Gulf Coast Small Stream Floodplain Forest

This is a predominantly forested system of the East Gulf Coastal Plain associated with small brownwater rivers and creeks. In contrast to East Gulf Coastal Plain Large River Floodplain Forest (CES203.489), it has fewer major geomorphic floodplain features typically associated with large river floodplains. Those features that are present tend to be smaller and more closely intermixed with one another, resulting in less obvious vegetational zonation. Bottomland hardwood tree species are typically important and diagnostic, although mesic hardwood species are also present in areas with less inundation, such as upper terraces and possibly second bottoms. As a whole, flooding occurs annually, but the water table usually is well below the soil surface throughout most of the growing season. Areas impacted by beaver impoundments are also included in this system.

Examples of this system may include a number of different plant communities, each with distinctive floristic compositions. Drew et al. (1998) described vegetation attributable to this systems as including the following species: Carya glabra, Magnolia grandiflora, Quercus virginiana, Liquidambar styraciflua, Acer floridanum (= Acer barbatum), Fraxinus americana, Fraxinus caroliniana, Celtis laevigata, Sabal minor, Ditrysinia fruticosa (= Sebastiania fruticosa), Serenoa repens, and Itea virginica. Smaller-scale features may be dominated by shrubs (Cephalanthus occidentalis, Decodon verticillatus) and/or perennial and annual herbs.

This system is associated with small brownwater rivers and creeks of the East Gulf Coastal Plain. It is confined to floodplains or terraces of streams and creeks. This system is dependent on a natural hydrologic regime, especially annual to episodic flooding. These landscapes usually encompass a variety of habitats resulting from natural hydrological spatial patterns (i.e., meander scars, sloughs, gravel bars, old depressions, and/or oxbows are present). Most component associations are temporarily flooded, with the possible addition of smaller-scale seasonally flooded features such as beaver-created herbaceous wetlands and shrub-dominated features. Some larger examples of this system include the Escambia, the Yellow (Alabama, Florida), the Choctawhatchee, the Chattahoochee, and the Flint rivers.

In pre-European settlement forests, community diversity in these bottomland systems was much more complex than in the modified landscapes of today. Fire, beaver activity, and flooding of varied intensity and frequency created a mosaic whose elements included canebrake, grass and young Betula-Platanus beds on reworked gravel or sand bars, beaver ponds, and grass-sedge meadows in abandoned beaver clearings, as well as the streamside zones and mixed hardwood and/or pine forests that make up more than 95% of the land cover that exists today.

Flooding is the principal disturbance in this system. When flooded, these systems may have a substantial aquatic faunal component, with high densities of invertebrates, and may play an important role in the life cycle of fish in the associated river. Unusually long or deep floods may stress vegetation or act as a disturbance for some species. Flood waters have significant energy. Larger floods cause local disturbance by scouring and depositing sediment along channels and occasionally causing channel shifts. There are two general types of floods: occasional catastrophic, prolonged floods (due to beaver activity or other severe event); and more frequent repeated minor flooding (i.e., several minor floods within a 10 year period). Flooding is more frequent on the lower terraces but frequently floods higher terraces (Wharton et al. (1982) zones IV and V). Catastrophic floods can cause the loss of canopy over large areas. Canopy decline and reproductive failure can create late-seral open stands. Duration of flooding varies with the placement of a site in the landscape and is a dominant process affecting vegetation on a given site. Flooding can deposit alluvium or scour the ground, depending on the landscape position of a site and the severity of the flood event. The sorting of plant communities by depositional landforms of different height suggest that wetness or depth of flood waters helps drive this process. Scouring and reworking of sediment make up an important factor in bar and bank communities. In addition to disturbance, floods bring nutrient input, deposit sediment, and disperse plant seeds (Landfire 2007a).

In addition to periodic flooding, the dominant ecological process in bottomland hardwood forests is the formation of windfall gaps, which can occur on the local scale (a single mature canopy tree) as well as the landscape scale (effects of tornadoes or hurricanes). Except for primary successional communities such as bars, most forests exist naturally as multi-aged old-growth forests driven by gap-phase regeneration. Windthrow is probably the most important cause of gaps, and is the primary cause of mortality in bottomlands. Major storms or hurricanes occurring at approximately 20-year intervals would have impacted whole stands. When canopy trees fall, seedlings in the understory are released and compete for a spot in the canopy. This leads to dense areas of herbaceous and woody vegetation in windfall gaps of all sizes. This is a major process in forest regeneration in bottomland hardwood forests.

