Ecological dynamics
The reference community of this site supports a grassland-shrub community. The potential grass understory is dominated by bluebunch wheatgrass (Pseudoroegneria spicata) with minor amounts of Sandberg bluegrass (Poa secunda) also common. Shadscale saltbush (Atriplex confertifolia) is the dominant shrub on this site and is a strong indicator of the site where it occurs on north aspects.
As a facultative halophyte, shadscale can tolerate soils with high salinity/sodicity (Simonin 2001). In MLRA 010X, shadscale is often associated with clayey textures in areas receiving 9 to 12 inches of annual precipitation. Shadscale communities historically experienced very infrequent fire due to low fuel loads and discontinuity, however, robust evidence of pre-settlement fire is lacking and estimates of fire return intervals range from 35 to greater than 600 years (Simonin 2001, Landfire 2007). This historical lack of fire may explain why the plant did not evolve fire adapted habits and is considered intolerant of fire, experiencing high mortality rates and relying on seed for post-fire recolonization.
Bluebunch wheatgrass is a drought tolerant bunchgrass with an extensive root system (Zlatnik 1999). Bluebunch is considered to be a highly fire adapted grass species with low growing buds often protected from fire. This morphology allows for rapid recovery following fire and bluebunch wheatgrass often increases relative to other plants post fire, especially after spring burning. Broom snakeweed is typically associated with disturbed habitats and early seral conditions in the sagebrush biome (Tirmstein 1999). It can rapidly invade following improperly managed grazing and as a highly drought tolerant species, may rapidly increase in density following drought periods. While broom snakeweed is typically killed by fire, seeds are often unharmed and rapid colonization from nearby sites is common. While it effectively compete with many grasses (similar rooting depth and possible allelopathy) it is often out-competed by other shrubs overtime.
Western juniper (Juniperus occidentalis) is a native conifer species in western North America, but its density and range have dramatically increased since the late 1800s. This is likely due to a combination of factors, principally: reductions in fire frequency; heavy livestock grazing; and increased atmospheric carbon dioxide (Fryer and Tirmenstein 2019). Juniper is sensitive to fire and most young trees are killed by even low severity fire. As Juniper trees mature and bark thickens, however, they become resistant to low severity fire yet are still killed by crown fires or high severity surface fires.
Climate cycles would have been an important driver of ecological dynamics historically, with drought periods potentially increasing relative composition of snakeweed and wet years increasing fire occurrence due to increased perennial grass production and fine fuels loads. Large die offs of shadscale in the Great Basin during periods of high precipitation (and associated fungal disease outbreaks) have been recorded (Simonin 2001). Further, decreases in shadscale density associated with extreme drought have also been recorded in regions receiving less annual precipitation than this site, yet attesting to the importance of climate in driving community composition (Lei 1999). This may be especially true when drought stress is combined with livestock grazing pressure (Chambers and Norton 1993). Livestock grazing may have altered the plant community composition of this site. Increases in broom snakeweed cover and decreases in perennial grass cover may result from chronic improperly managed grazing. Given an altered disturbance regime and degraded site conditions, invasions of exotic forb species, such as prickly lettuce (Lactuca serriola) and annual grass species such as annual bromes (Bromus spp.) and medusahead rye (Taeniatherum caput-medusae) may occur on this site. At high levels of invasion, exotic annual grasses may increase the frequency of fires and extend the season when fires are likely by augmenting early season fine fuel loads and fuel continuity. Sites may be particularly prone to fire following years of above average precipitation during which invasive annual grass production can increase dramatically (Pilliod et al. 2017). As site degradation progresses, either due to native vegetation loss or juniper encroachment, the highly erosive soils on this site may be lost due to wind and water erosion, causing a reduction in site potential which may be extremely difficult to restore.
An understanding of the site specific ecological dynamics for this site are incomplete. Thresholds between states and phases have yet to be quantified and restoration pathways and outcomes are poorly understood. Current and anticipated effects of climate change are not included in this model, yet this site may experience significant impacts as climate continues to change. The model below draws from the ecological dynamics described in disturbance response group 2 of Stringham et al. 2017, with several important modifications.
State 1
Historical Reference State
The Historical Reference State is representative of the natural range of variability for the site under pristine conditions. The reference state is a bunchgrass shrubland. 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 are maintained by ecosystem processes and structural elements such as the presence of all structural and functional plant groups, the retention of organic matter and the maintenance of plant community cover. Plant community phase changes are primarily driven by infrequent fire and/or periodic drought.
