Ecological dynamics
The Limy Upland ecological site 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 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
The Reference State (1) describes the range of vegetative community phases that occur on the Limy Upland ecological site where the natural processes are mostly intact. The Reference State (1) includes the Reference Community (1.1), the Degraded Native Grass Community (1.2), the At-Risk Community (1.3), and the Excessive Litter Community (1.4). 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 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 10. Limy Upland ecological site, Reference Community (1.1), in eastern Nebraska, MLRA 106.
The Reference Community or Native Mixedgrass Prairie Community (1.1) is dominated by native, warm-season tall and mid grass prairie species. This community 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 to15 percent forbs, and 0 to 5 percent shrubs. Big bluestem, little bluestem, and sideoats grama are the primary species in this community. Secondary species include switchgrass, Indiangrass, and blue grama. The site has a diverse forb population.
This plant community is less productive than similar upland sites and species diversity is limited. This is a resilient community and resistant to short term stresses such as drought and short periods of heavy stocking. The well-developed root systems support this resiliency when allowed adequate recovery periods between grazing events. When exposed to long term or frequent over grazing events without adequate rest, this plant community will degrade.
The annual vegetative production of this community averages about 3,000 lbs. per acre.
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Community 1.2
Degraded Native Grass Community
In the Degraded Native Grass Community (1.2), warm-season tall grasses such as big bluestem, switchgrass, and Indiangrass lose productive capacity through loss of vigor and reproductive potential. While the deep root systems of the tall grasses on this site help to sustain that component of the plant community, as growing season defoliation continues the tall grasses become subdominant. The more grazing evasive species such as sideoats grama and little bluestem increase and become dominant. Blue grama, Kentucky bluegrass and tall dropseed begin to increase. Forb diversity is reduced.
This community phase signals a significant loss of production. The change is often due to continuous season long grazing with inadequate recovery periods during the growing season. Warm-season short grasses and cool-season grasses increase. The composition of the forb component favors less palatable species and the potential for encroachment by invasive woody species becomes more likely. Fewer high biomass producing deep rooted species result in a reduced fire friendly fuel load.
While this plant community is less productive and less diverse than the representative plant community, the site/soil stability, hydrologic function, and biotic integrity remain sustainable.
Community 1.3
At-Risk Community
Figure 12. At-Risk Mid and Tallgrass Prairie- Eastern Nebraska
In At-Risk Community (1.3), the more palatable warm-season tall grasses have been reduced to a minor component by continued defoliation during their critical growth periods. Warm-season short grasses and cool-season grasses increase significantly. Sideoats grama, composite dropseed, and warm-season short grasses have become dominant. Nonnative, cool-season grasses such as smooth brome, tall fescue, and Kentucky bluegrass may be present as a minor component. Cheatgrass or other nonnative annual grasses may also be present in trace amounts.
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. This may result in formation of a compacted layer in the soil, and total annual vegetative production declines significantly. Without a management change, this community is at risk to transition 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 and 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
A shift from the Reference Community (1.1) to the Degraded Native Grass Community (1.2) occurs with continuous season long grazing or rotational grazing with inadequate recovery periods during the growing season. Repeated haying with inadequate recovery time can also cause this shift.
Pathway 1.1B
Community 1.1 to 1.4
Prolonged interruption (greater than 5 years) of the natural disturbances of herbivory and fire will convert the Reference Community (1.1) to the Excessive Litter Community (1.4).
Pathway 1.2A
Community 1.2 to 1.1
A shift from the Degraded Native Grass Community (1.2) toward the Reference Community (1.1) can be achieved through prescribed grazing. Applying grazing pressure during the rapid growth period of the undesirable cool-season grasses and allowing rest during the warm-season portion of the growing season favors the warm-season tall grass species. This grazing strategy will enable the deeply rooted warm-season tall grasses to outcompete the shallow rooted, 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
Continued season long grazing or rotational grazing with inadequate recovery periods during the growing season further degrades the site from the Degraded Native Grass Community (1.2) to the At-Risk Community (1.3). Repeated haying with inadequate recovery periods during the growing season can also cause this shift.
Pathway 1.2C
Community 1.2 to 1.4
Prolonged interruption (greater 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
Reversing the downward trend which converted the Degraded Native Grass Community (1.2) to the At-Risk Community (1.3) can be achieved with prescribed grazing early and late in the growing season to reduce cool-season grasses and increase the warm-season, tall and mid 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 rejuvenate. Appropriately timed prescribed fire will accelerate this process.
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Pathway 1.3B
Community 1.3 to 1.4
Prolonged interruption (greater than 5 years) of the natural disturbances of herbivory and fire 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 allow the vegetation to return to the Reference Community (1.1) from the Excessive Litter Community (1.4).
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Pathway 1.4B
Community 1.4 to 1.2
Reintroduction of the natural processes of herbivory and fire will allow the vegetation to return to the Degraded Native Grass Community (1.2) from the Excessive Litter Community (1.4).
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Pathway 1.4C
Community 1.4 to 1.3
Reintroduction of the natural processes of herbivory and fire will allow the vegetation to return to the At-Risk Community,
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State 2
Native/Invaded Grass State
The Native/Invaded Grass State (2) has been degraded from the Reference State (1) and much of the native warm-season, tall and mid grass community has been replaced by warm-season short grasses and cool-season grasses. 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 evasive plant communities. The Shortgrass Sod/Invaded Grass (2.1) and the Non-Native Cool-Season Grass (2.2) Communities are the components of the Native/Invaded Grass State (2).
