinyon and/or juniper may be present as a result of encroachment from neighboring sites and lack of disturbance. Annual non-native species are present to increasing.


Community Phase Pathway 3.1a, from phase 3.1 to 3.2:
Fire reduces or eliminates the overstory of sagebrush. Non-native annual species increase with higher-than-normal spring precipitation.

Community Phase 3.2:
Squirreltail dominates the site. Needlegrasses and other perennial grasses are reduced. Mountain big sagebrush is reduced or missing. Bitterbrush may be sprouting. Annual non-native species are increasing and may be co-dominant in the understory.

Community Phase Pathway 3.2a, from phase 3.2 to 3.1:
Absence of disturbance over time would allow for sagebrush and other shrubs to recover.

T3A: Transition from Shrub State 3.0 to Annual State 4.0:
Trigger: Fire or inappropriate grazing management can eliminate the squirreltail understory and transition to community phase 4.1 or 4.2.
Slow variable: Increased seed production and cover of annual non-native species.
Threshold: Increased, continuous fine fuels modify the fire regime by changing intensity, size and spatial variability of fires. Changes in plant community composition and spatial variability of vegetation due to the loss of perennial bunchgrasses and big sagebrush truncate energy capture and impact the nutrient cycling and distribution.

T3B: Transition from Shrub State 3.0 to Tree State 5.0:
Trigger: Time and lack of disturbance allows for maturation of the tree community. This may be coupled with grazing management that favors shrub and tree growth.
Slow variable: Over time, with lack of fire, the abundance and size of trees increase.
Threshold: Trees overtop mountain big sagebrush and out-compete shrubs for water and sunlight. Shrub skeletons exceed live shrubs with minimal recruitment of new cohorts. Bare ground areas are large and connected.

R3A: Restoration from Shrub State 3.0 to Seeded State 6.0:
Brush management and seeding of crested wheatgrass and/or other desired species. Presence of non-native annual species will make this restoration pathway difficult.

Annual State 4.0:
This state has two community phases, both characterized by a dominance of non-native annual grasses and forbs. Shrub cover is present in one phase, while the other is primarily annual grasses. Sagebrush and/or rabbitbrush may dominate the overstory. Annual non-native species dominate the understory. Ecological dynamics are significantly altered in this state. Annual non-native species create a highly combustible fuel bed that shortens the fire return interval. Nutrient cycling is spatially and temporally truncated as annual plants contribute significantly less to deep soil carbon. Because this is a productive site, some deep-rooted perennial grasses may remain, even in the annual state. Without management, it is unlikely these plants will be able to recruit in the presence of dominant annual grasses.

Community Phase 4.1:
Annual non-native plants such as cheatgrass dominate the site. This phase may have seeded species present if resulting from a failed seeding attempt. Needlegrasses, mountain big sagebrush, and other shrubs are only present in trace amounts and may be missing from the community.

Community Phase Pathway 4.1a, from phase 4.1 to phase 4.2:
Time and lack of disturbance allows for shrubs to reestablish. Probability of sagebrush reestablishment is extremely low.

Community Phase 4.2:
Annual non-native species, primarily cheatgrass, dominate the understory. Sprouting shrubs dominate the overstory. Perennial bunchgrasses are a minor component or missing. Seeded species may be present.

Community Phase Pathway 4.2a, from phase 4.2 to 4.1:
Fire kills non-sprouting shrubs and allows annual non-native species to dominate site.

Tree State 5.0:
This state is characterized by a dominance of pinyon and/or juniper in the overstory. It occurs where sagebrush sites exist adjacent to stands of trees. Big sagebrush and perennial bunchgrasses may still be present, but they are no longer controlling site resources. Skeletons of dead sagebrush plants are apparent. Soil moisture, soil nutrients, soil organic matter distribution and nutrient cycling have been spatially and temporally altered.

