Ecological dynamics
The plant communities and ecological dynamics associated with this site are presented in a State and Transition Model (STM) diagram and are described in more detail in the narratives that follow. These plant communities may not represent every possibility, but are probably the most prevalent and recurring plant communities. As more monitoring data are collected, some phases or states may be revised, removed, and/or new ones added. None of these plant communities should necessarily be thought of as “Desired Plant Communities.” The descriptions of plant communities in this document represent the current knowledge and experience at the time of this revision.
STMs are used to illustrate how a reference community (i.e. vegetation prior to Euro-American settlement but long after the arrival of Native Americans) and associated soil-vegetation relationships evolve when subjected to different environmental conditions and drivers of change (e.g. climate, land use, natural disturbances). The STM consists of the historic “Reference State” and potential “alternative states.” Each state may have one or more community “phases” to represent how system dynamics fluctuate within the “limits” of the state (Briske et al. 2008). The ecological resilience (i.e. stability) of a state can be maintained when negative feedbacks exist. Conversely, when system dynamics are disturbed enough through positive feedbacks, whether natural or human-caused, new conditions and processes may develop, resetting the trajectory of the site to an alternative stable state. Transitions between states occur when ecological “thresholds” are crossed. The ability to return to the prior state and reverse the transition would not occur without active restoration practices (i.e. “restoration pathways”). The drivers of change between phases and states are labeled in the diagram as codes (see legend beneath the diagram).
STMs provide a more mechanistic view of the ecological site, which land managers can use to distinguish current conditions from “desired” conditions, and to assess state vulnerability relative to ecological thresholds. Increased awareness of how soil-vegetation patterns and processes vary over space and time promotes greater opportunities to focus adaptive management and restoration efforts.
As ecological condition deteriorates due to overgrazing, Indian ricegrass, squirreltail, bud sagebrush, and winterfat decrease while shadscale and rabbitbrush increase.
When the potential natural plant community is burned, Indian ricegrass, shadscale, and winterfat decrease while rabbitbrush and Sandberg bluegrass increase.
Annual forbs and annual grasses are most likely to invade this site.
State 1
Reference State
The Reference State includes the plant communities best adapted to the unique combinations and factors associated with this ecological site prior to Euro-American settlement. Thus, the plant communities of the Reference State are believed to be non-existent or rare. The Desert Loam ecological site is driven primarily by climatic changes ranging from multi-year drought to years when precipitation levels are high. In response, the vegetation vacillates between a community co-dominated by shadscale saltbush (Atriplex confertifolia) and Indian ricegrass (Achnatherum hymenoides) to one dominated by a mix of perennial grasses and forbs. Historically, fire was not part of this system (West 1983, Brooks and Chambers 2011). The environmental conditions are hot and dry (i.e. 8-10” precipitation zone). Infiltration rates can be slow and potential water loss through evaporation relatively high compared to other coarser-textured soils (i.e. “inverse texture principle,” Noy-Meir 1973). However, due to a small representation of fine rock fragments (0-15%) contained within and on the soil surface, and higher production of perennial grasses, the Desert Loam (Shadscale) site is less susceptible to erosion than the associated Desert Flat (Shadscale) site.
Community 1.1
Shadscale Saltbush
The dominant aspect of the plant community is Indian ricegrass and shadscale. The composition by air dry weight is approximately 45 percent perennial grasses, 15 percent forbs, and 40 percent shrubs.
Shadscale and Indian ricegrass represent the dominant aspect of this plant community. The composition by air dry weight is approximately 45 percent perennial grasses, 15 percent forbs, and 40 percent shrubs. Winterfat (Krascheninnikovia lanata), and bud sagebrush (Picrothamnus desertorum) are common, with fourwing saltbush (A. canenscens), black sagebrush (Artemisia nova), and horsebrush (Tetradymia spp.) occurring as sub-dominants. Other herbaceous species include squirreltail (Elymus elymoides), James’ galleta (Pleuraphis jamesii), globemallow (Sphaeralcea spp.), Torrey’s milkvetch (Astragalus calycosus) and cushion buckwheat (Eriogonum ovalifolium). Microphytic crusts commonly occur in the interspaces between shrubs, providing stabilization and nitrogen fixation to the soil (West 1990).
