Ecological dynamics

Ecological Dynamics and Disturbance Response:

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 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 sites in this group are driven primarily by hydrology (sites included in this group are R026XY037NV and R026XF062CA). Sites included in this group will have the same or similar response to disturbances and have very similar state and transition models. Various states or phases may exist at once, if some areas are ponded and some remain dry depending on annual precipitation. Within a state, these patterns should be considered natural and not necessarily a product of degradation.

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 (Snyder et al. 2019). Species composition and productivity can be altered by the timing of precipitation and water availability with the soil profile (Bates et al. 2006).

The Great Basin sagebrush communities have high spatial and temporal variability in precipitation, both among years and within growing seasons. The Mountain Basin ecological site is subject to both periodic drought and flooding, which influence the vegetative community from year to year. Many of these sites have been altered since settlement times through changes in the hydrologic function of the basin. Ditches or flow path development within the site can lower water table, potentially decreasing the silver sagebrush community and transitioning the site to a drier, Wyoming big sagebrush plant community.

Silver sagebrush is often found on deep, poorly drained, often flooded, alluvial soils high in clay with a seasonally high water table. Silver sagebrush is an evergreen shrub that often forms colonies from a system of extensive rhizomes (Stubbendieck et al. 1992). The root system of silver sagebrush consists of a taproot with lateral roots and rhizomes, usually located within a few inches of the soil surface. Silver sagebrush is the most vigorous sprouter of all sagebrush (Wright et al. 1979); it is able to sprout from roots, rhizomes, and the root crown after disturbance (Ellison and Woolfolk 1937, Whitson et al. 1999, Blaisdell et al. 1982). It has been known to readily layer, meaning it can generate adventitious roots from branches touching soil (Blaisdell et al. 1982). Silver sagebrush is also capable of reproducing by seeds (Whitson 1999).

Silver sagebrush is susceptible to the herbicides 2, 4-D and 2,4,5-T, which have been used to reduce silver sagebrush cover in order to increase native grass production (Cornelius and Graham 1958, Hormay et al. 1962, Kachergis et al. 2014). Kachergis et al. (2014) found silver sagebrush returned to pre-spray levels within 50 years after spraying. They also found an initial increase in palatable native perennial grasses shortly after herbicide spraying combined with reduced stocking rates.

Silver sagebrush is a host species for the sagebrush defoliator, Aroga moth (Aroga websteri) (Henry 1961, Gates 1964, Hall 1965), but it remains unclear whether the moth causes significant damage or mortality to individual or entire stands of plants. Severe drought has been known to kill the crowns of entire stands of silver sagebrush, however after release from drought it can rapidly regrow due to its vigorous sprouting ability (Ellison and Woolfolk 1937).

Letterman needlegrass is an erect, densely-tufted perennial bunchgrass that forms large clumps. It is found on dry to moist soils in a variety of vegetation communities, including high elevation meadows, subalpine grasslands, the understory of aspen stands, and in sagebrush communities. It grows best on loamy soils with greater than 20 cm depth (Dittberner and Olson 1983).

Western wheatgrass is a rhizomatous grass that is capable of spreading vegetatively and thrives in disturbed soil (Cronquist et al. 1994). Mat muhly, a warm-season strongly rhizomatous perennial grass is also highly resistant to disturbance and usually grows in loose clumps or mats (USDA 1988, Penskar and Higman 1999, Schultz 2002). Mat muhly reproduces by seed or rhizomes. Mat muhly can be found on dry to moist sites and often persists in an area for many years after hydrological modifications lower the water table (USDA 1988).

This ecological site has moderate resilience to disturbance and resistance to invasion. Significant year-to-year variation in ponding and depth to water table are primary drivers for above-ground biomass production. Surface alteration, prolonged drought, or prolonged flooding decreases resilience and increases the probability of annual or perennial weed invasion. Three possible stable states have been identified for this DRG.

