Ecological dynamics

The distribution of the Northern Non-acid Moderately Wet Floodplain is primarily restricted to portions of the MLRA that are covered by a very thick mantle of loess. Development of this site is greatest within and along the edges of the highly dissected Loess Hills and western boundary of the Loess Plains. Based on the distribution of soils, occurrence of this site is primarily located in the northern half of the MLRA, however a small extension of nonacid soils occur in the extreme southern end of the site in Mississippi.

This site occurs in a number of geomorphic situations that include small to broad floodplains and alluvial fans. The site’s occurrence within small to moderately broad floodplains are generally associated with larger stream systems that drain the Loess Hills. Here, the site forms a close association with the Northern Non-acid Floodplain ecological site. The principal difference between the two is that the somewhat poorly drained soils of this site occur on slightly lower and wetter positions within the respective floodplains. In areas of western Kentucky and Tennessee, this site occurs on broad floodplains of larger stream systems that head and flow along the transition of the Loess Hills and Loess Plains. Alluvial fans of this site are fairly common, but they are mostly restricted to the interface of the Loess Hills and the edge or boundary of the Southern Mississippi Valley Alluvium (MLRA 131A) and secondarily on old fluvial terraces and active floodplains of large streams and medium-sized rivers.

A key characteristic of this site pertains to a distinct gradation of soil wetness. When viewed from the perspective of a soil catena, this site lies in the middle of the classic floodplain moisture gradient. This gradient is also reflected in the plant community. Species requiring drier conditions (e.g., tuliptree, American beech, and walnut) begin dropping out and are replaced by plants more tolerant of anaerobic conditions. Overstory components generally include a broad range of hardwood species that may consist of eastern cottonwood, green ash, sweetgum, American sycamore, sugarberry, bitternut hickory, and several oak species including Nuttall, water, willow, overcup, and cherrybark, the latter restricted to micro-highs or drier positions within the site. Understory density and cover is typically lower than better drained sites and ground cover may consist of a mixture of facultative wetland and wetland obligate species, depending on local conditions.

This ecological site is provisionally separated from similar floodplain sites that have acid soils. It is hypothesized that the nonacid reactions and active to superactive cation exchange capacity collectively contributes to very high productivity of these soils. The central factor believed to contribute to these important properties is the deep loess soils that cover the upland landscapes. The eroded loess collects on the low-lying floodplains and alluvial fans that ultimately define this site. On the most favorable sites, potential productivity (i.e., site index) of individual trees can be impressive. Site indices (represents 50 years of growth; cottonwood, 30 years) of some bottomland hardwoods on this site (e.g., American sycamore) are estimated to range from 110 to 130 feet (Broadfoot, 1976).

Forest stand disturbances vary in both size and type. Disturbances range from gap-scale (single tree to small group) to stand-initiating events that are greater than one acre (per Johnson et al., 2009). Smaller gaps or forest openings may result in the release of suppressed understory components, but the greatest response is often ingrowth or expansion of the surrounding canopy (Oliver and Larson, 1990). Understories of long-term, non-disturbed portions of the stand (i.e., complete canopy closure) are typically comprised of shade-tolerant woody and herbaceous species. Larger gaps often consist of heavy, downed woody debris and a dense concentration of shrubs, forbs, vines, and released saplings and young trees. Types of disturbances may include wind, severe ice storms, and beaver. The influence of the latter is perhaps the most dramatic as local hydrologic regimes are dramatically altered leading to wetter soils, different vegetation communities, and a different suite of ecological processes.

An additional disturbance factor that rarely occurs on this site today but is thought to have been an important historical influence is fire. This supposition is drawn from the presence of a single species: cane. Cane grows readily on this site and historically, extended across many floodplains of the Southeast (Gagnon, 2009). The sheer presence of this species alone in the historic community suggests disturbance beyond flooding alone. Fire may have been an important disturbance factor in the pre-settlement bottomland community (see Gagnon and Platt, 2008; Gagnon, 2009), which suggests that the structure of this site may have been more open where dense canebrakes existed. However, any vestige of that system is long past. Those areas that have been allowed to revert naturally are now best characterized as closed-canopied, bottomland hardwoods.

The principal land use, today, is agriculture production with some areas in pasture and/or forage production. The fertile soils of the broader floodplains are almost exclusively cropland where channelization and levee construction have occurred. Timber production is a minor land use, especially within protected areas but may be more common on floodplains that have not been channelized or leveed.

Perhaps the largest and most significant alteration to this site has been channelization and levee (or spoil bank) construction. Such hydrologic alterations have become the norm for floodplains in this MLRA. This action results in a disconnection between the stream - floodplain environment, which interrupts and alters the ecological processes and functions of the system as a whole.

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 this ecological site. This model is based on limited inventories, literature, expert knowledge, and interpretations. Plant communities will differ across MLRA 134 due to natural variability in climate, soils, and physiography. 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.