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Grass/Grasslike |
158 |
214 |
270 |
| Shrub/Vine |
140 |
190 |
240 |
| Forb |
53 |
71 |
90 |
| Total |
351 |
475 |
600 |
Table 6. Ground cover
| Tree foliar cover |
0%
|
| Shrub/vine/liana foliar cover |
5-15%
|
| Grass/grasslike foliar cover |
10-20%
|
| Forb foliar cover |
2-5%
|
| Non-vascular plants |
0%
|
| Biological crusts |
0%
|
| Litter |
0%
|
| Surface fragments >0.25" and <=3" |
0%
|
| Surface fragments >3" |
0%
|
| Bedrock |
0%
|
| Water |
0%
|
| Bare ground |
0%
|
Table 7. Canopy structure (% cover)
| Height Above Ground (ft) |
Tree |
Shrub/Vine |
Grass/
Grasslike |
Forb |
| <0.5 |
– |
– |
– |
– |
| >0.5 <= 1 |
– |
10-20% |
15-25% |
0-10% |
| >1 <= 2 |
– |
– |
– |
– |
| >2 <= 4.5 |
– |
– |
– |
– |
| >4.5 <= 13 |
– |
– |
– |
– |
| >13 <= 40 |
– |
– |
– |
– |
| >40 <= 80 |
– |
– |
– |
– |
| >80 <= 120 |
– |
– |
– |
– |
| >120 |
– |
– |
– |
– |
| Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
| J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
Community 1.2
Mixed Herbs
Grasses including Indian ricegrass, squirreltail, and James’ galleta (Pleuraphis jamesii) and forbs such as globemallow (Sphaeralcea spp.), will be the first to decline after extended drought, but are better equipped to take advantage of any transient precipitation, and will dominate the site once a wet period returns. A mixed herb community characterized by Indian ricegrass, squirreltail, western wheatgrass (Pascopyrum smithii), and globemallow, will re-occupy the recently opened niches.
Pathway 1.1a
Community 1.1 to 1.2
Although shadscale is able to tolerate some drought through partial shedding of leaves, multiple years of drought will kill shadscale, winterfat, and bud sagebrush, particularly in more decadent stands (West 1979). Any shrubs remaining following an extreme droughty period are typically weakened and thus more susceptible to invasion by insects, such as round-headed root borers, scales, grasshoppers, and other pathogens (Haws et al. 1990). During drought, native forbs and grasses will also decrease, but once a wet period returns to a drought-denuded stand, herbs are the first plants to re-occupy recently opened niches.
Pathway 1.1b
Community 1.1 to 1.2
An extremely wet period such as an El Nino-Southern Oscillation event and subsequent anoxic soil conditions will kill shadscale, and any other secondary shrubs. Prolonged periods of high soil moisture are believed to increase the susceptibility of shadscale to parasites and disease, such as water mold, root rot, and vascular wilt fungi (Wallace and Nelson 1990).
Pathway 1.2a
Community 1.2 to 1.1
Time, without significant disturbance and a return to a favorable climate, will allow shadscale and bud sagebrush to re-establish.
State 2
Shadscale Satlbush/Introduced Non-native Herbs State
State 2 is a description of the ecological site following Euro-American settlement, and the subsequent introduction of several non-native plants and animals. Climate change may also cause State 1 to shift into State 2, regardless of the land management practice. The plant community is very similar to State 1 with the exception that several introduced species including cheatgrass (Bromus tectorum), Russian thistle (Salsola spp.), halogeton (Halogeton glomeratus), tall tumblemustard (Sisymbrium altissimum), redstem stork's bill (Erodium cicutarium), and curveseed butterwort (Ceratocephala testiculata) are likely to be present. Once established, there is no practical method to effectively remove these plants from the site. Additionally, microphytic crusts are likely to be diminished or absent depending on disturbance history.
