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 invasives. 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 et al. 2013). Biotic factors that that influence resilience include site productivity, species composition and structure, and population regulation and regeneration (Chambers et al 2013).
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 2 meters in alluvial soils in Utah (Richards and Caldwell 1987). These shrubs have a flexible generalized root system with development of both deep taproots and laterals near the surface (Comstock and Ehleringer 1992).
In the Great Basin, the majority of annual precipitation is received during the winter and early spring. This continental semiarid climate regime favors growth and development of deep-rooted shrubs and herbaceous cool season plants using the C3 photosynthetic pathway (Comstock and Ehleringer 1992). Winter precipitation and slow melting of snow results in deeper percolation of moisture into the soil profile. Herbaceous plants, more shallow-rooted than shrubs, grow earlier in the growing season and thrive on spring rains, while the deeper rooted shrubs lag in phenological development because they draw from deeply infiltrating moisture from snowmelt the previous winter. 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).
Variability in plant community composition and production depends on soil surface texture and depth. Indian ricegrass increases with sandy soil surfaces. The amount of sagebrush in the plant community is dependent upon disturbances such as fire, Aroga moth infestations and grazing. Sandberg bluegrass more easily dominates sites where surface soils are gravelly loams than those where surface soils are silt loams.
Native insect outbreaks are also important drivers of ecosystem dynamics in sagebrush communities. Climate is generally believed to influence 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).
The perennial bunchgrasses 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 but taper off more rapidly than shrubs. General differences in root depth distributions between grasses and shrubs results in resource partitioning in these shrub/grass systems.
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 can increase resource pools by the decomposition of dead plant material following disturbance. The invasion of sagebrush communities by cheatgrass has been linked to disturbances (fire, abusive grazing) that have resulted in fluctuations in resources (Chambers et al 2007).
The introduction of annual weedy species, like cheatgrass, may cause an increase in fire frequency and eventually lead to an annual state. Conversely, as fire frequency decreases, sagebrush will increase and with inappropriate grazing management the perennial bunchgrasses and forbs may be reduced.
The ecological sites in this DRG have low resilience to disturbance and low resistance to invasion. Resilience increases with elevation, aspect, increased precipitation and increased nutrient availability. Five possible alternative stable states have been identified for this DRG.

Fire Ecology:
Fire is the principal means of renewal of decadent stands of Wyoming big sagebrush. Wyoming big sagebrush communities historically had low fuel loads, and patchy fires that burned in a mosaic pattern were common at 10 to 70 year return intervals (West and Hassan 1985, Bunting et al. 1987). Davies et al. (2007) suggest fire return intervals in Wyoming big sagebrush communities were around 50 to 100 years. More recently, Baker (2011) estimates fire rotation to be 200-350 years in Wyoming big sagebrush communities. Wyoming big sagebrush is killed by fire and only regenerates from seed. Recovery time for Wyoming big sagebrush may require 50 to 120 or more years (Baker 2006). 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. 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). However, the introduction and expansion of cheatgrass has dramatically altered the fire regime (Balch et al. 2013) and restoration potential of Wyoming big sagebrush communities.
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 related 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).
Burning has been found to decrease the vegetative and reproductive vigor of Thurber’s needlegrass (Uresk et al. 1976). Fire can cause high mortality, in addition to reducing basal area and yield of Thurber’s needlegrass (Britton et al. 1990a). The fine leaves and densely tufted growth form make this grass susceptible to subsurface charring of the crowns (Wright and Klemmedson 1965). Although timing of fire highly influenced the response and mortality of Thurber’s needlegrass, smaller bunch sizes were less likely to be damaged by fire (Wright and Klemmedson 1965). Thurber’s needlegrass often survives fire and will continue growth or regenerate from tillers when conditions are favorable (Koniak 1985, Britton et al. 1990a). Reestablishment on burned sites has been found to be relatively slow due to low germination and competitive ability (Koniak 1985). Cheatgrass has been found to be a highly successful competitor with seedlings of this needlegrass and may preclude reestablishment (Evans and Young 1978).
Indian ricegrass is fairly fire tolerant (Wright 1985), which is likely due to its low culm density and below-ground root crowns. Vallentine (1989) cites several studies in the sagebrush zone that classified Indian ricegrass as being slightly damaged from late summer burning. Indian ricegrass has also been found to reestablish on burned sites through seed dispersed from adjacent unburned areas (Young 1983, West 1994). Thus the presence of surviving, seed producing plants facilitates the reestablishment of Indian ricegrass. Grazing management following fire to promote seed production and establishment of seedlings is important.
Squirreltail is considered more fire tolerant than Indian ricegrass due to its small size, coarse stems, broad leaves and generally sparse leafy material (Wright 1971, Britton et al. 1990). Postfire regeneration occurs from surviving root crowns and from on-and off-site seed sources. Bottlebrush squirreltail has the ability to produce large numbers of highly germinable seeds, with relatively rapid germination (Young and Evans 1977) when exposed to the correct environmental cues. Early spring growth and ability to grow at low temperatures contribute to the persistence of bottle brush squirreltail among cheatgrass dominated ranges (Hironaka and Tisdale 1972).
Sandberg bluegrass, a minor component of this ecological site, has been found to increase following fire likely due to its low stature and productivity (Daubenmire 1975). Reduced bunchgrass vigor or density provides an opportunity for Sandberg bluegrass expansion and/or cheatgrass and other invasive species to occupy interspaces, leading to increased fire frequency and potentially an annual plant community. Sandberg bluegrass increases under grazing pressure (Tisdale and Hironaka 1981) and is capable of co-existing with cheatgrass. Excessive sheep grazing favors Sandberg bluegrass; however, where cattle are the dominant grazers, cheatgrass often dominates (Daubenmire 1970). Thus, depending on the season of use, the grazer and site conditions, either Sandberg bluegrass or cheatgrass may become the dominant understory with inappropriate grazing management. Repeated frequent fire in this community will eliminate big sagebrush and severely decrease or eliminate the deep rooted perennial bunchgrasses from the site and facilitate the establishment of an annual weed community with varying amounts of Sandberg bluegrass and rabbitbrush.
Fire will remove aboveground biomass from bluebunch wheatgrass but plant mortality is generally low (Robberecht and Defossé 1995) because the buds are underground (Conrad and Poulton 1966) or protected by foliage. Uresk et al. (1976) reported burning increased vegetative and reproductive vigor of bluebunch wheatgrass. Thus, bluebunch wheatgrass is considered to experience slight damage to fire but is more susceptible in drought years (Young 1983). Plant response will vary depending on season, fire severity, fire intensity and post-fire soil moisture availability.
Wildfire in sites with cheatgrass present could transition to cheatgrass dominated communities. Without management cheatgrass and annual forbs are likely to invade and dominate the site, especially after fire.

