During landfall (2005), the Hurricane Katrina storm surge pushed seawater and floating debris several kilometers inland at the Grand Bay National Estuarine Research Reserve (Mississippi). Wrack bands of woody and herbaceous plant debris 1-2m thick buried groundcover vegetation in coastal savannas with slash pine (Pinus elliottii) in the overstory and a diverse groundcover dominated by cordgrass (Spartina patens). That wrack has decreased slowly over time. We conducted a field experiment to test two hypotheses regarding effects of burial by wrack and subsequent removal of that wrack on plant communities: 1) Deposition of wrack depresses plants present above-ground in the groundcover; 2) Removal of wrack facilitates colonization by plants present as dormant propagules below the wrack. We randomly established 1 m² subplots, 8 in areas with and 8 in adjacent areas without wrack in each of three separated plots (total 48 subplots). We recorded numbers of species in subplots in June, 2008, and then manipulated wrack in half of the subplots in each plot. We simulated a new hurricane that shifted wrack deposits, exposing the soil substrate in subplots where wrack was removed, and depositing wrack in subplots without wrack. We resampled species numbers in subplots after treatment (June 2009, 2010).

Results/Conclusions

Three years after Hurricane Katrina, wrack remained in coastal pine savannas. The number of pieces of wood was greater in areas with wrack (21 ± 4 (s.e.) per 2m²) than without wrack (9 ± 2 per 2m²). Debris depth was greater in area with (13 ± 4 cm) than without (1 ± 0.1 cm) wrack. Before treatment, fewer species were present in subplots with wrack (0.83 ± 0.3 per m²) than without wrack (7.67± 0.94 per m²). Over the subsequent year, numbers of species increased in subplots where wrack was removed (12.58 ± 3.72) and decreased in subplots where wrack was added (3.25 ± 0.25). Species numbers did not change significantly in control subplots with or without wrack. Our results indicate that hurricane wrack deposits depress numbers of species present aboveground and suggest that effects of wrack deposition should vary with the magnitude of deposits. Wrack removal resulted in rapid colonization by species whose propagules remained viable through the storm surge and burial for several years. Wrack deposition during hurricanes thus introduces local heterogeneity in aboveground states of pine savanna groundcover, generating high local biodiversity via alternate local states involving species that spend differing amounts of time above and below-ground.