Community 1.1
Reference Plant Community
Bunchgrasses and shrubs dominate the reference plant community of this site, which was likely the most common expression of the site historically. Dominant species include shadscale and bluebunch wheatgrass. Broom snakeweed may occur in minor amounts.
Table 6. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Grass/Grasslike |
480 |
640 |
800 |
| Shrub/Vine |
120 |
160 |
200 |
| Forb |
0 |
0 |
0 |
| Total |
600 |
800 |
1000 |
Community 1.2
Bunchgrass dominant community
This community represents an early seral phase following disturbance and is dominated by perennial grasses such as bluebunch wheatgrass. Given the historical scarcity of fire on this site, this was likely an uncommon community. Sprouting shrubs will decrease following fire but will soon increase in abundance while non-sprouting shrubs, such as shadscale, will take longer to recolonize.
Community 1.3
Shrub dominant community
This community is characterized by an increased dominance of shrubs, particularly shadscale saltbrush. Perennial grasses will be decreased relative to the other communities in this state.
Pathway P1.1a
Community 1.1 to 1.2
Fire occurs with enough severity to kill most of the shrub community or an extended wet period leads to a die-off of shadscale and an increase in bunchgrasses.
Pathway P1.1b
Community 1.1 to 1.3
Time and lack of fire, or a prolonged drought, facilitates an increase in the shrub community.
Pathway P1.2b
Community 1.2 to 1.1
Time and lack of fire, or a prolonged drought, allows for regeneration of the shrub community.
Pathway P1.3b
Community 1.3 to 1.1
An extended wet period leads to a partial die-off of shadscale and an increase in bunchgrasses.
Pathway P1.3a
Community 1.3 to 1.2
Fire occurs with enough severity to kill most of the shrub community, or an extended wet period leads to a die-off of shadscale and an increase in bunchgrasses..
State 2
Current Potential State
This state is similar to the Reference State. Ecological function has not changed fundamentally, however the resiliency of the site has been reduced by the presence of invasive plants. Additionally, livestock herbivory may be present as a disturbance process and changes in climate may be altering ecological dynamics. Non-native plant species may increase in abundance but will not become dominant or control ecological processes within this state. These species can be highly flammable and can promote fire where historically fire had been infrequent. Negative feedbacks enhance ecosystem resilience and contribute to the stability of the state. These are maintained by ecosystem processes and structural elements such as the presence of all structural and functional groups, and retention of organic matter and nutrients. Positive feedbacks driven by plant community invasion decrease ecosystem resilience and stability of the state. These include exotic plant species' high seed output, persistent seed bank, rapid growth rate, ability to cross pollinate, and adaptations for seed dispersal. Plant community phase changes are primarily driven by infrequent fire, periodic drought and ungulate herbivory.
Current potential plant communities mirror those of the above Historical Reference (State 1) yet with the addition of a low level of invasive exotic plant invasion and influences of livestock herbivory. Livestock herbivory may result in decreases in deep rooted perennial grasses, and related increases in shallow-rooted perennial grasses (such as Sandberg's bluegrass) and shrubs, among other changes.
State 3
Shrub/Tree State
Within this state, site resources are primarily controlled by shrub species and western juniper. Native perennial grass composition has been reduced considerably. Sprouting and non-sprouting shrubs as well as western juniper are present. Exotic herbaceous species are often present.
Multiple plant community phases are likely within this state, influenced by livestock herbivory and fire. Sprouting species such as rabbitbrush and broom snakeweed as well as non-sprouting species such as shadscale may be dominant but will be outcompeted by western juniper overtime as canopy closure and moisture competition progresses. Invasive annual grasses such as cheatgrass and medusahead are likely.
Juniper encroachment can decrease cover of grasses and shrubs by reducing light availability and altering site hydrology through increased interception of precipitation, reduced infiltration and increased erosion. Bare ground will increase and erosion may be increased rendering the site at risk of transitioning to an eroded state (State 6).
State 4
Exotic Annual/Perennial State
Within this state, site resources are primarily controlled by exotic annual and perennial herbaceous species. Native perennial grass composition has been greatly diminished. Shrub species and western juniper may also be present.
Multiple plant communities are possible within this state, all of which are dominated by invasive annual grasses such as cheatgrass and medusahead as well as invasive annual and perennial forbs, such as prickly lettuce, or Russian thistle (Salsola tragus). Native invader shrub species such as broom snakeweed may also be common
Overtime, with increasing invasion by annuals, soil stabilizing perennial root biomass will be decreased, and bare ground may increase outside of the growing season, elevating potential for erosion and rendering the site at risk of transitioning to an eroded state (State 6).
