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. Needle and thread grass is adapted to coarser textured soils whereas Indian ricegrass will increase with sandy soil surfaces, and bottlebrush squirreltail will increase with silty soil surfaces. Production generally increases with soil depth. The calcium carbonate content often found in the rooting zone of the grass species is thought to be the primary reason for the lack of Thurber’s needlegrass in this Loamy 8-10 as compared to the similar sites in MLRA 24 and 25. The amount of sagebrush in the plant community is dependent upon disturbances like fire, Aroga moth infestations, and grazing.

Wyoming big sagebrush, the most drought tolerant of the big sagebrushes, is 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 depended on adequate moisture conditions. The root system is deep and well-developed with many laterals and one or more taproots. The majority of the roots are in the upper foot of soil with tap roots extending up to 6 feet in depth. The roots are infected with the vesicular-arbuscular mycorrhizae (VAM) Glomus microcarpus and Gigaspora spp. Wyoming big sagebrush is a long-lived species with maximum ages to 150 years old (Howard 1999).
Mycorrhizas (‘fungus-roots’) are the result of a symbiotic relationship between specialized soil organisms and plants roots. Wyoming big sagebrush seedlings exhibit greater drought resistance when inoculated with VAM. Average soil water potential resulting in death of Wyoming big sagebrush seedlings infected with VAM was 0.45 MPa lower than seedlings that were not infected with VAM (Stahl et al. 1998). A number of beneficial changes in the water relations of arbuscular mycorrhizal plants including altered rates of water uptake, hydraulic conductivity, leaf and stem water potentials, stomatal resistance and transpiration rates have been observed by researchers. Stahl et al. (1998) found VAM to be vitally important during the early stages of seedling establishment. Improved ability to extract soil nutrients and improve drought tolerance in Wyoming big sagebrush seedlings may have important consequences for restoration of degraded sagebrush habitat.
Sagebrush species set seed in the late summer and fall. Seeds ripen from September through October and fall from the plant. Cold, moist conditions and exposure to light increase germination in the spring (Johnson 2000). Seeds of sagebrush species are best adapted to germinate in habitats with conditions similar to that of the collection site. Survival of sagebrush seedlings is dependent on adequate moisture conditions. Young plants are susceptible to less than desirable condition for several years following germination.
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). Mature properly functioning sagebrush communities have higher infiltration rates and lower sediment production, than degraded systems. Reoccurring disturbances, natural or anthropogenic, will result in decreased sagebrush cover and increased cover of disturbance tolerant shrubs and non-natives. Loss of structural and functional groups affects ecosystem functioning and can result in soil loss.

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 is 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 accumulation and decomposition of litter increase nutrient concentrations under sagebrush shrub canopies. The breakdown of aging roots also contributes to organic matter and nutrient cycling in the sagebrush system. Carbon and nitrogen concentration are higher under sagebrush canopies when compared to interspaces (Chen and Stark 2000). The root systems of sagebrush maximizes water uptake with a deep taproot and shallow branching roots. The combination of deep and shallow roots also provides excellent soil stabilization.

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.

Infilling by singleleaf pinyon and Utah juniper may also occur with an extended fire return interval. Eventually, singleleaf pinyon and Utah juniper will dominate the site and out-compete sagebrush for water and sunlight severely reducing both the shrub and herbaceous understory (Lett and Knapp 2005, Miller et al. 2000). Bluegrasses may remain underneath trees on north-facing slopes. The potential for soil erosion increases as woodland matures and the understory plant community cover declines (Pierson et al. 2010).

Variability in plant community composition and production depends on soil surface texture and depth. Thurber’s needlegrass will increase on gravelly surfaces, whereas Indian ricegrass will increase with sandy soil surfaces. An argillic horizon will promote production of bluebunch wheatgrass. Production increases with soil depth. The amount of sagebrush in the plant community is dependent upon fire frequency, which would be highly infrequent.
Inappropriate grazing can lead to an increase in sagebrush and a decline in understory plants like Thurber’s needlegrass and Indian ricegrass. Squirreltail (Elymus elymoides) will increase temporarily with further degradation. Invasion of annual non-native invasive forbs and cheatgrass could occur with further grazing degradation, leading to a decline in squirreltail and an increase in bare ground. Wetter sites are more resistant to degradation and may result in sagebrush and Sandberg’s bluegrass dominating the site. A combination of overgrazing and prolonged drought leads to soil erosion, increased bare ground and a loss in plant production.

Where site degradation has been fire-induced, broom snakeweed (Gutierrezia sarothrae) and rabbitbrush (Chrysothamnus viscidiflorus) often dominate the site. Repeated burning of the plant community at intervals less than 10 to 15 years results in complete site dominance by non-native annuals (primarily cheatgrass, halogeton (Halogeton glomeratus), Russian thistle (Salsola tragus), fiddleneck (Amsinkia spp.), and tansy mustard (Descurainia spp.)and the near total absence of woody plants, including sagebrush. This ecological site has low resilience to disturbance and low resistance to invasion. Resilience increases with elevation, aspect, increased precipitation and increased nutrient availability. Six possible alternative stable states have been identified for this site.

Fire 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 at 10 to 70 year return intervals (Young et al. 1979, West and Hassan 1985, Bunting et al. 1987). Davies et al. (2007) suggest pre Euro-American settlement fire return intervals in Wyoming big sagebrush communities were around 50 to 100 years. 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 soil stored seed 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).
Spiny hopsage (Grayia spinosa) is considered to be somewhat fire tolerant and often survives fires that kill sagebrush. Mature spiny hopsage generally resprout after fire. Spiny hopsage is reported to be less susceptible to fire during summer dormancy. Thurber’s needlegrass is classified as moderately resistant, but depending on season of burn, phenology, and fire severity, it is moderately to severely damaged by fire. Burning has been found to decrease the vegetation and reproductive vigor. Early season burning is more damaging to this needlegrass than late season burning. Indian ricegrass can be killed by fire, depending on severity and season of burn. Indian ricegrass reestablishes on burned sites through seed dispersed from adjacent unburned areas. There is some speculation that Indian ricegrass may sprout from tillers after fire, especially if plant mortality was incomplete. Due to low culm density and below ground plant crowns, this is a fairly fire tolerant species Webber’s needlegrass is damaged by burning due to dense plant material that can burn slowly and long, charring to the growing points. Late summer and early fall fires are the least harmful. Sandberg bluegrass is generally unharmed by fire. It produces little litter, and its small bunch size and sparse litter reduces the amount of heat transferred to perennating buds in the soil. Its rapid maturation in the spring also reduces fire damage, since it is dormant when most fires occur.