Ecological dynamics
The site developed under Northern Great Plains climatic conditions, and included natural influence of large herding herbivores and frequent fire. Changes will occur in the plant communities due to weather fluctuations and/or management actions. Under adverse impacts, a slow decline in vegetative vigor and composition will occur. Under favorable conditions the site has the potential to resemble the reference state. Interpretations for this site are based on the Reference State. The Reference State has been determined by study of rangeland relic areas, areas protected from excessive disturbance, and areas under long-term rotational grazing regimes. Trends in plant community dynamics ranging from heavily grazed to lightly grazed areas, seasonal use pastures, and historical accounts also have been considered. Community phases, community pathways, states, transitions, thresholds and restoration pathways have been determined through similar studies and experience.
The natural disturbance regime consisted of frequent fires caused both by natural and Native American ignition sources. These fires occurred during any season of the year, but were concentrated in the spring and late summer or early fall. Lightening fires occurred most frequently in July and August while fires started by Native Americans occurred in April, September and October. Large ungulate grazing was heavy and occurred often, but usually for short durations. Grazing may have been severe when occurring after a fire event, or in association with reliable water sources. The grazing and fire interaction especially when coupled with drought events, set up the dynamics discussed and displayed in the following state and transition diagram and descriptions.
This ecological site has been grazed by domestic livestock since introduced into the area. The introduction of domestic livestock and the use of fencing and reliable water sources have radically changed the disturbance regime of this site. Heavy continuous grazing and/or continuous seasonal (spring) grazing, without adequate recovery periods following each grazing occurrence causes this site to depart from the reference plant community. Blue grama and Kentucky bluegrass if present, will begin to increase. Western wheatgrass will increase initially and then begin to decrease. Green needlegrass will decrease in frequency and production. In time, heavy continuous grazing will likely cause upland sedges and blue grama and/or Kentucky bluegrass if present to dominate and pioneer perennials and annuals to increase. The resulting plant community is relatively stable and competitive advantage prevents other species from establishing. Extended periods of non-use and/or lack of fire will result in a plant community having high litter levels, which favors an increase in Kentucky bluegrass and/or smooth bromegrass. Shrubs such as western snowberry increase in this situation, especially in areas prone to snow accumulation and drift.
Due to a general invasion of exotic species (such as Kentucky bluegrass and smooth bromegrass) across the MLRA within this site, returning to the 1.1 Western/Wheatgrass/Green Needlegrass Plant Community Phase may not be possible. Today, the 2.1 Western Wheatgrass/Green Needlegrass Plant Community Phases most resembles the 1.1 Reference Plant Community Phase in appearance and function.
Following the state and transition diagram are narratives for each of the described states and community phases. These may not represent every possibility, but they are the most prevalent and repeatable states/community phases. The plant composition tables shown below have been developed from the best available knowledge at the time of this revision. As more data are collected, some of these community phases and/or states may be revised or removed, and new ones may be added. The main purpose for including the descriptions here is to capture the current knowledge and experience at the time of this revision.
State 1
Reference
This state represents the natural range of variability that dominates the dynamics of this ecological site. This state is dominated by cool season grasses. The primary disturbance mechanisms for this site in the reference condition include frequent fire and grazing by large herding ungulates. Timing of fires and grazing coupled with weather events dictate the dynamics that occur within the natural range of variability. Cool season species can decline and a corresponding increase in short warm season grasses will occur.
Community 1.1
Western Wheatgrass/Green Needlegrass
This community phase was the most dominate both temporally and spatially. The prevailing climate and weather patterns favored the development of this community phase dominated by cool season grasses such as green needlegrass and western wheatgrass. There were also other needlegrasses and wheatgrasses present as well as small amounts of warm season grasses such as blue grama, sideoats grama, and possibly big bluestem. A variety of leguminous and non-leguminous perennial forbs were present but only in slight amounts. This is the reference plant community phase and is described in the “Plant Community Composition and Group Annual Production” portion of this ecological site description. This was a naturally nitrogen deficient plant community with a carbon to nitrogen ratio of approximately 40:1.
The potential vegetation was about 85% grasses or grass-like plants, 10% forbs, and 5% shrubs. The plant community was dominated by western wheatgrass and green needlegrass. Other grasses and grass-like plants included needleandthread, blue grama, porcupine grass, bearded wheatgrass and sedges. Significant forbs included American vetch, green sagewort, silverleaf scurfpea and Missouri goldenrod. In many areas western snowberry was the principal shrub and occurred in patchy mosaic. In other areas, silver sagebrush was the dominant shrub and occurred more evenly dispersed across the site. Other shrubs included prairie rose, leadplant, winterfat, and fringed sagewort.
