Ecological dynamics
The Reference State for this ecological site consists of a hardwood forest characterized by mixed hardwood species and a vegetative ground cover. The characteristic tree species for this state are hardwoods such as oak, beech, and hickory (Foti, 2004). Native grass species such as big bluestem, switchgrass, little bluestem, and Indiangrass grow in canopy openings and throughout the understory (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 (Spetich and He, 2008; Engle and Bidwell, 2001). Fires on similar upland ecological sites today are likely of moderate to low severity due to forested conditions and lower amounts of ground vegetation (Carey, 1992).
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 were much greater than in areas not affected by the hailstorm (Gower et al., 2015). Tornados can change plant community compositions in savanna 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).
Grazing and farming are feasible for this ecological site. Changes to the ecological dynamics are proportional to the intensity of livestock grazing and can accelerate through overgrazing (Angerer et al., 2016). For example, desirable grasses and forbs repeatedly grazed by livestock become weak and can die, and less desirable species may replace preferable ones (Smith, 1940).
The state-and-transition model for the Shallow Upland Ecological Site consists of three identified states: Reference, Encroached, and Pasture. Because of sparse data availability, the model 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 influences.
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 vegetation growth; wildlife grazing or browsing reduces the amount of grass available. Reduced grass availability lessens fire intensity and causes wildlife migration.
Characteristics and indicators. The Reference State consists of a hardwood forest characterized by mixed hardwoods and sporadic gaps where other vegetation grows. Hardwood species include oak, beech, and hickory (Foti, 2004). Native grass species, such as big bluestem, switchgrass, little bluestem, and Indiangrass, grow in canopy openings and throughout the understory (Arkansas Geological Survey, 2005).
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 from the denser canopy leads to a reduction in the herbaceous understory.
Pathway 1.1A
Community 1.1 to 1.2
The main drivers for this community pathway are the absence of fire and the growth 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 amount of woody vegetation. Drought and outbreaks of insects, diseases, or both may also result in a reduced woody canopy.
| Prescribed Burning |
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| Prescribed Grazing |
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State 2
Encroached
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 woody species. Woody species shade and outcompete herbaceous species. Fire intensity diminishes, and woody species take control of nutrient and water cycling.
Characteristics and indicators. The Encroached State consists of many woody species (especially eastern redcedar) and significant canopy closure. Time and fire intensity determine the community phases and species abundance and variation. As the woody canopy increases and the Encroached State develops, the hydrology of the ecological site alters. The denser canopy intercepts most precipitation and changes hydrological patterns to favor woody species over herbaceous species. Understory species, therefore, have less available water for growth and must compete with an extensive overstory root system (Zou et al., 2018).
Community 2.1
Post Oak-Eastern Redcedar
This community phase consists of oak, hickory, beech, and eastern redcedar. The canopy cover increases, thereby causing a reduction in the herbaceous ground cover. Eastern redcedar increases in size and quantity.
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 woody vegetation growth.
Pathway 2.2A
Community 2.2 to 2.1
The main driver for this community pathway is a reduction in canopy cover. As canopy cover decreases, more sunlight reaches the ground, and herbaceous vegetation increases. Increased herbaceous vegetation can lead to fires that reduce woody vegetation. Excessive fire and the mechanical removal of trees contribute to reduced 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 and chemicals to increase forage. Fertilizer inputs and brush management are essential for maintaining high productivity. Wildlife or livestock grazing or browsing reduces the amount of available forage.
Characteristics and indicators. The Pasture State consists of species grown for specific management goals, mainly livestock grazing. Common pasture species include buffalograss, western wheatgrass, little bluestem, sideoats grama, Bermudagrass, and bahiagrass. The quality and quantity of forbs, grasses, and legume species within this state depend on the level of management inputs (seeding, weed management, and land use). Both warm-season and cool-season grasses are suitable for this ecological site.
Community 3.1
Bermudagrass
This community phase consists of herbaceous species planted to maximize forage production for grazing livestock.
Community 3.2
Bermudagrass/ Post Oak-Eastern Redcedar
This community phase consists of herbaceous species planted to maximize forage production for grazing livestock. Improper pasture management allows for the growth of woody species, such as eastern redcedar and oaks.
Community 3.3
Eastern Redcedar-American Elm-Post Oak/ Bermudagrass
This community phase consists of herbaceous species planted to maximize forage production for grazing livestock. Improper pasture management over time allows the growth of woody species, such as eastern redcedar, oaks, and American elm. Woody species encroach on the pasture and dominate resources previously used by forage species.
Pathway 3.1A
Community 3.1 to 3.2
The main drivers for this community pathway are the absence of fire, improper management, 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 species.
Pathway 3.2B
Community 3.2 to 3.3
The main drivers for this community pathway are the absence of fire, improper management, 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 species.
Pathway 3.3B
Community 3.3 to 3.2
The main driver for this community pathway is the removal or reduction of woody species.
Transition T1A
State 1 to 2
The main trigger for this transition is the absence of wildfire, which allows woody species to grow and compete for nutrients, water, and sunlight.
The main slow variable for this transition is the 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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Restoration pathway R2A
State 2 to 1
Restoration efforts for this pathway begin with mechanical and chemical treatment of undesirable woody vegetation and seeding 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 |
|
Transition T3B
State 3 to 2
The main trigger for this transition is the 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, causing shading and increased competition for soil moisture, nutrients, and sunlight. Woody species dominance reduces the vigor and reproductive capacity of herbaceous species in the understory.
