Ecological dynamics
The Rarely Flooded Terrace reference state consists of a savanna, characterized by open grassland with trees and shrubs present in low numbers. The trees species for this state are mainly post oak, blackjack oak, black hickory, bitternut hickory, dwarf chinquapin oak, cedar elm, and other oak species. Dominate grasses consist of little bluestem, switchgrass, indiangrass, and big bluestem (Landfire 2010; NatureServe 2009).
Fire has significant influence on this ecological site. The historical average fire return interval was likely between 1 to 10 years (Hallgren, 2011). These wildfires would occur naturally through lightning strikes, but the majority were probably ignited by anthropogenic sources (DeSantis, 2010). Native grass species evolved with and responded well to fires, gaining an advantage compared with other plant species (Engle, 2001).
Grazing was also important to these ecological sites. As the practices of fencing and livestock husbandry replaced herds of bison, elk, and deer, the ecological dynamics of this site were altered (Kohl, 2013). Changes were usually proportional to the season and intensity of livestock grazing behavior and were accelerated by a combination of drought and overgrazing (when the consumption of vegetation biomass by livestock and other grazers exceeds the vegetations ability to recover in a timely fashion, thus exposing the soil and reducing the vegetations productive capacity (Angerer, 2013)). For example, palatable grasses and forbs are repeatedly grazed by livestock, weakening and potentially killing or replacing these species with less desirable species (Smith, 1940).
A variety of climate related events can occur that affect these ecological states such as hail storms, tornados, thunder storms, and extreme precipitation. Hail storms can reduce canopy size, increase litter deposition, and increase tree bark removal. When paired with other disturbances such as fire, the effects on tree species were much greater than in areas not affected by the hail storm (Gower, 2015). Tornados have been shown to change plant community compositions in savanna ecosystems, favoring hardwoods and eliminating softwoods (Liu, 1997). Lightning storms greatly effect ecosystems and while they generally occur during summer months, they can occur during every season. If a fire is started by a lightning strike and allowed to burn there will be different effects in the ecosystem depending on the season (Hiers, 2000). Drought occurs on 5 to 10 year intervals (Oklahoma Water Resource Board, 2011).
The Rarely Flooded Terrace ecological site consists of four identified states: Reference, Encroached, Pasture, and Eroded.
The encroached state consists of a many tree species, especially eastern redcedar, where there is significant canopy closure. Depending on how long this state has been present on the ecological site, the plant community will vary from oak, to hickory, to eastern redcedar. As the woody canopy increases and an encroached state occurs, the hydrology of the site is altered. The increased canopy intercepts most of the precipitation and changes hydrological patterns. Understory species will generally have less available water for growth and will have to compete with an extensive redcedar root system (Zou, 2018).
The pasture state will comprise species that are planted and grown for specific management goals, mainly livestock grazing. Common pasture species include buffalograss, western wheatgrass, little bluestem, sideoats grama, composite dropseed, silver beardgrass, winter bentgrass, purple lovegrass, kentucky bluegrass, tumblegrass, fall panicgrass, little barley, white sagebrush, slimflower scurfpea, and missouri goldenrod. Quality and quantity of forb, grass and legume species within this state will depend on the level of management inputs including seeding, weed management, and land uses. Species of both warm-season and cool-season grasses are feasible for these sites.
The eroded state consists of an area where the soil and plant communities are not stable due to wind and water erosion. In addition to destroying the original plant community, over plowing (degrading soil through cultivation) has resulted in major soil condition changes. Reductions in organic matter, mineral levels, soil structure, oxygen levels, water holding capacity, and populations of soil dwelling organisms are common on these sites. The extent of these changes depend upon duration of over plowing, as well as the species of crops grown and other management practices. Where vegetation is able to grow, this states plant community is predominately prairie threeawn interspersed in remnants of perennial grasses such as alkali sacaton, blue grama, sideoats grama and buffalograss. Numerous annuals are usually found on this state.
