Ecological dynamics
Subirrigated ecological sites developed under Central Great Plains climatic conditions, light to severe grazing by bison and other large herbivores, sporadic natural or man caused wildfires, and other biotic and abiotic factors which typically influence soil/site development. This continues to be a disturbance driven site, by herbivory, fire, and variable climate. Changes occur in the plant communities due to weather variations, impacts of native and/or exotic plant and animal species, and management actions.
One of the primary impacts to this site introduced by European settlers is season long, continuous grazing by domestic livestock. This management practice causes the repeated removal of the growing point and excessive defoliation of the leaf area of individual tall warm-season grasses. The resulting reduction of the ability of the plants to harvest sunlight depletes the root reserves, subsequently decreasing the root mass. This negatively impacts the ability of the plants to compete for life sustaining nutrients, resulting in declining vigor and eventual mortality. The space created in the vegetative community is then occupied by a species that evades the negative grazing impacts by a growing season adaptation such as a cool-season, shorter structure, or reduced palatability mechanism.
The State and Transition Model (STM) is depicted following this section and includes a Reference State (1), a Native/Invaded Grass State (2), a Sod-busted State (3), and an Invaded Woody State (4). Each state represents the crossing of a major ecological threshold due to alteration of the functional dynamic properties of the ecosystem. The main properties observed to determine this change are the soil and vegetative communities and the hydrologic cycle. The STM illustrates the common plant communities that can occur on the site and the transition pathways between communities (Bestelmeyer, 2010). The ecological processes will be discussed in more detail in the plant community descriptions following the diagram.
Each state may have one or more vegetative communities which fluctuate in species composition and abundance within the normal parameters of the state. Within each state, communities may degrade or recover in response to natural and man caused disturbances such as variation in the degree and timing of herbivory, presence or absence of fire, and climatic and local fluctuations in the precipitation regime.
Interpretations are primarily based on the Reference State and have 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 have been interpreted from heavily grazed to lightly grazed areas, seasonal use pastures, and historical accounts. Plant communities, states, transitional pathways, and thresholds have been determined through similar studies and experience.
State 1
Reference State
This state describes the range of vegetative community phases that occur on the Subirrigated ecological site where the natural processes are mostly intact. The Reference Community (1.1) is a representation of the native plant community phase that occupies a site that has been minimally altered by management. The Degraded Native Grass (1.2), the At-Risk (1.3), and the Excessive Litter (1.4) Communities are the phases that result from management decisions that are unfavorable for a healthy Reference Community (1.1).
High perennial grass cover and production allows for increased soil moisture retention, vegetative production, and overall soil quality.
Community 1.1
Reference Community
Figure 8. Subirrigated Ecological Site, Reference Community (1.1) in Eastern Nebraska, MLRA 106.
The Reference or Native Tall Grass Prairie Community (1.1) serves as a description of the native plant community that naturally occurs on the site when the natural disturbance regimes are intact or closely mimicked by management practices. This phase is dynamic, with fluid relative abundance and spatial boundaries between the dominant structural vegetative groups. These fluctuations are primarily driven by different responses of the species to changes in precipitation timing and abundance, and to fire and grazing events.
The potential vegetation consists of approximately 70 to 85 percent grasses and grass-likes, 5 to 15 percent forbs, and 0 to 5 percent shrubs. Big bluestem, Indiangrass, prairie cordgrass, and switchgrass are the primary species in this community. Secondary species include little bluestem, sedges, and Canada wildrye. The site has a diverse forb population.
This plant community is highly productive, diverse, and resistant to short term stresses such as drought and short periods of heavy stocking. The well-developed root systems support resiliency when allowed adequate recovery periods between grazing events.
When exposed to long term or frequent overgrazing events without adequate rest, this plant community will degrade. Grazing during wet periods can cause excessive soil compaction, and lead to hummocking.
The average annual vegetative production of this community ranges from 3,500 lbs. per acre in the northern portion of the MLRA to 7,000 lbs. in the southern portion of the MLRA.
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Community 1.2
Degraded Native Grass Community
Figure 10. Subirrigated Ecological Site, Degraded Native Grass Community (1.2), MLRA 106.
In the Degraded Native Grass Community (1.2), big bluestem, switchgrass, Indiangrass, prairie cordgrass, and other desirable species lose productive capacity through loss of vigor and reproductive potential. Forb diversity is reduced. Subirrigation of the deep-rooted, tall grasses on this site helps to sustain the warm-season, tall grass community. However, mid grasses such as little bluestem, and various sedges will increase to become dominant species. Foxtail barley, Kentucky bluegrass and tall dropseed begin to increase.
