Ecological dynamics
The plant community on this ecological site is dominated by fourwing saltbush, Wyoming sagebrush, Indian ricegrass, and needle and thread (Hesperostipa comata). James’ galleta (Pleuraphis jamesii), sand dropseed (Sporobolus cryptandrus) and yellow rabbitbrush (Chrysothamnus viscidiflorus) are also found frequently on this site. Location on the valley floor as made this site susceptible to human influences, such as grazing. Potentially, grazing has been an influence on plant community dynamics since European settlement in the mid-1800s. Fourwing saltbush is fairly palatable to livestock and can be eliminated or reduced in density by overgrazing. Fire can also reduce the density of fourwing saltbush. The introduction of livestock, along with alteration of fire frequency, and spread of non-native plants can cause this site to depart from reference condition. Shifting from reference is marked by a decrease in native perennial grasses and shrubs, while annual forbs and grasses increase. Because this site is located on fairly unstable landforms, such as dunes, removal of native vegetation can increase erosion and active movement of dunes. Non-native plants can become established after disturbances, cheatgrass (Bromus tectorum), Russian thistle (Salsola iberica) and halogeton (Hologetom glomeratus) are common invasive plants on this site.
As plant communities respond to changes in management or natural influences, recovery to previous states may not be possible without major energy inputs. The amount of energy needed to affect vegetative shifts depends on present biotic and abiotic features and the desired results. The following diagram does not necessarily depict all possible transitions and states, but it does present the most prevalent and repeatable plant communities. Ongoing data is collection may warrant revision or removal of plant communities and new ones may be added. This model was developed using rangeland data collected from 1984 to 2014. Both ocular estimated and measured data was collected and utilized.
State 1
Reference State
This state represents the natural range of variability that dominates the dynamics of this ecological site. The biotic communities that persist when all successional sequences occur within the natural disturbance regime. Perennial cool and warm season grasses and a diverse shrub community are dominant in this State. Primary disturbance mechanisms include fire, grazing by native herbivores, and active alluvial deposits that change drainage patterns. These disturbances coupled with weather events dictate the dynamics that occur within the natural range of variability. The reference state is self-sustaining and hold a high resistance to natural disturbances and high resilience after natural disturbances. When natural disturbances occur, the rate of recovery is relatively rapid due to niches being filled with highly adapted native vegetation. The reference state has been determined by study of literature reviews, trends in plant community dynamics, and historical accounts.
Community 1.1
Fourwing saltbush/Wyoming sagebrush/Indian ricegrass
This plant community is characterized by both native shrubs and perennial cool season grasses and some warm season grasses. Indian ricegrass, Needleandthread, sand dropseed, and galleta may be present. Fourwing saltbush and/or Wyoming sagebrush form the dominant visual aspect, shadscale and rabbitbrush are generally present. Other grass, shrubs, and forbs may or may not be present and cover is variable. The composition by air-dry weight is approximately 55 percent perennial grasses, 5 percent forbs, and 40 percent shrubs.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Grass/Grasslike |
83 |
358 |
605 |
| Shrub/Vine |
60 |
260 |
440 |
| Forb |
8 |
33 |
55 |
| Total |
151 |
651 |
1100 |
Table 6. Ground cover
| Tree foliar cover |
0%
|
| Shrub/vine/liana foliar cover |
15-30%
|
| Grass/grasslike foliar cover |
15-30%
|
| Forb foliar cover |
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 |
– |
– |
– |
0-10% |
| >1 <= 2 |
– |
– |
25-35% |
– |
| >2 <= 4.5 |
– |
25-35% |
– |
– |
| >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
Indian ricegrass/needle and thread/rabbitbrush
This community phase is characterized by perennial grasses where Indian ricegrass is the dominant plant species. Needle and thread, sand dropseed, sandhill muhly, and galleta may also be present. Perennial warm and cool season grasses (30-50% cover) are very common while shrubs make up only 1-9% of the canopy cover, measured as first raindrop impact. Fourwing saltbush and/or Wyoming sagebrush form the dominant visual cover.
Community 1.3
Wyoming sagebrush/four-wing saltbush/Indian ricegrass/needle and thread
This plant community is characterized by a dominance of native perennial shrubs, where cool season perennial grasses are still present in the understory. Indian ricegrass, Needleandthread, sand dropseed, and galleta may be present. Fourwing saltbush and/or Wyoming sagebrush form the dominant visual cover. Other grass, shrubs, and forbs may or may not be present and cover is variable. This plant community is more at risk that other plant communities in the reference state due to the lack of understory.
