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Chapter I

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Introduction

Lightning gaps are a common disturbance in mangroves throughout the world, including Papua New Guinea (Paijmans and Rollet 1977), Panama (Smith 1992, Sousa and Mitchell 1999), Dominican Republic (Sherman et al. 2000), and the United States of America (Odum et al. 1982; Smith et al. 1994). Lightning strikes within the mangrove canopy create a relatively circular to elliptical clearing from the top to the bottom of the forest canopy (Duke 2001, Sherman et al. 2000). These strikes kill several trees, instead of just one or two as is often seen in upland terrestrial forests (Anderson 1966). The mechanism by which lightning strikes kill multiple trees is not well understood, but it occurs in many ecosystems (Peace 1940, Anderson 1964, Brunig 1964, 1972, Paijmans and Rollet 1977, Magnusson et al. 1996, Sherman et al. 2000, Duke 2001). Florida, in particular, has one of the highest rates of cloud to ground lightning strikes in the United States (Changnon 1989). Lightning gaps are common in the mangrove forest of Everglades National Park due to the high rate of strikes (7 to 9 flashes/km2 yr-1, Huffines and Orville 1999).

Mangroves comprise an extensive expanse at the junction of the terrestrial forest and nearshore marine ecosystems in the tropics. Mangroves are generally highly productive ecosystems, which have extremely stressful environmental conditions (i.e. high salinity, high temperatures, extreme tidal flooding, anaerobic soils, etc. Odum et al. 1982). Mangrove forests worldwide are noted for a sparse understory and few sapling-size individuals (Janzen 1985, Corlett 1986, Lugo 1986, Tomlinson 1986). There have been numerous studies to determine how the mangrove forests develop. Classical mangrove investigations have reported species-specific zonation patterns to the forest (Davis 1940, Lugo and Snedaker 1974, Chapman 1976). These zonation patterns have typically been linked to environmental stressors that may facilitate specific species at the expense of others. Evidence for and against species sorting has been reported (Rabinowitz 1978, Clarke and Allaway 1993, Smith 1992, Smith et al 1994, Chen and Twilley 1999, Clarke and Kerrigan 2000). More recently, mangroves have been being investigated to determining the role disturbance (hurricanes/typhoons, tidal waves/Tsunamis, hydrological diversions) plays in the forest dynamics (Smith et al. 1994, Duke 2001, Cahoon et al. 2003, Baldwin et al. 2001).

Observations of numerous small gaps and a meager understory within closed canopy forest have inspired investigations to determine the role gaps play in mangrove community structure and diversity by applying concepts from upland terrestrial systems to mangrove forest dynamics (Smith 1992, Feller and McKee 1999, Clarke and Kerrigan 2000, Sherman et al. 2000, Duke 2001, Ellison 2002). Gaps provide an altered environment both above and below ground. Typically gaps have increased light (quantity), temperature, humidity, soil temperatures (Fetcher et al. 1985), soil water (Denslow et al. 1987, Becker et al. 1988), and change the quality of light, and decreased root formation (Denslow et al. 1987). Despite the importance of soil processes during succession, most canopy gap investigations have concentrated on only aboveground effects. Specifically, mangrove canopy gaps have been found to alter several physical factors and processes important for mangrove regeneration: humidity, evapotranspiration, light levels, and soil properties (salinity, temperatures, and nutrients) (Smith 1987a, Smith 1992). These changes can also lead to modifications in the crab community (Osborne and Smith 1990, Smith 1987b). Crabs play a key role in these ecosystems; their burrows increase soil aeration, reduce sulfides and ammonium, and increase mangrove sapling productivity (Smith et al. 1991). Mangrove forest structure and productivity have also been found to influence fiddler crab size (Colpo and Negreiros-Fransozo 2004). Thus the relationship between crab and mangrove population structure is a complex feedback that will likely change during gap succession.

The physical environment within the gaps may facilitate favorable conditions that can shift species-specific survivorship, recruitment, and growth of the flora and fauna both among and within a species across life stages (Brokaw 1985, Denslow 1987, Hubbell et al. 1999). Additionally, as succession progresses within the gap the environmental conditions will change which may allow a specific species to have favorable conditions only at certain stages during the successional trajectory. The regenerative processes within lightning-initiated gaps can potentially drive mangrove forest diversity and structure in South Florida. Chapman (1976) suggested the idea of “cyclical succession” with mangrove forest oscillating between two stages of development due to physical disturbances. Lugo (1980) argued for the “arrested succession” of mangroves due to physical disturbances such as hurricanes, winds, waves, fire, etc. Finally, Duke (2001) hypothesized that recruitment within small canopy gaps can prevent mangroves from reaching a senescence stage. The conditions within these lightning gaps may facilitate recruitment of certain species at the expense of others.

A comprehensive understanding of the dynamics of this mangrove forest is of considerable importance. The forest is located in the Shark River estuary, downstream of the Shark River Slough, and receives freshwater inputs from the greater Everglades drainage and thus is under the influence of upstream water management practices of the Greater Everglades. The Everglades drainage is currently undergoing an ecosystem restoration concentrating on modifying water deliveries to mimic pre-drainage flows. In addition to the changing freshwater flows linked to restoration, this mangrove forest is impacted by sea level rise. The hydrological conditions of a site are known to substantially affect soil processes including sedimentation, erosion, and the shrink and swell of soil materials. Additionally, soil elevation and surface flooding have been identified as important factors in mangrove species recruitment and survival (McKee 1993, 1995, Ellison and Farnsworth 1993, Rabinowitz 1978ab, McMillan 1971). For example, under more flooded conditions survival of Rhizophora mangle is greater than that of Avicennia germinans and Laguncularia racemosa (McKee 1993). Therefore, a comprehensive understanding of the successional dynamics of lightning initiated gap in the mangrove forest of Shark River must take into account current and future hydrological conditions.


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