Ecological dynamics
The site developed under Northern Great Plains climatic conditions, and included natural influence of large herding herbivores and sporadic 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 this site has the potential to resemble the Reference State. Interpretations for this site are based on the Little Bluestem/Big Bluestem/Needlegrass Plant Community Phase (1.1). The natural disturbance regime consisted of sporadic 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. 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. Continuous grazing without adequate recovery periods causes this site to depart from the Little Bluestem/Big Bluestem/Needlegrass Plant Community Phase (2.1). Initially little bluestem will increase. Kentucky bluegrass will increase in frequency and density. Kentucky bluegrass may eventually form into a dense sod under heavy continuous grazing. Grasses such as little bluestem, big bluestem, switchgrass and Indiangrass will decrease in frequency and production and can eventually be removed from the site. A lack of disturbance (i.e, non- use and fire suppression) will cause litter levels and plant decadence/mortality to increase resulting in an increase of Kentucky bluegrass and smooth bromegrass.
The following diagram illustrates the common states, community phases, community pathways, transitions and restoration pathways that can occur on the site. 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. These are the most common plant community phases and states based on current knowledge and experience, and changes may be made as more data is collected. Narratives following the diagram contain more detail pertaining to the ecological processes.
State 1
Reference
This state represents the natural range of variability that dominated the dynamics of this ecological site. This state was diverse, stable, productive and well adapted to the Northern Great Plains. The high water table supplied much of the moisture for plant growth. Plant litter was properly distributed with little movement and natural plant mortality was very low. This was a sustainable state in terms of soil stability, watershed function and biologic integrity. This state was dominated by warm-season grasses, with lesser amounts of cool-season grasses. The primary disturbance mechanisms for this site in the reference condition included sporadic fire and grazing by large herding ungulates. Timing of fires and grazing coupled with weather events dictated the dynamics that occurred within the natural range of variability. Mid and tall statured grass species could have declined with a corresponding increase in short statured warm-season grasses and cool-season grass-like species occurring.
Community 1.1
Little Bluestem/Big Bluestem/Needlegrass
This community phase was the most dominant both temporally and spatially. The prevailing climate and weather patterns favored the development of this community phase dominated by tall and mid warm-season and mid cool-season grasses such as little bluestem, big bluestem, and porcupine grass and/or green needlegrass. Other grass and grass-like species included switchgrass, Indiangrass, sideoats grama, western wheatgrass, slender wheatgrass and sedge. A variety of native perennial forbs were present but only in slight amounts. Interpretations are based primarily on this plant community phase. It is further described in the “Plant Community Composition and Group Annual Production” portion of this ecological site description. This was a naturally nitrogen deficient plant community.
Table 5. Annual production by plant type
| Plant type |
Low
(lb/acre) |
Representative value
(lb/acre) |
High
(lb/acre) |
| Grass/Grasslike |
2805 |
3600 |
4870 |
| Forb |
195 |
300 |
425 |
| Shrub/Vine |
0 |
100 |
205 |
| Total |
3000 |
4000 |
5500 |
| Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
| J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
State 2
Native/Invaded
This state is very similar to the reference state. The invasion of introduced species has altered the natural range of variability for this ecological site. This state still has a strong component of warm-season grass species, 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. Grazing coupled with weather events dictate the dynamics that occur within this state. Fire could still play an important role, but is typically not utilized or is suppressed. The warm-season native grasses can decline and an increase in introduced sod grasses will occur. Many times, this state appears as a mosaic of community phases caused primarily by continuous season-long grazing.
Community 2.1
Little Bluestem/Big Bluestem/Needlegrass
This community phase most closely resembles the Reference State in appearance and ecological functions (e.g., hydrologic, biotic and soil/site stability). The warm-season grass 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 is dominated by tall and mid warm-season grasses, and mid cool-season grasses such as little bluestem, big bluestem, and porcupine grass and/or green needlegrass. Other grass and grass-like species include switchgrass, Indiangrass, sideoats grama, western wheatgrass, slender wheatgrass and sedge. A variety of native perennial forbs are present but only in slight amounts. 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 and biological activity on this ecological site possibly due to the introduction of non-native species may be a causative factor leading to the eventual dominance of cool-season introduced grasses in the Invaded State (State 3).
| 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 2.2
Little Bluestem/Kentucky Bluegrass
Grazing pressure reduces the tall, less grazing tolerant species, while the shorter more grazing tolerant species increase. Litter amounts are reduced, and energy capture shifts to slightly earlier in the growing season due to a decline in the warm-season grass component. Non-native grasses, such as Kentucky bluegrass and smooth bromegrass tend to increase and may begin to dominate this community phase. In the early stages of this community phase, little bluestem will initially increase along with the increase of introduced cool-season grasses. In many situations with inadequate recovery periods, the little bluestem will also begin to decline over time, facilitating the change to the Invaded State. Significant grass and grass-like species include little bluestem, green needlegrass, western wheatgrass, slender wheatgrass, Kentucky bluegrass and sedge. Other grasses present include big bluestem, switchgrass, porcupine grass, and sideoats grama. Indiangrass and northern reedgrass are largely absent. The common forbs include American licorice, cudweed sagewort, goldenrod, silverleaf scurfpea, and western yarrow. Western snowberry and prairie rose are the principal shrubs.