Pathway 1.1b
Community 1.1 to 1.2
This pathway occurs as events favor the increased establishment of perennial cool season grasses with a decrease in shrub cover. Events typically include fire, which reduces shrub cover and allows for increased establishment of grasses, and insect herbivory.
Pathway 1.1a
Community 1.1 to 1.3
This pathway occurs as events favor the increased establishment of shrubs. Events may include drought or time without fire disturbances. Drought favors the establishment of shrubs due to its ability to out-compete native perennial grasses for moisture. Time without fire allows shrubs, which have longer recovery periods, to establish.
Pathway 1.2a
Community 1.2 to 1.1
This pathway occurs as natural events favor the increased establishment of shrubs, including drought or time without disturbances. Drought favors the establishment of shrubs due to their ability to out-compete native perennial grasses for moisture. Time allows shrubs, which have longer recovery periods, to establish.
Pathway 1.3a
Community 1.3 to 1.2
This pathway occurs as natural events favor the increased establishment of perennial cool season grasses with a decrease in shrub cover. Events typically include patchy fire, which reduces shrub cover and allows for increased establishment of grasses, and insect herbivory.
State 2
Current Potential State
This state is similar to the reference state except that non-native plants are now present in all plant community phases. The primary disturbance mechanisms can be natural or human induced. Events, such as improper livestock grazing, prolonged rodent activity, changes in historic fire regimes, OHV overuse, insect herbivory, or drought may influence this site. A shift in species composition will affect the nutrient cycling, soil-water relationships, hydrology, and soil stability. Dominant grasses include both cool season and some warm season grasses; however many times heavy spring grazing will remove the cool season grasses and heavy late summer and early fall grazing will remove the warm season grasses. This state is losing resistance to disturbances and resilience after disturbance. Invasive plants are beginning to fill the niches and establish on the site.
Community 2.1
Four-wing saltbush/Wyoming sagebrush/Indian ricegrass/non-native
This plant community is characterized by both native shrubs and perennial cool season grasses. Indian ricegrass, Needleandthread, sand dropseed, galleta and invasive non-native species may be present.
Community 2.2
Indian ricegrass/needle and thread/rabbitbrush/non-natives
This community phase is characterized by perennial grassland where Indian ricegrass is the dominant plant species. Needle and thread, sand dropseed, blue grama, and galleta may also be present. Perennial cool season grasses are very common while shrubs make up only a small percentage of the canopy cover, measured as first raindrop impact. Fourwing saltbush and/or Wyoming sagebrush form the dominant visual cover. Commonly seen invasive plant species include cheatgrass, Russian thistle, and halogeton. Other grasses, shrubs, and forbs may or may not be present and cover is variable.
Community 2.3
Wyoming sagebrush/Four-wind saltbush/Indian ricegrass/needle and thread/non-natives
This plant community is characterized by a dominance of native perennial shrubs, where cool and warm season perennial grasses are still present in the understory. Needleandthread, Indian ricegrass, sand dropseed, and galleta may be present. Fourwing saltbush and/or winterfat form the dominant visual cover, and generally cutler mormontea is present. Commonly seen invasive plant species include cheatgrass and Russian thistle. Other grass, shrubs, and forbs may or may not be present and cover is variable.
Pathway 2.1b
Community 2.1 to 2.2
This pathway occurs as natural events favor the increased establishment of perennial cool season grasses with a decrease in shrub cover. Events typically include fire, insect herbivory, and grazing in such a way to reduce shrub cover and production, resulting in an increase in the grass and forb understory.
Pathway 2.1a
Community 2.1 to 2.3
This pathway occurs as natural events favor the increased establishment of shrubs. Events may include drought, time without a fire disturbance, and grazing livestock in such a way that removes the grass and forb understory and causes an increase in shrub cover and production.
Pathway 2.2a
Community 2.2 to 2.1
This pathway occurs as events favor the increased establishment of shrubs, including drought time without fire disturbances, and grazing livestock in such a way that removes the perennial grass and forb understory and facilitates the increased establishment of shrubs.
Pathway 2.3a
Community 2.3 to 2.2
This pathway occurs as events, such as fire, insect herbivory, or grazing livestock in such a way to decrease shrub cover and production, favor the increased establishment of perennial warm and cool season grasses with a decrease in shrub cover.
State 3
Sprouting Shrub/Native Grass/Invasive Annual State
This state occurs after repeated fires or over grazing. Fourwing saltbush is susceptible to overgrazing and can leave a system if pressured by grazing. Fire can also remove fourwing saltbush from a site. It can then be replaced by fire tolerant shrubs such as rabbitbrush. Invasive annual, like cheatgrass and Russian thistle can also become dominant in the understory. Community phase 3.2 is the "At risk" phase.