State 5
Seeded State
Within this state, site resources are primarily controlled by introduced range grasses such as crested wheatgrass (Agropyron cristatum). Multiple community phases may occur within this state and will include different compositions of native and invasive shrub and grass species and western juniper. Plant composition will be dependent on seeded species mixes used and the effectiveness of the revegetation effort. Similar to the ecological dynamics within other states of this site, shrub species and western juniper will increase with greater time since fire and herbivory pressure. While introduced range grasses may provide some analogous ecological functions to native perennial grasses, they may competitively exclude native grasses in the long-term.
State 6
Eroded State
This state is characterized by the loss of soil through wind and water erosion. Low herbaceous cover is available to stabilize the soil and renders the soil surface vulnerable to raindrop impacts, runoff and entrainment of soil particles by wind. The potential for a shift to this state is greatly increased toward the steeper slope range of this site. Positive feedbacks develop as low vegetative cover leads: to decreased litter and organic matter production, higher soil temperature variability, and decreased nutrient cycling, further destabilizing soil and decreasing potential plant establishment. Grass and forb vegetative cover is low yet invasive annual plants and western juniper are likely. Rehabilitation of this state is unknown but would likely pose significant challenges due to degradation of abiotic function of the site.
Transition T1A
State 1 to 2
Introduction of non-native, invasive plants.
Transition T2A
State 2 to 3
Chronic improperly managed grazing or sufficient time without shrub and tree controlling fire, allowing an increase in grazing resistant and/or fire intolerant woody species such as shadscale and western juniper, and a decrease in the perennial grass seedbank.
Transition T2B
State 2 to 4
Catastrophic fire, soil disturbing treatments or prolonged improperly managed grazing in the presence of non-native, invasive species.
Restoration pathway R3A
State 3 to 2
Shrub/Tree management and seeding of native species coupled with minimal soil disturbance, potentially requiring the use of herbicide.
Context dependence. Generally, drill seeding should not be attempted on slopes steeper than 30% (Doerr 1986), yet broadcast, hydroseeding or aerial application may still be possible. Risk of failure may be high due to low site resilience as a result of the droughty nature of the site, steep slopes, and shallow, clayey soils.
Transition T3A
State 3 to 4
Catastrophic fire or failed woody species treatment, possibly in combination with prolonged improperly managed grazing in the presence of invasive plant species.
Restoration pathway R3B
State 3 to 5
Shrub/Tree management and seeding of desired species coupled with minimal soil disturbance, potentially requiring the use of herbicide.
Context dependence. Generally, range drill seeding should not be attempted on slopes steeper than 30% (Doerr 1986), yet broadcast, hydroseeding or aerial application may still be possible. Risk of failure may be high due to low site resilience as a result of the droughty nature of the site, steep slope angles, and shallow, clayey soils.
Transition T3B
State 3 to 6
Prolonged time without stand replacing fire allowing juniper encroachment to advance and outcompete other functional plant groups, possibly followed by additional disturbance such as a low intensity fire or excessive grazing that diminishes understory vegetation but leaves trees intact. Stand replacing fire occurring after advanced juniper encroachment has eliminated understory species and associated seedbanks may also trigger this transition.
Restoration pathway R4A
State 4 to 5
Reduction of exotic plant species through management actions such as herbicide, prescribed grazing, or mowing, and seeding of desired species coupled with minimal soil disturbance.
Context dependence. Generally, range drill seeding should not be attempted on slopes steeper than 30% (Doerr 1986), yet broadcast, hydroseeding or aerial application may still be possible. Risk of failure may be high due to low site resilience as a result of the droughty nature of the site, steep slopes, persistent exotic species seedbanks and shallow, clayey soils.
Transition T4A
State 4 to 6
Catastrophic fire leading to a loss of vegetative cover, large increase in bareground and potential augmentation of soil hydrophobicity, creating highly erosive conditions. Additionally, a failed rehabilitation attempt with excessive soil disturbance can trigger this transition.
Transition T5A
State 5 to 3
Prolonged improperly managed grazing, or lack of fire with sufficient intensity to remove woody species, for an extended interval in the presence of shrubs and western juniper.
Transition T5B
State 5 to 4
Catastrophic fire, multiple fires or failed rehabilitation attempt in the presence of invasive plant species.
Transition T5C
State 5 to 6
Catastrophic fire, or multiple fires in short succession, possibly in combination with prior prolonged disturbance such as successive grazing, especially on steeper slopes within the range of this site.
Context dependence. This transition may be especially likely on steeper slopes within the range of this site.