Ecological dynamics:
The plant communities of this site are dynamic in response to changing weather patterns and disturbance regimes. The reference community phase is dominated by big sagebrush (Artemisia tridentata) and deep-rooted, cool season perennial bunchgrasses such as Thurber’s needlegrass and bluebunch wheatgrass. Indian ricegrass (Achnatherum hymenoides) and spiny hopsage (Grayia spinosa) are always found on this site, but at low percentages.

Sagebrush species 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 sagebrush seedlings is dependent on adequate moisture conditions. Young plants are susceptible to less than desirable conditions for several years following germination. Density and age of sagebrush and other woody perennials in the community is largely dependent upon fire frequency.

It is well known that sagebrush species naturally hybridize (McArthur et al., 1988, Richardson et al., 2012, and others). Natural hybridization has been important in the differentiation and success of sagebrush as a landscape dominant (McArthur et al. 1988). The sagebrush found on this site is most likely a hybrid. It frequently displays intermediate morphological characteristics between basin big sagebrush (A. tridentata spp. tridentata) and Wyoming big sagebrush (A. tridentata spp. wyomingesis) including mature plant height, growth form, and leaf shape (B. Perryman, personal communication, May 19, 2014).
Following this discussion of hybridization you will find references to Wyoming big sagebrush throughout the document. At this time, research is lacking regarding the ecological potential of all possible hybridized sagebrush species/subspecies. The suspected hybrid dominating this ecological site is found in the same elevation zone and precipitation zone and on the same landscape position and soils as Wyoming big sagebrush, therefore making it appropriate to existing literature about Wyoming big sagebrush to describe ecological dynamics of the site. Updates and revisions will be made as more information is available.

Wyoming big sagebrush is the most drought-tolerant of the three major big sagebrush subspecies. The root system is deep and well-developed with many laterals and one or more taproots, maximizing water uptake. The majority of the roots are in the upper foot of soil with tap roots extending up to 6 feet in depth (Howard 1999). The combination of deep and shallow roots also provides excellent soil stabilization. The roots are inoculated with the vesicular-arbuscular mycorrhizae (VAM) Glomus microcarpus and Gigaspora spp., which help to mitigate nutrient and moisture limitations. Mycorrhizas or ‘fungus-roots’ are the result of a symbiotic relationship between specialized soil organisms and plant roots. Beneficial changes in the water relations of plants inoculated with VAM include altered rates of water uptake, hydraulic conductivity, leaf and stem water potentials, stomatal resistance and transpiration rates (Stahl 1998).

Dominant perennial bunchgrasses, Thurber’s needlegrass and bluebunch wheatgrass, tolerate droughty conditions found on this ecological site by extending their root system into subsoil horizons maximizing water uptake. The rooting depth of Thurber’s needlegrass associated with sagebrush in Idaho is reported to be 24” (Archer 2000). The maximum rooting depth of bluebunch wheatgrass in Colorado is reported to be 52”, with the highest concentration of roots around 18” (Zlatnik 1999). The fine textured argillic horizon found in typical soils associated to this ecological site is located between 14” and 28” increasing available moisture in the prime rooting depth for these perennial grasses. Ecological significance of deep roots in arid environments helps to extend growth into the dry season and increases survival.

