Ecological dynamics
The Northern Ponded Loess Terrace is characterized by shallow depressions on old fluvial terraces. The site typically occurs as inclusions within the larger hydroxeric flatwoods community (the latter defined here as the matrix community; also described as the Northern Wet Loess Terrace ecological site). These features typically pond during the wetter times of the year, which generally occurs from winter into spring. Hydrologically, these depressions may be classed as open or “flow-through” systems or closed, meaning no outlet. Most are thought to represent the latter, but based on topographic examinations of these features, a few are indicated as having “blue line” streams running through or beginning within their location.
Most ecological descriptions of flatwoods systems make mention of shallow depressions and their influence on vegetation (e.g., NatureServe, 2009; Bryant, 1999), but they are rarely defined or distinguished as a separate community. Terrace depressions are recognized here as a distinct ecological site given their conservation significance as essential breeding habitat for amphibian and invertebrate communities and for their overall contribution to species and site diversity. Plant communities occurring within terrace depressions generally support a greater concentration of wetland species than the surrounding matrix flatwoods community.
Dimensions or aerial extent of individual depressions vary widely with some encompassing an area less than 0.5 acre to others spanning 50 acres (USDA-NRCS, 1995). Depths within and among these features vary widely as well. Some sites are shallow and remain ponded only during the wettest periods of the year, whereas others attain depths over three feet and remain ponded for longer durations, often through the spring and sometimes into early summer. These variations in size and depth have tremendous influences on associated vegetation. Shallower sites or zones within a depression may support species generally associated with better drainage such as Shumard’s oak and cherrybark oak. Deeper areas often consists of species often associated with wetter conditions such as willow oak, pin oak, and overcup oak. Although rare, the deepest sites may consists mainly of wetland obligates such as bald cypress, overcup oak, buttonbush, and swamp cottonwood.
An additional contributor to the range of variability for this site is its geographic extent. The distribution of this site from western Kentucky to the Southern Rolling Plains in southwest Mississippi contributes to species differences north to south. For example, in western Kentucky swamp white oak occurs locally and to the south, loblolly pine may occur in some shallow depressions.
Since settlement of the region, this site has incurred a number of alterations and impacts. Most impacts probably resulted from a rippling effect as the surrounding matrix (flatwoods) community was being converted to agriculture, pastureland, and/or timberland. Seasonally wet depressions were often recognized as low spots and were simply avoided. As demands for more intensive production increased, these sites were sometimes targeted for drainage and conversion to cropland. Today, many local occurrences of this site are recognizable from a distance because they generally consist of a circle of trees situated in the middle of large agriculture fields.
This ecological site is as rare today as the matrix community within which it is inextricably associated. Impacts that are incurred to the surrounding flatwoods system are filtered into this site. Accordingly, reference conditions of this ecological site are entirely dependent upon the conditions of its matrix system, the Northern Wet Loess Terrace.
A single community phase is identified to represent the range of natural variability of this site and is entirely contingent upon the existence and functionality of the surrounding flatwoods community. Reference conditions of this site are not found in isolated patches of trees that are situated in open fields.
This ecological site has limitations of varying severity for all production purposes (i.e., cropland, pastureland, and timberland), mainly due to seasonal wetness. Two major altered states are recognized for this site: 1) the altered matrix community with depressions undrained; and 2) altered hydrology whereby some form of drainage system has been constructed to limit seasonal ponding. This approach is somewhat unconventional and should be regarded as “provisional”. Future inventories and project development may lead to complete readjustments of the currently defined reference state and associated altered states.
Following this narrative, a “provisional” state and transition model is provided that includes the “perceived” reference state and several alternative (or altered) vegetation states that have been observed and/or projected for the Northern Ponded Loess Terrace ecological site. This model is based on limited reconnaissance, literature, expert knowledge, and interpretations. Plant communities will differ across MLRA 134 due to natural variability in climate, soils, and physiography. Some of the presented alternate states may warrant removal as additional information is learned and/or becomes available. Depending on objectives, the reference plant community may not necessarily be the management goal.
The environmental and biological characteristics of this site are complex and dynamic. As such, the following diagram suggests pathways that the vegetation on this site might take, given that the modal concepts of climate and soils are met within an area of interest. Specific locations with unique soils and disturbance histories may have alternate pathways that are not represented in the model. This information is intended to show the possibilities within a given set of circumstances and represents the initial steps toward developing a defensible description and model. The model and associated information are subject to change as knowledge increases and new information is garnered. This is an iterative process. Most importantly, local and/or state professional guidance should always be sought before pursuing a treatment scenario.
State 1
Hydroxeric Flatwoods Depressions
The Northern Ponded Loess Terrace is inextricably linked to the larger matrix community within which it belongs, the Northern Wet Loess Terrace or hydroxeric flatwoods. Any severe alteration that changes composition or functionality of the matrix system affects this site. Therefore, reference conditions of these seasonally ponded flatwood depressions are defined by and occur within reference sites of the matrix community.