This plant community was well adapted to the Northern Great Plains climatic conditions. Individual species varied greatly in production depending on growing conditions (timing and amount of precipitation and temperature). Community dynamics, nutrient cycle, water cycle and energy flow were functioning properly. Plant litter was properly distributed with very little movement off-site and natural plant mortality was very low. Due to the cool-season dominance of this phase, majority of energy capture was primarily in mid- to late spring and early summer. The diversity in plant species allowed for high drought tolerance. Run-off from adjacent sites and moderate or high available water capacity provided a favorable soil-water-plant relationship.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Grass/Grasslike |
1265 |
2148 |
3025 |
| Forb |
115 |
180 |
250 |
| Shrub/Vine |
20 |
72 |
125 |
| Total |
1400 |
2400 |
3400 |
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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
Western Wheatgrass/Blue Grama/Sedges
This plant community was the result of long-term, heavy, continuous grazing and/or annual, early spring seasonal grazing. Repeated spring grazing would have depleted stored carbohydrates, resulting in weakening and eventual decline of the cool-season mid statured bunchgrasses. Blue grama and western wheatgrass were the dominant species, with the appearance of this phase sometimes shifting from a blue grama/western wheatgrass to a western wheatgrass/blue grama. Other grasses and grass-likes included sedges, needleandthread, prairie junegrass and annual grasses. Forbs such as western ragweed, scurfpea, cudweed sagewort and scarlet globemallow may also have been present. This plant community can occur throughout the pasture, on spot grazed areas, and around water sources where season-long grazing patterns occur.
Due to a decline in the cool-season dominance, energy capture shifted slightly towards early to mid summer. Hydrologic function declined slightly due to reduction in mid statured bunch grasses and increase in blue grama. Bare ground would have increased slightly and litter amounts decreased, resulting in elevated soil surface temperatures. Production was reduced to 85 to 95% of phase 1.1.
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N |
D |
Community 1.3
Western Wheatgrass/Annual Forbs
This plant community phase is characterized by grazing tolerant species and annual forbs. This phase was approximately 30 percent less productive than phase 1.1. Bare ground increased dramatically, litter amounts declined and soil surface temperatures increased. Infiltration rates declined. Less grazing tolerant species were still present but in greatly reduced amounts so this phase still had adequate resilience to return to the 1.1.
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D |
Pathway 1.1a
Community 1.1 to 1.2
This pathway occurred when events favored the decrease of cool season bunch grasses and the increase of cool season rhizomatous grasses and short statured warm season grasses. Such events included late fall or early spring fires followed by severe grazing. This may have been a common occurrence in the natural range of variability. Spring and early summer drought, especially combined with early season burns or grazing could have also initiated this pathway. Continuous early season burning or continuous early season grazing would also favor this pathway.
Pathway 1.2a
Community 1.2 to 1.1
A reduction in grazing and fire frequency would initiate this pathway. Cool season mid statured bunchgrasses would have regained vigor and the plant community would once again resemble the 1.1 plant community phase.
Pathway 1.2b
Community 1.2 to 1.3
Continued heavy grazing, like that associated with a more permanent water site or short term prairie dog occupation, would have continued to shift the plant community from the one described in phase 1.1 to one characterized by very grazing tolerant western wheatgrass and annual forbs.
Pathway 1.3a
Community 1.3 to 1.1
Removal of the excessive grazing disturbance permitted this plant community to recover the mid statured cool season bunchgrass component and shift towards phase 1.1. This pathway may have resulted from the natural movement of prairie dogs, elimination of the prairie dogs (e.g. plague), or a change grazing patterns relative to the proximity to water.
State 2
Native/Invaded
This state is very similar to the reference state. The invasion of introduced cool season sodgrasses has altered the natural range of variability for this ecological site. This state is still dominated by native cool season grasses, but invasive introduced cool season sodgrasses are now present in all community phases of this state. The primary disturbance mechanisms for this state include grazing by domestic livestock and infrequent fires. Timing and duration of grazing coupled with weather events dictate the dynamics that occur within this state. The cool season native grass can decline and an increase in introduced sod grasses will occur. Often this state appears as a mosaic of community phases caused primarily by continuous season-long grazing.
Community 2.1
Western Wheatgrass/ Green Needlegrass
This community phase most closely resembles the Reference State in appearance and ecological functions (e.g., hydrologic, biotic and soil/site stability). The cool-season dominated community is maintained with grazing systems that allow for adequate recovery periods following grazing events, and potentially the combination of grazing and prescribed burning which closely mimics the natural disturbance regime. This community phase closely resembles the Reference State community phase 1.1 (see narrative for 1.1 Western Wheatgrass/Needlegrass). The vegetation is about 85% grasses or grass-like plants, 10% forbs, and 5% shrubs (by weight) with western wheatgrass constituting about 30% of the canopy cover and green needle grass making up about 18% of the canopy cover.
