Floodplain ecosystems are sustained through connections with their rivers. The hydrologic regime is a primary driver of the dynamics of forests occupying these floodplains. In coastal areas, floodplain forests are also strongly influenced by tidal forces, deltaic processes, and tropical storm events. Forests that occur at the southernmost extent of the great bottomland hardwood forest expanse of the Lower Mississippi Alluvial Valley are undergoing rapid relative sea level rise that extends the hydroperiod. With chronic, increased flooding, regeneration of less flood-tolerant species is inhibited, mature trees gradually die, and coastal forests eventually retreat. Disturbances that open the canopy, such as hurricanes, may ameliorate conditions and provide a reprieve for trees experiencing the dual stresses of flooding and shade. Resilience of coastal floodplain forests to climate change will depend upon the rate and trajectory of community disassembly as species exhibit differential sensitivity to environmental changes. Species that can benefit from pulses of resource availability, e.g., enhanced light availability, may be able to persist longer against the backdrop of sea level rise. Identifying these opportunistic species will help to predict the resilience capacity and longevity of the system. The objective of this study was to assess patterns of growth of the dominant tree species in a subsiding bottomland forest that has experienced periodic tropical storm disturbances. I predicted that relative growth rates of trees in this forest would exhibit a pattern of decline, consistent with the chronic effects of sea level rise but that growth trends of some species would be punctuated by spurts initiated by hurricane events. I used tree data collected over a 17-year period from a five-hectare monitoring plot established in the floodplain forest at Barataria Preserve, Jefferson Parish, Louisiana.
Results/Conclusions
The forest community did not exhibit detectable shifts in composition. Rather, there was dramatic loss of individuals across the entire flooding gradient, and the forest showed signs of disassembly. Contrary to my hypothesis, growth rates were not affected by hurricane disturbances, and mortality was higher in years that coincided with major storms (e.g., Hurricane Katrina in 2005). Hurricanes appear to accelerate the demise of these systems. In contrast, some species responded favorably during drought years, suggesting that these extreme events may offset, albeit temporarily, the effects of lengthened hydroperiods from relative sea level rise. Resiliency of this system will depend, in part, on the ability of species to migrate upslope with climate change and sea level rise.