This community phase signals a significant loss of production. This is due to continuous season long grazing with inadequate recovery periods. Grazing evasive warm-season and cool-season grasses increase. The composition of the forb component remains diverse, but the potential for encroachment by invasive woody species becomes more likely, due to fewer deep-rooted species and a reduced fuel load to carry fire. While this plant community is less productive and less diverse than the representative plant community, it remains sustainable in regard to soil and site stability, hydrologic function, and biotic integrity.
Community 1.3
At-Risk Community
Figure 11. Subirrigated Ecological Site, At-Risk Community (1.3), MLRA 106.
In the At-Risk Community (1.3), the more palatable warm-season, tall grasses have been reduced to a minor component of the plant community due to continued defoliation during their critical growth periods. Warm-season, mid grasses and cool-season grasses increase significantly. Little bluestem, composite dropseed, and foxtail barley are the dominant grass species. Non-native, cool-season grasses have invaded the plant community and increased to minor abundance.
Soil health is affected by reduced efficiency in the nutrient, mineral, and hydrologic cycles as a result of decreases in plant litter and rooting depths. Total annual vegetative production declines significantly. Without a management change, this community is at-risk to degrade to the Native/Invaded Grass State (2).
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Community 1.4
Excessive Litter Community
The Excessive Litter Community (1.4) develops when the natural disturbances of herbivory and fire are removed from the system for extended periods of time (five years or more). Litter significantly exceeds the amount expected on the site and the species present can tolerate a thatch layer. Individual plants tend to be clumped when there is an excessive amount of litter. Species that cannot tolerate an extensive litter layer have low vigor and reduced productivity.
Once the undisturbed litter layer develops to a certain level, a significant amount of precipitation is held in this layer increasing evaporation, limiting soil available moisture, and simulating drought conditions. If herbivory or fire are not reintroduced, the plant community will experience a significant amount of death loss.
Pathway 1.1A
Community 1.1 to 1.2
Degraded Native Grass Community
A shift from the Reference Community (1.1) to the Degraded Native Grass Community (1.2) occurs with continuous season long grazing, rotational grazing with inadequate growing season recovery periods (deferment), or haying with inadequate recovery time between cuttings.
Pathway 1.1B
Community 1.1 to 1.4
Prolonged interruption (five years or more) of the natural disturbances of herbivory and fire will move the Reference Community (1.1) to the Excessive Litter Community (1.4).
Pathway 1.2A
Community 1.2 to 1.1
Degraded Native Grass Community
The Degraded Native Grass Community (1.2) can be shifted toward the Reference Community (1.1) by implementing prescribed grazing. Applying grazing pressure during the growth period of the undesirable cool-season grasses and grazing deferment during the warm season portion of the growing season favors the desirable warm-season grasses. This grazing strategy will enable the deeply rooted, warm-season, tall grasses to out compete warm-season, mid and short grasses and the cool-season grasses. Appropriately timed prescribed fire will accelerate this process.
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Pathway 1.2B
Community 1.2 to 1.3
Degraded Native Grass Community
Continued grazing with inadequate growing season recovery time (deferment) utilizing either season long, continuous grazing, rotational grazing, or haying further degrades the site and moves the Degraded Native Grass Community (1.2) to the At-Risk Community (1.3).
Pathway 1.2C
Community 1.2 to 1.4
Prolonged interruption (more than 5 years) of the natural disturbances of herbivory and fire will convert the Degraded Native Grass Community (1.2) to the Excessive Litter Community (1.4).
Pathway 1.3A
Community 1.3 to 1.2
Degraded Native Grass Community
Reversing the downward trend and moving the plant community from the At-Risk Community (1.3) to the Degraded Native Grass Community (1.2) can be achieved with prescribed grazing early and late in the growing season to reduce the less productive cool-season grasses. Targeting the peak growth period of cool-season grasses with high intensity grazing events followed by rest will allow the native, warm-season, tall grasses to increase. Appropriately timed prescribed fire will accelerate this process.
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Pathway 1.3B
Community 1.3 to 1.4
Prolonged interruption (more than five years) of the natural disturbances of herbivory and fire will convert the At-Risk Community (1.3) to the Excessive Litter Community (1.4).
Pathway 1.4A
Community 1.4 to 1.1
Reintroduction of the natural processes of herbivory and fire will return the Excessive Litter Community (1.4) to the Reference Community (1.1).
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Pathway 1.4B
Community 1.4 to 1.2
Reintroduction of the natural processes of herbivory and fire will return the Excessive Litter Community (1.4) to the Degraded Native Grass Community (1.2).
