Ecological dynamics
Sandy Flood Plain sites developed under Northern 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-man 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 warm-season tallgrasses. 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), a shorter structure, or a reduced palatability mechanism.
The State-and-Transition Model (STM) is depicted below, and is made up of a Reference State, a Native/Invaded Grass State, a Sod-busted State, and an Invaded Woody State. 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 hydrological cycle.
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.
Growth of native cool-season plants begins about April 1 and continues to about June 15. Native warm-season plants begin growth about May 15 and continue to about August 15. Green-up of cool-season plants may occur in September and October if adequate moisture is available.
The following is a diagram illustrating the common plant communities that can occur on the site and the transition pathways between communities.
State 1
Reference State
This state describes the range of vegetative community phases that occur on the Sandy Floodplain site where the natural processes are mostly intact.
The Reference Community is a representation of the native plant community phase that occupies a site that has been minimally altered by management. The Degraded Native Grass, the At-Risk Native Grass, and the Excessive Litter Communities are the phases that result from management decisions that are unfavorable for a healthy Reference Community. The Ephemeral Forb Community is the result of a high intensity disturbance event.
High perennial grass cover and production allows for increased soil moisture retention, vegetative production, and overall soil quality.
Community 1.1
Reference Community
The Reference Community serves as a description of the native mixed-grass 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 fire and grazing events.
The potential vegetation consists of approximately 70 to 90 percent grasses and grass-like plants, 5 to 10 percent forbs, and 5 to 10 percent shrubs. Sand bluestem, little bluestem, Indiangrass and switchgrass are the primary species in this community. Secondary species include prairie sandreed, sideoats grama, western wheatgrass, sand lovegrass, and Canada wildrye. The site has a very 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 over-grazing events without adequate rest, this plant community will degrade.
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Community 1.2
Degraded Native Grass Community
Sand bluestem, switchgrass, Indiangrass, prairie sandreed, and other desirable species lose productive capacity through loss of vigor and reproductive potential. Forb diversity is reduced. Periodic flooding and a fluctuating water table on this site help to sustain the tall grasses in the plant community. However, midgrasses such as western wheatgrass, sideoats grama, and various sedges will increase to become the dominant species.
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 grasses 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 site/soil stability, watershed function, and biologic integrity.
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Community 1.3
At-Risk Native Grass Community
In this plant community, the more palatable warm-season tallgrasses have been reduced to remnant populations by continued defoliation during their critical growth periods. Grazing-evasive warm-season grasses and cool-season grasses increase significantly. Blue grama, western wheatgrass, and sand dropseed are the dominant warm-season grasses. Kentucky bluegrass encroachment also occurs on flatter slopes.
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.
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Community 1.4
Excessive Litter Community
The Excessive Litter Community describes the response of the community to the removal of the natural disturbances of herbivory and fire. As the undisturbed duff layer deepens, infiltration of the precipitation is interrupted and evaporation increases significantly, simulating drought-like conditions.
Community 1.5
Ephemeral Forb Community
This community describes the flush of forbs that occurs in response to a major disturbance, or combination of disturbances. Growing season wildfire followed by hail, extreme prolonged drought, or extreme defoliation by herbivores are all examples of these disturbances. The native warm-season grasses reestablish dominance within a few years of the event.
Pathway 1.1A
Community 1.1 to 1.2
A shift from the Reference Community to the Degraded Native Grass Community occurs with continuous season-long grazing and inadequate recovery periods during the growing season.
Pathway 1.1B
Community 1.1 to 1.4
Prolonged interruption of the natural disturbances of herbivory and fire will result in conversion from this community to the Excessive Litter Community.
Pathway 1.1C
Community 1.1 to 1.5
A high-impact disturbance event or combination of events causing excessive defoliation of the vegetation, i.e., a growing season wildfire followed by a significant hailstorm, or a prolonged intensive grazing event or long-term drought, etc.
