Ecological dynamics
The Reference State for this ecological site consists of a hardwood forest characterized by a hardwood species overstory and an herbaceous forest floor. Overstory hardwood species include oak and hickory (Eldredge, 1937). Softwood species, such as loblolly pine and shortleaf pine, are also on similar upland sites. Native grass species, such as big bluestem, switchgrass, little bluestem, and Indiangrass, are common (Owen, 1858).
Fire significantly influences this ecological site. Historically, the average fire-return interval was likely between 3 and 25 years (Guyette and Spetich, 2003; Hallgren et al., 2012). Some of these wildfires occurred naturally through lightning strikes, but human activities probably caused most of these fires (DeSantis et al., 2010). Native species evolved with and responded well to fire, gaining an advantage over other plant species (Spetich and He, 2008; Engle and Bidwell, 2001). Fires on similar upland ecological sites today likely have moderate to low severity due to forested conditions and reduced amounts of ground vegetation (Carey, 1992).
Historically, grazing animals, such as elk and bison, were present in the Reference State of this ecological site (Heikens, 2007). As fencing and livestock husbandry practices replaced herds of bison, elk, and deer, the ecological dynamics of this site shifted (Kohl et al., 2013). Changes to the ecological dynamics were commonly proportional to the season and intensity of livestock grazing and accelerated through a combination of drought and overgrazing (Angerer et al., 2016). For example, palatable grasses and forbs repeatedly grazed by livestock become weak and can die, and less desirable vegetation may replace preferable species (Smith, 1940).
Climate-related events, such as hailstorms, tornados, thunderstorms, and extreme precipitation, occur on this site. Hailstorms 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 are much greater than in areas not affected by the hailstorm (Gower et al., 2015). Tornados can change plant community compositions throughout ecosystems, favoring hardwoods and eliminating softwoods (Liu et al., 1997). Lightning storms greatly affect ecosystems. Lightning storms generally occur during summer months but can occur during any season. If a fire starts with a lightning strike, the effects on the ecosystem vary depending on the season (Hiers et al., 2000). According to the Oklahoma Water Resource Board, drought occurs every 5 to 10 years (OWRB, 2022).
Because of sparse data availability, the state-and-transition model created for this site only explored basic principles and included only a small number of species. Further data collection from this ecological site would provide a greater understanding of ecological form and function and of resource consumption and distribution.
State 1
Reference
The Reference State represents the natural range of variability for the ecological site without major human influence.
The main drivers for community pathways within the Reference State are fire frequency intervals between 3 and 25 years (Guyette and Spetich, 2003; Hallgren et al., 2012), climate effects (decadal scale), insect or disease presence or establishment, and wildlife grazing or browsing.
Fire is the main feedback mechanism within this state, and fire-tolerant species dominate the ecological site. Fire intervals suppress herbaceous vegetation growth; wildlife grazing or browsing reduces the amount of herbaceous vegetation available. Reduced grass availability lessens fire intensity and causes wildlife migration.
Characteristics and indicators. The Reference State consists of a hardwood forest characterized by a mixed hardwood overstory and an herbaceous forest floor. Hardwood species include oak and hickory (Eldredge, 1937). Softwood species, such as loblolly pine and shortleaf pine, are also on similar upland sites. Native grass species, such as big bluestem, switchgrass, little bluestem, and Indiangrass, are common (Arkansas Geological Survey, 2005).
Dominant plant species
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oak (Quercus), tree
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hybrid hickory (Carya), tree
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loblolly pine (Pinus taeda), tree
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shortleaf pine (Pinus echinata), tree
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sumac (Rhus), shrub
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dogwood (Cornus), shrub
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big bluestem (Andropogon gerardii), grass
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little bluestem (Schizachyrium), grass
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Indiangrass (Sorghastrum), grass
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switchgrass (Panicum virgatum), grass
Community 1.1
Post Oak/Little Bluestem
Oak trees and warm-season, perennial tallgrasses dominate this community phase. Dominant grasses are little bluestem, big bluestem, Indiangrass, and switchgrass. Common trees include post oak, blackjack oak, white oak, and red oak.
Community 1.2
Post Oak-Eastern Redcedar (At-Risk Community)
This community phase has a moderately closed canopy and an understory of tallgrasses and midgrasses. The absence of fire allows post oak, blackjack oak, and eastern redcedar densities to increase. Competition due to the increased canopy cover leads to a reduction in the herbaceous understory cover.
Pathway 1.1A
Community 1.1 to 1.2
The main drivers for this community pathway are the absence of fire and the natural regeneration of woody species. Excessive grazing pressure may also accompany this transition.