Although shadscale-dominated sites are typically characterized by lower precipitation, in some years the amount and timing of precipitation fluctuates. When precipitation is higher than normal, these sparsely vegetated sites are less effective at utilizing the increased resource (i.e. available water), and therefore are considered more susceptible to invasive species (i.e. fluctuating resource hypothesis) (Davis et al. 2000, Brooks and Chambers 2011). In some areas, invasive grasses have increased enough to fill interspaces between shrubs allowing the site to carry fire, an extremely unusual event in historic salt desert shrublands (West 1994, Brooks and Chambers 2011).
Shadscale is used for forage by all classes of livestock, as well as mule deer and pronghorn antelope. Historically, lower desert communities including the Desert Flats were used for grazing of sheep during winter months. Prior to the Taylor Grazing Act of 1934, many of these areas were overgrazed and depending on the intensity, type of livestock, and season of use, the community was set onto various trajectories (Kitchen and Hall 1996).
Community 2.1
Shadscale Saltbush
Shadscale saltbush is the dominant component of this community. Bud sagebrush, Indian ricegrass, and squirreltail may be sparsely represented in the understory. Non-native species are present but do not dominate. Microphytic crusts are scarce.
Community 2.2
Mixed herbs
In this community Indian ricegrass is likely to be dominant. Squirreltail and a mix of annual forbs may also be present.
Community 2.3
Root-sprouting shrubs
Root-sprouting shrubs such as rubber rabbitbrush, yellow rabbitbrush, and green molly, will characterize this community. Indian ricegrass along with invasive annuals are likely to be present in the understory. Microphytic crusts may persist following low intensity fires provided there is enough precipitation and the burned area is protected from subsequent disturbance (Johansen et al 1984).
Community 2.4
Increasing Shadscale
Following a period of intensive fall grazing, shadscale will dominate the plant community with only a minor component of mixed herbs present in the understory.
Community 2.5
Increasing non-native annuals/Diminished shrubs
Some shrubs may be present, but invasive annuals (e.g. cheatgrass, Russian thistle, halogeton) are increasing and could potentially dominate the site without management attention. Other annuals such as tall tumblemustard, redstem stork's bill, and curveseed butterwort are also common. Native grasses and forbs are minimal or absent. Sites may have compacted soils and/or experienced soil loss. These degraded conditions put the community at greater risk for crossing an ecological threshold.
Pathway 2.1a
Community 2.1 to 2.2
With prolonged drought, shadscale’s resistance to insects (e.g. round-headed root borers, scales, grasshoppers) and other pathogens will decrease, causing the shrub component to decline (Haws et al. 1990). Although the herbaceous component will be the first to go with drought, grasses and forbs are also the first to re-occupy the site when the rains return.
Pathway 2.1b
Community 2.1 to 2.2
An extremely wet period such as an El Nino-Southern Oscillation event and subsequent anoxic soil conditions can kill shadscale and other shrubs due to exposure to soil-borne parasites and diseases such as water mold, root rot, and vascular wilt fungi (Wallace and Nelson 1990).
Pathway 2.1c
Community 2.1 to 2.3
An increase of cheatgrass and other annual forbs can produce enough fine fuels to carry wildfires across these landscapes (West 1994, Brooks and Chambers 2011). Fire will temporarily remove shadscale and bud sagebrush. Other root-sprouting shrubs such as rubber rabbitbrush (Ericameria nauseosa), yellow rabbitbrush (Chrysothamnus viscidiflorus), and green molly (Bassia americana) will increase with invasive annual species dominating the understory.
Pathway 2.1d
Community 2.1 to 2.4
Uncontrolled fall grazing while shadscale is dormant will favor shadscale, particularly if the grazing is done by sheep. Shadscale will increase to nearly pure stands. Some annual herbs may be present in the understory, but bud sagebrush will decrease (Harper et al. 1990, Kitchen and Hall 1996). In heavily trampled areas, microphytic crusts are reduced or absent. Topsoil is easily lost and infiltration is reduced.
Pathway 2.1e
Community 2.1 to 2.5
Uncontrolled spring or summer grazing is injurious to shadscale. Shadscale and bud sagebrush will diminish, while non-native annuals such as cheatgrass, halogeton, and Russian thistle will increase and potentially dominate the site (Harper et al. 1990, Kitchen and Hall 1996). Microphytic crusts are reduced or absent. Topsoil may be lost and infiltration reduced from the compaction of soils.