Pathway 1.2A
Community 1.2 to 1.1
A shift from the Degraded Native Grass Community toward the Reference Community can be achieved through prescribed grazing. Applying grazing pressure during the growth period of the undesirable cool-season grasses and allowing rest during the warm-season growing season favors our desired species. This grazing regime will enable the deeply rooted warm-season tallgrasses to outcompete the shallow rooted grazing evasive warm-season 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
Maintaining continuous season-long grazing with inadequate recovery periods during the growing season further degrades the site to the At-Risk Native Grass Community.
Pathway 1.2C
Community 1.2 to 1.4
Prolonged interruption of the natural disturbances of herbivory and fire will result in conversion from this community to the Excessive Litter Community.
Pathway 1.2D
Community 1.2 to 1.5
A high-impact disturbance event, or combination of events causing excessive defoliation of the vegetation, i.e., a growing season wildfire followed by a significant hailstorm, or a prolonged intensive grazing event, or long-term drought, etc.
Pathway 1.3A
Community 1.3 to 1.2
Reversing the downward trend to the previous community can be achieved with prescribed grazing early and late in the growing season to reduce undesirable 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 tallgrasses to rejuvenate. Appropriately timed prescribed fire will accelerate this process.
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Pathway 1.3B
Community 1.3 to 1.4
Interruption of the natural disturbances of herbivory and fire will result in conversion from this community to the Excessive Litter Community.
Pathway 1.3C
Community 1.3 to 1.5
A high-impact disturbance event, or combination of events causing excessive defoliation of the vegetation, i.e., a growing season wildfire followed by a significant hailstorm, or a prolonged intensive grazing event, or long-term drought, etc.
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 previous community.
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 previous community.
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 previous community.
Pathway 1.4D
Community 1.4 to 1.5
A high-impact disturbance event, or combination of events causing excessive defoliation of the vegetation, i.e., a growing season wildfire followed by a significant hailstorm, or a prolonged intensive grazing event, or long-term drought, etc.
Pathway 1.5A
Community 1.5 to 1.1
Restoration occurs naturally once the disturbance event has subsided. Allowing growing season rest will accelerate the recovery.
Pathway 1.5B
Community 1.5 to 1.2
Restoration occurs naturally once the disturbance event has subsided. Allowing growing season rest will accelerate the recovery.
Pathway 1.5C
Community 1.5 to 1.3
Restoration occurs naturally once the disturbance event has subsided. Allowing growing season rest will accelerate the recovery.
State 2
Native/Invaded Grass State
This state has been degraded from the Reference State and much of the native warm-season grass community has been replaced by less desirable plants. The loss of warm-season tall- and midgrasses 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 Native Evaders/Invaded Grass and the Smooth Brome Communities are the components of the Native/Invaded Grass State.
Community 2.1
Native Evaders/Invaded Grass Community
This plant community represents a shift from the Reference State across a plant community threshold. With continued grazing pressure, Kentucky bluegrass, and composite dropseed will become the dominant plant species, with only trace remnants of the more palatable warm-season midgrasses such as sideoats grama and little bluestem. Composite dropseed is a grazing-evasive warm-season midgrass with low palatability. 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 more shallow replacement root systems.
Grazing management practices that allow for adequate periods of recovery between grazing events will favor warm-season tall- and midgrasses. Appropriately timed prescribed fire will accelerate the restoration process.
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Community 2.2
Smooth Brome Community
This plant community contains predominately smooth brome but also contains some native warm-season grass remnants. Production of plant communities dominated by smooth brome 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
This community will be converted to a Smooth Bromegrass Community through the following practices:
introduced grass seeding, excessive warm season grazing, inadequate warm season rest, multi season haying and nitrogen fertilizing in spring and/or fall.
Pathway 2.2A
Community 2.2 to 2.1
Restoration can be achieved by 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.
State 3
Sod-busted State
This threshold is crossed as a result of mechanical disturbance to facilitate production agriculture. If farming operations are suspended, the site can be abandoned, which will result in the Natural Reclamation Community, or be reseeded to a desired perennial forage mixture, which is described as the Reseeded Grass Community. Permanent alterations of the soil community and the hydrological cycle make restoration to the original native Reference State extremely difficult, if not impossible. Formation of a compacted plow pan in the soil profile is likely.
