In the northern Gulf of Mexico, tropical storms are predicted to intensify with climate change. More powerful storm surges impacting shorelines eroded by chronic sea-level rise will transport saline water and deposit wrack progressively further inland. We hypothesized that wrack deposited by storm surges facilitates landward shifts in species distributions, which may be necessary for long-term survival in shrinking coastal zones. Landward migration of species occurs by two means. Propagules disperse inland by wrack rafting during storms and surge-deposited wrack buries standing vegetation opening up space for colonization. Following Hurricane Katrina, we initiated a study of wrack effects on diversity and community dynamics at Weeks Bay, Alabama, USA to test these hypotheses. In 2006, we quantified wrack along the ~ 400 m coastal transition from salt marsh to mesic hardwood forest and established permanent vegetation plots in and immediately adjacent to the primary deposition zone. We also collected wrack samples at random locations to determine composition of the germinable propagules within the wrack. The second part of the study consisted of an experiment in which we manipulated Katrina wrack to determine its short-term effect on diversity and plant community composition.
Results/Conclusions
Hurricane Katrina wrack was highly concentrated in the shrub-dominated marsh/forest ecotone, and vegetation in the deposition area differed significantly from adjacent vegetation. The germination and experimental studies demonstrated that wrack can serve as a dispersal vector and, coupled with the burial of standing vegetation, can drive changes in community composition. Diversity increases when wrack decomposes or is removed by subsequent disturbance and germination from the “wrack bank” and seedbank occurs. Wrack rafting during powerful storm surges may be an important means of long-distance dispersal and the patches it creates may function as ephemeral “stepping stones” for landward migration of coastal species.