The basic difference between this community phase and 1.1 of the Reference State is the presence of minor amounts of introduced cool-season grasses and forbs. This is likely a naturally nitrogen deficient plant community, but perhaps less so than the Reference State. A change in the nutrient cycle on this ecological site is possibly due to the introduction of non-native species and may be a causative factor leading to the eventual dominance of cool-season introduced grasses in the Invaded State.
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D |
Community 2.2
Western Wheatgrass/Blue Grama/Sedge/Sagewort
This community phase occurs when natural or management actions favor the development of a sodgrass community. It is dominated by western wheatgrass, blue grama, upland sedges and fringed sagewort. Western wheatgrass would make up approximately 25 – 30 percent of the canopy cover, blue grama 20 – 25 percent, needleandthread 15 – 20 percent and threadleaf sedge 5 – 10 percent of the canopy cover.
Both tap rooted and fibrous rooted perennial forbs increase in this phase, but remain a minor component. Nutrient cycling declines due to a lack of deep root grasses, higher soil surface temperatures due to lack of plant cover, and lack of leguminous forbs, Water cycling also declines due to a decrease in the rooting depth of the plant community, increase in percent of bare ground, and increased soil surface temperatures. These changes result in a plant community producing approximately 60 – 80 percent of biomass produced by phase 1.1.
Community 2.3
Western Wheatgrass/Kentucky Bluegrass
This community phase is characterized by an increase in the introduced cool season sodgrass, Kentucky bluegrass. Western wheatgrass and green needlegrass are secondary species. Warm season grasses are present in minor amounts and tap rooted perennial forbs have decreased. Production and infiltration both decrease and this community phase is at risk of transitioning across a state threshold. With natural or management actions that decrease the composition of the cool season bunchgrasses and increase the composition of Kentucky bluegrass, transition T2 will be initiated.
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D |
Pathway 2.1a
Community 2.1 to 2.2
Western Wheatgrass/ Green Needlegrass
Western Wheatgrass/Blue Grama/Sedge/Sagewort
Several combinations of events can occur to initiate this pathway. Continuous, heavy season-long grazing or heavy seasonal grazing will favor the shift to sod forming grasses and sedges. Along this pathway, the timing of energy capture shifts from spring and early summer to early spring and mid summer. The change in plant functional and structural groups and the composition and distribution of the vegetation causes a decrease in production and an increase in runoff with a corresponding decrease in infiltration. Nutrient cycling is restricted as the rooting depth of the vegetation decreases with the change in functional and structural groups. Plant community diversity is reduced with a loss of leguminous forbs and minor grass components.
Pathway 2.1b
Community 2.1 to 2.3
This pathway is initiated with any management action that allows the introduced Kentucky bluegrass to increase. Heavy late season or chronic season long grazing will favor this change. Total rest from grazing and no fire events will also initiate this pathway. The change in plant functional and structural groups and the composition and distribution of the vegetation causes a decrease in production and an increase in runoff with a corresponding decrease in infiltration. Nutrient cycling in restricted as the rooting depth of the vegetation decreases with the change in functional and structural groups. Available nitrogen increases due to invasive legumes (black medic and sweetclover) altering the carbon to nitrogen ratio, favoring non-native, nitrogen dependent species such as Kentucky bluegrass.
Pathway 2.2a
Community 2.2 to 2.1
Western Wheatgrass/Blue Grama/Sedge/Sagewort
Western Wheatgrass/ Green Needlegrass
This community pathway is initiated by implementation of prescribed grazing management which includes adequate recovery periods following each grazing event, and stocking levels which match the available resources. If properly implemented, this will shift the competitive advantage from the introduced cool-season species to the native cool-season bunchgrasses. The addition of prescribed burning may expedite this shift. As this pathway continues, the change in composition and distribution of the plant community will increase infiltration and reduce runoff.
Pathway 2.2b
Community 2.2 to 2.3
Complete rest from grazing and no fire events will initiate this pathway. As plant litter accumulates, the competitive advantage is shifted from the native species to the invasive cool season grasses. Shrubs will also begin to increase within this phase. Infiltration rates will begin to decline as deeper rooted native species are replaced by the shallow rooted non-native cool season grasses.
State 3
Invaded
This state is the result of invasion and dominance of Kentucky bluegrass and/or smooth brome. This state is characterized by these two species and an increasing thatch layer that effectively blocks introduction of other plants into the system. Once the state is well established, single disturbance events such as high intensity fires or severe grazing, will not result in more than a very short term reduction of these two species. These events may reduce the dominance of the sodgrasses, but due to the large amount of rhizomes in the soil there is no opportunity for the native species to establish and dominate before the sodgrasses rebound and again dominate the system.