A state and transition model has been created to explain this ecological site. However, sparse data availability only allowed basic principles to be explored and a small number of species to be recorded. More data should be collected from this ecological site to provide a greater understanding of the ecological form and function, as well as the resources consumption and distribution.
State 1
Reference
The reference state is considered to be representative of the natural range of variability without major anthropogenic influences. This state is characterized by a warm-season, perennial, tallgrass savanna with scattered oak trees. Total overstory canopy cover is less than 25 percent.
Drivers- Fire frequency between 1 and 10 years (Hallgren, 2011), climate (decadal scale), insect and disease presence or establishment (oak wilt, beetles), and wildlife grazing or browsing.
Feedbacks- Fire tolerant grasses dominate the ecological site, fire intervals suppress woody vegetation growth. Wildlife grazing/browsing decreases the amount of grass available, decreasing fire intensity and causing wildlife migration to a new grazing location.
Characteristics and indicators. The reference state is characterized by a savanna, an open grassland with trees and shrubs present in low numbers. The trees species for this state are mainly post oak, blackjack oak, black hickory, bitternut hickory, chinquapin oak, cedar elm, and other oak species. Dominate grasses consist of little bluestem, switchgrass, indiangrass, and big bluestem (Landfire 2010; NatureServe 2009).
Community 1.1
Post Oak/Little Bluestem
This community phase is dominated by warm-season, perennial, tall grasses with scattered oak trees. Grasses are estimated to accounted for 55 to 65 percent of vegetative production by weight. Dominant grasses are little bluestem, big bluestem, Indiangrass, switchgrass, and Canada wildrye. Common trees species include post oak and blackjack oak.
Community 1.2
Post Oak-Eastern Redcedar (at risk community)
This community phase has a moderately closed canopy with an understory of tallgrasses and midgrasses. The absence of fire has allowed post oak, blackjack oak, and eastern redcedar densities to increase. The overstory tree canopy is around 25%. The competition from the increased canopy has led to a decrease in herbaceous understory plants.
Pathway 1.1A
Community 1.1 to 1.2
Possible reasons for increased canopy cover are the absence of fire (less fire=more woody vegetation growth), overgrazing (overgrazing grasses= less fuel for fires), and the natural regeneration of woody species. This transition may also be coupled with prolonged drought.
Pathway 1.2A
Community 1.2 to 1.1
Possible reasons for decreased canopy cover are excessive fire behavior (higher intensity fire=more woody vegetation consumption), climatic shifts, and insect/disease outbreaks. Periodic wildfire reduces size and number of trees and shrubs. This community phase pathway may follow years with above average herbaceous production, resulting in more fine fuels.
| Prescribed Burning |
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| Prescribed Grazing |
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State 2
Encroached
The encroached state is dominated by tree species. Canopy cover is greater than 25%.
Driver- Absence of wildfire, seed dispersal by wildlife, climate (decadal scale), and canopy density.
Feedbacks- Tree species dominate the ecological site, causing shading of grass species and shorter woody species. Less ground cover will decrease fire risk. Nutrient and water cycling will be controlled by tree species.
Characteristics and indicators. The encroached state consists of many tree species, especially eastern redcedar, where there is significant canopy closure. Depending on how long this state has been present on the ecological site, the plant community will vary from oak, to hickory, to eastern redcedar. As the woody canopy increases and an encroached state occurs, the hydrology of the site is altered. The increased canopy intercepts most of the precipitation and changes hydrological patterns to favor tree species over grass species. Understory species will generally have less available water for growth and will have to compete with an extensive redcedar root system (Zou, 2018).
Community 2.1
Post Oak- Eastern Redcedar
This community phase is characterized by overstory canopy cover ranging from 25 to 50 percent. Overstory consists mainly of oak species with hickory with eastern red cedar present.
Community 2.2
Eastern Redcedar- Post Oak
This community phase is characterized by an overstory canopy cover ranging from 50 to 75 percent. Overstory consists mainly of eastern redcedar. Oak and hickory trees may be present, however, ecosystem dynamics are dominated by eastern redcedar.