Pathway 2.2a
Community 2.2 to 2.1
Time, without significant disturbance and a return to a favorable climate, will allow shadscale and bud sagebrush to re-establish.
Pathway 2.2b
Community 2.2 to 2.5
Uncontrolled spring grazing will reduce perennial bunchgrasses allowing non-native annuals (e.g. cheatgrass, halogeton, and Russian thistle) to increase and potentially dominate the site (Harper et al. 1990, Kitchen and Hall 1996). Additionally, topsoil can be lost and infiltration reduced from the compaction of soils.
Pathway 2.3a
Community 2.3 to 2.1
Time, without significant disturbance and a return to a favorable climate, will allow shadscale and bud sagebrush to re-establish.
Pathway 2.3b
Community 2.3 to 2.2
An extremely wet period such as an El Nino-Southern Oscillation event and subsequent anoxic soil conditions will kill most shrubs due to exposure to soil-borne parasites and diseases such as water mold, root rot, and vascular wilt fungi (Wallace and Nelson 1990).
Pathway 2.4a
Community 2.4 to 2.1
If grazing practices are adjusted by matching stocking rates to current site productivity, shifting the season of use to later season (spring) grazing to reduce shadscale, and if the site has not experienced accelerated soil erosion, warm-season bunchgrasses such as Indian ricegrass and James’ galleta (Pleuraphis jamesii) will increase.
Pathway 2.4b
Community 2.4 to 2.5
Provided the buildup of fine annual fuel is sufficient, disturbance by wildfire will kill shadscale. Post-fire, cheatgrass and other early spring annuals will increase outcompeting the perennial warm season grasses and push the community toward a more degraded phase (Community Phase 2.5), the most “at risk” for crossing an ecological threshold. Subsequent fires may become more frequent and intense, removing any remaining shrubs.
It is possible however, if invasive annuals are kept under control and enough root-sprouting shrubs are present, the community may move toward a root-sprouting shrub phase (Community Phase 2.3) post-fire.
Pathway 2.5a
Community 2.5 to 2.1
If grazing practices are adjusted by matching stocking rate to current site productivity, shifting the season of use to early and/or mid-winter grazing when shadscale is dormant, and if the site has not experienced accelerated soil erosion, both shadscale and the warm-season bunchgrasses (e.g. Indian ricegrass and James’ galleta) will increase. Protecting the site from unnecessary disturbance may limit further encroachment by annuals.
State 3
Seeded Range State
The Seeded Range State exists where managers decided to re-seed in order to address management concerns such as the need to increase forage production, control soil erosion, and/or suppress wildfire. Re-vegetation efforts often utilize non-native perennial herbs that mimic the structure and function of but are more competitive than native species (Brooks and Chambers 2011, Davies et al. 2010, DiTomaso and Smith 2012, Hirsch and Monaco 2011, Newhall et al. 2004). Therefore, these areas are often dominated with non-native perennial grasses (e.g. crested wheatgrass). Forage kochia is an introduced semi-shrub that is commonly used in revegetation and fire suppression (Monaco et al. 2003). In some cases, monocultures of the seeded species were created. However, more recent seedings have included a broader suite of species (and cultivars) to increase germination rate and assist succession and recovery of the native shadscale community (State 2). Depending on grazing levels, drought, and seed source availability, shadscale and a mix of other native species will re-establish over time. State 3 can be maintained when livestock grazing is compatible with current site productivity, but continued heavy livestock grazing will negatively impact resiliency. Non-native annuals such as cheatgrass and tall tumblemustard are common invaders in these areas.
Community 3.1
Seeded perennials
Depending on the objectives of the seeding effort, this community may be characterized by a single species (e.g. crested wheatgrass, forgage kochia), or by a mix of seeded perennial species (native and/or non-native) including a variety of cool and/or warm season grasses (e.g. wheatgrasses, wildryes, Indian ricegrass, squirreltail, and sand dropseed (Sporobolus cryptandrus)). Provided the disturbances are minimal, livestock stocking rates match current site productivity, and seed sources exist, shadscale, rubber rabbitbrush, and other native species may begin to re-establish. Non-native annuals are likely to be present. If the climate is conducive, seeded ranges can be maintained when grazing practices are compatible with current site productivity.
