Mangrove community response to sea-level rise in the Florida Keys, USA: Potential limitations on upslope migration
The Florida Keys are a low elevation island chain representing the southernmost extent of the continental United States. Red mangrove (Rhizophora mangle), white mangrove (Languncularia racemosa), and black mangrove (Avicennia germinans) are the primary intertidal coastal plant species in this area. Red mangrove typically dominates the lower intertidal zone, whereas black and white mangroves occur landward at higher, less frequently flooded elevations. The Florida Keys have a unique geologic history, with many of the mid and upper intertidal areas exhibiting exposed surface layers of limestone. Importantly, significant soil development is frequently limited to lower intertidal areas and the higher elevation supratidal forests. As a result, black and white mangroves are often stunted in the mid and upper intertidal areas. We addressed the following primary questions: 1) Does the low elevation of the Florida Keys in combination with limited soil development for upslope migration make these mangrove habitats particularly susceptible to sea-level rise? 2) What are the key environmental factors that are currently controlling mangrove species occurrence and zonation? To address these questions we initiated a long-term study in which transects were established in different hydrogeomorphic settings on two Keys. Transects began at the lower intertidal, red mangrove interface, and extended upslope to the furthest landward extent of mangrove habitat. Plant community composition, edaphic factors, and elevation gradients were determined along each transect. Sediment elevation tables (RSETs) were installed at critical points along each transect (four per transect) to monitor accretion and net elevation change through time.
Results from a principal components analysis indicate that red mangrove cover is positively associated with soil depth, whereas black mangrove cover is positively associated with extractable soil salinity. White mangrove cover was not strongly associated with any of the environmental variables measured, suggesting that this species may establish opportunistically, but then may be replaced through time. Data from RSETs indicate that sediment accretion is variable and can be transient but that pulsed disturbance events, such as tropical storms and hurricanes, can play an important role in depositing sediment to upslope areas. Without these pulsed sediment deposition events, we speculate that accelerated rates of rising sea levels may exceed the rate of soil development, thereby limiting the ability of these mangrove communities to successfully move upslope.