Ecological dynamics

An ecological site is the product of all the environmental factors responsible for its development and it has a set of key characteristics that influence a site’s resilience to disturbance and resistance to invasive species. Key characteristics include 1) climate (precipitation, temperature), 2) topography (aspect, slope, elevation, and landform), 3) hydrology (infiltration, runoff), 4) soils (depth, texture, structure, organic matter), 5) plant communities (functional groups, productivity), and 6) natural disturbance regime (fire, herbivory, etc.) (Caudle 2013). Biotic factors that influence resilience include site productivity, species composition and structure, and population regulation and regeneration (Chambers et al. 2013).

The Great Basin vegetation 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 increase resource uptake by the decomposition of dead plant material following disturbance. The invasion of 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; in competitive environments cheatgrass roots were found to penetrate only 15 cm, while isolated plants and pure stands were found to root up to 1.7 m. Mahogany stands are susceptible to drought, frost, and invasion by non-native species, especially cheatgrass. Cheatgrass affects mahogany seedling growth by competing for water resources and nutrients in an area (Ross 1999).

Periodic drought regularly influences Great Basin ecosystems, and drought duration and severity has increased throughout the 20th century in much of the Intermountain West. Major shifts away from historic 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).

Long-lived curl-leaf mountain mahogany, deep-rooted cool season perennial bunchgrasses, and long-lived shrubs (50+ years) with high root-to-shoot ratios dominate the ecological sites in this DRG. Curl-leaf mountain mahogany (hereafter, mahogany) is a widespread species in Nevada, occurring on mountain slopes spanning a wide elevation range, from the sagebrush and pinyon-juniper vegetation zones into the mountain shrub communities that border mixed conifer and even subalpine ecosystems (Tueller 1989). Mahogany is a multi-branched, evergreen shrub or tree reaching 3 to over 20 feet in height. Mahogany plants are long-lived; it is common to find plants over 200 years old, however there are some reports of plants over 1,000 years old (Ex et al. 2011, Schultz 1987, Schultz et al. 1990). As mahogany stands increase in average age, average canopy volume and height of the individuals present also increases. As average canopy height and volume increase, stand density declines (Schultz et al. 1991). Stands with a closed or nearly closed canopy often have little recruitment in the understory (Schultz et al. 1990, 1991), despite high seed density beneath trees (Russell and Schupp 1998, Ibáñez et al. 2002). Intraspecific competition reduces the growth rates of all age classes and may increase mortality in the younger plants.

The species plays an important role in biogeochemical cycles, since its roots can host nitrogen-fixing nodules (Youngberg and Hu 1972, Freund et al., 2018), possibly allowing for successional processes on poor soils in stressful environments (Kratsch and Graves, 2004). Seedlings of mahogany exhibit rapid root growth in relation to top growth, providing some resistance to drought and competition with invasive species (Dealy 1974). Dealy (1974) reported that curl-leaf mahogany seedlings have a mean taproot length of 0.97 m after 120 days, while the mean top height was slightly less than 2.5 cm. Ibáñez et al. (1999) and Schultz et al. (1996) found that mahogany seedlings germinate abundantly under the canopy of adult plants, but rarely successfully establish there due to shading and higher litter amounts. In addition, Schultz et al. (1996) found that seedlings had significantly higher long term success in areas dominated by sagebrush canopy than in areas under mahogany canopy or in interspaces. The shading and hydraulic lift provided by adult sagebrush may create a microsite that facilitates mahogany recruitment (Gruell et al. 1985, Ibáñez et al. 1999).

Mountain big sagebrush is a minor component of this site. Infrequent large recruitment events and simultaneous low, continuous recruitment is the foundation of population maintenance for this plant (Noy-Meir 1973). It is not tolerant of shade and may be missing in dense stands of mahogany.