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Pathway 1.4C
Community 1.4 to 1.3
Reintroduction of the natural processes of herbivory and fire will return the Excessive Litter Community (1.4) to the At-Risk Community (1.3).
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State 2
Native/Invaded Grass State
The Native/Invaded Grass State (2) occurs when an ecological threshold has been crossed and the majority of the native, warm-season tall and mid grasses of the Reference State (1) have been replaced by less desirable native grasses and non-native, invasive grasses. The plant community consists of the more grazing tolerant native species and a significant component of introduced, cool-season grasses such as reed canary grass, smooth brome, tall fescue, and Kentucky bluegrass. Forb diversity is limited to species with low palatability. Plant diversity is low. The Native/Invaded Grass State includes the Native/Invaded Grass Community (2.1) and the Invaded Cool-Season Grass Community (2.2).
The loss of warm-season, tall and mid grasses has negatively impacted energy flow and nutrient cycling. Water infiltration is reduced due to the shallow root system and rapid runoff characteristics of the grazing tolerant plant communities. Impaired energy capture and altered hydrologic function are reflected in reduced vegetative productivity, shallower rooting depth and degraded soil quality. Continuous and heavy grazing pressure will maintain this plant community in a sod bound condition.
Grazing management practices that allow for adequate periods of recovery between grazing events and grazing timed early and late during the growing season will reduce the vigor of the non-native, cool-season grasses and may allow the warm-season, tall grasses to increase in abundance and productivity. Return to the Reference State (1) is unlikely due to the loss of plant diversity, overall soil disturbance, and alterations to hydrologic function.
Community 2.1
Native/Invaded Grass Community
The Native/Invaded Grass Community (2.1) occurs when an ecological threshold is crossed, and the plant community moves from the Reference State (1) to the Native/Invaded Grass State (2). Native, cool-season, mid and short grasses are codominant with non-native, invasive, cool-season grasses. The invasive grasses present include reed canary grass, Kentucky bluegrass, and smooth brome in the northern portion of the MLRA or tall fescue in the southern portion of the MLRA. Only trace remnants of the more palatable warm-season, tall grasses are present.
Continuous, heavy grazing pressure will maintain this plant community in a sod bound condition. Forb richness and diversity have decreased. With the decline and loss of deeper penetrating root systems, a compacted layer may form in the soil profile below the shallower replacement root systems. Grazing management practices that allow for adequate periods of growing season recovery (deferment) between grazing events will favor warm-season, tall and mid grasses. Appropriately timed prescribed fire will accelerate this process.
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Community 2.2
Invaded Cool-Season Grass Community
Figure 14. Subirrigated ecological site, Invaded Cool-Season Grass Community (2.2), MLRA 106.
The Invaded Cool-Season Grass Community (2.2) consists of a monoculture or near monoculture of a non-native, cool-season grass. Typically, reed canary grass will dominate the site but, in some cases, the dominant grass will be smooth brome or tall fescue. Native, warm-season, tall grasses or native, cool-season grasses may be present in trace amounts. Production of non-native cool-season dominated plant communities is highly variable, depending upon the percentages of species present and outside inputs such as fertilizer and weed control.
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Pathway 2.1A
Community 2.1 to 2.2
The Native/Invaded Grass Community (2.1) will be converted to an Invaded Cool-Season Grass Community (2.2) through the following practices: introduced grass seeding, excessive warm season grazing, inadequate warm season rest, multiple hay cuttings, and nitrogen fertilization in spring or fall.
Pathway 2.2B
Community 2.2 to 2.1
The Invaded Cool-Season Grass Community (2.2) can be moved to the Native/Invaded Grass Community (2.1) by increasing the native, warm-season, tall grasses through herbicide treatment and reseeding. If adequate native remnants are present, appropriately timed prescribed fire and a follow up prescribed grazing program may achieve the desired results.
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State 3
Sodbusted State
The threshold to the Sodbusted State (3) is crossed as a result of mechanical disturbance to facilitate production agriculture. Extensive areas of this ecological site were plowed and converted to crop production by early European settlers and their subsequent generations. In addition to permanently altering the existing vegetative community, repeated tillage negatively impacted soil properties. Reductions in organic matter, mineral levels, soil structure, oxygen levels, and water-holding capacity along with increased runoff and erosion as well as shifts in the populations of soil-dwelling organisms were common on these sites. The extent of these changes depended upon the duration of cropping as well as crops grown and other management practices.