Fire Ecology:

Silver sagebrush is an evergreen shrub that often forms colonies from a system of extensive rhizomes (Stubbendieck et al. 1992). Silver sagebrush has been found to be less sensitive to fire than other sagebrush species due to its ability to sprout. The root system of silver sagebrush consists of a taproot with lateral roots and rhizomes, usually located within a few inches of the soil surface. Rhizome length of plains silver sagebrush in Montana averaged 1.1 m (3.4 ft). Silver sagebrush is the most vigorous sprouter of all sagebrush (Britton and Wright 1979). It is able to sprout from roots, rhizomes, and the root crown after disturbance (Ellison and Woolfolk 1937, Whitson 1999, (Blaisdell et al. 1982). Silver sagebrush has spreading rhizomes underground and sprouts after fire (Cronquist et al. 1994, Blaisdell 1982). Silver sagebrush is also capable of reproducing by seed (Whitson 1999). Seedling establishment can occur in the years after fire if the growing season is favorably wet (Wambolt et al. 1989). Survival and resprouting ability of silver sagebrush is considerably greater in the spring versus the fall (White and Currie 1983). As burn intensity increases, regrowth of silver sagebrush plants decreases (White and Currie 1983). Fall burning resulted in mortality of 40 to >70% of the silver sagebrush plants, suggesting summer wildfires could cause substantial stand death. Post-fire 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 or other weedy species (Miller et al. 2013).

Fire return intervals for silver sagebrush largely depend on the fire intervals of surrounding vegetation communities. Usually this silver sagebrush ecological site is a smaller pocket in a large landscape of Wyoming big sagebrush. Thus, fire return intervals for Ashy Mountain Basin are probably similar to those estimated for Wyoming big sagebrush. Wyoming big sagebrush communities historically had low fuel loads, and patchy fires that burned in a mosaic pattern were common at 10-70 year return intervals (Young and Evans 1978, West and Hassan 1985, Bunting et al. 1987). Davies et al. (2006) suggest fire return intervals in Wyoming big sagebrush communities were around 50-100 years.

The non-modal site, Ashy Mountain Basin (R026XF062CA), is typically found in a larger setting of mountain big sagebrush (Artemisia tridentata var. vaseyana). Mountain big sagebrush systems are estimated to have burned more often than lower-elevation Wyoming big sagebrush sites. Pre-settlement fire return intervals in mountain big sagebrush communities varied from 15 to 25 years (Burkhardt and Tisdale 1969, Houston 1973, Miller and Tausch 2000).

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).

The rhizomatous growth form of western wheatgrass makes it capable of surviving fire and may increase vegetative growth afterward (Bushey 1987, Wasser 1982). Mat muhly is resistant to damage from fire because the rhizome buds are insulated by soil (Benedict 1984). A few studies have observed that fire in the spring has stimulated flowering (Anderson and Bailey 1980, Pemble et al. 1981), however there is little other documentation of this plant’s fire response.

Livestock/Wildlife Grazing Interpretations:

Silver sagebrush, as with other sagebrush species, has been known to increase with grazing (Kachergis et al. 2014). The reduction of the herbaceous understory allows this shrub to increase and dominate these sites. Silver sagebrush can provide an important source of browse and is used by livestock and big game when other food sources are scarce (Kufeld et al. 1973, Wasser 1982, Cronquist et al. 1994). In fall and winter feeding trials, silver sagebrush was among the most preferred sagebrush species for mule deer and sheep (Sheehy and Winward 1981). However, silver sagebrush is an aggressive colonizer and can occupy areas at high densities, due to its ability to resprout from the crown and to spread by rhizomes (Monsen et al. 2004). Therefore, silver sagebrush can increase significantly under inappropriate grazing management on this site.

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). Letterman’s needlegrass increases under grazing by sheep and decreases with cattle grazing (Ellison 1954, Ellison and Aldous 1952, Bowns and Bagley 1986).

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, and is especially important fall forage for big game. (Monsen et al. 2004). Letterman’s needlegrass has been shown to increase under grazing by sheep and decreases under light grazing by cattle and horses (Bowns and Bagley 1986). It also declines when grazing is excluded for a long time (Turner 1969).

Western wheatgrass is a preferred feed for livestock and wildlife, but is not a very productive plant (Enevoldsen and Lewis 1978, Hafenrichter et al. 1968). It is short in stature and has sparse growth in low-water conditions. Compared to native bunchgrasses, western wheatgrass is not as palatable (Hafenrichter et al. 1968).