Classifying this system with a single plant association name is extremely difficult, if not impossible, due to the variability of species dominance from one depression to another. Composition of this site appears to be directly related to aerial extant and more importantly, depth of individual depressions. Collectively, size and depth of these surface concavities ultimately determine ponding duration – that and the amount of precipitation that has fallen within the wet season. Sites that are relatively shallow and ponded from winter into mid-spring typically support a mixed-oak stand that often consist of species found in drier habitats. Deeper sites that remain ponded for longer periods (some into early summer, precipitation depending) may support a greater number of wetland obligate species.
From a conservation and biodiversity perspective, seasonally ponded (closed) depressions or vernal pools are recognized for the critically important functions and values they provide. This site contributes to the biodiversity of the overall flatwoods system by supporting a number of additional wetland components that may not occur otherwise. These features may also support rare and vanishing plant species and serve as vital habitat for an entire zoological system.
Of concern, portions of this site have incurred tremendous alteration due to drainage. Any attempt to reestablish perceived reference conditions of a stand or a local site must first restore the natural hydrology of that location, which may entail removing drainage structures. If not, management may improve stand structure and even composition to a degree, but the site, overall, will remain in an altered state relative to reference conditions.
Community 1.1
Mixed Oak Seasonally Ponded Depression
This community phase represents the compositional, hydrological, and structural complexity of stands supporting perceived reference conditions. With no intact example of a pre-settlement community remaining, this phase is arbitrarily chosen to represent the range of conditions that exist.
Composition of this community is quite variable and highly dependent upon ponding depth and duration. Canopy components of shallow sites (includes the wetted margin of deeper depressions) may consist of Shumard’s oak, cherrybark oak, willow oak, water oak, pin oak, swamp chestnut oak, sweetgum, green ash, red maple, and American elm with overcup oak occurring in deeper spots. Locales in the northern extent of this site may include swamp white oak and occasionally, northern red oak. Understory vegetation may include black willow, possumhaw, American snowbell, sawtooth blackberry, smallspike false nettle, catchfly grass, woolgrass, blunt broomsedge, hairy sedge, along with many additional sedges, rushes, and mosses. Although more rare, deeper sites (greater than 2 feet deep) have been observed to support stands of bald cypress, buttonbush, and small patches of swamp cottonwood; overcup oak is generally present in most all deeper depressions. Loblolly pine may be an additional component of this site to the south in Mississippi.
Ground cover is highly dependent on ponding duration. Sites that are ponded for longer periods (e.g., winter into early summer) generally have a ground cover mainly comprised of leaf litter and detritus.
State 2
Altered Matrix Community – Undrained
This ecological site is inextricably associated to the matrix community within which it occurs. Impacts to the surrounding flatwoods system are filtered into this site. Therefore, this state represents many of the early impacts to the flatwoods system during and after settlement. This state is believed to represent the prevailing condition of this site, today.
A key criterion of this state is that the geomorphological characteristics of the depressions are not destroyed and that these surface concavities still pond seasonally. The surrounding matrix community may sustain varying levels of impacts from land use actions that include forestry practices, pasturage, and cropland. It is acknowledged that each of the preceding land uses will exert varying levels of impacts on isolated depressions, and each land practice may transition this site to completely different altered states. However, adequate information to describe those potential differences (or altered states) are unknown at this time. Future inventories may lead to readjustments of this state.
It is worth noting that all areas currently supporting perceived reference conditions were once in this particular altered state. There are no known examples of a pre-settlement reference site. Conceivably, if formerly impacted sites (from timber, grazing, and cropland) can support perceived reference conditions, today, then surely areas currently under varying land uses have that same potential, provided that soil properties, drainage patterns, and geomorphic characteristics have not been destroyed or severely altered.
Community 2.1
Mixed Oak – Vine Depression
This community phase represents the composition and condition of many isolated depressions that occur within timberland, pastureland, and/or cropland. Obviously, composition may vary under the different land uses, especially if particular tree species within the depressions are harvested. The community phase will also change if trees are removed and the area kept in herbaceous cover. However, there are a number of examples where trees have been retained in these isolated depressions.
Examples of forested depressions in open fields often consist of species found in reference sites, except there are usually some select species harvesting. Species occurring in these sites may consist of pin oak, willow oak, water oak, overcup oak, red maple, black cherry, winged elm, American elm, and green ash. The most notable difference in these sites surrounded by open areas is the profuse entanglement of greenbrier. Greenbrier is often the dominant vine in these sites, providing as much as 90 percent ground cover locally.