This community phase is often dispersed throughout the pasture, in an overgrazed/undergrazed pattern, typically referred to as patch grazing. Some areas (overgrazed) will exhibit the impacts of heavy use, while other areas (undergrazed) will have a build-up of litter and a high amount of plant decadence. This is a typical pattern found in properly stocked pastures grazed season-long. In the undergrazed patches, 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 and native rhizomatous grasses are limited to small colonies. In the overgrazed patches, plant vigor is reduced and the competitive advantage goes towards the grazing tolerant short statured species such as Kentucky bluegrass.
This community phase is approaching the threshold which would readily lead to the Invaded State. If management is significantly altered, this community phase can still be reverted back to the Little Bluestem/Big Bluestem/Needlegrass community. Grazing management that allows for adequate recovery periods will tend to restore the ecological functions of this site. Fire can play a role in reducing the introduced cool-season species. The combination of grazing and fire may be the most effective method to move this community phase towards a community resembling the interpretive plant community. Soil erosion is low. Infiltration is reduced, while runoff is increased compared to the interpretive plant community.
| Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
| J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
Pathway 2.1a
Community 2.1 to 2.2
This community pathway is triggered by a change in the natural disturbance regime, most often caused by continuous grazing without adequate recovery periods. Chronic heavy grazing for extended periods during the growing season will also favor this shift. Included with areas affected by a lack of adequate recovery periods may be areas that receive little or no grazing, which may also lead to the increase of introduced cool-season species. Along this pathway, the timing of energy capture shifts from early to mid summer to spring and early 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 native forb diversity and minor grass components.
Pathway 2.2a
Community 2.2 to 2.1
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 tall and mid warm-season grass species. The addition of prescribed burning may expedite this shift.
State 3
Introduced Species
This state is the result of invasion and dominance of introduced species. This state is characterized by the dominance of Kentucky bluegrass and an increasing thatch layer that effectively blocks introduction of other plants into the system. Once the state is well established, even drastic events such as high intensity fires driven by high fuel loads of litter and thatch will not result in more than a very short term reduction of Kentucky bluegrass. These events may reduce the dominance of Kentucky bluegrass, but due to the large amount of rhizomes in the soil there is no opportunity for the native species to establish and dominate before Kentucky bluegrass rebounds and again dominates the system. This State also includes the Annual, Pioneer Perennial community phase which is highly variable depending on the disturbance which causes this transition (T4). Over time, the Annual, Pioneer Perennial community phase will likely become dominated by introduced cool-season grasses, and shift to the Kentucky Bluegrass Sod, Forbs Plant Community Phase (3.2).
Community 3.1
Kentucky Bluegrass/Decadent Bluestem/Shrubs
This community phase is dominated by Kentucky bluegrass. Big bluestem and other native warm- and cool-season native grass species are still present, but much reduced in vigor and production. Western snowberry can increase and become a major component in this community phase. Common forbs include American licorice, cudweed sagewort, western yarrow, and silverleaf scurfpea. Infiltration is reduced and runoff is increased when compared to the Reference State. Soil erosion is low. Much of the plant nutrients are tied up in the excessive litter. Organic matter oxidizes in the air rather than being incorporated into the soil due to the absence of animal impact and reduced soil biological activity. Typically, bunchgrasses (little bluestem) develop dead centers and rhizomatous grasses form small colonies because of a lack of tiller stimulation. Nutrient cycling is limited by the rooting depth of the Kentucky bluegrass and the alteration of the soil biotic community. Energy capture into the system is restricted to a short window provided by the early season species and the high amount of dead, standing plant material.