Fire is infrequent and of limited importance in lower, wetter areas, but was historically important in the older and higher terraces, and also crept into the floodplains. Putnam (1951 as cited in Wharton et al. 1982) states that a serious fire season occurs on an average of about every 5 to 8 years in the bottomland hardwood forests of the Mississippi Alluvial Plain. It is conjectured that Native Americans maintained canebrakes by deliberate fall burning. Infrequent, mild surface fires would occur in the system; however, they would not alter species composition or structure. Except in canebrake, most fires were very light surface fires, creeping in hardwood or pine litter with some thin, patchy cover of bottomland grasses such as Chasmanthium laxum and Chasmanthium latifolium. Flame lengths were mostly 15 to 30 cm (6-12 inches). Fire-scarred trees can be found in most small stream sites except in the wettest microsites. Stand-replacement fires are unknown in this type. Except where Native American burning was involved, fires likely occurred primarily during drought conditions and then often only when fire spread into bottomlands from more pyrophytic uplands. Trees may be partially girdled by fire in duff, followed by bark sloughing. While fire rarely killed the tree, this allowed entry of rot, which, in the moist environment, often resulted in hollow trees, providing nesting and denning habitat for many species of birds and animals. Surface fires occurred on a frequency ranging from about 3 to 8 years in streamside canebrake, streamside hardwood/canebrake, or pine, to 25 years or more in hardwood litter. Low areas having a long hydroperiod, islands, and areas protected from fire by backswamps and oxbows were virtually fire-free. Fire effects were largely limited to top-kill of shrubs and tree saplings less than 5 cm (2 inches) in diameter, and formation of hollow trees (Landfire 2007a).

Changes in hydrology due to the activities of beaver is also an important ecological process in bottomland hardwood forests. Beaver impoundments kill trees (sometimes over large areas) and may create open water habitat, cypress-tupelo stands, or cause stand replacement. Meandering streams are dynamic and frequently change course, eroding into the floodplain and depositing new point bars, thus creating new habitat for early-seral plant communities. In addition, insect outbreaks would occur infrequently in closed canopy states.

The distinctive dynamics of stream flooding and protected topographic position dominate the forming of the distinctive vegetation of this system. Not all of the factors are well known. Gradients of most of these rivers limit floods to fairly short duration. Flooding is most common in the winter, but may occur in other seasons.

The most critical anthropogenic threats to bottomland hardwood systems are fragmentation and hydrological alteration. Clearing, impoundment, drainage through channelization, levee building, and other forms of ecosystem alteration have been and continue to be extensive in these systems. Fragmentation of forest stands into smaller and smaller patches leads to disruption of natural processes such as plant succession, nutrient cycling, and litter accumulation. Viable forest patches must be large enough to allow for processes that maintain floral and faunal species composition and structure at the landscape scale (Harris 1989, Sharitz and Mitsch 1993). Increases in edges versus interior of patches favor common and weedy species over specialists. In terms of generalists, this includes white-tailed deer, raccoons, opossums, and brown-headed cowbirds. These species affect others through activities such as increased browsing, nest predation, etc. (Harris 1989). Edge effects occur around forest patches, and are more intense and disruptive in small patches, and with more abrupt edges. Patches with natural edges are probably fairly functional, but sharp artificial edges lead to increased mortality of the trees and more severe deterioration of ecological processes. If intact natural forest patches are buffered by areas of low-intensity forest management, that is, for example, preferable to being adjacent to agricultural land (Harris 1989). Alteration of natural hydrologic processes through impoundment and channelization have severely disrupted the function, structure, and species composition of large areas of bottomland forest. The most significant potential climate change effects over the next 50 years include alteration of water flow, most likely periods of drought alternating with more intense storms.

This system is found in the East Gulf Coastal Plain, from the coast northward and inland to the extent of unconsolidated sediments in Kentucky.