Ecological dynamics
The Poorly Drained Flood Plain reference state consists of a bottomland hardwood forest that is periodically flooded throughout the year. The common trees species for this state are sweetgum, sycamore, hickory, oak, hackberry, elm, and loblolly pine (Eldredge, 1937).
Flooding is a major ecosystem disturbance on this ecological site. Ecosystems are affected differently by flooding depending on the duration, time of year, and water stagnation. Species diversity has been shown to decrease with increased flooding duration. Nutrients and seeds are transported and distributed throughout the landscape during flooding events (Smith and Callahan, 1983). Flooding during the dormant season does not have negative effects on species diversity and growth (Bedinger, 1979).
Fire has some influence on this ecological site during dry years. High precipitation throughout the year will decrease fire behavior due to proximity with riparian areas. The historical average fire return interval was likely between 3 and 25 years (Guyette and Spetich, 2003; Hallgren, DeSantic, and Burton, 2012). These wildfires would occur naturally through lightning strikes, but the majority were probably ignited by anthropogenic sources (DeSantis, Hallgren, and Stahle, 2010).
Climate related events, such as hail-storms, tornados, thunderstorms, and extreme precipitation, occur on these sites. 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 hail-storms (Gower et al., 2015). Tornados have been shown to change plant community compositions in savanna ecosystems, favoring hardwoods and eliminating softwoods (Liu et al., 1997). Thunderstorms greatly effect ecosystem dynamics. Thunderstorms generally occur during summer months but can occur during every season. If a fire is started by a lightning strike, there will be different effects in the ecosystem depending on the season (Hiers, Wyatt, and Mitchell, 2000).
Grazing and farming can occur on this ecological site. Changes to the ecological dynamics are proportional to the intensity of livestock grazing and can be accelerated by overgrazing (Angerer, Fox, and Wolfe, 2013; Kohl, 2016). For example, desirable grasses and forbs are repeatedly grazed by livestock, weakening, and potentially killing or replacing these species with less desirable species (Smith, 1940).
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 will be collected to provide a greater understanding of the ecological dynamics, as well as the resources consumption and distribution.
State 1
Reference
The Reference State is representative of the natural range of variability without major anthropogenic influences.
Drivers: Flooding duration and frequency, climate (decadal scale), insect and disease presence or establishment, wildlife grazing or browsing, and fire frequency.
Feedbacks: Water tolerant tree species dominate this ecological site. Flooding events limit what species can grow and survive inundation.
Characteristics and indicators. The reference state consists of a bottomland hardwood forest that is periodically flooded throughout the year.
Community 1.1
Water Hickory-Overcup Oak
Water Hickory and Overcup Oak.
Community 1.2
Nuttall Oak- Southern Hackberry- Sweetgum
Nuttall Oak, Southern Hackberry, and Sweetgum.
Community 1.3
Shagbark Hickory- Southern Red Oak
Shagbark Hickory and Southern Red Oak.
Pathway 1.1A
Community 1.1 to 1.2
This pathway is characterized by periods of lower precipitation and decreased flooding.
Pathway 1.2B
Community 1.2 to 1.1
This pathway is characterized by periods of higher precipitation and increased flooding.
Pathway 1.2A
Community 1.2 to 1.3
This pathway is characterized by periods of lower precipitation and decreased flooding.
Pathway 1.3A
Community 1.3 to 1.1
This pathway is characterized by periods of higher precipitation and increased flooding.
Pathway 1.3B
Community 1.3 to 1.2
This pathway is characterized by periods of higher precipitation and increased flooding.
State 2
Encroached
The encroached state is dominated by woody species.
Driver: Absence of fire, seed dispersal by wildlife, climate (decadal scale), and canopy density.
Feedbacks: Woody species dominate the ecological site, shading herbaceous species. As herbaceous species are outcompeted for resources, fire frequency decreases. Nutrient and water cycling are controlled by woody species.
Characteristics and indicators. The Encroached State consists of many woody species, especially eastern redcedar, where there is significant canopy closure. Time and fire frequency determine the community phases and species abundance and variation. As the woody canopy increases the hydrology of the site is altered. The increased canopy intercepts most of the precipitation. Understory species have less available water for growth and must compete with an extensive overstory root system.
State 3
Pasture
The Pasture State is characterized by the dominance of improved forage species. The 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.
Drivers: Mechanical soil disturbance and seed planting, climate (decadal scale), seed dispersal, and wildlife or livestock grazing or browsing.
