Ecological dynamics
The Reference state is the plant community in which interpretations are primarily based and is used as a reference in order to understand the original potential of the site. The Reference state evolved under the combined influences of climatic conditions, periodic fire activity, grazing by large herbivores, and impacts from small mammals and insects. Changes may occur to the Reference state due to management actions such as continuous season-long or year-long grazing, increased stocking rates, climatic conditions such as drought, and natural events such as multiple fires in close succession. The reference plant community for this ecological site is dominated by a diversity of tall and medium height, cool-season and warm-season grasses which are tightly intermixed and well distributed over the site. Various forbs, half-shrubs, and shrubs are common on this site. The Reference state is not necessarily the management goal, as other vegetative states may be considered desired plant communities as long as critical resource concerns are met.
In addition to the Reference state, other plant communities can occur on this site and are usually the result of historic management practices. Grazing practices such as continuous season-long or year-long grazing, heavier stocking rates, or a combination of these factors on this ecological site results in a decrease of tallgrasses, mid-grasses, and more palatable forbs and in an increase of shortgrasses, sedges, and less palatable forbs. Half-shrubs and shrubs increase in the absence of prescribed fire and wildfire. More frequent fire intervals decreases the shrub component resulting in a site dominated by grasses along with a substantial increase in the forb component (Porensky et al. 2018). There are various transitional stages which may occur on this ecological site. The information presented is representative of a dynamic set of plant communities that illustrate the complex interaction of several ecological processes.
State 1
Reference
The Reference state evolved under the combined influences of climatic conditions, periodic fire activity, grazing by large herbivores, and impacts from small mammals and insects. The Reference state is the plant community in which interpretations are primarily based and is used as a reference in order to understand the original potential of the site. The plant communities and various successional stages represent the natural range of variability. The Reference state for this ecological site consists of four communities.
Community 1.1
Wyoming Big Sagebrush, Green Needlegrass, and Western Wheatgrass
Community 1.1 is characterized by a mixed-grass community and Wyoming big sagebrush. The predominant species are cool-season grasses such as green needlegrass and western wheatgrass, with a smaller component of warm-season grasses. Shortgrasses and sedges such as blue grama, prairie Junegrass, Sandberg bluegrass, threadleaf sedge, and needleleaf sedge are present at low cover. Forbs such as American vetch, upright prairie coneflower, large Indian breadroot, milkvetch, sulphur-flower buckwheat, purple prairie clover, white prairie clover, and tapertip hawksbeard are present. The dominant shrub species is Wyoming big sagebrush, although other shrubs such as fourwing saltbush and sub-shrubs such as winterfat occur at lower canopy covers. The potential vegetation (air-dry weight) is approximately 71 percent grasses, 10 percent forbs, and 19 percent shrubs. The total annual production (air-dry weight) is approximately 1,900 pounds per acre during an average year, but can range from approximately 1,500 pounds per acre in below average years to approximately 2,300 pounds per acre in above average years.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Grass/Grasslike |
1065 |
1349 |
1633 |
| Shrub/Vine |
285 |
361 |
437 |
| Forb |
150 |
190 |
230 |
| Total |
1500 |
1900 |
2300 |
| 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
Green Needlegrass and Western Wheatgrass
Community 1.2 is characterized by a reduction in Wyoming big sagebrush and an increase in cool-season, rhizomatous wheatgrasses such as western wheatgrass and thickspike wheatgrass and mid-statured bunchgrasses such as green needlegrass and bluebunch wheatgrass. Shortgrasses and sedges such as blue grama, prairie Junegrass, Sandberg bluegrass, buffalograss, threadleaf sedge, and needleleaf sedge occur at low cover. This community results in a substantial increase in the forb component with species such as American vetch, upright prairie coneflower, large Indian breadroot, milkvetch, sulphur-flower buckwheat, purple prairie clover, white prairie clover, and tapertip hawksbeard (Porensky et al. 2018). Half-shrubs such as winterfat may occur. Wyoming big sagebrush is rare or absent. This community occurs following sagebrush mortality due to wildfire, prescribed fire, mechanical and chemical treatments, or biological processes and may persist for 80 years or longer (Porensky et al. 2018).
Community 1.3
Wyoming Big Sagebrush, Western Wheatgrass, and Blue Grama
Community Phase 1.3 is characterized by an increase in shortgrass species and sedges such as blue grama, prairie junegrass, Sandberg bluegrass, threadleaf sedge, and needleleaf sedge, and a decrease in mid-statured grasses such as western wheatgrass and green needlegrass. Shortgrass species dominate the plant community and mid-statured grasses are less common and contribute little to overall production. This plant community occurs due to long-term drought, grazing management such as continuous season-long or year-long grazing, increased stocking rates, or a combination of these factors.
