Lightning gaps have been identified as a common disturbance in mangroves throughout the world. Lightning created canopy gaps are abundant in the mangrove forests of Florida. 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 is the Greater Everglades restoration effort, known as the Comprehensive Everglades Restoration Plan (CERP). CERP 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.

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 signal for 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.

The mean canopy gap area was 212 m² and the expanded gap was 299 m² for the general gap survey. Gaps were slightly elliptical in shape with an average eccentricity of 1.28. There was a preferential directional bias to the longest axis of the gaps. However, there was no evidence of wind extensions to the gaps. Thirty-three percent of the variation in gap size was explained by average surrounding canopy height.

To understand the successional process it is important to determine a relative age of the recovering gap. The aging of tropical gaps has been a problem in a number of environments. Here we have developed a hypothetical relative aging of gaps based on the ratio of the coarse woody debris of the gap compared to the average of coarse woody debris amount for the study island. The average coarse woody debris was 52.52 tons ha^-1 for the study island. The gap coarse woody debris varied from 16.7 to 182 tons ha^-1. There is a linear relationship between relative gap age and the amount of coarse woody debris present with older gaps having a higher ratio of coarse woody debris relative to younger gaps (fig. 1).

Figure 1. Hypothesized relation between gap age and the amount of coarse woody debris. [larger image]

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 assessed sediment parameters for six gaps of different successional age and found that compaction is significantly lower in gaps (0.094 kg/cm²) and higher in the surrounding closed canopy forest (0.124 kg/cm²). The same was true for sediment shear strength (gap mean = 7.4 kg/cm² and forest mean = 8.8 kg/cm²). Shear strength and soil compaction taken together indicates soil cohesiveness. There is a linear relation between these parameters and our data supports this linear relation; however, all samples from gaps are consistently grouped towards lower end of this relation (fig. 2). Sediment shear strength and compaction are usually correlated with bulk density, however, our study does not support this finding. In addition, there was no difference in sediment bulk density between gaps and the surrounding forest. 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.

Figure 2. Relation between soil compaction and soil shear strength for mangrove forested sites (squares) and gap sites (diamonds). [larger image]

Contact: Whelan, Kevin R.T., USGS, c/o Department of Biological Sciences, OE Building, Room 167, Florida International University, Miami, FL 33199. Phone: 305-348-6047; FAX 305-348-3096; Email: whelank@fiu.edu, Poster, Ecology and Ecological Modeling

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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