Community 3.1
Introduced Bluegrass/Smooth Bromegrass/Shrubs
This plant community develops after an extended period of 10 or more years of non-use by herbivores and exclusion of fire. Non-native grasses, such as Kentucky bluegrass, crested wheatgrass, and smooth brome tend to invade and may dominate this plant community. Other grasses present in greatly reduced amounts include western wheatgrass, porcupine grass, green needlegrass, and bearded wheatgrass. The common forbs include sweetclover, green sagewort, cudweed sagewort, and American vetch. Western snowberry is the principal shrub and tends to increase in density and cover.
Litter buildup reduces plant vigor and density, and native seedling recruitment declines. Due to a lack of tiller stimulation and sunlight, native bunchgrasses typically develop dead centers, native rhizomatous grasses are limited to small colonies, and short statured warm season grasses essentially disappear. Production is reduced to 70 – 80 percent of phase 2.1. This plant community phase is very resistant to change. Soil erosion is low.
This plant community can occur in grazed pastures, and is most commonly found in areas most distant from water. This is a typical pattern found in properly stocked pastures grazed season-long.
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State 4
Shortgrass
This state is the result of long term repeated disturbances such as long-term drought coupled with severe defoliation or severe long-term defoliation when site is located next to perennial water sources. Repeated disturbances deplete stored carbohydrates, resulting in weakening and eventual death of the cool season mid-grasses resulting is a shift to community phases dominated by short grasses and forbs.
Community 4.1
Blue Grama/Sedge Plant Community
This plant community is the result of heavy, continuous grazing and/or annual, early spring seasonal grazing. Repeated spring grazing depletes stored carbohydrates, resulting in weakening and eventual death of the cool season mid-grasses. Visually and by weight, blue grama and sedge are the dominant species. By canopy cover, blue grama would constitute 3 – 5% while sedges would be 5 -10%. Some western wheatgrass may still be present in the community but the needlegrasses have been greatly reduced. In the western portion of the MLRA, clubmoss may be subdominant (15 – 20 % cover) or possibly dominate by cover (40 – 60%). Other grasses and grass-likes include western wheatgrass (5 – 10% cover), needleandthread (5 – 10% cover), prairie junegrass and annual grasses. Forbs such as western ragweed, scurfpea, cudweed sagewort and scarlet globemallow may also be present. This plant community can occur throughout the pasture, on spot grazed areas, and around water sources where season-long grazing patterns occur.
This plant community is 40 to 50% less productive than the reference plant community. Reduced litter and plant vigor result in higher soil temperatures, lowered infiltration rates, and high evapotranspiration rates which gives blue grama a competitive advantage over cool season mid-grasses.
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D |
Transition T1
State 1 to 2
This is the transition from the native grass dominated reference state to a state that has been invaded by introduced cool-season grass species. When propagules of Kentucky bluegrass are present, this transition occurs as natural and/or management actions favor a decline in the composition of warm and cool season bunch grasses and an increase in cool-season sodgrasses. This transition is compounded by a change in the historic grazing and fire regime where native herbivores would follow periodic fires with grazing. This historic grazing/fire sequence has largely been replaced by chronic season-long or heavy late season grazing. Complete rest from grazing and suppression of fire can also lead to this transition. The threshold between states is crossed when Kentucky bluegrass, smooth brome, and other introduced species become established on the site. These species typically are part of functional/structural groups that were not present in the Reference State.
Transition T2
State 2 to 3
Complete rest from grazing and elimination of fire are the two major contributors to this transition, especially when smooth brome is present. Preliminary studies indicate this threshold may exist when Kentucky bluegrass exceeds 30% of the plant community and native grasses represent less than 40% of the plant community composition. The opportunity for high intensity spring burns is severely reduced by early green up and increased moisture and humidity at the soil surface and grazing pressure cannot cause a reduction in sodgrass dominance. Production is limited to the sod forming species. Infiltration continues to decrease and runoff increases, energy capture into the system is restricted to early season low producing species. Nutrient cycling is limited by root depth of the dominate species.
Transition T3
State 2 to 4
Heavy, continuous season-long grazing shifts the competitive advantage to the highly grazing tolerant short grass and grass-like species. Reduced litter amounts, elevated surface temps and a reduced infiltration combine to favor drought tolerant, short statured species.
Restoration pathway R1
State 3 to 2
This restoration pathway may be initiated with the combination of prescribed burning followed by high levels of prescribed grazing management. The success of this restoration pathway depends on the presence of a remnant population of native grasses in community phase 3.1. This remnant population may not be readily apparent without close inspection. The application of prescribed burning may be needed at relatively short intervals in the early phases of this restoration process. Some previous efforts have shown promise with early season prescribed burning; however, fall burning may also be effective under certain circumstances. Both prescribed grazing and prescribed burning are necessary to successfully initiate this restoration pathway.
Restoration pathway R2
State 4 to 2
Due to the absence of cool season bunchgrasses within phase 4.1, restoration of this community phase to one resembling that of State 2 would require range seeding possibly combined with some type of mechanical treatment.
