Southern Coastal Plain Blackwater River Floodplain

This ecological system occurs along certain river and stream drainages of the southern Coastal Plain of Florida, Alabama, Mississippi, and southwestern Georgia that are characterized by dark waters high in particulate and dissolved organic materials, and that generally lack floodplain development. In most cases these are streams that have their headwaters in sandy portions of the Outer Coastal Plain. Consequently, they carry little mineral sediment or suspended clay particles and are not turbid except after the heaviest rain events. The water is classically dark in color due to concentrations of tannins, particulates, and other materials derived from drainage through swamps or marshes. In comparison with spring-fed rivers and brownwater rivers of the region, this system tends to be much more acidic in nature and generally lacks extensive and continuous floodplains and levees. Steep banks alternating with floodplain swamps are more characteristic. This system includes mixed rivers, with a mixture of blackwater and spring-fed tributaries such as the Suwannee River. Canopy trees typical of this system are obligate to facultative wetland species such as Taxodium distichum, Nyssa aquatica, and Chamaecyparis thyoides.

The rivers in which this system occurs are characterized by dark waters high in particulate and dissolved organic materials, and that generally lack floodplain development. In most cases these are streams that have their headwaters in sandy portions of the Outer Coastal Plain (Smock and Gilinsky 1992). Consequently, they carry little mineral sediment or suspended clay particles and are not turbid except after the heaviest rain events. The water is classically dark in color due to concentrations of tannins, particulates, and other materials derived from drainage through swamps or marshes (FNAI 1990). In comparison with spring-fed rivers and brownwater rivers of the region, this system tends to be much more acidic in nature and generally lacks extensive and continuous floodplain and levees; steep banks alternating with floodplain swamps are more characteristic (FNAI 1990). This system includes mixed rivers, with a mixture of blackwater and spring-fed tributaries such as the Suwannee River.

This is a linear to large-patch ecological system; stands may be contiguous over thousands of acres. The largest examples could be called matrix examples of this ecological system. Examples are by nature linear and tend to be narrow. The Satilla River in Georgia is about 375 km in length and may be the largest example. The lower floodplain is about 2 km across, an approximate size of 750 square km could be used as a working upper bound. There may be limited areas with trees greater than 150 years. Probably there are many stands aged 70-100 years, and many that are younger than 70 years. Stands that have not had extensive timber removal will probably have more woody debris and constitute better habitat for component animal and plant species. Areas that have been logged may become dominated by Pinus taeda, Liquidambar styraciflua, and Acer rubrum with a common shrub being Morella cerifera.

Flooding is the most important ecological factor in this system. Frequency and duration of flooding determine the occurrences of different associations and separate the system from other kinds of wetlands. Flooding brings nutrients and excludes non-flood-tolerant species. When flooded, the system 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. Larger floods cause local disturbance by scouring and depositing sediment along channels, and occasionally causing channel shifts. However, the low gradient and binding of sediment by vegetation generally makes these processes much slower and less frequent than in river systems of most other regions. The areas flooded for the longest durations tend to have high amounts of total organic carbon in the soil and sediments which deplete levels of aquatic dissolved oxygen (Todd et al. 2010).

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. In addition to periodic flooding, the formation of windfall gaps is a dominant ecological processes in bottomland hardwood forests. Windfall gaps occur from the local scale (a single mature canopy tree) to the landscape scale (effects of tornadoes and hurricanes). 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.

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.

Fire is not believed to be important, due to low flammability of much of the vegetation, wetness, and abundance of natural firebreaks. Fire is infrequent on the lower terraces, but was frequent historically on older terraces outside the floodplain and 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. Some areas of bottomlands apparently were once occupied by canebrakes, which presumably were maintained through deliberate fall burning by Native Americans. Infrequent, mild surface fires would occur in the system; however, they would not alter species composition or structure.

Changes in hydrology due to the activities of beaver are 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. Insect outbreaks would occur infrequently in closed canopy states.

Conversion of this type has primarily resulted from removal of characteristic canopy species through logging, intensive forestry management, fragmentation, hydrological alteration, and runoff of fertilizers (nitrogen and phosphorus), animal waste, pesticides, and other chemicals (Smock and Gilinsky 1992). Clearing, impoundment, drainage through channelization, levee building, and other forms of ecosystem alteration have been and continue to be extensive in these systems. These alterations have severely disrupted the function, structure, and species composition of large areas of bottomland forest, with local to global implications (Sharitz and Mitsch 1993).

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). Feral hogs (Sus scrofa) conduct rooting of the soil, destroying native vegetation and soil-dwelling animals. 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).

Hydrologic functioning may also be impaired by upstream impoundments, water withdrawals, interbasin transfers, etc. These reduce the frequency and magnitude of flooding events. In general, this which would lessen the dynamism of the flooding regime, altering the formation of microtopographic features, scouring and deposition, etc. Invasive exotic plants are a threat, timber removal can change the species composition. Erosion from cleared and regularly plowed uplands has led to significant siltation in the past. Today, forest best management practices and streamside management zones can control or reduce erosion which reaches floodplains, rivers and creeks. In addition, the widespread introduction of Ligustrum sinense, Microstegium vimineum, and other exotic invasives has dramatically reduced native diversity in the understory. The most significant potential climate change effects over the next 50 years include alteration of waterflow, most likely periods of drought alternating with more intense storms.

This system is found in the East Gulf Coastal Plain of Alabama, Mississippi, southwestern Georgia, Florida, and adjacent portions of central Florida.