State 3
Altered Hydrology - Drained
This state is representative of areas that have had drainage structures installed to meet production objectives. Many of the drained depressions have been cleared of vegetation and included within the greater production system, which was either cropland or pastureland. In pastureland settings, some of these depressions were not drained but retained as stock or cattle ponds. Draining these seasonal ponds also assisted with forest or timberland production.
Areas that have been simply ditched may have restorative capacity. However, those areas that have been drained and had the micro-topographic features of the depression removed (bulldozing, leveling, filling, etc.), are lost. Such former sites have been essentially erased.
Community 3.1
Production – Cropland / Pastureland / Timberland
This phase is representative of the varying land uses associated with depressions that have been drained. Although the intensity of each land use differs, impacts to the site are similar: draining these depressions removes their ecological functions and values. Sites that have not been filled or physically leveled may be reparable. For those that have, this state and phase serves as an endpoint.
State 4
Conservation
This alternative state is included to represent the range or breadth of conservation actions that may be implemented and established should other land uses be discontinued within a given location. Several actions may be chosen including the restoration of the site’s natural hydrology (removal of drainage structures) and establishing native warm season grasses, suitable forbs for pollinators, and/or native trees and managing for forest or woodland conditions. If at all possible, the herbaceous species established should be derived from the “wild types” (genetic stock) from the Loess Plains or from adjoining ecoregions. This action would help preserve the unique genetic material from the area and would help to reintroduce the native herbaceous varieties back into a portion of their former range.
Community 4.1
Native Herbaceous or Woodland
Maintaining this site’s capacity for seasonal ponding is key to any conservation action associated with this particular site. Beyond that, this community phase represents the establishment of select native plants to meet conservation objectives into the larger matrix flatwoods community (Northern Wet Loess Terrace). As alluded to above, the best case scenario is the establishment of native species selected from the genetic stock of the Loess Plains or neighboring ecoregions. Herbaceous species suitable for establishing on the larger flatwoods community include Indian grass, little bluestem, gama-grass, threeawn, wild oat grasses, panic grass, blazing star, evening-primrose, asters, sunflowers, goldenrod, tickseed, coneflowers, rattlesnake master, mountain mints, agave, milkweeds, sedges, among many others. Additional study is needed on this site before a complete set of plants can be generated. Key to the perpetuation and maintenance of an herbaceous community is frequent fire, generally on a 1 to 3 year return interval (judgement based on early accounts of frequent burning; e.g., Loughridge, 1888).
Transition T1A
State 1 to 2
This pathway represents what generally occurred historically where well-defined flatwoods depressions were left mostly intact. Some timber harvests to remove desirable species was certainly conducted, but the ponding capabilities of the depressions were not altered (i.e., drained). However, the surrounding matrix community, the hydroxeric flatwoods, incurred alteration from heavy cutting, grazing, and then a final conversion to open cropland and/or pastureland.
Transition T1B
State 1 to 3
This pathway represents dramatic alteration to the function and characteristics of this site by ditching and draining depressions in addition to alteration of the matrix community.
Restoration pathway R2A
State 2 to 1
This pathway signifies a return to perceived reference conditions where the hydrologic characteristics of the flatwoods community and associated depressions were never severely altered. Natural succession is the principal process leading back to the perceived reference state. This pathway is representative of most “protected” flatwoods in existence, today.
Transition T2A
State 2 to 3
This pathway represents an attempt to drain these depressions for wider coverage of and increase in production (includes timberland, pastureland, and cropland).
Transition T2B
State 2 to 4
This pathway represents the decision to discontinue production of the surrounding matrix community and to establish native grasses/forbs or trees on this site. This action also includes management activities to “guide” natural succession and conservation maintenance. Actions may include prescribed fire for maintaining and enhancing herbaceous establishment of the matrix community and herbicide treatments for controlling exotic species invasions and to ensure select tree establishment.
Transition T3A
State 3 to 4
This pathway represents the decision to discontinue production of the surrounding matrix community and to establish native grasses/forbs or trees on this site. This action also includes management activities to “guide” natural succession and conservation maintenance. Actions may include prescribed fire for maintaining and enhancing herbaceous establishment of the matrix community and herbicide treatments for controlling exotic species invasions and to ensure select tree establishment. This action may also include the removal of drainage ditches, tiles, or other water control structures.
Restoration pathway R4A
State 4 to 1
This pathway signifies a return to perceived reference conditions where the hydrologic characteristics of the flatwoods community and associated depressions were never severely altered. Natural succession is the principal process leading back to the perceived reference state. This pathway is representative of most “protected” flatwoods in existence, today.
Transition T4A
State 4 to 2
This pathway represents the discontinuation of conservation practices and a return to production with isolated depressions remaining intact.
Transition T4B
State 4 to 3
This pathway represents the discontinuation of conservation practices and a return to production with isolated depressions being altered by draining and/or mechanically erased.