Mat muhly withstands heavy grazing because of its sod-forming growth form (USDA 1988). It is a short-statured plant with stems typically 3 to 8 inches long and many basal and stem leaves between one-half and two or more inches long (USDA 1988).
In general, inappropriate grazing by domestic livestock or feral horses can cause needlegrasses to decrease and mat muhly or western wheatgrass to initially increase. Continued deterioration may lead to a decrease in all deep-rooted grasses and an increase silver sagebrush.

State and Transition Model Narrative Group 5

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 5.

Reference State 1.0:
The reference state 1.0 is a representative of 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, hydrology 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 silver sagebrush and needlegrasses. Mat muhly and western wheatgrass can be significant components. Forbs and other grasses make up smaller components.

Community Phase Pathway 1.1a, from Phase 1.1 to 1.2:
Fire will top-kill silver sagebrush and allow for the perennial bunchgrasses and mat-forming grasses to increase. Fire severity is dependent on amount of fine fuels in the understory.

Community Phase Pathway 1.1b, from Phase 1.1 to 1.3:
Ponding reduces plant productivity and may allow rabbitbrush to dominate.

Community Phase 1.2:
This community phase is characteristic of a post-disturbance, early- to mid-seral community. Needlegrasses and other perennial grasses dominate. Silver sagebrush is reduced within the community after fire, but will be sprouting. Rabbitbrush and other sprouting shrubs may increase. Perennial forbs may be a significant component for a number of years following fire. If coming from a Phase 1.3 (post-flood), silver sagebrush will reestablish by seed.

Community Phase Pathway 1.2a, from Phase 1.2 to 1.1:
Time and lack of disturbance will allow sagebrush to increase.

Community Phase Pathway 1.2b, from Phase 1.2 to 1.3:
Prolonged ponding reduces plant productivity, causes silver sagebrush stress, and may allow rabbitbrush to dominate once the site dries.

Community Phase 1.3:
Rubber rabbitbrush becomes dominant after a wet year or years that result in ponded conditions. Bare ground increases and may dominate the visual aspect. Silver sagebrush and grasses are reduced.

Community Phase Pathway 1.3a, from Phase 1.3 to 1.1:
Release from ponded conditions allows silver sagebrush to dominate.
T1A: Transition from the Reference State 1.0 to Current Potential State 2.0
Trigger: This transition is caused by the introduction of non-native annual plants.
Slow variables: Over time the annual non-native species will increase within the community.
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 the Reference State 1.0 to Sagebrush State 3.0
Trigger: Long term drought, incision, or other significant hydrological change that lowers the water table. May be coupled with lack of fire and inappropriate grazing management. Transition not associated with introduction of annual non-native species.
Slow Variables: Silver sagebrush is not capable of surviving with a low water table. Over time, plants die off and are not capable of reproducing in the drier soil conditions. Wyoming big sagebrush is able to populate the area. If coupled with inappropriate grazing management, needlegrasses are lost from excessive long-term use.
Threshold: Permanent lowering of the water table beyond the reach of silver sagebrush that results in mortality of adult plants.

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: a shrub-grass dominant phase, a perennial grass dominant phase, a shrub dominant phase and a sprouting shrub dominant phase. These non-native species can be highly flammable and promote fire where historically fire had been infrequent. Negative feedbacks enhance ecosystem resilience and contribute to the stability of the state. These feedbacks 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.

Community Phase 2.1:
This community phase is similar to the Reference State Community Phase 1.1, with the presence of non-native species in trace amounts. Silver sagebrush, needlegrasses and mat muhly dominate the site. Forbs and other shrubs and grasses make up smaller components of this site.

Community Phase Pathway 2.1a, from Phase 2.1 to 2.2:
Fire will top kill silver sagebrush and allow for the herbaceous community to increase. Fire severity is dependent on amount of fine fuels in the understory. Annual non-native species are likely to increase after fire.

Community Phase Pathway 2.1b, from Phase 2.1 to 2.3:
Ponding reduces plant productivity and may allow rabbitbrush to dominate.

Community Phase Pathway 2.1c, from Phase 2.1 to 2.4:
Time without disturbance such as fire. May be coupled with inappropriate grazing management.