Community 3.1
Reseeded Grass Community
This plant community does not contain native remnants, and varies considerably depending on the seed mixture, the degree of soil erosion, the age of the stand, nitrogen fertilizer use, and past grazing management. Prescribed grazing with adequate recovery periods will be needed to maintain productivity and desirable species.
Native range and seeded grasslands 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 on precipitation, management and grass species seeded.
Community 3.2
Natural Reclamation Community
This plant community consists of annual and perennial weeds and less desirable grasses. These sites have been farmed and abandoned without being reseeded. Soil organic matter/carbon reserves are reduced, soil structure is changed, and a plow-pan or compacted layer can be formed which decreases water infiltration. Residual synthetic chemicals may remain from farming operations. In early successional stages, this community is not stable. 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 tree canopy cover reaches 15 percent with an average tree height exceeding 5 feet, the threshold is crossed to the Invaded Woody State. The primary coniferous interloper is eastern redcedar. Honeylocust and green ash number among the deciduous native trees, along with several exotic introduced species, including Siberian elm. These woody species are encroaching due to lack of prescribed fire and 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 Eastern Redcedar/ Honeylocust Community.
Community 4.1
Eastern Redcedar/Honeylocust Community
This community has at least a 15 percent canopy of Eastern redcedar. Honeylocust encroachment may occur as you move east within the MLRA, when brush management and prescribed burning is absent over an extended period of time. Generally, this site is very conducive to eastern redcedar seedling invasion especially when adjacent to a seed source. Eastern redcedars will eventually dominate the site, resulting in a closed canopy, reduced forage production and limited livestock grazing and wildlife habitat value.
Eastern redcedar control can usually be accomplished with prescribed burning while the trees are six foot tall or less and fine fuel production is over 1,500 pounds per acre. Trees of all heights can be controlled with the use of specifically adapted preparation, and ignition and holding techniques. Mechanical removal followed by a chemical treatment on stumps is effective on honeylocust and other root sprouting species.
Total annual production during an average year varies significantly, depending on the production level prior to encroachment and the percentage of canopy cover.
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Transition T1A
State 1 to 2
Heavy grazing without adequate recovery periods will cause this state to lose a significant proportion of warm-season, tall- and midgrass species and cross a threshold to the Native/Invaded Grass State. 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.
Transition T1B
State 1 to 3
The Reference State is significantly altered by mechanical tillage to allow the site to be placed into production agriculture. The disruption to the plant community, the soil and the hydrology of the system make restoration to a true reference state unlikely.
Transition T1C
State 1 to 4
Disruption of the natural fire regime and the planting of invasive exotic and native woody species can cause this state to shift to the Invaded Woody State.
Transition T2A
State 2 to 3
The state is significantly altered by mechanical tillage to allow the site to be placed into production agriculture. The disruption to the plant community, the soil and the hydrology of the system make restoration to a true reference state unlikely.
Transition T2B
State 2 to 4
Disruption of the natural fire regime and the planting of invasive exotic and native woody species can cause this state to shift to the Invaded Woody State.
Transition T3A
State 3 to 4
Disruption of the natural fire regime and the planting of invasive exotic and native woody species can cause this state to shift to the Invaded Woody State.
Restoration pathway R4A
State 4 to 1
Prescribed burning, wildfire, harvest, and brush management will move this plant community toward one of the herbaceous dominated plant communities. 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 or trees such as honeylocust or Siberian elm are 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.
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Restoration pathway R4B
State 4 to 2
Prescribed burning, wildfire, harvest, and brush management will move this plant community toward one of the herbaceous dominated plant communities. 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 or trees such as honeylocust or Siberian elm are 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.
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Restoration pathway R4C
State 4 to 3
Prescribed burning, wildfire, harvest, and brush management will move this plant community toward one of the herbaceous dominated plant communities. 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 or trees such as honeylocust or Siberian elm are 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.
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