Perennial bunchgrasses generally have shallower root systems than 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. Letterman needlegrass, the dominant grass on the non-modal ecological site, 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 with greater than 20 cm depth (Dittberner and Olson 1983).

Western needlegrass is a strongly tufted perennial grass that grows up to 4 dm in height (Cronquist et al. 1994). It grows in dry, well-drained soils from upper foothills up into the higher areas of the mountains in the western United States (USDA Forest Service 1988). The roots of this grass are deep, fibrous and spreading, which allows it to be more resistant to trampling and drought (USDA Forest Service 1988).

Cusick’s bluegrass and/or muttongrass are found on this site. There is evidence that these two common names have been used interchangeably or are sometimes misidentified (Monsen et al. 2004), but they occupy similar ecological niches (Cronquist et al. 1972). Cusick’s bluegrass is a strongly tufted perennial grass but may be somewhat rhizomatous in loose soils (Cronquist et al. 1972). It begins growth very early in the season and may produce two crops of inflorescences in a growing season (Cronquist et al. 1972). Muttongrass persists well in open areas and under canopies of oak and other shrubs (Monsen et al. 2004). Muttongrass may be more shade tolerant than other perennial bunchgrasses and may persist in the understory as the canopy closes (Erdman 1970).

There is potential for infilling by singleleaf pinyon (Pinus monophylla) on these sites. Infilling may occur if the site is adjacent to woodland sites or other ecological sites with conifers present. Without disturbance, such as low-intensity fire, pinyon will eventually dominate and out-compete mahogany for water and sunlight. The authors have observed this phenomenon and there is ongoing research to evaluate this process. One study found that mahogany seedlings responded best to mechanical juniper removal compared to burning (O’Connor et al. 2013).

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

Fire Ecology:

The fire return interval in mahogany-dominated sites is not well documented, however a study by Arno and Wilson (1986) suggested mahogany with ponderosa pine communities had fire return intervals of 13 to 22 years before 1900. Fire frequency most likely depends on surrounding vegetation. Most often mahogany stands occur on warm, dry, rocky ridges or outcrops where fire would be an infrequent occurrence (USDA 1937). Dealy (1974) and Scheldt (1969) found that mahogany trees were larger and older on fire-resistant rocky sites and were the seed source if fire destroyed the non-rocky portion of the site. Mahogany is considered a weak sprouter, and is usually moderately to severely damaged by severe fires. Because of their thicker bark, mature trees can often survive low-severity fires (Gruell et al. 1985). The recovery time of these sites is variable; some measurements show that stands lack recruitment for up to 30 years post-fire (Gruell et al. 1985, Ross 1999). Mahogany seeds germinate and have the highest survival rates with moderate litter amounts; litter depths over 0.25 inches can impede recruitment (Gruell et al. 1985, Ibáñez et al. 2002, Ibáñez et al., 1999, Schultz et al. 1991, Schultz et al. 1990). Since these plants germinate well under the protection of adult mahogany and sagebrush, germination rates may be quite low immediately after fire (Shultz et al. 1996, Ross 1999).

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 (Wright 1971, Young 1983). In addition, season and severity of the fire will influence plant response as will post-fire soil moisture availability.

Emergence of western needlegrass seeds was shown to significantly improve with additions of smoke and burned soil (Blank et al. 1996).

Mountain big sagebrush is killed by fire (Neuenschwander 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 15-20 years following fire, but establishment after severe fires may proceed more slowly (Bunting et al. 1987).

Depending on fire severity, snowberry and other sprouting shrubs may increase after fire. Snowberry is top-killed by fire, but resprouts after fire from rhizomes (Leege and Hickey 1971, Noste and Bushey 1987). Snowberry has been noted to regenerate well and exceed pre-burn biomass in the third season after a fire (Merrill et al. 1982). Douglas rabbitbrush (Chrysothamnus viscidiflorus) is also found in these sites. It has a large taproot root system and is known to be shorter lived and less competitive than sagebrush. Seedling density, flower production, and shoot growth decline as competition from other species increases (McKell and Chilcote 1957, Miller et al. 2013). Douglas rabbitbrush is top-killed by fire, but sprouts vigorously after fire (Kuntz 1982, Akinsoji 1988).

