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Poster presented October 2003 at the Annual Technical Presentations Meeting - SFWMD/USGS Cooperative Program

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


map showing study area
Study Area [larger image]
Florida has the highest level of cloud to ground lightning strikes in the United States. Consequently, gap formation in the mangroves due to lightning strikes is a common disturbance phenomenon. We investigated some basic characteristics of lightning gaps in the mangroves of Southwest Florida. The 39 lightning gaps censused were comprised of mixed species and age classes of mangroves. The mean canopy gap area was 212 m2 and the extended gap size was 299 m2. Gaps were slightly elliptical in shape with an average eccentricity of 1.28. However, there was no evidence of wind extensions to the gaps. Gap size was partially explained (33%) by surrounding tree height. In a subset of six gaps of differing age we have found that coarse woody debris is generally higher than the surrounding forest. We propose to develop a relative aging formula based on this relationship. Our investigation of sediment parameters of these same gaps has found that there is no difference in soil bulk density; however, the soil cohesiveness in the gaps is lower than the closed canopy forest. We additionally found differences in the mean abundance of crab burrows depending on gap successional status.

Keywords: Lightning strike, gaps, extended gap area, mangroves


aerial photo of lightning gap
Photo 1: Aerial view of a recently created lightning gap, Shark River area. [larger image]
Lightning gaps have been identified as a common disturbance in mangroves throughout the world; including Papua New Guinea (Paijmans and Rollet 1977), Malaysia (Anderson 1964), Panama (Smith 1992), Florida (Odum et al. 1982, Smith et al. 1994), and the Dominican Republic (Sherman et al. 2000). Lightning created canopy gaps are abundant in the mangrove forests of Florida (Photo 1, 2, 3). An average 9,900 cloud to ground strikes occur annually in Florida, the highest level found in the United States. The dynamics of Florida mangrove systems may result from the interactions of small-scale lightning gaps, large-scale hurricane disturbances and sea level rise. In addition to these ecosystem forcing functions are the modifications due to the Greater Everglades Restoration effort. This effort will greatly influence hydrological flow through much of the 100,000 ha of mangroves in South Florida. Clearly, to understand how the mangrove ecosystems may change in response to restoration it is important to determine how these mangrove ecosystems respond to small- and large-scale disturbance along with sea level rise. To understand lightning gaps in mangroves it is imperative to determine some basic characteristics of these types of gaps. This is only the second time lightning created gaps in the mangrove ecosystem have been extensively studied.
photo of elliptic patch of dead standing trees
Photo 2: Clearly visible, elliptic patch of dead standing trees, Shark River area. [larger image]

The goal of this project is to characterize the role of lightning generated gaps within South Florida mangrove ecosystems. This is being accomplished by: following short-term changes in community level and environmental processes; evaluating community characteristics in a time series of gaps along a known salinity gradient; and appraising the regional impact of mangrove gap dynamics. Here we present findings of 39 gaps from the general region for canopy and expanded gap size and direction of orientation. Additionally, we present initial findings of habitat characteristics for six gaps of differing successional age located in the lower Shark River Region.


photo of lightning bolt
  • Determine basic gap characteristics.
    • Compare canopy gap area and extended gap area.
    • Size distribution, gap orientation.
    • Relationship of gap area to surrounding forest height.
  • Investigate differences due to gap age.
    • Relate relative gap aging based to the amount of coarse woody debris amount.
    • Determine sediment parameters:
  • Sediment compaction, sediment shear strength, and bulk density.
  • Crab abundance.
[larger image]



comparison graph of canopy and extended gap area for 39 gaps
Figure 1 Comparison of canopy and extended gap area for 39 gaps. [larger image]

graph of size distribution of gap area
Figure 2 Size distribution of gap area. [larger image]

graph of expanded gap size versus surrounding tree height
Figure 3 Expanded gap size versus surrounding tree height. [larger image]

graph of relative aging of gaps
Figure 4 Relative aging of gaps. [larger image]

graph of soil compaction versus soil shear strength
Figure 5 Soil compaction versus soil shear strength. [larger image]