Community 1.4
Western Wheatgrass and Blue Grama
Community 1.4 is characterized by an increase in shortgrass species and sedges such as blue grama, prairie Junegrass, Sandberg bluegrass, threadleaf sedge, and needleleaf sedge, and a decrease in mid-statured grasses such as western wheatgrass and green needlegrass. Wyoming big sagebrush is rare or absent. This plant community occurs due to long-term drought, grazing management such as continuous season-long or year-long grazing, increased stocking rates, or a combination of these factors. Sagebrush mortality is evident due to wildfire, prescribed fire, mechanical and chemical treatments, or biological processes.
Pathway 1.1A
Community 1.1 to 1.2
Community 1.1 will transition to community 1.2 through prescribed fire and wildfire, mechanical and chemical treatments, and biological processes. Wyoming big sagebrush is greatly reduced and perennial grasses will dominate the site.
Pathway 1.1B
Community 1.1 to 1.3
Drought, grazing practices such as continuous season-long or year-long grazing, heavier stocking rates, or a combination of these factors can shift community 1.1 to community 1.3. These factors favor an increase in shortgrasses such as blue grama and a decrease in cool-season mid-statured grasses. Wyoming big sagebrush cover will be similar to community 1.1.
Pathway 1.2A
Community 1.2 to 1.1
Approximately 80 years or more of natural vegetative regrowth will transition community 1.2 to community 1.1. Approximately 80 years or more without fire allows Wyoming big sagebrush to recolonize the site (Porensky et al. 2018).
Pathway 1.2B
Community 1.2 to 1.4
Drought, grazing practices such as continuous season-long or year-long grazing, heavier stocking rates, multiple fires in close succession, or a combination of these factors can shift community 1.2 to community 1.3. These factors favor an increase in blue grama and a decrease in cool-season mid-statured grasses. Wyoming big sagebrush cover will be similar to community 1.2.
Pathway 1.3B
Community 1.3 to 1.1
Approximately 80 years or more post-fire (Porensky et al. 2018), normal or above average precipitation, and lighter stocking rates transitions community 1.3 to community 1.1. This transition may also occur through natural succession.
Pathway 1.3A
Community 1.3 to 1.4
Community 1.3 will transition to community 1.4 through prescribed fire and wildfire, mechanical and chemical treatments, and biological processes. Wyoming big sagebrush is greatly reduced and perennial grasses will dominate the site.
Pathway 1.4B
Community 1.4 to 1.2
Normal or above average precipitation and lower stocking rates transitions community 1.4 to community 1.2.
Pathway 1.4A
Community 1.4 to 1.3
Approximately 80 years or more of natural shrub regrowth will transition community 1.4 to community 1.3. Approximately 80 years or more without prescribed fire or wildfire allows Wyoming big sagebrush to recolonize the site (Porensky et al. 2018).
State 2
Shortgrass
The dynamics of the Shortgrass state are driven by long-term drought, grazing management such as continuous season-long or year-long grazing, or a combination of these factors. The Shortgrass state for this ecological site consists of two communities.
Community 2.1
Wyoming Big Sagebrush, Blue Grama, and Threadleaf Sedge
Community 2.1 is characterized by a dominance of shortgrasses and sedges such as blue grama, prairie Junegrass, Sandberg bluegrass, and threadleaf sedge. Mid-statured grasses such as western wheatgrass and green needlegrass are rare or absent. Prairie sagewort, broom snakeweed, and Wyoming big sagebrush are common. This plant community occurs when site conditions decline due to long-term drought, grazing management such as continuous season-long or year-long grazing, or a combination of these factors. This community phase results in a reduction of soil surface litter, soil organic matter, and infiltration and an increase of soil surface runoff. This plant community is capable of tolerating season-long, heavy grazing and therefore is highly resistant to change.
Community 2.2
Blue Grama and Threadleaf Sedge
Community 2.2 is characterized by a complete dominance of shortgrasses and sedges such as blue grama, prairie junegrass, Sandberg bluegrass, threadleaf sedge, and needleleaf sedge. Cool-season, mid-statured grasses such as western wheatgrass and green needlegrass are rare or absent. Prairie sagewort is common and Wyoming big sagebrush is rare. This plant community occurs when site conditions decline due to long-term drought, grazing management such as continuous season-long or year-long grazing, or a combination of these factors, and a fire has occurred on the site less than approximately 30 years prior. Sagebrush mortality is evident due to wildfire, prescribed fire, mechanical and chemical treatments, or biological processes. This community results in a reduction of soil surface litter, soil organic matter, and infiltration and an increase of soil surface runoff. This plant community is capable of tolerating season-long, heavy grazing and therefore is highly resistant to change.
Pathway 2.1A
Community 2.1 to 2.2
Community 2.1 will transition to community 2.2 through prescribed fire and wildfire, mechanical and chemical treatments, and biological processes. Wyoming big sagebrush is greatly reduced and perennial grasses will dominate the site.
Pathway 2.2A
Community 2.2 to 2.1
It is estimated that approximately 80 years or more of natural post-fire vegetative regrowth (Porensky et al. 2018) could transition community 2.2 to community 2.1. It is possible that this transition could occur over time, however, the processes are not fully understood at this time. Therefore, this pathway is considered hypothetical until further investigation can be completed.