This plant community is somewhat resistant to change without a combination of prescribed grazing and prescribed burning. The combination of both grazing and fire is most effective in moving this plant community towards the Native/Invaded State. Once this plant community is reached, time and external resources will be needed to see any immediate recovery in diversity.
| 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 3.2
Kentucky Bluegrass Sod, Forbs
This community phase is dominated by Kentucky bluegrass with lesser amounts of sedge. Some native and non-native forbs can increase in production and cover as well. The dominant grass is Kentucky bluegrass, with common forbs including cudweed sagewort, goldenrod, aster, western ragweed, western yarrow, and a variety of introduced forbs. The longer this community phase exists the more resistant it becomes. Natural or management disturbances that reduce the cover of Kentucky bluegrass are very short lived due to the abundance of rhizomes of Kentucky bluegrass in the soil and the lack of propagules of other species. Production is limited to the sod forming species. Energy capture into this system is limited to one early growing species. Runoff increases and is the highest of any plant community phase on this ecological site. Nutrient cycling is severely limited to the rooting depth of the Kentucky bluegrass and production is limited.
Community 3.3
Annual/Pioneer/Non-Native Perennial
This community phase is highly variable depending on the level and duration of disturbance related to the T4 transitional pathway. In this MLRA, the most probable origin of this phase is plant succession following cropland abandonment. This plant community will initially include a variety of annual forbs and grasses. Over time, the introduced cool-season perennial grasses will begin to establish on this site.
Pathway 3.1a
Community 3.1 to 3.2
This pathway is initiated by heavy continuous season-long grazing. The heavy continuous grazing favors those plants which can tolerate repeated defoliation (Kentucky bluegrass and sedges). Western snowberry will experience mechanical damage and will decrease in production and cover. Grazing pressure reduces litter cover resulting in elevated soil surface temperatures increasing evaporation rates and further reduction of biological activity.
Pathway 3.3a
Community 3.3 to 3.2
With grazing and time, the grazing tolerant Kentucky bluegrass will continue to increase leading to community phase 3.2. In the absence of grazing, this pathway will lead to a community phase resembling 3.1 with the primary difference being the lack of western snowberry and remnant native grass species.
State 4
Cropland
This state is the result of annual cropping
Transition T1a
State 1 to 2
This was the transition from the native warm-season grass dominated reference state to a state that has been invaded by introduced species. When propagules of Kentucky bluegrass are present, this transition occurs as natural and/or management actions favored a decline in the composition of warm-season rhizomatous grasses and an increase in cool-season sodgrasses. This transition was 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 hasten this transition. The threshold between states was crossed when Kentucky bluegrass, smooth bromegrass, and other introduced species became established on the site. These species occupy functional/structural groups that were not present in the Reference State.
Transition T2a/b
State 2 to 3
Constraints to recovery. T2a - Complete rest from grazing or extended periods of very light grazing and elimination of fire are the two major contributors to this transition. The opportunity for high intensity spring burns is severely reduced by early green up, and increased moisture and humidity at the soil surface. Plant litter accumulation tends to favor the more shade tolerant introduced grass species. The nutrient cycle is also impaired, and the result is typically a higher level of nitrogen which also favors the introduced species. Increasing plant litter decreases the amount of sunlight reaching plant crowns thereby shifting competitive advantage to shade tolerant introduced grass species. Studies indicate that soil biological activity is altered, and this shift apparently exploits the soil microclimate and encourages growth of the introduced grass species. Once the threshold is crossed, a change in grazing management alone cannot cause a reduction in sodgrass dominance. Preliminary studies would tend to 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.
T2b- Heavy continuous season-long grazing is the primary driver of this transition. The very grazing tolerant species have the competitive advantage during this transition. The opportunity for high intensity spring burns (which can serve to reduce the introduced cool-season species) is severely reduced by early green up and the lack of fuel. The nutrient cycle is impaired due to the lack of available carbon for soil biota due to accumulation in the surface layer root mat. This results in reduced soil biological activity. Studies indicate that soil biological activity is altered, and this shift apparently exploits the soil microclimate and encourages growth of the introduced grass species. Once the threshold is crossed, a change in grazing management alone cannot cause a reduction in sodgrass dominance. Preliminary studies would tend to 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.
Restoration pathway R3a/b
State 3 to 2
R3a - 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. However, the initial application of prescribed fire can have detrimental effects on remnant native bunchgrass crowns. Damage may be reduced by adjusting prescription parameters. 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. If successful, the resultant plant community may have a relatively higher amount of warm-season grasses than the interpretive plant community.
R3b- It may be possible using selected plant materials and agronomic practices to approach something very near the functioning of the Native/Invaded State (State 2). Application of chemical herbicides and the use of mechanical seeding methods using adapted varieties of the dominant native grasses are possible and can be successful. After establishment of the native grasses, management objectives must include the maintenance of those species, the associated reference state functions and continued treatment of the introduced sodgrasses.
Transition T4a
State 4 to 3
This transition occurs with cessation of cropping practices being applied.
