COS 68-5 - Success of Mangrove Colonization and Persistence at its Northeastern Atlantic Boundary: Theory

Background/Question/Methods

To explore how mangrove and marshes compare in their ability to build elevation in response to rising sea levels, numerical experiments were performed using the Marsh Equilibrium Model (MEM), which was developed for saltmarshes (Morris et al. 2002) and modified for growth of the black mangrove Avicennia germinans in the transition zone in NE Florida. The model was parameterized with data from ecotonal mangrove and saltmarsh plots in NE FL at the Guana Tolomato Matanzas National Estuarine Research Reserve (GTMNERR; www.wetfeetproject.com). At sites in the GTMNERR where elevation gain has been monitored, only the sites where mangroves are encroaching have sufficient rates of elevation gain to keep pace with current rates of sea-level rise (SLR).

Results/Conclusions

The success of mangrove colonists depends on relative elevation and the acceleration of SLR. At low elevation it is a race against time. Success depends on the ability of young mangroves to ‘out-grow’ SLR through biogenic accretion. Counterintuitively, mature mangroves may not survive as long as young mangroves, because mature mangroves do not possess the same growth advantage. Moreover, the survival of mangroves is dependent on their growth strategies with respect to their vertical distributions. Several growth strategies are possible: They can have a fixed vertical range throughout their development, or they can have an expanding range driven by SLR. The most successful strategy is one in which the vertical growth range expands to lower elevations as the mangroves mature. Starting the simulation with mature black mangroves reduces survivorship, vertical accretion rates, and carbon sequestration. Young mangroves appear to be superior to salt marshes in their ability to keep up with sea level. Mangroves produce tissue with higher lignin concentrations than Spartina alternifora and have higher biomass production. Consequently, mangroves are superior contributors to the biogenic component of marsh surface accretion. In terms of coastal resilience, the climate-driven, northward migration of mangroves may be beneficial for Florida wetlands.