Livestock/Wildlife Grazing Interpretations:
Mountain mahogany is an important cover and browse species for big game such as elk (Cervus canadensis), mule deer (Odocoileus heminous), pronghorn antelope (Antilocarpra americana), and bighorn sheep (Ovis canadensis) (Lanner 1984, Furniss et al. 1988, Sabo et al. 2005). Sampson and Jespersen (1963) state that mahogany is excellent browse for mule deer, and domestic livestock will browse this plant to varying degrees in all seasons except summer. It is not uncommon for these trees to develop a “hedged” appearance after years of regular browsing by wildlife. According to (Olsen 1992) mahogany is consumed widely by mule deer throughout the year. In fact, mule deer fecal pellets were observed to contain mahogany year-round, with the highest frequency of leaves found in winter (Gucker 2006). Mule deer will use mahogany for cover as well (Steele et al. 1981).

Despite low palatability, mountain big sagebrush is eaten in small amounts by sheep, cattle, goats, and horses. Chemical analysis indicates that the leaves of big sagebrush equal alfalfa meal in protein, have a higher carbohydrate content, and yield twelvefold more fat (USDA 1937).


Antelope bitterbrush is a small component of these sites, but is a critical browse species for mule deer, antelope and elk and is often utilized heavily by domestic livestock (Wood et al. 1995). Grazing tolerance is dependent on site conditions (Garrison 1953) and the shrub can be severely hedged during the dormant season for grasses and forbs.

Letterman’s needlegrass provides valuable forage for both livestock and wildlife (Taylor 2000). It begins growth early in the year and is available to be utilized when other grasses are not yet palatable. It is especially important fall forage for big game (Monsen et al. 2004). Letterman’s needlegrass appears to tolerate sheep grazing, however time and timing of grazing is not well documented for this species (Bowns and Bagley 1986). It also declines when grazing is excluded for a long time (Turner 1969).

Western needlegrass is slow to mature and remains green through most of the growing season. Since it can remain green into fall, it is higher quality forage compared to other species that have senesced by then (USDA Forest Service 1988). For livestock this grass has good forage value, and it has fair forage value for wildlife (Stubbendieck et al. 1992). Seeds of this grass are avoided by grazing animals, but are not necessarily injurious. Since seeds are avoided by grazing animals, a large amount of the seed produced grows to maturity (USDA Forest Service 1988).

Cusick’s bluegrass was the most palatable and preferred grass compared to Thurber’s needlegrass and bluebunch wheatgrass in a 1975 grazing study, and was the most negatively affected by grazing (Rickard et al. 1975). Uresk and Rickard (1976) found Cusick’s bluegrass to be a highly preferred grass, especially in the spring, even when it is a minor component of the plant community.

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 for several reasons: 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 alderleaf 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.

State and Transition Model Narrative for Group 15:
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 15. Site included in this Disturbance Response Group are R026XY009NV and R026XY015CA.

Reference State 1.0:

The Reference State 1.0 represents the natural range of variability 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:
This community is dominated by curl-leaf mountain mahogany. Mountain big sagebrush and snowberry make up the shrub components of the understory. Needlegrasses and bluegrasses are dominant perennial bunchgrasses. A diversity of other grasses and forbs exist in the understory.

Community Phase Pathway 1.1a from phase 1.1 to 1.2:
Fire will reduce the mahogany overstory and allow the understory species to dominate the site. Due to low fuel loads, fires will typically be low severity, resulting in a mosaic pattern.

Community Phase Pathway 1.1b from phase 1.1 to 1.3:
Time and lack of disturbance or fire, drought, herbivory, or combinations of these causes mountain mahogany to increase. The shrub and herbaceous understory components decline due to increased shading from the trees. Muttongrass increases with more shade.