Community Phase 5.1:
Pinyon and juniper dominate. Trees are actively growing with noticeable leader growth. Mountain big sagebrush is stressed and dying. Needlegrass and other perennial grasses reduced. Annual non-native species are present under tree canopies. Bare ground interspaces are large and connected.

Community Phase Pathway 5.1a, from phase 5.1 to 5.2:
Absence of disturbance over time allows for tree cover and density to further increase and out-compete the herbaceous understory species for sunlight and water.

Community Phase 5.2:
Pinyon and/or juniper dominate the site and tree leader growth is minimal. Annual non-native species may be the dominant understory species and will typically be found under tree canopies. Trace amounts of sagebrush may be present, however dead skeletons will be more numerous than living sagebrush. Bunchgrass may or may not be present. Bare ground areas are large and connected, and soil redistribution is apparent.

Community Phase Pathway 5.2a, from phase 5.2 to 5.1:
Tree thinning treatment (typically for fuels management) removes some tree cover and may allow sagebrush to survive. Without further management this pathway is temporary.

T5A: Transition from Tree State 5.0 to Annual State 4.0:
Trigger: Catastrophic crown fire would reduce or eliminate trees to transition the site to 4.1. Tree removal when annual non-natives such as cheatgrass are present would also transition the site to state 4.0.
Slow variable: Increased seed production and cover of annual non-native species.
Threshold: Increased, continuous fine fuels modify the fire regime by changing intensity, size and spatial variability of fires. Changes in plant community composition and spatial variability of vegetation due to the loss of perennial bunchgrasses and sagebrush truncate energy capture and impact the nutrient cycling and distribution.

R5A: Restoration from Tree State 5.0 to Shrub State 6.0:
Tree removal with minimum soil disturbance such as hand felling or mastication within community phase 5.1 when native grasses are still present.

R5B: Restoration from Tree State 5.0 to Seeded State 6.0:
Tree removal and seeding of desired species. Tree removal practices that minimize soil disturbance are recommended. Probability of success declines with increased presence of non-native annual species.

Seeded State 6.0:
This state has two community phases: a grass-dominated phase and a shrub dominated phase. This state is characterized by the dominance of seeded introduced wheatgrass species in the understory. Crested wheatgrass is a dominant plant in this phase. Conservation practices such as brush management and prescribed grazing should be used to maintain the perennial bunchgrasses and other desirable species.

Community Phase 6.1:
Seeded wheatgrasses and/or other seeded species dominate the community. Non-native annual species are present. Trace amounts of mountain big sagebrush may be present, especially if seeded.

Community Phase Pathway 6.1a, from phase 6.1 to 6.2:
Time and lack of disturbance allows shrubs to dominate. This process may be accelerated through inappropriate grazing management.

Community Phase 6.2:
Mountain big sagebrush and/or bitterbrush increases and dominates the overstory. Seeded wheatgrass species are a minor component. Annual non-native species may be present in trace amounts. Pinyon and/or juniper may be present.

Community Phase Pathway 6.2a, from phase 6.2 to 6.1:
Fire, brush management and/or Aroga moth infestation reduces sagebrush overstory and allows for seeded wheatgrasses or other seeded grasses to increase.

T6A: Transition from Seeded State 6.0 to Annual State 4.0:
Trigger: Catastrophic fire.
Slow variables: Increased production and cover of non-native annual species.
Threshold: Increased continuous fine fuels modify the fire regime by changing intensity, size, and spatial variability of fires. Changes in plant community composition and spatial variability of vegetation due to the loss of perennial bunchgrasses and sagebrush truncate energy capture spatially and temporally, thus impacting nutrient cycling and distribution.

T6B: Transition from Seeded State 6.0 to Tree State 5.0:
Trigger: Time and lack of disturbance or management action allows for pinyon and/or juniper to dominate. This may be coupled with grazing management that favors tree establishment by reducing understory herbaceous competition for site resources.
Slow variables: Over time, the abundance and size of trees will increase.
Threshold: Trees dominate ecological processes and number of shrub skeletons exceed number of live shrubs.