State 4
Invasive Annuals State
The Invasive Annuals State is a degraded condition as a result of long-term heavy grazing and invasion by annuals. Halogeton and/or cheatgrass are typically dominant.
Community 4.1
Halogeton and/or cheatgrass dominant
Halogeton and/or cheatgrass are the primary species in this community. Russian thistle may also be present after fire, if topsoil has remained intact, but will likely diminish within 3 to 4 years post-fire without further disturbance (Young 1991). Conversely, if topsoil has been lost, Russian thistle can persist for nearly a decade (Allen 1989).
Once established, exotic species can permanently alter soil properties and processes, even after the invasive species is removed or controlled (Allen et al. 2011, Kulmatiski et al. 2008). Soils invaded by cheatgrass typically have faster decomposition rates, with varying responses in total phosphorus (P) and nitrogen (N) availability and (N) cycling (Ehrenfeld 2003). Belnap et al. (2005) also noted shifts in and lower biomass of key taxa within the microbial community in cheatgrass dominated areas that can directly affect nutrient cycling and ultimately modify soil structure and plant growth. Halogeton, the succulent halophytic summer annual, will translocate and accumulate salts in plant tissues, which then leach back into near-surface soils from leaf litter and dead roots. Secondary effects from the increased soil salinity may limit infiltration rates and capillary rise of water, inhibit nitrifying microorganisms, and ultimately cause the soil crust to harden and depress plant growth (Eckert and Kinsinger 1960, Kitchen and Jorgensen 2001, Allen et al. 2011). Under such impoverished soil conditions, natural regeneration of native shrubs is unlikely and may impede the outcome of revegetation efforts (Harper et al. 1996, Grant and Paschke 2012).
Due to the abundance of fine fuels cycling through this system annually, the site will become more prone to wildfire. Fire return intervals can shorten and perpetuate an invasive annuals state.
State 5
Eroded State
The Eroded State is a highly degraded condition as a result of extreme wildfire and very powerful, persistent winds removing most if not all vegetation. In some areas, multiple inches of soil may be lost, while deposition and dunning may occur in other areas.
Community 5.1
Barren lands
Extreme fire will remove most if not all vegetation. Microphytic crusts are very rare, but if present before the fire, may be found beneath surviving shrubs. High winds could cause further erosion or burial in some areas.
Until some vegetation is in place to hold or catch less stable soil particles, persistently high winds can create positive feedbacks of continued soil erosion through the process of saltation. If coarser, less stable sediments from upwind are blown over finer-grained (e.g. lacustrine) deposits, the abrasion to the surface can trigger soil particles downwind to detach and add to the total horizontal sediment flux (Gillette et al. 1996, Miller et al. 2012).
Transition T1A
State 1 to 2
The introduction of exotic flora and fauna, possible extinctions of native flora or fauna, along with climate change, will cause State 1 to transition to State 2. Reversal of this change back to State 1 is impracticable. These soils are easily disturbed by trampling. Compacted areas are more susceptible to wind or water erosion. Perennial vegetation decreases while bare ground increases leading to further runoff and soil loss.
Transition T2a
State 2 to 3
Over the years, land managers have seeded rangelands for a variety of reasons. Historical tilling and removal of shadscale through chemical or mechanical means, followed by seeding of non-native perennial grasses such as crested wheatgrass (Agropyron cristatum) was done to increase forage production. In areas where annuals have invaded and fire is of concern, drilling and re-seeding with forage kochia (Bassia prostrata) to create fuel-breaks (i.e. “greenstripping”) has also been employed (Monaco et al. 2003).