graph of small crab holes by gap successional status
graph of medium crab holes by gap successional status
graph of large crab holes by gap successional status
graph of total crab holes by gap successional status
Figure 6 Abundance of crab burrows by gap successional status. [click on each of the graphs above to view a larger image]


aerial photo of newly created lightning gaps and older gaps
Photo 3: Aerial view of newly created lightning gaps and older gaps. [larger image]
Lightning created gaps in south Florida are approximately 299 m2 with a directional bias of the longest axis in the 80-90 degree direction. The surrounding forest height affects the size of the lightning gap created. The lightning gaps are elliptical in shape (1.28 eccentricity). These are on average smaller but of similar shape to other lightning created gaps in the mangroves reported by Sherman et al. 2000. We have proposed a possible relative aging technique based on the amount of coarse woody debris in a gap compared to average coarse woody debris in the surrounding forest. In the further we will use historical aerial photographs to age a subset of gaps and using this ratio of CWD will develop an equation for relative aging. It appears that roots within the lightning created gaps die as a consequence of above ground tree mortality and possibly from the lightning itself. Root death causes the physical properties of the sediment to differ from the surrounding forest type. We found that sediment compaction and shear strength is significantly lower in gaps than surrounding forest. Additionally, soil cohesiveness is lower in the gaps, however, there was no difference in sediment bulk density between gaps and the surrounding forest. Additionally, there are differences in the size distribution of crab burrows which may be related to these soil parameters. There are plans to extend this type of investigation of lightning gaps to the mid-stream and upstream areas of the Shark River Region. In this way we will investigate the effect of salinity on the recovery process.

photo of lightning bolt


We wish to thank the members of the mangrove field rat squad: G. Anderson, H. Barreras, M. Warren, L. Figaro, J. Akeung, L. Hadden, S. Beeler, D. Riggs, L. Romero, C. Walker, and C. Whelan for assistance. Everglades National Park for access sites. Financial support was provided by the Global Climate Change Program of USGS/BRD and the U.S. Dept. of Interior's “Critical Ecosystems Studies Initiative” administered by Everglades National Park under Interagency agreement #IA5280-7-9023.

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Anderson, J.A.R. 1964. Some observations on climatic damage in peat swamp forests in Sarawak. Commonwealth Forestry Review, 43: 145-158.

Allen, J. A. K. C. Ewel, B. D. Keeland, T. Tara, & T.J. Smith III, 2000. Downed wood in Micronesian mangrove Forests. Wetlands, Vol. 20, No 1, pp. 169-176.

Odum, W.E., McIvor, C.C. & Smith, T.J., 1982. The mangroves of South Florida: a community profile. FW/OBS-82/29, Fish and Wildlife Service. Washington, DC.

Paijmans, K. & Rollet, B. 1977. The mangroves of Golley Beach, Papua New Guinea. Forest Ecology and Management, 1:119-140.

Runkle, J.R. 1981. Gap regeneration in some old-growth forests of the eastern United States. Ecology, 62:4: 1041-1051.

Sherman, E.S., Fahey, J.T. & Battles, J.J., 2000. Small-scale disturbance and regeneration dynamics in a neotropical mangrove forest. Journal of Ecology, 88:165-178.

Smith, T.J. III. 1987. Seed predation in relation to tree dominance and distribution in mangrove forest. Ecology, 68, 266 – 273.

Smith, T.J. III, 1992. Forest structure. Tropical Mangrove Ecosystems (ed A.I. Robertsons & D.M. Alongi), pp 101-136. American Geophysical Union

Smith, T.J. III, Robblee, M.B., Wanless, H.R. & Doyle, T.W. 1994. Mangroves, hurricanes and lightning strikes. Biosience, 44:256-262.

1 Student Career Experience Program

1,2 U. S. Geological Survey
Center for Water and Restoration Studies.
C/o Biological Sciences, OE 167,
University Park
Florida International University
Miami, Florida, 33199. 305-348-6047,

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Related information:

SOFIA Project: Understanding and Predicting Global Climate Change Impacts on the Vegetation and Fauna of Mangrove Forested Wetlands in Florida

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