Ecotone resilience in a coastal system of mangroves and hardwood hammocks

Background/Question/Methods

Initial sea-level rise effects on low-lying coastline vegetation will likely result from an increase in the frequency and magnitude of storm surges. Feedbacks between vegetation and vadose zone pore-water salinity likely result in complex interactions between halophytic and glycophytic vegetation due to differential adaptive responses. In coastal Everglades National Park, relatively impermeable marl soils distributed in a ridge and swale topography overlie highly permeable karst limestone saturated with high salinity water. Soil salinity dynamics reflect pronounced rainfall seasonality. A model of MANgrove and hardwood HAMmock competition (MANHAM) has been integrated with a variable density Saturated/Unsaturated groundwater TRAnsport model (SUTRA). The combined model (MANTRA) is being used to estimate likely vegetative responses to various scenarios of changing sea-level and precipitation patterns.

The mangrove/hammock regime is characterized by the occurrence of sharp ecotones over relatively shallow elevation gradients that may be maintained by a vegetation switch. A disturbance such as an input of salinity from a storm surge could upset this meta-stable boundary, leading to a regime shift of halophytic vegetation inland. MANTRA allows simulation of the interaction of vegetation with subsurface salinity dynamics while examining the sensitivity of the vegetation switch to relevant variables.

Results/Conclusions

The response of the halophyte/glycophyte system to storm surge overwash is predicted to depend on factors such as amount and duration of the salinity increase in the soil, the water-table elevation and salinity of the groundwater, the amount and timing of precipitation, runoff and infiltration, the extent of wind induced storm damage, and the amount of mangrove propagules that are washed into the hardwood hammock.  In addition, direct mortality of hammock vegetation and increasing floating dispersal of mangrove propagules due to storm surge increase the likelihood of a regime shift.  For a pulse disturbance to be sufficient to result in a regime shift without these contributing factors the duration of high salinity must greatly exceed the duration of the storm surge.