Community 2.1
Shortgrass Sod/Invaded Grass Community
The Shortgrass Sod/Invaded Grass Community (2.1) represents a shift from the Reference State (1) across a plant community threshold to the Native/Invaded Grass State (2). With continued grazing pressure, the dominant plant species will include warm-season short grasses, such as blue grama, and nonnative cool-season grasses, such as Kentucky bluegrass and smooth bromegrass in the north, and tall fescue in the south. The more palatable warm-season grasses, such as little bluestem and sideoats grama, will be present only as remnants. The more grazing tolerant warm-season short grasses become dominant. Cheatgrass may become a significant component of the community on some sites. Continuous and heavy grazing pressure will maintain this plant community in a sod bound condition. Forb richness and diversity has 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 between grazing events will favor warm-season, tall and mid grasses. Appropriately timed prescribed fire will accelerate the process.
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Community 2.2
Invaded Cool-Season Grass Community
Figure 15. Limy Upland ecological site, Invaded Cool-Season Community- Eastern Nebraska
The Invaded Cool-Season Grass Community (2.2) is dominated by smooth bromegrass in the northern portion of the MLRA and by tall fescue in the southern portion. Kentucky bluegrass may also be dominant. Some warm-season remnants may be present. Annual grasses, such as cheatgrass, often have a significant presence. Production of nonnative cool season dominated plant communities is highly variable, depending upon the percentages of composition present and outside inputs such as fertilizer and weed control.
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Pathway 2.1A
Community 2.1 to 2.2
The Shortgrass Sod/Invaded Grass Community (2.1) will be converted to an Non-Native Cool-Season Grass Community (2.2) through the following practices: introduced grass seeding, excessive warm season grazing, inadequate growing season recovery periods, multi season haying, and nitrogen fertilizing in spring and/or fall.
Pathway 2.2A
Community 2.2 to 2.1
The Non-Native Cool-Season Grass Community (2.1) can return to the Shortgrass Sod/Invaded Grass Community (2.1) with 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
Sod-busted State
The threshold to the Sod-busted 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.
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
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
Figure 17. Limy Upland ecological site, Invaded Woody State (4), eastern Nebraska, MLRA 106.
The Invaded Woody State (4) is the result of woody encroachment. Once the tree canopy cover reaches 15 percent with an average tree height exceeding five feet, the threshold is crossed. Woody species are encroaching due to lack of prescribed fire and other brush management practices. Bur oak, eastern red cedar, coralberry, Osage orange, roughleaf dogwood, and honey locust are some of the first species to increase. Although these plants are native to North America, they were not historically a significant part of this reference state. If allowed to continue with little or no disturbance, eastern red cedar, oaks and elm trees will eventually dominate. If eastern red cedar is present, the transition to eastern red cedar forest is thought to occur when canopy cover reaches approximately 30 percent. As the overstory closes, various processes serve to increase woody dominance. Woody foliage shades the understory and intercepts rainfall increasing evaporative loss. Litterfall acts similarly and further reduces effective precipitation while also creating a less uniform resource distribution with nutrients concentrated under individual trees. At some critical point, the understory becomes incapable of carrying a fire of the intensity needed to kill the woody species, and the disturbance response is now dictated by the overstory. Mature oak forests are nearly invulnerable to stand replacement fire, while established eastern red cedar stands can only burn as crown fires. Typical ecological impacts are a loss of native grasses, degraded forage productivity, and reduced soil quality. This state consists of the Invaded Woody Community (4.1)
Prescribed burning, wildfire, timber harvest and brush management will move this state toward a grass dominated 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 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 to the Reference State (1).
Community 4.1
Invaded Woody Community
The Invaded Woody Community (4.1) has at least 15 percent canopy cover consisting of trees generally 5 feet or taller. Encroaching trees are primarily eastern red cedar. Additional woody cover from deciduous trees and shrubs may be present. 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 T1A
State 1 to 2
Heavy grazing or haying with inadequate recovery periods will cause the Reference State (1) to lose most 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 T1B
State 1 to 3
The Reference State (1) is significantly altered by mechanical tillage converting site to the Sod-busted State (3) to facilitate production agriculture. The disruption to the plant community, soils, and hydrology of the system make restoration to a true Reference State (1) unlikely.
Transition T1C
State 1 to 4
Disruption of the natural fire regime and encroachment of invasive exotic and native woody species can cause the Reference State (1) to transition to the Invaded Woody State (4).
Transition T2A
State 2 to 3
The Native/Invaded Grass State (2) is significantly altered by mechanical tillage converting site to the Sod-busted State (3) to facilitate production agriculture. The disruption to the plant community, soils, and hydrology of the system make restoration unlikely.
Transition T2B
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 transition to the Invaded Woody State (4).
Transition T3A
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 transition to the Invaded Woody State (4).
Restoration pathway R4A
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 may initially increase following tree removal through mechanical brush management treatments and prescribed fire. If re-sprouting brush 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 the Invaded Woody State (4).
Land that transitioned to the Invaded Woody State (4) from the Native/Invaded Grass State (2) or the Sod-busted State (3) cannot be restored to the Reference State (1) through removal of woody species.
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Restoration pathway R4B
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 may initially increase following tree removal through mechanical brush management treatments and prescribed fire. If resprouting brush 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 the Invaded Woody State (4).
Land that transitioned to the Invaded Woody State (4) from the Native/Invaded Grass State (2) or the Sod-busted State (3) cannot be restored to the Reference State (1) through removal of woody species.
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| Prescribed Burning |
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Restoration pathway R4C
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 may initially increase following tree removal through mechanical brush management treatments and prescribed fire. If resprouting brush 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 the Invaded Woody State (4).
Land that transitioned to the Invaded Woody State (4) from the Native/Invaded Grass State (2) or the Sod-busted State (3) cannot be restored to the Reference State (1) through removal of woody species.
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