Ecological dynamics
The Reference State for this ecological site is a bottomland hardwood forest. Common tree species include oak, sycamore, hickory, and hackberry (Bedinger, 1979). This ecological site is part of the South-Central Interior Large Floodplain System and the Arkansas Valley Prairie and Woodland System (NatureServe, 2009). These ecosystems are known as hardwood flats in the HGM (hydrogeomorphic) classification system (Klimas et.al., 2009).
Fire somewhat influences this ecological site during dry years. High precipitation throughout the year impacts fire behavior due to proximity to riparian areas (NatureServe, 2009). Historically, the average fire-return interval was likely between 3 and 25 years for the surrounding landscape (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).
Climate-related events, such as floods, hailstorms, tornados, thunderstorms, and extreme precipitation, occur on these sites. 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 species (Smith, 1940).
Timber harvesting alters the form and function of oak-pine ecosystems. Depending on the timber harvest method used (shelterwood, group selection, single-tree selection, seed tree, variable retention, or clearcutting), ecosystem dynamics can change very little or completely alter in form and function (Clark and Covey, 2012). Most of the currently forested sites have younger stands. Timber harvest practices alter the species composition and quality of these stands and reduce the ecological value of the site. (Guldin and Fitzpatrick, 1991).
Floods occur very rarely on this ecological site. Flooding affects ecosystems differently depending on flooding duration, the time of year, and ponding duration. Flooding during the dormant season does not negatively affect species diversity or growth (Bedinger, 1979). Flooding events transport and distribute nutrients and seeds throughout the landscape.
The state-and-transition model for the Terrace Ecological Site consists of three identified states: Reference, Timber Plantation, 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 climate effects (decadal scale), insect and disease presence or establishment, flooding events, wildlife grazing or browsing, and wildfire frequency and intensity.
High soil moisture and very rare flooding events are the main feedback mechanisms within this state. Because these factors limit the types of species that can grow and survive, water-tolerant trees dominate the ecological site.
Characteristics and indicators. The Reference State consists of a bottomland hardwood forest. Common tree species for this state are oak, sycamore, hickory, and hackberry (Bedinger, 1979).
Community 1.1
Oak-Sycamore
Community 1.2
Oak-Hickory-Sweetgum
Pathway 1.1A
Community 1.1 to 1.2
This pathway consists of lower precipitation and less soil moisture.
Pathway 1.2B
Community 1.2 to 1.1
This pathway consists of higher precipitation and more soil moisture.
Pathway 1.2A
Community 1.2 to 1.3
This pathway consists of lower precipitation and less soil moisture.
Pathway 1.3A
Community 1.3 to 1.1
This pathway consists of higher precipitation and more soil moisture.
Pathway 1.3B
Community 1.3 to 1.2
This pathway consists of higher precipitation and more soil moisture.
State 2
Timber Plantation
The Timber Plantation State consists of merchantable trees planted to maximize production and wood volume. Common species include loblolly pine and oak. Important ecological drivers in this state include timber type (softwood or hardwood) and harvest method.
The main drivers for community pathways in this state are prescribed fire, pest management, vegetation management, and canopy density.
Timber harvesting is the main feedback mechanism for this state. Management focuses on increasing the growth of cultivated species.
Community 2.1
Loblolly Pine
State 3
Pasture
Species that improve forage quality dominate the Pasture State. The quality and quantity of forbs and grass species within this state depend on the level of management inputs (seeding, weed management, and land uses). Both warm-season and cool-season forages are suitable for this ecological site.
The main drivers for community pathways in this state are the mechanical disturbance of the soil and seed planting, climate effects (decadal scale), seed dispersal, and wildlife or livestock grazing or browsing.
The main feedback mechanism in this state is the use of mechanical equipment or chemicals to maintain a productive pasture. Wildlife or livestock grazing or browsing reduces the amount of available forage. Fertilizer inputs and brush management are essential for maintaining high productivity.
Community 3.1
Bermudagrass
Transition T1A
State 1 to 2
The main triggers for this transition are the planting of merchantable trees, targeted suppression of vegetation, prescribed fire, and fertilization.
The main slow variables for this transition are increased production and management of merchantable trees, alongside tree thinning when appropriate.
The main thresholds for this transition are the removal of existing vegetation and the planting of timber 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.
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 a 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.
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.
Restoration pathway R3A
State 3 to 1
Restoration efforts for this pathway include the removal of introduced species, cessation of suppression activities affecting desirable species, planting of native species, and regeneration. Introduced species may need removing several times before native species can flourish.
Transition T3A
State 3 to 2
The main triggers for this transition are the planting of merchantable trees, targeted suppression of vegetation, prescribed fire, and fertilization.
The main slow variables for this transition are increased production and management of merchantable trees, alongside tree thinning when appropriate.
The main threshold for this transition is the suppression of introduced forage species due to management strategies and shading from the canopy.