Community Phase 1.2:
This community phase is characteristic of a post-disturbance, early- to mid-seral plant community. Snowberry and rabbitbrush are sprouting. Perennial grasses and forbs dominate. Mahogany and mountain big sagebrush may be present, but only in patches.

Community Phase Pathway 1.2a from phase 1.2 to 1.1:
Time and lack of disturbance or fire, drought, herbivory, or combinations of these allows the mountain mahogany and sagebrush to increase.

Community Phase 1.3 (At-Risk):
Mahogany density will increase in the absence of disturbance. Shrubs and deep-rooted perennial bunchgrasses will be shaded out by the dense mahogany. Bluegrasses are more shade tolerant, however, and increase in the understory. Mahogany in dense stands will lose lower branches due to shading and/or herbivory, resulting in a more tree-like appearance.

Community Phase Pathway 1.3a fro, phase 1.3 to 1.2:
A low-severity or spot fire, snow loading, or insect damage will decrease the overstory and allow for the herbaceous plants in the understory to increase.

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.
Slow variables: Over time, the annual non-native plants will increase within the community decreasing organic matter inputs from deep-rooted perennial bunchgrasses resulting 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 Tree State 3.0:
Trigger: Time and lack of disturbance allows pinyon to increase and overtop the mountain mahogany. Litter increases while understory plants decrease.
Slow variables: Over time, abundance and size of singleleaf pinyon will increase.
Threshold: Pinyon dominate(s) ecological processes. Trees overtop and outcompete mountain mahogany and shrubs for water and sunlight. Shrub skeletons exceed live shrubs with minimal recruitment of new cohorts.

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 the same three 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:
This community is dominated by curl-leaf mountain mahogany. Mountain big sagebrush and snowberry make up the shrub components of the understory. Needlegrasses and bluegrasses are dominant perennial bunchgrasses. A diversity of other grasses and forbs exist in the understory. Annual non-native species like cheatgrass are present.

Community Phase Pathway 2.1a from phase 2.1 to 2.2:
Fire will decrease or eliminate the overstory of mahogany and allow the perennial bunchgrasses to dominate the site. Fires will typically be small and patchy due to low fuel loads.

Community Phase Pathway 2.1b from phase 2.1 to 2.3:
Time and lack of disturbance or fire, drought, herbivory, or combinations of these causes mountain mahogany to increase. The shrub and herbaceous understory components decline due to increased shading from the mahogany and/or pinyon pine. Muttongrass increases with more shade.

Community Phase 2.2:
This community phase is characteristic of a post-disturbance, early to mid-seral community phase. Needlegrasses and other perennial grasses dominate the site. Snowberry and/or rubber rabbitbrush may be sprouting. Mountain mahogany and mountain big sagebrush are patchy. Annual non-native species are present.

Community Phase Pathway 2.2a from phase 2.2 to 2.1:
Time and lack of disturbance or fire, drought, herbivory, or combinations of these allows the mountain mahogany and sagebrush to increase.

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. Perennial bunchgrasses may also increase in production. Fire may also play a part in this pathway.

Community Phase 2.3 (At-Risk):
Mahogany density will increase in the absence of disturbance. Shrubs and deep-rooted perennial bunchgrasses will be shaded out by the dense mahogany. Bluegrasses are more shade tolerant, however, and increase in the understory. Mahogany in dense stands will lose lower branches due to shading and/or herbivory, resulting in a more tree-like appearance. Pinyon pine may be present.

Community Phase Pathway 2.3a from phase 2.3 to 2.2:
Fire reduces the shrub overstory and allows perennial bunchgrasses to dominate the site. Fires are typically low severity resulting in a mosaic pattern due to low fuel loads. A fire following an unusually wet spring or a change in management favoring an increase in fine fuels may be more severe and reduce sagebrush cover to trace amounts. Annual non-native species are likely to increase after fire.