Transition T2b
State 2 to 4
Heavy continuous season- (or year-) long grazing will worsen the fragile conditions of Community Phase 2.5 (Increasing non-native annuals/ Diminished shrubs) and create positive feedbacks to further degradation. Other Community Phases of State 2 (i.e. 2.1, 2.2, 2.3, and 2.4) could possibly sustain longer periods with heavy grazing, but this would eventually lead to Community Phase 2.5, and ultimately pass an ecological threshold into State 4. Annual species, particularly halogeton and cheatgrass, benefit from continued disturbance. Soils will become more compacted and susceptible to erosion.
Transition T2c
State 2 to 5
The unusual event of extreme wildfire and intense winds will move State 2 to a completely unvegetated and eroded state. If there is no plant cover and high winds continue, resident soils will be displaced and upwind soils may move in creating dune environments. Seed establishment is not possible under these conditions creating a positive feedback that is difficult to reverse.
Restoration pathway R3a
State 3 to 2
If a period of time passes without fire (or other significant disturbance), grazing practices are compatible with productivity, and there is evidence of re-establishment of native shrubs and perennial grasses, it is possible that the Seeded Range State can be restored to the former Shadscale Saltbush/ Introduced Non-native Herbs State.
Transition T3a
State 3 to 4
If the Seeded Range State receives heavy continuous season- (or year-) long grazing, this will reduce the seeded and/or re-establishing perennial species (herbs and shrubs) and continue to disturb the soils, potentially creating a positive feedback where annual species, such as halogeton and cheatgrass, are the few species able to persist. Soils will be more compacted and susceptible to erosion.
Transition T3b
State 3 to 4
Depending on the intensity, frequency, and availability of exotic annuals, wildfire may reduce the seeded and/or re-establishing perennial species (herbs and shrubs), and push the community into one dominated by invasive annuals.
Transition T3c
State 3 to 5
The unusual event of extreme wildfire and intense winds will move the Seeded Range State to a completely unvegetated and eroded state. If vegetation cover is low and winds are high, resident soils may continue to erode and upwind soils may move to create dune environments. Seed establishment is not possible under these conditions thus creating a positive feedback that is difficult to reverse.
Restoration pathway R4a
State 4 to 3
Depending on the degree plant-soil feedbacks have changed (see: Community Phase 4.1), the potential to transition the Invasive Annuals State to a seeded range state may be practicable if the species selected for seeding are known to tolerate the “invader-cultured soil” (Grant and Paschke 2012, Newhall et al. 2004), provided a favorable climate and appropriate grazing practices are imposed. “Greenstripping” with forage kochia to facilitate the establishment of persistent vegetation and fire-breaks has been successfully employed as well (Monaco et al. 2003). Regardless of the land treatment, the landscape setting including the juxtaposition of particular landforms and soil properties should be considered carefully. Depending on the spatial extent and connectivity of the disturbance, land treatments can have variable impacts on aeolian sediment fluxes (i.e. wind erosion), particularly in areas with persistent high winds and fragile soils (Miller et al. 2012). For example, upwind soil and vegetation conditions are important factors when predicting and mitigating potential downwind erosion dynamics (Miller et al. 2012).
Transition T4a
State 4 to 5
The unusual event of extreme wildfire and intense winds will move the Invasive Annuals State to a completely unvegetated and eroded state. If there is no vegetation cover and winds are high, resident soils will continue to erode, or in some cases other soils can be blown in creating dune environments. Seed establishment is not possible under these conditions thus creating a positive feedback that is difficult to reverse.
Restoration pathway R5a
State 5 to 3
It may be possible to transition the Eroded State to a seeded range state, provided the seeded species are capable of surviving in these extremely harsh conditions (Newhall et al. 2004), the area receives enough precipitation, and careful consideration is made to the landscape setting (i.e. juxtaposition of certain landforms and soil properties) (Miller et al. 2012). The spatial extent and connectivity of land treatments, particularly in areas with high winds and fragile soils, can directly influence aeolian sediment flux through saltation. Upwind soil and vegetation conditions are important factors when predicting and mitigating potential downwind erosion dynamics (Miller et al. 2012). Arranging drill rows perpendicularly to prevailing winds, installing fences to catch sediments, and limiting grazing disturbances would be important mitigation measures to reduce erosion. Herbicides should be used with caution, as the cover of exotic annual plants can provide protection for perennial seedlings as they re-establish.
