Hurricanes have long affected the Atlantic and gulf coasts of North America. Return times (how frequently a hurricane strikes an area) can average from 5 to 20 years along the northern gulf coast, depending on location. Hurricanes are widely viewed as destructive agents responsible for the loss of human lives and economic disruptions, but their impact on natural ecosystems is poorly known. On 26 August 1992, Hurricane Andrew made landfall along the south-central Louisiana coast with sustained winds of 54 meters per second (about 120 mph) and a storm surge of 1­2 meters, making it one of the more powerful storms to hit the gulf coast in recent memory (Fig. 1).

Coastal Wetlands

Louisiana's coastal marsh ecosystems contain 40% of the coastal wetlands found in the United States and are the result of geomorphic processes linked with the formation and degradation of deltas associated with the Mississippi River (see chapter on Coastal Louisiana). Plant species are distributed along dominant gradients of salinity and elevation, which result in broad zones of saltwater, brackish, and freshwater marsh paralleling the coast. Previous investigations in these coastal marshes have shown intrusion of saltwater, flooding, herbivory, and disturbance as dominant variables controlling plant species richness.

Storms such as Hurricane Andrew are part of the evolution of coastal systems. Hurricanes help form and alter the shapes of coastlines and play a role in maintaining plant species diversity. Fresh and brackish marshes along the Louisiana coast appear structured, in part, by the infrequent and intense salt-intrusion events associated with hurricane storm surges. Coastal wetlands in Louisiana are also increasingly at risk from various natural and human influences. Louisiana leads the nation in wetland loss, averaging some 65.6 square kilometers lost each year. Since the 1930's, the state has lost an estimated 3,950 square kilometers of coastal wetlands; this represents 80% of the nation's total coastal wetland loss. Subsidence, sea-level rise, human activities, and erosion caused by storms have all been implicated in these high rates of loss. A storm such as Andrew can result in a year's worth of loss in a single day.

Physical damage to coastal wetlands was evident following the passage of Hurricane Andrew. Sediment overwash, ripped and torn marsh, erosion of pond and lake margins, wrack (large amounts of plant debris) deposition, and lateral compression of marshes were common. Substantial sediment deposition was associated with the passage of the storm resulting, in some cases, in the burial of the prestorm surface and smothering vegetation. Extensive areas of marsh were pushed against firm barriers (for example, levees and firmly grounded marsh), resulting in a ridge and trough pattern with ridges 60­200 centimeters higher in elevation than the surrounding marsh surface. Areas of wrack completely buried the vegetation. Freshwater marsh species exposed to water half as saline as seawater (10­15 parts per thousand) were "burned," and the aboveground portions of these plants were killed. In scoured areas, unconsolidated or weakly rooted marsh was eroded.

No sites were without some impact. Sites that received some sediment but not enough to bury the dominant vegetation were least impacted. Hurricane Andrew created a heterogeneous landscape with different disturbance patches juxtaposed in complex configurations. These disturbance patches represented habitats for uncommon and less widely distributed plant species to invade. The relative abundances of species in the different disturbance patches also changed. Differences in vertical elevation (height above the water surface), the amount of organic material, and the amount of new sediment surface created complex gradients of soil moisture, salinity, and nutrient availability.

Marsh Vegetation

Vegetation loss and initial recovery differed in areas with different types of storm damage. At sites where prestorm data were available, one was able to see how Hurricane Andrew caused changes in the composition of the vegetation. In 1991, before the hurricane, 20 species of angiosperms were present at these sites, which were dominated by salt meadow cordgrass and American bulrush. The dominant species were distributed broadly throughout the marsh, with salt meadow cordgrass generally more abundant than American bulrush. Permanent plots sampled in 1991 before the hurricane differed in plant species composition from the same plots sampled in 1992 after the hurricane, and were different still when sampled again in 1993. Total plant cover decreased sharply in all damage categories except compressed marsh sites. The surface of the compressed marsh was elevated, creating drier and nonflooded habitat. Plant cover increased significantly there. There was, however, a shift in species composition from a community dominated by perennial grasses to one dominated by forbs (Fig. 2). Areas smothered by thick sediment deposits were quickly recolonized by short-stature wetland species, such as small spikerush, but later developed a dense plant cover as taller species became established. Plants were very slow to colonize wrack-covered areas because the wrack had to decay or be removed before plants could grow through the debris. Significant numbers of species did not become established until late in 1993. By October 1993, most damage types showed comparable levels of total plant cover except for the wrack sites. Scour areas were generally devoid of vegetation and represent permanent marsh loss. Areas where the aboveground parts of plants suffered from salt burn and died back simply regrew within weeks after the storm.

