Doctor of Philosophy (PhD)
Tropical cyclones recurrently influence coastal forests worldwide. Tree survival (resistance) and post-cyclone recruitment (resilience) can vary with cyclone intensity, producing differences in composition, arboreal structure, and dynamics among affected forests. Studies of tropical cyclone wind effects on coastal forests typically emphasize damage more than post-cyclone responses. We hypothesized that intense cyclones might produce large, stratum-dependent effects that prevent affected forests from returning to pre-storm conditions. We explored direct effects of major Hurricane Katrina and post-hurricane changes in oak-dominated bottomland and cypress/tupelo-dominated swamp forests within the inactive portion of the Mississippi River deltaic plain. Overall mortality was high (14-25%) but concentrated within overstory strata (22-70%). Most dominant species were either killed or heavily damaged but less abundant overstory species (red maple, sweetgum, elm, ash) survived and are reforming the overstory via resprouting and enhanced recruitment. Understory trees responded more intensely, with one usurper species, dwarf palmetto, forming a dense arboreal layer. Post-Katrina forests resemble pre-Katrina forests but possess dissimilar overstories and closed understories that might suppress woody recruitment and establishment, suggesting movement of forests toward an alternate state, impeding their return to earlier conditions.
Response syndromes categorizing combinations of resistance and resilience can help predict post-cyclone changes in forest composition, structure, and dynamics. High survival and recruitment by usurper species might influence forest composition by rapidly forming dense arboreal layers post-cyclone. We examined effects of a major usurper species (Sabal minor, dwarf palmetto) on germination and establishment of woody seedlings in post-Katrina forests using an experimental palm frond removal study. We hypothesized that woody seedling abundance: 1) should increase with either partial or complete frond removal 2) should differ with elevation and 3) should increase over time. Our results suggest that: 1) palm frond removal increases woody seedling abundance during spring germination; 2) wetter, lower elevation habitats contain more woody seedlings than drier, higher elevation habitats; and 3) high post-germination mortality reduces seedling abundances. High dwarf palmetto densities also appear to constrain the abundance of palmetto seedlings over time. Finally, these relationships display seasonal fluctuations which appear to be driven by abiotic factors such as temperature or water availability. The post-cyclone responses of this usurper species are directly influencing woody establishment and recruitment within post-Katrina forest communities, inhibiting their return to pre-cyclone conditions.
Umlang, Lance C., "Effects of a Major Hurricane on Dynamics, Structure, and Composition of Mississippi River Delta Forests" (2023). LSU Doctoral Dissertations. 6287.
Platt III, William J.