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). 

Figure 1. A picture of a typical lightning strike (photo by 

Michael Fewings). 

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.

Figure 2. Aerial vew of a recently created lightning 

gap in the Shark River area

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. 

Lightning Gaps in the Mangrove Forest of Everglades National Park

U.S. Department of Interior USGS Fact sheet FS-2004-3016

U.S. Geological Survey
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).

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. 

Figure 4. Density of Rhizophora saplings (number per 

m2), by size class (height in m), for relatively old gaps 

(solid) versus younger gaps (dashed).

Figure 5. Soil compaction versus soil shear strength 

(both in kg per m2) for gaps (red) and forest canopy 

(blue) locations. 

Gap versus Canopy Soils

0.00

0.05

0.10

0.15

0.20

0.25

0 5 10 15

Torsion

C

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m

p

a

c

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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.

 

For more information, please contact

Kevin R.T. Whelan1 or Thomas J. Smith III2

U. S. Geological Survey

 1 Biological Sciences Department (OE 167)

 Florida International University

 University Park

 Miami, FL 33199

 Phone-305-348-6047

 Fax-305-348-1986

 Email: whelank@u.edu

2

 600 4th Street South

 St. Petersburg, FL 33701

 Phone-727-803-8747 

 Fax-727-803-2030

 Email: Tom_J_Smith@usgs.gov