Individual plant species responded differently to Hurricane Andrew. Coverage of the dominant species showed quite different responses to each disturbance type (Fig. 3). The compressed sites were dominated by salt meadow cordgrass and creeping waterprimrose with only modest amounts of American bulrush. In contrast, wrack areas showed a slow recolonization by salt meadow cordgrass and creeping waterprimrose. Areas of thick sediment showed a strong recolonization by salt meadow cordgrass, American bulrush, salt marsh camphor-weed, and small spikerush. Least-impacted sites were characterized by nearly equal amounts of salt meadow cordgrass and American bulrush, with lesser amounts of salt marsh camphor-weed and creeping waterprimrose. Compressed marsh also provided habitat for terrestrial weeds uncommon in the coastal wetlands, and because the soils were not waterlogged, these sites may ultimately provide habitat for woody shrubs and trees more commonly found on the tops of levees.

Perhaps as important as the habitat heterogeneity created by the storm is the effect on sediment supply to coastal wetlands. Hurricanes represent an important mechanism by which coastal wetlands cut off from normal riverine sediment supplies may receive significant amounts of sediment that might partially offset the effects of coastal subsidence and marsh deterioration. Along much of the Louisiana coast, sediment accretion rates are often insufficient to maintain the elevation of the marsh surface relative to sea level. Although Andrew deposited sediment along large areas of the coast, these accumulations varied. Areas closer to the path of the storm and near a ready supply of sediments accumulated more sediment than areas distant from the track of the storm or from a sediment supply. The thickest sediments ranged from 10 to 16 centimeters and were deposited in marsh areas corresponding to the northeast quadrant of the storm track as it passed near the sediment-rich Atchafalaya River and delta. As the storm passed the delta, the accompanying storm surge mixed these sediments into the water column and deposited them onto the marsh surface. The hurricane-deposited sediments corresponded to a 100%­200% increase over prestorm deposition rates.

Barrier Islands

Andrew's impact was not confined to coastal wetlands. Barrier islands are the outermost land exposed to hurricanes and often lose significant areas of beach and marsh to erosion. Storm waves associated with tropical storms continuously alter the shape and profile of these islands. On some islands, up to 68 meters of beach were lost to erosion and significant overwashing and deposition of sand on back barrier wetlands occurred. Recent photo interpretation has documented that between 1990 and 1992 (after Hurricane Andrew passed near them), the Isle Dernieres barrier chain lost 30% of its land area. This is particularly devastating because over the past 130 years, nearly 78% of the land area in the Isle Dernieres chain had already been lost. Overwash and sand movement also damaged many island plant communities. Because plants on barrier islands are generally adapted to sand movement and salt spray, it is not surprising that despite burial by sand and exposure to saltwater, the vegetation on these islands is recovering. Changes in elevation caused by the movement and accumulation of sand, however, resulted in varying environmental conditions and a redistribution of plant species on the landscape.

Hardwood Forests

Hurricane Andrew diminished in strength after making landfall. The storm, though, passed through the Atchafalaya basin with sufficient force that more than 450 square kilometers of forested wetland were put at risk. This area contains 35% of the remaining bottomland hardwood forest and swamp forest of the Lower Mississippi floodplain. The impact of Hurricane Andrew on this forested landscape varied greatly with forest type, canopy structure, topography, and location relative to the storm's path. Most of the initial loss of tree density and canopy was restricted to bottomland hardwood forest. Stands lost between 10% and 60% of their basal area (the cross sectional-area of their trunks). Willow trees were particularly susceptible to damaging winds and in certain sites more than 85% of them were toppled. Surprisingly, baldcypress and tupelo trees were largely unaffected except for the loss of an occasional branch. The recovery of the forest will depend on the previous forest cover, the type of damage, the specific environmental conditions created, and availability of seeds and seedlings. Understory trees and saplings were unaffected by the hurricane despite the loss of canopy trees. These survivors will grow rapidly because of the removal of the canopy, which shaded them and suppressed their growth. In other cases, seeds and new tree seedlings will become established and form the new forest.

Hurricanes are a major factor affecting coastal ecosystems along the northern Gulf of Mexico. Not only are they responsible for habitat loss, but their frequent landfalls also create a mosaic of different disturbed areas resulting in a heterogeneous landscape. The plant communities that develop in the different patches are dynamic and respond to changing environmental conditions resulting from disturbance. Louisiana's coastal ecosystems are increasingly at risk from human activities. Most climate models predict a period of increased hurricane activity and a tendency toward stronger storms as we approach the new century. Continued research and monitoring are needed to determine the extent to which these ecosystems will become more vulnerable to disturbance from hurricanes.