Northern Gulf Coast Loess Bluff Forest

This system is largely confined to steep bluffs bordering the northern portion of the eastern edge of the Mississippi River Alluvial Plain. The geology is typically mapped as the Jackson Formation. These bluffs extend up to 150 m (500 feet) in elevation and from 30 to 60 m (100-200 feet) above the adjacent plain. They consist of a belt of Pleistocene and Tertiary eolian deposits that are often deeply eroded and very steep, with fertile topsoil and abundant moisture. The vegetation is often richer than surrounding non-loessal areas, or those with only thin loess deposits. The forests found on these bluffs are intermediate in soil moisture for the region and may best be thought of as mesic. The vegetation may sometimes be referred to as western mesophytic forest and may share some superficial similarities with cove forests of the Interior Highlands. In many cases, these bluffs provide habitat for plant species that are rare or absent from other parts of the Coastal Plain. The composition of these forests changes from north to south along the bluffs; more southerly examples are represented by the East Gulf Coastal Plain Southern Loess Bluff Forest (CES203.556), and these would contain Magnolia grandiflora as an important component. As currently defined this system ranges northward from about 32°N latitude (where the Big Black River cuts through the bluffs), and occurs only in the westernmost portions of the Upper East Gulf Coastal Plain, including northern and central Mississippi, western Tennessee, and western Kentucky, being restricted to the northern part of the Loess Bluff Hills (EPA Ecoregion 74a).

Examples of this system have deciduous canopies dominated by Fagus grandifolia or this species in combination with Quercus alba. The most mesic stands may lack codominance by Quercus spp. In addition, a variety of other hardwood species may also be found in the overstory, including Liriodendron tulipifera, Liquidambar styraciflua, Acer rubrum, Nyssa sylvatica, Fraxinus americana, Magnolia acuminata (of local distribution), and Pinus taeda (in more southern stands). This system is defined as being north of the range of Magnolia grandiflora, which excludes the "Beech-Magnolia" forests of the southern loess bluffs. Some subcanopy components (in addition to canopy species) include Carpinus caroliniana, Diospyros virginiana, Oxydendrum arboreum, Cornus florida, Acer floridanum (= Acer barbatum), Magnolia macrophylla, Ostrya virginiana, Ulmus alata, and Ilex opaca. Other shrubs and woody vines include Decumaria barbara, Rhododendron canescens, Toxicodendron radicans, Vitis rotundifolia, and Smilax glauca. Important herbs include Polystichum acrostichoides, Woodwardia areolata, Osmunda cinnamomea, Mitchella repens, and Hexastylis arifolia. In many cases, these bluffs provide habitat for plant species that are rare or absent from other parts of the Coastal Plain, such as Magnolia acuminata, Aralia racemosa, and Hydrophyllum canadense (Chester et al. 1997).

This system is largely confined to the lower portions of steep bluffs east of the Mississippi River. These bluffs consist of a belt of Pleistocene and Tertiary eolian deposits (Braun 1950) that are often deeply eroded and very steep, with fertile topsoil and abundant moisture (Miller and Neiswender 1987). The core of this is mapped as the Jackson Formation (Hardeman 1966) and corresponds more broadly with Ecoregion 74a (Bluff Hills) (EPA 2004). These bluffs border the eastern edge of the Mississippi River Alluvial Plain from about 32°N latitude (where the Big Black River cuts through the bluffs) northward to western Tennessee and Kentucky. Examples may extend up to 150 m (500 feet) in elevation and from 30 to 60 m (100-200 feet) above the adjacent Mississippi Alluvial Plain. In Tennessee the loess soils may be 9-27.5 m (30-90 feet deep) (Springer and Elder 1980).

These are stable, generally fire-sheltered forests. These forests probably generally exist naturally as old-growth forests, with canopy dynamics dominated by gap-phase regeneration. As modeled here, replacement disturbance is over 60% and more likely due to weather-related events than fire. Included among these are windthrow, lightning, and ice damage, as well as the inclusion of the erosion and mass wastage (Bryant et al. 1993) that give the bluffs their characteristic steepness. Widespread insect or disease mortality has not been reported. Wind/weather/stress replacement frequency is modeled near 240 years, replacement fire return at approximately 385 years, and all fire return frequency at about 85 years. "Open" structure is uncommon, even when defined as canopy closure less than 81%, and may be created by mixed-severity fire. Surface fire may maintain open conditions, but it does not transition closed classes. Disturbance is presumed to mirror mixed mesophytic forest, occurring primarily in small gaps (less than one-quarter acre), although the occurrence of aggregates of intolerant species suggests that larger scale disturbances occasionally play a role (Landfire 2007a). In addition, periodic droughts may cause death of or stress to moisture-requiring canopy trees. There is presumably some natural disturbance from the effects of windstorms, which are relatively frequent in the range of this system.

Conversion of this type has primarily resulted from repeated canopy removal through logging, which is also the most critical anthropogenic threat. These sites were historically less frequently logged than the adjacent pine-dominated uplands, with more desirable species being removed in preference to Fagus grandifolia, which is less desirable in the lumber trade. In addition, some mesic hardwood forests in more moderately dissected terrain have been converted to pine plantations or impacted (destroyed or fragmented) by agriculture. Bluff habitats are often prime sites for development, especially along major rivers. Complete devastation by natural agents was probably very rare in this forest type (Batista and Platt 1997). These forests also suffer the effects of ozone and acidic atmospheric deposition.

Aside from actual site conversion, feral hogs (Sus scrofa) represent one of the greatest threats to biodiversity in these forests (Engeman et al. 2007). They can be especially difficult to control in sensitive slope forests (Edwards et al. 2013). Invasive exotic species, including Lonicera japonica and Ligustrum sinense, can become dominant in the ground and shrub layers following canopy disturbance. The most significant potential climate change effects over the next 50 years include periods of drought, which will affect the health and survival of the moisture-requiring trees, as well as increase the probability of damaging wildfire.

This system is endemic to the loess bluffs ("Bluff Hills" [Ecoregion 74a] of EPA (2004)) along the eastern edge of the Mississippi River Alluvial Plain in Mississippi, Tennessee, and Kentucky.