U.S. Department of the Interior
U.S. Geological Survey
FS 2004-3016

Figure 1. (above) A picture of a typical lightning strike (photo by Michael Fewings). [larger image] Figure 2. (above) Aerial view of a recently created lightning gap in the Shark River area. [larger image]

With an average of 9,900 cloud-to-ground lightning strikes occurring annually in Florida (Figure 1), lightning-created canopy gaps are a common and important component of the mangrove ecosystem (Figure 2).

These gaps produce a mosaic of different successional stages that may be vital as a living seed bank in the mangrove ecosystem. Species dynamics in the mangrove ecosystem of Everglades National Park are affected by the Everglades' hydrological restoration, sea-level rise, hurricanes, and lightning gaps. Clearly, to understand the dynamics of future mangrove ecosystems, determining how these ecosystems respond to small- and large-scale disturbances will be crucial. Here we characterize the role of lightning gaps within south Florida mangrove ecosystems. By using a serial sampling of recent and closing gaps, we will be able to determine the way these small-scale disturbances affect current and future forest structure in the larger context.

Our study area is the mangrove forest of the Shark River region of Everglades National Park (Figure 3). This forest is approximately 15 km wide. From the coast upstream toward the freshwater marshes, canopy height declines from an average of 10.1 m to 5.4 m. The forest consists of stands of mixed mangroves with Rhizophora mangle L. (red mangrove), Laguncularia racemosa (L.) Gaertn (white mangrove) and Avicennia germinans (L.) Stearn (black mangrove). Historically, catastrophic hurricanes occur approximately every 30 years in south Florida. Our study sites have not been substantially disturbed by a hurricane since Hurricane Donna in 1960.

We examined 54 gaps that were randomly located by boat and helicopter. The average gap size is 332 ± 193 m². In general the gaps were elliptical in shape. The gaps varied in age from newly created to extensively recovered (< 1 month to > 10 years).

Figure 3. A color infrared aerial photo of the southern Florida peninsula. The border of the Everglades National Park (ENP) is outlined in black. Our study region is outline in red. [larger image]

Figure 4. (above) Density of Rhizophora saplings (number per m2, by size class (height in m), for relatively old gaps (solid) versus younger gaps (dashed). [larger image] Figure 5. (above) Soil compaction versus soil shear strength (both in kg per m2) for gaps (red) and forest canopy (blue) locations. [larger image]

Within a subset of the gaps, we surveyed tree species, saplings, seedlings, crab burrow density, and we assessed the light environment, soil physical structure, and amount of coarse woody debris. Preliminary results indicate a general trend for red mangroves to dominate recovery in the short term (Figure 4). We found differences in the relative abundance of crab burrows depending on gap status.

The mean number of crab burrows is 99.57 per m² with the values ranging from 48.25 to 158.75. Soil shear strength and compaction are lower in gaps than in the surrounding forest (Figure 5). There is higher transmittance of photosynthetically active radiation (PAR) in new gaps than in the recovering gaps and closed canopy forest. The results of this study will be incorporated into current modeling efforts that are attempting to predict mangrove forest structure in response to Everglades restoration efforts. These models include the Across Trophic Level System Simulation package (ATLSS) and the Everglades Landscape Model (ELM).

Acknowledgments

We thank G. Anderson, H. Berreras, M. Warren, L. Figaro, J. Akeung, L. Haddon, S. Beeler, D. Riggs, L. Romero, C. Walker, and C. Whelan for assistance and Everglades National Park for access to sites. Financial support was provided by the Global Climate Change Program of USGS/BRD and the U.S. Dept. of Interior's "Critical Ecosystem Studies Initiative" administered by Everglades National Park under interagency agreement #IA5280-7-9023.

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