State 3
Invaded
The Invaded state occurs when invasive plant species invade native plant communities and displace the native species. The Invaded state consists of one community.
Community 3.1
Cheatgrass and Field Brome
Observations suggest that native species diversity declines significantly when invasive or noxious species exceed approximately 30 percent of the plant community. Non-native, perennial, drought tolerant grasses such as crested wheatgrass, non-native, annual, invasive species such as cheatgrass and field brome, and noxious weed species can eventually dominate the seedbank of this site and displace native species. Reduced plant species diversity, simplified structural complexity, and altered ecological processes result in a state that is substantially departed from the Reference state. The dominance of annual, invasive grasses such as cheatgrass and field brome increases the fire cycle frequency.
State 4
Cropland
The Cropland state occurs when cultivation occurs to the land. The Cropland state consists of one community.
Community 4.1
Cropland
The land is cultivated and converted to crop production. Annual, cool-season cereal grains such as spring wheat, winter wheat, and barley are common crops which replace native plant communities.
State 5
Post Cropland
The Post Cropland state occurs when cultivated cropland is abandoned and allowed to either re-vegetate naturally or is seeded back to perennial species for livestock grazing or wildlife use. This state can transition back to the Cropland state if the site is returned to cultivation. No formal studies have been obtained regarding Wyoming big sagebrush recovery following cultivation and further investigation is needed to assess Wyoming big sagebrush recovery in the Post Cropland state. The Post-Cropland State (5) has two communities.
Community 5.1
Abandoned Cropland
In the absence of active management, the site can re-vegetate naturally and potentially return to a perennial grassland community over time. Shortly after cropland is abandoned, annual and biennial forbs and annual brome grasses invade the site. The site is highly susceptible to erosion due to the absence of perennial species. Eventually, these pioneering annual species are replaced by perennial forbs and perennial shortgrasses. Depending on the historical management of the site, mid-statured perennial grasses may also return; however, species composition will depend upon the seed bank. Invasion of the site by exotic species, such as crested wheatgrass and annual bromes, will depend upon the site’s proximity to a seed source. Approximately 50 or more years after cultivation, these sites may have species composition similar to communities in the Reference state (Dormaar, J.F., and S. Smoliak. 1985). However, soil quality is consistently lower than conditions prior to cultivation and a shift to the Reference state is unlikely.
Community 5.2
Perennial Grass
When the site is seeded to perennial forage species this community can persist for several decades. Introduced perennial grasses, in particular, may form monocultures that persist for approximately 60 years or more (Samuel, M.J., and R.H. Hart. 1994). A mixture of native species may also be seeded to provide species composition and structural complexity similar to that of the Reference state. However, soil quality conditions have been substantially altered and will not return to pre-cultivation conditions.
Transition T1A
State 1 to 2
Prolonged drought, grazing management such as continuous season-long or year-long grazing, heavier stocking rates, or a combination of these factors weaken the resilience of the Reference state and drive its transition to the Shortgrass state. The Reference state transitions to the Shortgrass state when mid-statured grasses are greatly reduced and shortgrasses and sedges such as blue grama, Sandberg bluegrass, prairie Junegrass, and threadleaf sedge dominate the plant community.
Transition T1B
State 1 to 3
The Reference state transitions to the Invaded state when non-native grasses or noxious weeds invade the plant community. Exotic plant species dominate the site in terms of cover and production and site resilience has been substantially reduced. In addition, other rangeland health attributes, such as reproductive capacity of native grasses and soil quality, have been substantially altered from the Reference state.
Transition T1C
State 1 to 4
Tillage or application of herbicide followed by seeding of cultivated crops, such as winter wheat, spring wheat, and barley, transitions the Reference state to the Cropland state.
Restoration pathway R2A
State 2 to 1
Blue grama can resist displacement by other species. A reduction in stocking rates alone may not be sufficient to reduce the cover of blue grama in the Shortgrass state and mechanical treatments may be necessary. Therefore, returning the Shortgrass state to the Reference state could require considerable cost, energy, and time.
Transition T2A
State 2 to 3
The Shortgrass state transitions to the Invaded state when non-native grasses, noxious weeds, and other invasive plants invade the Shortgrass state. Exotic plant species dominate the site in terms of cover and production. Site resilience has been substantially reduced.
Transition T2B
State 2 to 4
Tillage or application of herbicide followed by seeding of cultivated crops, such as winter wheat, spring wheat, and barley, transitions the Shortgrass state to the Cropland state.
Transition T3A
State 3 to 4
The Invaded state will transition to the Cropland state when the site is placed under cultivation.
Transition T4A
State 4 to 5
The transition from the Cropland state to the Post Cropland state occurs with the cessation of cultivation. The site may also be seeded to perennial forage species, such as crested wheatgrass and alfalfa, or a mix of native species.
Transition T5A
State 5 to 4
Tillage or application of herbicide followed by seeding of cultivated crops, such as winter wheat, spring wheat, and barley, transitions the Post Cropland state to the Cropland state.
