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Chapter II

Succession of lightning-initiated canopy gaps in a Neo-subtropical mangrove forest.

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Abstract

Lightning strikes are a important disturbance mechanism in the mangrove forest of Everglades National Park. I studied the successional dynamics of lightning-initiated gaps to determine their influence on mangrove forest stand structure and community dynamics. I determined the environmental characteristics of gap size, gap shape, light environment, soil bulk density, soil torsion, soil compaction, and fiddler crab (Uca thayeri) abundance. I additionally determined the vegetative composition in a chronosequence of gap stages (new, recruiting, and growing gaps, closed canopy intact forest). Canopy opening size averaged 202 ± 16 m2 (± 1SE) and expanded gap size averaged 289 ± 20 m2 (± 1SE) (sensu Runkle). As gaps filled with saplings, light transmittance at seedling height (1.3 m) decreased exponentially. Gaps had greater fine woody debris but less coarse woody debris than the surrounding forest. Soil torsion and soil compaction were lower in the gaps than the forest. The abundance of fiddler crab burrows decreased with distance upstream from the Gulf of Mexico, and large and medium burrow abundance increased linearly with total seedling abundance. A distinct two-cohort recruitment pattern was evident in the seedling/sapling surveys, suggesting a partitioning of the succession between individuals present pre-lightning strike and individuals recruited post-strike. High densities of Rhizophora mangle imply that lightning strike disturbances in these mangroves favors their recruitment and does not favor Avicennia germinans and Laguncularia racemosa. However, average A. germinans seedling height was found to increase in later gap stages, suggesting an increase in the transition probability from seedling to sapling stage, perhaps related to gap successional development. This study does not support L. racemosa pioneering status in the mangrove forest as has been suggested in the literature. Overall, vegetative dynamics in lightning-initiated canopy gaps indicate that this disturbance may maintain South Florida mangroves in a cyclical or arrested successional state of development.


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