If farming operations are suspended, the site can be abandoned or seeded to permanent vegetation. Seedings are either a tame pasture forage mixture, the Seeded Pasture Community (3.2), or a mixture of native grasses and forbs, the Reseeded Native Grass Community (3.1). Abandonment results in the Natural Reclamation Community (3.3). Permanent alterations of the soil, plant community, and the hydrologic cycle make restoration to the Reference State (1) extremely difficult, if not impossible.
Community 3.1
Reseeded Native Grass Community
The Reseeded Native Grass Community (3.1) does not contain native remnants, and varies considerably depending upon the seed mixture, the degree of soil erosion, the age of the stand, fertility management, and past grazing management. Prescribed grazing with adequate recovery periods will be required to maintain productivity and desirable species.
Native range and grasslands seeded to native species are ecologically different and should be managed separately. Factors such as functional group, species, stand density, and improved varieties all impact the production level and palatability of the seedings. Species diversity is often limited, and when grazed in conjunction with native rangelands, uneven forage utilization may occur.
Total annual production during an average year varies significantly depending upon precipitation, management, grass species seeded, and the extent to which soils and hydrologic function have been impacted by previous farming operations.
Community 3.2
Seeded Pasture Community
The Seeded Pasture Community (3.2) does not contain native remnants and varies considerably depending upon the extent of soil erosion, the species seeded, the quality of the stand that was established, the age of the stand, and management of the stand since establishment.
Several factors make seeded tame pasture a different grazing resource than native rangeland and land seeded to a native grass mixture. Factors such as species selected, stand density, improved varieties, and harvest efficiency all impact production levels and palatability. Species diversity on seeded tame pasture is often limited to a few species. When seeded pasture and native rangelands or seeded pasture and seeded rangeland are in the same grazing unit, uneven forage utilization will occur. Improve forage utilization and stand longevity by managing this community separately from native rangelands or land seeded to native grass species.
Total annual production during an average year varies significantly depending upon precipitation, management, grass species seeded, and the extent to which soils and hydrologic function have been impacted by the previous farming operations. Improved varieties of warm-season or cool-season grasses are recommended for optimum forage production.
Community 3.3
Natural Reclamation Community
Figure 16. Subirrigated Ecological Site, Natural Reclamation Community (3.3), MLRA 106.
The Natural Reclamation Community (3.3) consists of annual and perennial weeds and less desirable grasses. These sites have been farmed and abandoned without being reseeded. Soil organic matter and carbon reserves are reduced, soil structure is changed, and a plow pan or compacted layer can form, which decreases water infiltration. Residual synthetic chemicals may remain from farming operations. In early successional stages, this community is not stable. The hazard of erosion is a concern.
Total annual production during an average year varies significantly depending on the succession stage of the plant community and any management applied to the system.
State 4
Invaded Woody State
Once the woody canopy cover reaches 15 percent, the threshold is crossed to the Invaded Woody State (4). Woody species are encroaching due to heavy grazing with the lack of prescribed fire or other brush management practices. Typical ecological impacts are a loss of native warm-season grasses, degraded forage productivity, and reduced soil quality. This state consists of the Invaded Woody Community (4.1).
Community 4.1
Invaded Woody Community
Figure 17. Subirrigated ecological site, Invaded Woody Community (4.1), eastern Nebraska, MLRA 106.
The Invaded Woody Community (4.1) has at least 15 percent of woody cover canopy. Locust, willow, ash, dogwood, and cottonwood are some of the deciduous invaders. Eastern red cedar is often present in the understory. In the absence of fire and brush management, this ecological site is very susceptible to eastern red cedar seedling invasion, especially when adjacent to a seed source. Eastern red cedar can eventually dominate the site resulting in a closed canopy monoculture which drastically reduces forage production, and which has limited value for either livestock grazing or wildlife habitat. Due to fire suppression over many years, this plant community will develop extensive ladder fuels which can lead to a removal of most tree species with a wildfire. With properly managed intensive grazing, encroachment of deciduous trees will be minimal; however, this will not impact encroachment of conifer species. The herbaceous component decreases proportionately in relation to the percent canopy cover, with the reduction being greater under a conifer overstory. Eastern red cedar control can usually be accomplished with prescribed burning while the trees are six feet tall or less and fine fuel production is greater than 1,500 pounds per acres. Larger red cedars can also be controlled with prescribed burning, but successful application requires the use of specifically designed ignition and holding techniques (https://www.loesscanyonsburngroup.com).