Vegetative cover of perennial plants on this ecological site is generally sparse, even under reference conditions. However, soil space not occupied by living plants is usually covered in biological soil crusts. In Wyoming big sagebrush communities of southeastern Idaho, biological soil crust were found to occupy between 40-60% of the soil surface in an undisturbed setting (Memmott et al. 1998, Kaltenecker et al. 1999). Biological soil crusts are formed by living organisms, cyanobacteria, green algae, lichens, mosses, microfungi, etc., and their by-products, creating a crust of soil particles bound together by organic materials. In rangelands they have several important functions including; helping to retain soil moisture, reducing wind and water erosion, fixing atmospheric nitrogen and contributing to soil organic matter (USDI-BLM 2001). Soil crusts are also good indicators of physical disturbance. Disturbances such as off-road vehicles and trampling by humans and livestock destroy the physical structure of soil crusts. Once destroyed the pieces of crust are blown or washed away, reducing soil stability and fertility (Belnap 2003). Extent of impact is determined by severity, frequency, size and timing of disturbance. Recovery of biological crust may take decades to hundreds of years. Therefore, it is important to prevent degradation.

This site inherently has low resistance to invasion by non-natives and low resilience following invasion by non-natives, due to a combination of factors such as disturbance history, relatively low elevation, and the semi-arid climate. In Great Basin ecosystems, inherent resilience typically increases with elevation due to higher levels of water, nutrients, and annual biomass production. Wyoming sagebrush ecosystems are the most vulnerable to cheatgrass (Bromus tectorum) invasion due to the combination of low resilience to disturbances such as fire and low resistance to cheatgrass invasion (Chambers et al. 2012). Management activities should be prioritized based on the relative resilience and resistance of a specific ecological site.

Inappropriate management; grazing, recreation, etc., on this site can lead to an increase in sagebrush and a decline in herbaceous species, namely Thurber’s needlegrass and other deep rooted perennial bunchgrasses. Reoccurring disturbances, natural or anthropogenic, will result in decreased sagebrush cover and increased cover of disturbance-tolerant shrubs and non-natives. A combination of inappropriate management and prolonged drought often leads to an increase in bare ground and a decrease in plant production, all contributing to increased soil erosion. The loss of structural and functional groups affects ecosystem functioning and can result in soil surface instability and soil loss. Additional discussion regarding how functional groups and disturbance impact the hydrologic functions of this ecological site can be found in the Ecological Site Interpretations under Hydrology Functions.


Disturbance Ecology:
Prior to Euro-American settlement, Wyoming big sagebrush communities historically had low fuel loads, and patchy fires that burned in a mosaic pattern were common (Young et al. 1979, West and Hassan 1985, and Bunting et al. 1987). Estimates of pre Euro-American settlement fire return intervals in Wyoming big sagebrush communities vary widely; 10 to 40 years (Young and Evans 1981), 50 to 100 years (Davies et al. 2007), 100 to 240 years (Baker 2006). Actual fire return intervals were largely dependent on landscape position and year to year variation in herbaceous fuel loading. There is, however, general agreement among authors that infrequent fire probably maintained Wyoming big sagebrush communities as open, seral stands of sagebrush with productive herbaceous understories. The introduction and expansion of cheatgrass has dramatically altered the fire return intervals and restoration potential of Wyoming big sagebrush communities.

Fire is the principal means of renewal for decadent stands of Wyoming big sagebrush. Wyoming big sagebrush plants of all ages are killed by fire. Depending on site conditions prior to wildfire, perennial grasses and forbs will dominate initially after wildfire. Wyoming big sagebrush establishes afterwards from seed stored in the soil and from seed produced by remnant plants that escaped fire. Prolific seed production from nearby unburned plants coupled with high germination and survival rates is required to ensure establishment following fire. The VAM upon which Wyoming big sagebrush depends on for healthy growth are usually harmed by fire and may take several years to recover. Typically, fewer VAM are killed by low-intensity wildfire than by more severe fire intensities (Howard 1999).

This ecological site can also be impacted by the native sagebrush defoliating insect the aroga moth (Aroga websteri). Individual plants are partially or entirely killed through defoliation under low to moderate moth population densities. Under high population densities, entire stands involving thousands of acres can be eliminated (Furniss and Barr 1975). Adult moths lay eggs in the late summer/early fall on small cracks in the bark or on the leaves of sagebrush plants. After approximately two weeks larvae hatch and mine into the leaves and remain there through the winter. The following spring larvae resume work mining the leaves. Leaves are cut off the plant and used to build cocoons. In the later part of June or July larvae go in to the pupil stage and adult moths emerge in approximately two weeks (Gates 1964). The aroga moth is a native insect and therefore has native predators. It is impacted by 20 or more species of parasites. The moth and the larvae are also preyed upon by two species of beetles (Furniss and Barr 1975).