Pathway 1.2A
Community 1.2 to 1.1
The main drivers for this community pathway are periodic wildfires that reduce the size and number of trees and shrubs. This community phase may follow years of above-average herbaceous production, resulting in more fine fuels. Drought and outbreaks of insects, diseases, or both may also contribute to a reduction in woody canopy cover.
| Prescribed Burning |
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| Prescribed Grazing |
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State 2
Encroached
Tree species and a closed canopy dominate the Encroached State.
The main drivers for community pathways within the Encroached State are the absence of wildfire, seed dispersal by wildlife, climate effects (decadal scale), and canopy density.
The main feedback mechanism for community pathways in this state is the dominance of tree species. Tree species shade and outcompete herbaceous species and shorter woody species. Less ground cover reduces fire risk. Tree species take control of nutrient and water cycling.
Characteristics and indicators. The Encroached State consists of many tree species (oak, eastern redcedar, hickory, beech) and significant canopy closure. Time, fire, and tree density determine community phases and species abundance or variation. The woody canopy cover increases, and the Encroached State develops. The denser canopy intercepts most of the precipitation and changes the hydrology of the site. Understory species likely have less available water for growth and must compete with extensive woody plant root systems (Zou et al., 2018).
Community 2.1
Post Oak- Eastern Redcedar
This community phase consists of oak, hickory, beech, and eastern redcedar. An increase in canopy cover causes a reduction in herbaceous species. Eastern redcedar trees become more abundant.
Community 2.2
Eastern Redcedar- Post Oak
Eastern redcedar dominates this community phase. Oak, hickory, and beech trees may be present. Oak, hickory, and beech trees experience reduced vigor and reproductive capacity due to shading and competition from eastern redcedar.
Pathway 2.1A
Community 2.1 to 2.2
The absence of wildfire is the main driver for this community pathway and contributes to natural regeneration and increased tree and shrub cover.
Pathway 2.2A
Community 2.2 to 2.1
The main driver for this community pathway is a reduced canopy cover. As canopy cover decreases, the amount of sunlight reaching the ground increases, and understory vegetation increases. Denser ground vegetation can lead to fires that control woody vegetation. This community is present when the canopy cover is between 25 to 50 percent. Excessive fire and mechanical removal of trees can contribute to the reduction in canopy cover.
State 3
Pasture
The Pasture State consists of introduced herbaceous species planted to maximize livestock forage production.
The main drivers for community pathways within the Pasture State are the mechanical disturbance of the soil and seed planting, climate effects (decadal scale), seed dispersal, and wildlife and livestock grazing or browsing.
The main feedback mechanism for community pathways in this state is the use of mechanical equipment or chemicals to manipulate the site. Wildlife and livestock grazing or browsing reduces the amount of available forage. Fertilizer inputs and brush management are essential for maintaining high productivity.
Characteristics and indicators. The Pasture State consists of species grown for specific management goals, mainly livestock grazing. Common pasture species include clover, tall fescue, ryegrass, small grains, vetches, peas, lespedeza, alfalfa, Bermudagrass, bahiagrass, dallisgrass, Johnsongrass, crabgrass, millet, big bluestem, little bluestem, Indiangrass, eastern gamagrass, and switchgrass (UAES, 2022). The quality and quantity of forbs, grasses, and legume species within this state depend on the level of management inputs (seeding, weed management, nutrient inputs, and land use). Both warm-season and cool-season grasses grow in the Pasture State.
Community 3.1
Bermudagrass
This community phase consists of grass and forb species planted to maximize production for grazing livestock.
Community 3.2
Bermudagrass/ Post Oak- Eastern Redcedar
This community phase consists of grass and forb species planted to maximize production for grazing livestock. Inadequate pasture maintenance allows the growth of tree species, such as eastern redcedar. Generally, eastern redcedar first starts growing along fence lines and areas under electric utility wires.
Community 3.3
Eastern Redcedar- American Elm- Post Oak/ Bermudagrass
This community phase consists of grass and forb species planted to maximize production for grazing livestock. Inadequate pasture maintenance over time allows the growth of woody species, such as eastern redcedar, oak, and American elm. These woody species encroach on the pasture and dominate significant areas of land and resources previously used by palatable forage species.
Pathway 3.1A
Community 3.1 to 3.2
The main drivers for this community pathway are the absence of fire, seed dispersal by wildlife, and natural regeneration.
Pathway 3.2A
Community 3.2 to 3.1
The main driver for this community pathway is the removal or reduction of woody vegetation.
Pathway 3.2B
Community 3.2 to 3.3
The main drivers for this community pathway are the absence of fire, seed dispersal by wildlife, and natural regeneration.
Pathway 3.3A
Community 3.3 to 3.1
The main driver for this community pathway is the removal or reduction of woody vegetation.
Pathway 3.3B
Community 3.3 to 3.2
The main driver for this community pathway is the removal or reduction of woody vegetation.
State 4
Plantation
The Plantation State consists of merchantable trees planted to maximize timber production and wood volume. Common species in timber plantations include loblolly pine and oak.