Resprouting brush must be chemically treated immediately after mechanical removal to achieve effective treatment. The forb component will initially increase following tree removal. To prevent return to a woody dominated community, ongoing brush management such as hand cutting, chemical spot treatments, or periodic prescribed burning is required. This plant community is resistant to change and resilient given normal disturbances. In higher canopy cover situations, the soil erosion will increase in relation to most of the plant communities from which this plant community originated. The water cycle is also significantly altered under higher canopy cover. Infiltration is reduced and runoff is typically increased because of a lack of herbaceous cover and the rooting structure provided by the herbaceous species. Total annual production during an average year varies significantly, depending on the production level prior to encroachment and the percentage of canopy cover.
Transition 1A
State 1 to 2
Heavy grazing or haying with inadequate, growing season recovery periods will cause the Reference State (1) to lose a significant proportion of warm-season, tall and mid grass species and cross a threshold to the Native/Invaded State (2). Water infiltration and other hydrologic functions will be reduced due to the root matting presence of sod forming grasses. With the decline and loss of deeper penetrating root systems, soil structure and biological integrity are catastrophically degraded to the point that recovery is unlikely. Once this occurs, it is highly unlikely that grazing management alone will return the community to the Reference State (1).
Transition 1B
State 1 to 3
The Reference State (1) is significantly altered by mechanical tillage to facilitate production agriculture shifting site to the Sod-busted State (3). The disruption to the plant community, the soil, and the hydrology of the system make restoration to a true reference state unlikely.
Transition 1C
State 1 to 4
Disruption of the natural fire regime and the encroachment of invasive exotic and native woody species can cause the Reference State (1) to shift to the Invaded Woody State (4).
Transition 2A
State 2 to 3
The Native/Invaded Grass State (2) is significantly altered by mechanical tillage to facilitate production agriculture shifting site to the Sod-busted State (3). The disruption to the plant community, the soil, and the hydrology of the system make restoration unlikely.
Transition 2B
State 2 to 4
Disruption of the natural fire regime and encroachment of invasive exotic and native woody species can cause the Native/Invaded Grass State (2) to shift to the Invaded Woody State (4).
Transition 3A
State 3 to 4
Disruption of the natural fire regime and encroachment of invasive exotic and native woody species can cause the Sodbusted State (3) to shift to the Invaded Woody State (4).
Restoration pathway 4A
State 4 to 1
Prescribed burning, wildfire, timber harvest, and brush management will move the Invaded Woody State (4) toward the Reference State (1). The forb component of a site with heavy tree density or canopy cover will initially increase following tree removal through mechanical brush management treatments and prescribed fire. If resprouting brush such as honey locust or Siberian elm is present, stumps must be chemically treated immediately after mechanical removal. Ongoing brush management such as hand cutting, chemical spot treatments, or periodic prescribed burning is required to prevent a return to this state.
If the Invaded Woody State transitioned from the Native/Invaded Grass State (2) or the Sod-busted State (3), the land cannot be restored back to the Reference State (1) as the native plant community, soils, and hydrologic function have been too severely impacted prior to the woody encroachment to allow restoration back to the Reference State (1).
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Restoration pathway 4B
State 4 to 2
Prescribed burning, wildfire, timber harvest, and brush management will move the Invaded Woody State (4) toward the Native/Invaded Grass State (2). The forb component of a site with heavy tree density or canopy cover will initially increase following tree removal through mechanical brush management treatments and prescribed fire. If resprouting brush such as honey locust or Siberian elm is present, stumps must be chemically treated immediately after mechanical removal. Ongoing brush management such as hand cutting, chemical spot treatments, or periodic prescribed burning is required to prevent a return to this state.
If the Invaded Woody State (4) transitioned from the Native/Invaded Grass State (2) or the Sod-busted State (3), the land cannot be restored back to the Reference State (1) as the native plant community, soils, and hydrologic function have been too severely impacted prior to the woody encroachment to allow restoration back to the Reference State (1).
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Restoration pathway 4C
State 4 to 3
Prescribed burning, wildfire, timber harvest, and brush management will move the Invaded Woody State (4) toward the Sod-busted State (3). The forb component of a site with heavy tree density or canopy cover will initially increase following tree removal through mechanical brush management treatments and prescribed fire. If resprouting brush such as honey locust or Siberian elm is present, stumps must be chemically treated immediately after mechanical removal. Ongoing brush management such as hand cutting, chemical spot treatments, or periodic prescribed burning is required to prevent a return to this state.
If the Invaded Woody State (4) transitioned from the Native/Invaded Grass State (2) or the Sod-busted State (3), the land cannot be restored back to the Reference State (1) as the native plant community, soils, and hydrologic function have been too severely impacted prior to the woody encroachment to allow restoration back to the Reference State (1).
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