Hurricane disturbances accelerate region-wide disassembly of coastal bottomland forests in the face of sea level rise
Coastal floodplain forests at the southernmost extent of the great bottomland hardwood forest expanse of the Lower Mississippi Alluvial Valley are experiencing relative sea level rise rates that are among the most rapid in the world. With increased flooding, forests typically retreat and convert to marsh or open water. The rate and trajectory of community disassembly/reassembly, as species exhibit differential sensitivity and capitalize on regeneration opportunities, reflect the degree to which coastal floodplain communities are resilient to these chronic forces. We hypothesized that canopy-opening disturbances such as hurricanes may enhance resilience of these communities by providing a reprieve for forest communities experiencing the dual stresses of flooding and shade. We used long-term data (1998-2012) collected from a 5 ha plot spanning the elevation gradient from the natural levee to the backswamp of Bayou des Familles at Jean Lafitte National Park, Jefferson Parish, LA. The plot was established in 1998 to examine patterns of tree distribution, regeneration, growth, and mortality, in Barataria Basin, a highly vulnerable portion of the Mississippi River Delta. In 2013, we supplemented the plot data with tree core information by collecting cores from multiple individuals of several dominant species to assess pre- and post-hurricane growth patterns.
Overall tree mortality greatly exceeded regeneration throughout the study period. Comparisons of tree and sapling distributions indicated limited upslope migration of a few species, while others exhibited complete regeneration failure. We found no support for our hypothesis that hurricanes provided an ecological reprieve. In fact, mortality was much greater than regeneration following major hurricane events when compared to “background” trends. Tree-ring analysis revealed suppression of dominant species and no growth responses to known tropical storms. Our results indicate that this forest is in a disassembly phase with diminished tree densities and basal area, a pattern that is exacerbated by windstorm events. Thus, the forest is increasingly a subset of its former ecological self as diversity and tree abundance decline. Community reassembly is quite limited but does include upslope migration of some species; however, this trend does not appear to be triggered by canopy-opening disturbances. In conclusion, this coastal forest ecosystem appears to have low resilience as it is steadily diminished by high tree mortality with little offset through regeneration. Recent hurricanes do not substantially deflect the chronic effects of relative sea level rise on forest structure and persistence.