Pathway 2.1A
Community 2.1 to 2.2
Canopy cover increases, decreasing the amount of sunlight that reaches the ground and understory vegetation. A decrease in the ground vegetation can lead to fires that are not intense enough to control woody vegetation. This community will generally occur when canopy cover is greater than 75%. Possible reasons for increased canopy cover are fire suppressions (less fire=more woody vegetation growth).
Pathway 2.2A
Community 2.2 to 2.1
Canopy cover decreases, increasing the amount of sunlight that reaches the ground and understory vegetation. An increase in the ground vegetation can lead to fires that are able to control woody vegetation. This community will generally occur when canopy cover is greater between 25-50%. Possible reasons for decreased canopy cover are excessive fire (more fire= less woody vegetation) and mechanical tree removal.
State 3
Pasture
The pasture state consists of introduced grass species that are planted to maximize livestock forage production.
Drivers- Mechanical soil disturbance and seed planting, climate (decadal scale), seed dispersal, and wildlife/livestock grazing or browsing.
Feedbacks- Land managers use mechanical or chemical equipment to manipulate the ecological site. Wildlife and livestock grazing/browsing decrease the amount of available forage. Inputs of fertilizer and brush management are required to maintain high productivity across this ecological state.
Characteristics and indicators. The pasture state will comprise species that are planted and grown for specific management goals, mainly livestock grazing. Common pasture species include buffalograss, western wheatgrass, little bluestem, sideoats grama, composite dropseed, silver beardgrass, winter bentgrass, purple lovegrass, kentucky bluegrass, tumblegrass, fall panicgrass, little barley, white sagebrush, slimflower scurfpea, and missouri goldenrod. Quality and quantity of forb, grass and legume species within this state will depend on the level of management inputs including seeding, weed management, and land uses. Species of both warm-season and cool-season grasses are feasible for these sites.
Community 3.1
Bermuda Grass
Grass and forb species have been planted to maximize production for grazing livestock.
Community 3.2
Bermuda Grass/ Post Oak- Eastern Redcedar
Grass and forb species have been planted to maximize production for grazing livestock. Tree species are encroaching.
Community 3.3
Eastern Redcedar- American Elm- Post Oak
Grass and forb species have been planted to maximize production for grazing livestock. Tree species have encroached this site and cover significant amounts of grazing land.
Pathway 3.1A
Community 3.1 to 3.2
Fire Suppression
Pathway 3.2A
Community 3.2 to 3.1
Tree Removal, Brush Management
Pathway 3.2B
Community 3.2 to 3.3
Fire Suppression
Pathway 3.3A
Community 3.3 to 3.1
Tree Removal, Brush Management
Pathway 3.3B
Community 3.3 to 3.2
Tree Removal, Brush Management
State 4
Eroded
The eroded state consists of an area where the soil and plant communities are not stable due to wind and water erosion.
Drivers- Loss of soil and site stability, active soil erosion, climate (decadal scale), and wildlife/livestock grazing or browsing.
Feedbacks- Reduced basal cover and increased bare ground resulting in increased overland flow leading to rills and gullies.
Characteristics and indicators. The eroded state consists of an area where the soil and plant communities are not stable due to wind and water erosion. In addition to destroying the original plant community, over plowing (degrading soil through cultivation) has resulted in major soil condition changes. Reductions in organic matter, mineral levels, soil structure, oxygen levels, water holding capacity, and populations of soil dwelling organisms are common in this state. The extent of these changes depend upon duration of over plowing, as well as the species of crops grown and other management practices. Where vegetation is able to grow, this states plant community is predominately prairie threeawn interspersed in remnants of perennial grasses such as alkali sacaton, blue grama, sideoats grama and buffalograss. Numerous annuals are usually found on this state.
Community 4.1
Yellow Bluestem
Significant amounts of bare ground are present with grasses and forbs that establish quickly in poor soil conditions.
Transition T1A
State 1 to 2
Trigger- Fire suppression, overgrazing (when the consumption of vegetation biomass by livestock and other grazers exceeds the vegetations ability to recover in a timely fashion, thus exposing the soil and reducing the vegetations productive capacity (Angerer, 2013)), shading, and increased woody species seedling success.
