Mississippi Delta Island Forest

This system includes forests on barrier islands and spits formed during the deltaic shifts of the Mississippi River. It also includes the woody vegetation of salt domes in the Mississippi River deltaic plain. Since natural deltaic processes have been altered, barrier islands are no longer being formed in the Mississippi Delta region and existing barrier islands are undergoing subsidence and beach erosion. Some documented stands that apparently pertain to this system are found on Native American middens (shell mounds) located in the salt marshes of Hancock County, Mississippi. This system currently includes one forested beach ridge located at Grande Isle in Louisiana.

This system includes forests on barrier islands and spits formed during the deltaic shifts of the Mississippi River. It also includes the woody vegetation of salt domes in the Mississippi River deltaic plain. Some documented stands that apparently pertain to this system are found on Native American middens (shell mounds) located in the salt marshes of Hancock County, Mississippi (Eleuterius and Otvos 1979). This system also includes one forested beach ridge located at Grande Isle in Louisiana.

The maritime environment for these forests is extremely dynamic and may be severely altered or destroyed by geologic processes, including catastrophic destruction by storms. Fire may have naturally occurred infrequently in this system, but probably was not an important factor. Mature Quercus virginiana trees are fire-resistant when mature, and their litter also does not easily burn (Stalter and Odum 1993). Maritime forest systems remain subject to periodic severe physical stresses, although less than coastal dune and grassland systems. Vegetation structure and composition are influenced by salt spray (sea salt aerosol) and extreme disturbance events such as hurricanes, erosion, accretion and sand burial. Chronic salt spray, as well as intense salt spray during storms are important influences on vegetation structure and composition; however, the extent to which plant communities found in this system are shaped by salt spray varies. The most heavily salt-influenced examples of these systems may appear pruned or sculpted. Extreme salt spray or saltwater flooding in storms can severely disturb vegetation, though it recovers if the landforms have not been altered.

Hurricanes frequently make landfall in the northern Gulf of Mexico region, and have a significant impact on coastal systems. Even when they do not make landfall, the storm surge and wave action generated by an off-shore storm can have a significant impact. A total of 112 hurricanes made landfall from Wakulla County, Florida, to Hancock County, Mississippi, during the period 1926 to 2005 (Jarrell et al. 1992 with updates). From the period 1851 to 2004, 36 major hurricanes (Category 3 or higher) made landfall along the Gulf Coast from Louisiana to the Florida Panhandle (Blake et al. 2005). Hurricane-associated storm surges can overwash the dune system and cause significant erosion and/or sand burial of maritime forests. Personal observations along coastal areas of the Florida panhandle region in 2005 revealed large areas of vegetation extending several hundred yards inland that were killed or significantly impacted by saltwater inundation (Landfire 2007a).

The role of fire in this system is poorly documented. The majority of this system occurs on narrow barrier islands along the northern Gulf of Mexico. FNAI (1990) indicates that the mesic conditions and insular locations of well-developed maritime hammock communities inhibit natural fires, which occur no more frequently than once every 26 to 100 years. Liu et. al. (2003), in their study of sediment cores from Little Lake, Alabama, suggested that wildfires have been common in the coastal ecosystems in Alabama; however, they offered no frequency estimates. They did suggest a correlation between hurricanes and fire. This correlation was also supported by Meyers and van Lear (1998) who suggest that hurricane-fire interactions once played a major role in the development of ecosystems in the southern U.S., influencing their composition, structure, and pattern on the landscape (Landfire 2007a).

The following fire-return interval estimates were based on the Huffman and Platt (2004) study of fire scars on slash pines on Little St. George Island (Florida) and the return interval in similar ecological systems on the mainland. Fires were primarily light surface fires occurring every four years during the growing season. More intense replacement fires may have occurred following hurricanes, when more fuel was available as a result of storm damage. There is little fuel to sustain surface fires in the Quercus/Ceratiola ericoides-dominated ridges. Fires in this vegetation type are typically replacement fires that burn through the shrub crowns. The return interval here was estimated at 25 to 100 years, and may have occurred following a hurricane or other intense storm-related event when more fuel was available and fire intensity was higher (Landfire 2007a).

The most critical anthropogenic threat is that, due to the alteration of the natural deltaic processes, barrier islands are no longer being formed in the Mississippi Delta region and existing barrier islands are undergoing subsidence and beach erosion. Conversion of this type has primarily resulted from habitat fragmentation. Maritime forests occur on the most stable portions of barrier islands and are very attractive building sites. Clearing lots for houses involves disturbing or destroying most, if not all, the natural vegetative cover to make space for homes, parking areas, drainage fields, and septic systems. Following construction, native vegetation is often replaced by lawns and ornamental shrubs, many of which are exotic and possibly invasive (Bellis 1995).

Roads threaten the growth patterns and species composition because opening the forest canopy allows increased salt penetration to the forest interior. Areas without extensive fragmentation into small lots will still suffer degradation from construction of roads, even those that parallel the axis of the barrier island (Bellis 1995). Any kind of canopy opening exposes the uncleared areas of forest vegetation to increases in salt aerosol impact, wind shear, and altered drainage (Gaddy and Kohlsaat 1987). Generally, at least one main road is constructed along the entire length of a barrier island, above the dune ridge at the perimeter of maritime forests, to permit easy access to beaches. Other roads are built laterally to the trunk road for access to developments and private residences. These feeder roads, such as those constructed to provide beach access, are typically parallel to the direction of on shore winds, and serve to intensify the effects of salt spray and wind shear, further degrading the canopy. These would be regarded as having moderately severe degradation. The presence of only roads parallel to the axis of the barrier island represent low severity of degradation, although these are cumulative processes and the degradation will continue to increase with time.

An additional stressor to wetland communities within maritime forests is the removal of groundwater from barrier island aquifers. Rainfall is generally the only source of freshwater on barrier islands, and the maritime forest community acts as the primary watershed. Precipitation entering the watershed is rapidly drawn deep into a freshwater lens, which floats above the denser saltwater in the permeable sediments beneath barrier islands. Excessive pumping of freshwater from the lens for residential and commercial purposes can lead to loss of the hydrostatic head in the freshwater lens, which could, in turn, increase the rate of saltwater intrusion into surface waters on the island (Ward 1975, Winner 1975, 1979, Bellis 1995).

The most significant potential climate change effects over the next 50 years include rising sea level and an increase in storms. The climate is expected to be warmer, and estimates of changes in rainfall amounts vary widely. Under possible conditions of climate change, increased natural disturbance by wind, salt spray, and storm surge intrusion will be significant, but the magnitude is quite uncertain. Examples of this system contain species that can recover from these disturbances, but increased frequency will result in younger canopies, more time spent in recovery stages, and shifts toward the most tolerant species. Some maritime forests will likely become maritime shrub and some maritime shrub will become grassland (NCDENR 2010).

This system is apparently restricted to Louisiana. It is found on barrier islands and spits formed during the deltaic shifts of the Mississippi River.