Community Phase 2.2:
This community phase is characteristic of a post-disturbance, early/mid-seral community. Needlegrasses and other perennial bunchgrasses dominate. Silver sagebrush is reduced within the community post-fire, but will resprout. Rabbitbrush and other sprouting shrubs may increase. Perennial forbs may be a significant component for a number of years following fire. If coming from a Phase 2.3 (post-flood), silver sagebrush will reestablish by seed. Annual non-native species are stable or increasing within the community.

Community Phase Pathway 2.2a, from Phase 2.2 to 2.1:
Time and lack of disturbance and/or grazing management that favors the establishment and growth of sagebrush allows the shrub component to recover. Silver sagebrush sprouts and will be able to return to pre-burn levels quickly.

Community Phase Pathway 2.2b, from Phase 2.2 to 2.3:
Prolonged ponding reduces plant productivity, causes silver sagebrush stress, and may allow rabbitbrush to dominate once the site dries.

Community Phase 2.3:
Rubber rabbitbrush becomes dominant after a wet year or years that result in ponded conditions. Bare ground increases and may dominate the visual aspect. Silver sagebrush and bunchgrasses are reduced.

Community Phase Pathway 2.3a, from Phase 2.3 to 2.1:
Release from ponded conditions allows silver sagebrush to dominate.

Community Phase 2.4:
Sagebrush dominates the overstory and perennial bunchgrasses in the understory are reduced, either from competition with shrubs or from inappropriate grazing, or from both. Rabbitbrush may be a significant component. Mat muhly may increase. This site is susceptible to further degradation from grazing, drought, and fire.
Community Phase Pathway 2.4a, from Phase 2.4 to 2.2:
Fire will top kill silver sagebrush and allow for the herbaceous community to increase. Fire severity is dependent on amount of fine fuels in the understory.

T2A: Transition from Current Potential State 2.0 to Sagebrush State 3.0
Trigger: Long term drought, incision, or other significant hydrological change that lowers the water table. May be coupled with lack of fire and inappropriate grazing management. Transition not associated with introduction of annual non-native species.
Slow Variables: Silver sagebrush is not capable of surviving with a water table below the rooting zone during spring growing season. Over time, plants die off and are not capable of reproducing in the drier soil conditions. Big sagebrush (Artemisia tridentata) is able to populate the area. If coupled with inappropriate grazing management, needlegrasses are lost from excessive long-term use. Rhizomatous grasses or dryland sedge may become the dominant understory.
Threshold: Permanent lowering of the water table beyond the reach of silver sagebrush that results in mortality of adult plants.

Shrub State 3.0:
This state has two community phases, a silver sagebrush-dominated phase and a post-fire phase. Long-term inappropriate grazing management reduces or eliminates grazing-intolerant grasses like needlegrasses and basin wildrye. Repeated heavy utilization in the spring or season-long use is damaging to the bunchgrass community on this site. Shrubs and grazing-tolerant grasses and grass-likes become dominant. The loss of deep-rooted grasses reduces the amount and depth of organic matter that is cycled in the soil. Shrub cover exceeds site concept and may be decadent, reflecting stand maturity and lack of seedling establishment due to competition with mature plants. The shrub overstory and rhizomatous grass and/or sedge 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:
Decadent silver sagebrush dominates the overstory. Deep-rooted perennial bunchgrasses are present in only trace amounts or are absent from the community. Mat muhly, western wheatgrass, and dryland sedges increase. Bare ground may be significant. Annual non-native species may be present.

Community Phase Pathway 3.1a, from Phase 3.1 to 3.2:
Fire reduces cover and production of silver sagebrush. Rabbitbrush sprouts after fire and becomes the dominant shrub. Mat muhly, western wheatgrass, and sedges survive fire and increase in the understory.

Community Phase 3.2:
Mat muhly, western wheatgrass, and/or Douglas sedge dominate. Rubber rabbitbrush may be a significant component. Basin wildrye and needlegrasses are missing. Silver sagebrush may be sprouting.

Community Phase Pathway 3.2a, from Phase 3.2 to 3.1:
Time without disturbance allows silver sagebrush to again become dominant.