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 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 shallow and 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 (Dobrowolski et al. 1990). However, community types with low sagebrush as the dominant shrub were found to have soil depths and thus available rooting depths of 71 to 81 cm in a study in northeast Nevada (Jensen 1990). These shrubs have a flexible generalized root system with development of both deep taproots and laterals near the surface (Comstock and Ehleringer 1992). The ecological sites in this group have very shallow soil depths, resulting in low available water holding capacity and therefore low productivity. Additionally, these sites occur on harsh windswept ridgeline positions that favor low-stature vegetation.

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 with the soil profile (Bates et al. 2006).
Low sagebrush is fairly drought tolerant but also tolerates periodic wetness during the early growing season. Low sagebrush is also susceptible to the sagebrush defoliator, Aroga moth. Aroga moth can partially or entirely kill individual plants or entire stands of big sagebrush (Furniss and Barr 1975), but the research is inconclusive of the damage sustained by low sagebrush populations.

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 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 (Bromus tectorum) has been linked to disturbances (fire, abusive grazing) that have resulted in fluctuations in resources (Chambers et al. 2007).

At the time of publication of this document, no literature could be found about the ecological dynamics of pine needlegrass (Achnatherum pinetorum), but it likely responds similarly to Letterman’s needlegrass (Achnatherum lettermanii), another densely tufted needlegrass which reproduces by seed and tillering.

The ecological sites in this DRG have moderate to high resilience to disturbance and resistance to invasion. Resilience increases with elevation, aspect, increased precipitation and increased nutrient availability. Three possible alternative stable states have been identified for this DRG.

Fire Ecology:
Fire is not a major ecological component of these community types (Winward 2001), and would be patchy and infrequent when they occur due to the low productivity of the sites. Fire return intervals have been estimated at 100 to 200 years (Kitchen and McArthur 2007). Low sagebrush is killed by fire and does not resprout (Tisdale and Hironaka 1981). Establishment after fire is from seed, generally blown in and not from the seed bank (Bradley et al. 1992). Fire risk is greatest following a wet, productive year when there is greater production of fine fuels (Beardall and Sylvester 1976). Fine fuel loads generally average 100 to 400 pounds per acre (110- 450 kg/ha) but are occasionally as high as 600 pounds per acre (680 kg/ha) in low sagebrush habitat types (Bradley et al. 1992). Recovery time of low sagebrush following fire is variable (Young 1983). After fire, if regeneration conditions are favorable, low sagebrush recovers in 2 to 5 years, however, on harsh sites where cover is low to begin with and/or erosion occurs after fire, recovery may require more than 10 years (Young 1983). Slow regeneration may subsequently worsen erosion (Blaisdell et al. 1982).

The effect of fire on bunchgrasses relates to culm density, culm-leaf morphology, and the size of the plant. 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) and may retard reestablishment of deeper-rooted bunchgrasses.
Prairie junegrass can be a significant component on these sites. It is a short-statured cool season perennial bunchgrass that is relatively shallow rooted (Albertson and Weaver 1944) but is found throughout the western United States and can vary morphologically. Prairie junegrass is moderately resistant to fire, likely due to its low stature and loosely tufted growth form (Young 1983). It is typically not a dominant grass in the Great Basin but is found in early-seral and climax communities and occurs on sites with coarse to medium textured soils (Friesen 2002). Prairie junegrass is widely documented as a drought tolerant species that is often used for reclamation but one study found that its drought tolerance significantly decreased in populations at higher elevations (Zhang et al. 2011).
Needlegrasses are slightly to moderately damaged by fire depending on season of burn. They tend to be more susceptible when burned during mid-summer (Wright and Klemmedson 1965).

Livestock/Wildlife Grazing Interpretations:
Domestic sheep and, to a much lesser degree, cattle consume low sagebrush, particularly during the spring, fall, and winter (Sheehy and Winward 1981). Heavy dormant season grazing by sheep will reduce sagebrush cover and increase grass production (Laycock 1967). Severe trampling damage to supersaturated soils could occur if sites are used in early spring when there is abundant snowmelt. Trampling damage, particularly from cattle or horses, in low sagebrush habitat types is greatest when high clay content soils are wet. In drier areas with more gravelly soils, no serious trampling damage occurs, even when the soils are wet (Hironaka et al. 1983). Bunchgrasses, in general, best tolerate light grazing after seed formation. Britton et al. (1990) observed the effects of clipping date on basal area of 5 bunchgrasses in eastern Oregon, and found grazing from August to October (after seed set) has the least impact. Heavy grazing during the growing season will reduce perennial bunchgrasses and increase sagebrush (Laycock 1967). Abusive grazing by cattle or horses will likely increase low sagebrush, rabbitbrush and some forbs such as arrowleaf balsamroot (Balsamorhiza sagittata). Annual non-native weedy species such as cheatgrass and mustards, and potentially medusahead (Taeniatherum caput-medusae), may invade.
Needlegrasses are widely distributed throughout the U.S. but are most common in the Great Basin and Southwest. They have a high forage value specifically in the western ranges. When mature the foliage can become coarse and reduce the palatability of these grasses, however, they remain green longer than other grasses and mature well, making them valuable forage for late fall and winter. The seeds of these grasses are mechanically injurious to grazing animals and can sometimes work into the tissues of the mouth, tongue, ears, and nose of livestock and game animals (USDA 1988).

Reduced bunchgrass vigor or density provides an opportunity for Sandberg bluegrass expansion and/or cheatgrass and other invasive species to occupy interspaces. Sandberg bluegrass increases under grazing pressure (Tisdale and Hironaka 1981) and is capable of co-existing with cheatgrass or other weedy species.

Prairie junegrass is palatable to all livestock and many wildlife species including deer, antelope, elk, bighorn sheep, small mammals and upland birds. It is valuable forage in the early spring as it develops earlier than most species and flowers in April to June. Palatability decreases during seed development but then returns in the fall after curing. It is tolerant to grazing as long as adequate soil moisture is available (Fernandez-Gimenez and Allen-Diaz 2001).

Low sagebrush sites are often used for strutting grounds for sage grouse (Centrocercus urophasianus) because the low cover allows for high visibility of strutting males (McAdoo and Back 2001). Sage grouse also use these high wind-swept sites during the winter where sagebrush provides food and cover (Braun, Connelly and Schroeder 2005).