Community Phase Pathway 2.3b from phase 2.3 to 2.4:
Fall, winter, and spring precipitation and temperatures mediate the ability for annual grasses and perennial grasses to germinate and/or survive. Higher than normal spring precipitation creates high annual production of annual grasses (Bradley et al. 2016). Non-native annual species increase in production and density throughout the site. Perennial bunchgrasses may also increase in production.

Community Phase 2.4 (At-Risk):
This community is at risk of crossing into an annual state. Native bunchgrasses dominate; however, annual non-native species such as cheatgrass may be sub- or co-dominant in the understory. 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. Pinyon pine may be present.

T2A: Transition from Current Potential State 2.0 to Tree State 3.0:
Trigger: Time and lack of disturbance allows pinyon to increase and overtop the mountain mahogany. Litter increases while understory plants decrease.
Slow variables: Over time, abundance and size of pinyon will increase.
Threshold: Pinyon pine dominate(s) ecological processes. Trees overtop and outcompete mountain mahogany and shrubs for water and sunlight. Shrub skeletons exceed live shrubs with minimal recruitment of new cohorts.

T2B: Transition from Current Potential State 2.0 to Annual State 4.0:
Trigger: Fire or a failed range seeding leads 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: Cheatgrass or other non-native annuals dominate understory.

Tree State 3.0:
This state has two community phases that are characterized by the dominance of singleleaf pinyon in the overstory. Mountain big sagebrush and perennial bunchgrasses may still be present, but they are no longer controlling site resources. Soil moisture, soil nutrients, soil organic matter distribution and nutrient cycling have been spatially and temporally altered.

Community Phase 3.1:
Pinyon pine and mountain mahogany dominate the site. Mountain big sagebrush and snowberry are minor component. Bluegrasses dominate understory. Annual non-native species may be present or dominant.

Community Phase Pathway 3.1a from phase 3.1 to 3.2:
Time and lack of disturbance or fire, drought, inappropriate grazing management, or combinations of these allows for maturation of the pinyon/juniper community.

Community Phase 3.2:
Pinyon pine dominates the site. Mountain mahogany is decadent and the stand lacks recruitment. Bluegrasses are present. Understory is reduced overall. Annual non-native species may be present.

T3A: Transition from Tree State 3.0 to Annual State 4.0:
Trigger: To community phase 4.1: Overgrazing in the presence of non-native annual species can cause a decrease in perennial bunchgrasses and an increase in annual species. Spring and/or fall moisture may also increase annual species. To community phase 4.2: Fire in the presence of annual invasive grasses.
Slow variables: Cover and production of annual non-native species increase in the understory.
Threshold: Loss of mahogany overstory, mountain big sagebrush, and deep-rooted perennial bunchgrasses changes nutrient cycling, nutrient redistribution, and reduces soil organic matter. Increased, continuous fine fuels modify the fire regime by changing intensity, size and spatial variability of fires.

R3A: Restoration from Tree State 3.0 to Current Potential State 2.0:
Removal of pinyon from site will allow mountain mahogany to again become the dominant overstory.

Annual State 4.0:
This state has two community phases: one with annual invasive plants in the understory of an intact mahogany stand, and another post-fire phase where mahogany is a minor component or missing from the site. This state is characterized by the dominance of annual non-native species such as cheatgrass and/or tansy mustard in 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:
Mountain mahogany dominates the overstory and annual non-native plants such as cheatgrass dominate the understory. Native perennial grasses and forbs are significantly reduced. Sagebrush and snowberry may or may not be present.

Community Phase Pathway 4.1a:
Catastrophic fire reduces the mountain mahogany overstory and allows annual species to dominate.

Community Phase 4.2:
Annual non-native species dominate the site. The open canopy may allow sprouting shrubs and bluegrasses to increase.