Community 3.1
Yellow rabbitbrush/native grass/forb/non-natvie species
This plant community is characterized by a shrub overstory with an invasive forb and grass understory. The native and/or introduced perennial grass canopy cover is minimal. Invasive forbs make up a large portion of the understory. Russian thistle is present and the most productive species on the site. Other invasive species include stickseed, annual Cryptantha, tansy mustard, and broom snakeweed. Minimal amounts of cheatgrass are present.
Community 3.2
Yellow rabbitbrush/annual forb/annual grass
This plant community is characterized by a minimal shrub overstory with an invasive forb/grass understory and it is the “at-risk” community.
Pathway 3.1a
Community 3.1 to 3.2
Improper grazing management or fire reducing perennial bunchgrass cover and increasing annual forbs and grass cover.
Pathway 3.2a
Community 3.2 to 3.1
Grazing pressure alleviated allowing perennial bunchgrasses to establish
State 4
Invasive Annual State
This state is characterized by the dominance of invasive grasses/forbs. These species may include, but are not limited to cheatgrass, Russian thistle, tansy mustard, broom snakeweed, annual stickseed, or annual Cryptantha. One or more invasive species has increased to a point where they influence or drive the disturbance regime and nutrient cycle. Russian thistle and/or cheatgrass are the most likely of these species to establish and dominate. Russian thistle is a prolific reproducer—one plant can produce up to 250,000 seeds, which are dispersed by the wind causing the dead plant to “tumble” across the landscape. While generally considered an unwanted weed, Russian thistle may actually help disturbed sites recover more quickly. If topsoil is present, Russian thistle roots are invaded by mycorrhizal fungi and because this plant does not form associations with the fungi the root and plant are killed. This causes increased mycorrhizal fungi in the soil and an increased chance for other plants to establish through the aid of these fungi associations. Yet, managers must be aware that that without proper management other invasive annuals, such is cheatgrass may become established instead of the desirable native species. If topsoil is missing the mycorrhizal fungi, Russian thistle can persist for relatively long periods of time (Howard, 1992). Cheatgrass has an entirely other effect on the soil/plant/water relationships. Research has shown that plant species differ substantially in the effects on soil water content and temperature, and in their effects on the frequency and intensity of disturbance. After cheatgrass has invaded a site fundamental nutrient cycling processes, root pores, mycorrhizal associations, microbial species, and soil organic material changes (Chapin et al. 1997; Belnap and Phillips, 2001). These alterations may eventually create ecologically impoverished sites that are very difficult to restore to functionally diverse perennial herbaceous and woody communities. The competitiveness of the invasive species and the ability of these species to quickly establish after a disturbance make this state extremely resistance to change and resilient after a disturbance.
Invasive Annual State: Community phases influenced by improper grazing and weather cycles.
Indicators: A complete understory of invasive annuals, where native perennial shrubs, grasses, and forbs are minimally present.
Feedbacks: Improper livestock grazing and weather cycles that maintain the dominance of invasive forbs/shrubs with minimal decrease in occurring native perennial grasses, shrubs, and forbs. Increased occurrence of cheatgrass, decreasing the fire return interval. Flooding and alluvial deposits that facilitates the establishment of sand sagebrush.
Trigger – The increased establishment of cheatgrass, caused by a decrease in the fire return interval, which facilitates the continued removal of other grasses, shrubs, and forbs.
Community 4.1
Invasive forbs/invasive grasses
This plant community is characterized by a shrub overstory with an invasive forb and grass understory. The native and/or introduced perennial grass canopy cover is minimal. Invasive forbs make up a large portion of the understory. Russian thistle is present and the most productive species on the site. Other invasive species include stickseed, annual Cryptantha, tansy mustard, and broom snakeweed. Minimal amounts of cheatgrass are present.
Transition T1A
State 1 to 2
This transition is from the native perennial bunchgrass understory in the reference state to a state that has non-native invasive plants present in the understory. This transition occurs as natural and/or management actions favor a decline in native perennial grasses and an increase in invasive plant species. Possible events include poorly managed domestic livestock, extended droughts, fire, etc. Cheatgrass, although has been known to establish in intact perennial vegetation communities that have experienced little disturbances either caused naturally or by human activities. Once non-native plants have established, a threshold had been crossed.
Transition T2A
State 2 to 3
Inappropriate grazing management favoring shrub dominance and reducing perennial bunchgrasses and/or drought or fire.
Transition T3A
State 3 to 4
Continued improper grazing or large fire