Slow Variables: Increasing amounts of shade (reducing ground cover species), decreasing amounts of native grass species (both annual and perennial) changing nutrient cycles from grass dominated to leaf dominated, and increased woody vegetation changes hydrology and precipitation through percolation and transpiration.
Thresholds: Canopy cover becoming greater than 25%.
Transition T1B
State 1 to 3
Trigger- Mechanical tree removal, mechanical and chemical woody vegetation suppression (removing brush by use of mechanical cutter, chopper, or other equipment followed by an application of chemicals in order to reduce fuel loading and improve ecological site condition, (NRCS 314)), introducing annual/ perennial grass and forb species, prescribed fire (applying controlled fire to a predetermined area of land, (NRCS 338)), and prescribed grazing ( managing the harvest of vegetation with grazing and-or browsing animals, (NRCS 528)).
Slow Variables: Increased production and management of introduced species.
Thresholds: Native seed sources are removed and suppressed from the ecological site.
| Brush Management |
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| Prescribed Burning |
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| Land Clearing |
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| Prescribed Grazing |
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Restoration pathway R2A
State 2 to 1
Trigger- Mechanical tree removal, mechanical and chemical woody vegetation suppression (removing brush by use of mechanical cutter, chopper, or other equipment followed by an application of chemicals to reduce fuel loading and improve ecological site condition, (NRCS 314)), and prescribed fire that will kill trees and suppress woody vegetative growth (applying controlled fire to a predetermined area of land, (NRCS 338)).
Slow Variables: Removal of tree species decreases the amount of shade, allowing grass species to receive nutrients. Nutrient and water cycles shift from complete tree domination to a combination of tree and grass dominated.
Thresholds: Canopy cover becomes less than 25%.
| Brush Management |
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| Prescribed Burning |
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Transition T2A
State 2 to 3
Trigger- Mechanical tree removal, mechanical and chemical woody vegetation suppression (removing brush by use of mechanical cutter, chopper, or other equipment followed by an application of chemicals to reduce fuel loading and improve ecological site condition, (NRCS 314)), prescribed fire that will suppress woody vegetative growth (applying controlled fire to a predetermined area of land, (NRCS 338)), prescribed grazing ( managing the harvest of vegetation with grazing and-or browsing animals, (NRCS 528)), and introduced species planting/establishment.
Slow Variables: Removal of tree species decreases the amount of shade, allowing grass species to receive nutrients. Nutrient and water cycles shift from complete tree domination to a combination of tree and grass dominated. Introduced species are established.
Thresholds: Introduced species become a significant environmental factor at the ecological site.
| Brush Management |
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| Prescribed Burning |
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| Land Clearing |
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| Prescribed Grazing |
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Transition T3A
State 3 to 4
Trigger- Drought, over-plowing (degrading soil through cultivation), and overgrazing (when the consumption of vegetation biomass by livestock and other grazers exceeds the vegetations ability to recover in a timely fashion, thus exposing the soil and reducing the vegetations productive capacity (Angerer, 2013)).
Slow Variables: Vegetation quantity and quality will decrease over time. Ground cover will decrease, increasing water and wind erosion. New vegetation will quickly be consumed by livestock/wildlife when it becomes available.
Thresholds: Significantly reduced ground cover, increasing water and wind erosion.
Feedbacks- Any vegetation that is grown will quickly be consumed by livestock, destabilizing the soil and continuing erosion.
Restoration pathway R4A
State 4 to 3
Stop practices that are causing harm such as overgrazing (when the consumption of vegetation biomass by livestock and other grazers exceeds the vegetations ability to recover in a timely fashion, thus exposing the soil and reducing the vegetations productive capacity (Angerer, 2013)), over-plowing (degrading soil through cultivation), and other unsustainable agricultural practices. Establish ground cover, preferably using non-invasive species. Following ground cover establishment, plant species that will increase site stabilization. Manage for desirable site conditions.