Community phases differ by timber type (softwood or hardwood) and harvest method.
The main drivers for community pathways in the Plantation State are prescribed fire, pest management, vegetation management, and canopy density.
The main feedback mechanism for community pathways in this state is timber harvesting. Cultivated tree species dominate this ecological site and shade other vegetation. Timber management reduces competition with other species and assists the growth of desirable timber species.
Characteristics and indicators. The Plantation State consists of trees planted and managed to maximize merchantable timber production. Loblolly pine is the most common species for planting, but hardwood trees are also prevalent. Community phases differ by tree type (softwood or hardwood), timber harvest method, and reforesting practices.
Community 4.1
Loblolly Pine
This community phase consists of loblolly pine planted to maximize timber production.
Dominant plant species
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loblolly pine (Pinus taeda), tree
Transition T1A
State 1 to 2
The main triggers for this transition are the absence of fire and excessive grazing pressure, both of which allow woody seedlings to grow and outcompete herbaceous vegetation.
The main slow variable for this transition is increased competition for sunlight, nutrients, and moisture resources. Increased overstory competition reduces the vigor and reproductive capacity of the herbaceous understory.
The main threshold for this transition is a shift in nutrient cycling (from grass and leaf dominance to leaf and needle dominance). Increased woody canopy cover alters hydrologic cycles, potentially reducing runoff and infiltration and increasing precipitation interception by woody species.
Transition T1B
State 1 to 3
The main triggers for this transition are tree removal, mechanical and chemical suppression of woody vegetation, tillage, and the introduction of annual or perennial forage species.
The main slow variables for this transition are increased production and management of forage species.
The main thresholds for this transition are changes in soil properties, such as structure, organic matter, and nutrient cycling, and changes in type and frequency of disturbance.
| 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 T1C
State 1 to 4
The main triggers for this transition are the removal of native trees, mechanical and chemical suppression of woody vegetation, and the introduction of plantation tree species.
The main slow variables for this transition are increased production and management of introduced species.
The main thresholds for this transition are changes in soil properties, such as structure, organic matter, and nutrient cycling, and changes in type and frequency of disturbance.
Restoration pathway R2A
State 2 to 1
Restoration efforts for this pathway begin with the mechanical and chemical treatment of undesirable woody vegetation and seeding of native species. A grazing management plan and the reintroduction of historical disturbance regimes must accompany these initial treatments. Returning to a historical fire interval through prescribed burning helps to suppress woody vegetation and manage invasive species.
| Brush Management |
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| Prescribed Burning |
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Transition T2A
State 2 to 3
The main triggers for this transition are tree removal, mechanical and chemical suppression of woody vegetation, tillage, and the introduction of annual or perennial forage species.
The main slow variables for this transition are increased production and management of forage species.
The main thresholds for this transition are changes in soil properties, such as structure, organic matter, and nutrient cycling, and changes in type and frequency of disturbance.
| 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 T2B
State 2 to 4
The main triggers for this transition are the removal of native trees, mechanical and chemical suppression of woody vegetation, and the introduction of plantation tree species.
The main slow variables for this transition are increased production and management of introduced species.
The main thresholds for this transition are changes in soil properties, such as structure, organic matter, and nutrient cycling, and changes in type and frequency of disturbance.
Transition T3B
State 3 to 2
The main trigger for this transition is a lack of management or abandonment.
The main slow variables for this transition are increased establishment and increased size of woody species.
The main threshold for this transition is woody species dominance. Woody species dominate ecological processes by shading other species and increasing competition for soil moisture, nutrients, and sunlight. Woody species dominance reduces the vigor and reproductive capacity of herbaceous species in the understory.
Transition T3A
State 3 to 4
The main triggers for this transition are the removal and suppression of forage species, mechanical and chemical suppression of woody vegetation, and the introduction and management of plantation tree species.
The main slow variables for this transition are the increased production and management of plantation species.
The main thresholds for this transition are changes in soil properties, such as structure, organic matter, and nutrient cycling, and changes in kind and frequency of disturbance.
Transition T4A
State 4 to 2
The main trigger for this transition is a lack of management or abandonment.
The main slow variables for this transition are the increased establishment and increased size of woody species.
The main threshold for this transition is woody species dominance. Woody species dominate ecological processes by shading other species and increasing competition for soil moisture, nutrients, and sunlight. Woody species dominance reduces the vigor and reproductive capacity of herbaceous species in the understory.
Transition T4B
State 4 to 3
The main triggers for this transition are tree removal, mechanical and chemical suppression of woody vegetation, tillage, and the introduction of annual or perennial forage species.
The main slow variables for this transition are increased production and management of forage species.
The main thresholds for this transition are changes in soil properties, such as structure, organic matter, and nutrient cycling, and changes in type and frequency of disturbance.