Feedbacks: Land managers use mechanical and chemical equipment to increase forage. Inputs of fertilizer and brush management are required to maintain high productivity. Wildlife and livestock grazing and browsing decrease the amount of available forage.
Characteristics and indicators. The Pasture State consists of species that are grown for specific management goals, mainly livestock grazing. Common pasture species include buffalograss, western wheatgrass, little bluestem, sideoats grama, Bermudagrass, and bahiagrass. Quality and quantity of forb, grass, and legume species within this state depend on the level of management inputs (seeding, weed management, and land uses). Species of both warm-season and cool-season grasses are feasible for these sites.
State 4
Plantation
The plantation state is characterized by the planting of merchantable trees species. The most common species for a plantation is loblolly pine. Community phases differ by tree type (softwood or hardwood) and the harvesting process.
Drivers: Prescribed fires, pest management, vegetation management, canopy density.
Feedbacks: Timber harvesting. Planted tree species dominate this ecological site, shading out other vegetation. Anthropogenic management decreases competition with other species and assists in growth.
Characteristics and indicators. A plantation state consists of tree species that are planted and managed to maximize the production of merchantable timber. The most common plantation species is loblolly pine, followed by hardwood trees. Community phases differ by tree type (softwood or hardwood), timber harvest method, management, and reforesting practices.
Transition T1C
State 1 to 2
Trigger: The absence of wildfire allows woody species to increase and outcompete herbaceous species for nutrients, water, and sunlight.
Slow variables: Increased competition for sunlight, nutrients, and moisture resources. Increased overstory competition results in decreased vigor and reproductive capacity of herbaceous understory species.
Thresholds: Nutrient cycles shift from grass-and-leaf dominance to leaf-and-needle dominance. Increased woody canopy cover alters hydrologic cycles, potentially increasing runoff, decreasing infiltration, and increasing precipitation interception to woody species.
Transition T1B
State 1 to 3
Trigger: Mechanical and chemical woody vegetation suppression, tillage, and annual forage species introduction.
Slow Variables: Increase production and management of forage species.
Thresholds: Changes in soil properties, such as structure, organic matter, and nutrient cycling, as well as changes in type and frequency of disturbance.
Transition T1A
State 1 to 4
Trigger: Native tree removal, mechanical and chemical woody vegetation suppression, introduce plantation tree species.
Slow Variables: Increased production and management of plantation species.
Thresholds: Changes in soil properties such as structure, organic matter, and nutrient cycling as well as changes in type and frequency of disturbance.
Transition T2A
State 2 to 3
Trigger: Mechanical and chemical woody vegetation treatment, tillage, and forage species introduction.
Slow Variables: Increase production and management of forage species.
Thresholds: Changes in soil properties, such as structure, organic matter, and nutrient cycling, as well as changes in type and frequency of disturbance.
Transition T2B
State 2 to 4
Trigger: Native tree removal, mechanical and chemical woody vegetation suppression, introduce plantation tree species.
Slow Variables: Increased production and management of plantation species.
Thresholds: Changes in soil properties such as structure, organic matter, and nutrient cycling as well as changes in type and frequency of disturbance.
Transition T3B
State 3 to 2
Triggers: Lack of management or abandonment.
Slow Variables: Increase in the establishment and size of woody species.
Thresholds: Woody species dominate ecological processes. This reduces vigor and reproduction of understory species due to shading and increased competition for soil moisture, nutrients, and sunlight.
Transition T3A
State 3 to 4
Trigger: Forage species removal and suppression, mechanical and chemical woody vegetation suppression, introduce and manage plantation tree species.
Slow Variables: Increased production and management of plantation species.
Thresholds: Changes in soil properties such as structure, organic matter, and nutrient cycling as well as changes in kind and frequency of disturbance.
Transition T4B
State 4 to 2
Triggers: Lack of management or abandonment.
Slow Variables: Increase in the establishment and size of woody species.
Thresholds: Woody species dominant ecological processes resulting in reduced vigor and reproduction of herbaceous species in the understory due to shading and increased competition for soil moisture, nutrients, and sunlight.
Transition T4A
State 4 to 3
Trigger: Tree removal, mechanical and chemical woody vegetation suppression, tillage, introduce annual or perennial forage species.
Slow Variables: Increase production and management of forage species.
Thresholds: Changes in soil properties such as structure, organic matter, and nutrient cycling as well as changes in type and frequency of disturbance.
