Dispersal patterns of black mangrove (Avicennia germinans) propagules at a mangrove-saltmarsh ecotone

Background/Question/Methods

Intertidal mangrove forests are expected to adapt to climate-induced sea-level rise via landward migration; therefore, empirical data on dispersal processes at mangrove boundaries will advance our understanding of mangrove adaptability to climate change. Landward transport of propagules is limited by the magnitude and frequency of high water events. During high tide events, propagule dispersal may be limited by the permeability of the landward ecotone. In this study, we empirically examined dispersal patterns of Avicennia germinans (black mangrove) propagules at the landward margin of a mangrove forest. At our site, the landward mangrove boundary is characterized by an ecotone between succulent saltmarsh vegetation and the saltmarsh grass, Sporobolus virginicus, which dominates the interior of the island. Experimental plots (n=10) were established along the conspicuous interface between these saltmarsh zones, which was designated as the 0m position, denoting the seaward and landward portions of the plots. Five plots were assigned to a grass only treatment, created by weeding plots so that they contained only Sporobolus virginicus. The other 5 plots were manipulated to create a distinct succulent-grass ecotone treatment, which was achieved by removing grass seaward of the 0m position and by removing all vegetation except the grass from the landward portion of the plots. Propagules were collected in situ from mature A. germinans trees, marked, and emplaced into each plot at the 0m position. Subsequently, the position of propagules was observed after plots had been inundated by high tides.

Results/Conclusions

We were able to relocate 97% of propagules that were emplaced into our experimental plots. Regardless of treatment, a substantial proportion of emplaced propagules moved a negligible distance from where they had been emplaced; a mean (±SE) 37.4% (±6.0) of propagules moved less than 5 cm (either landward or seaward). Most propagules were dispersed at least 5 cm in the seaward direction (45.0% ± 6.2 SE). Propagules emplaced into plots containing Sporobolus virginicus moved a mean (±SE) distance of 3.4 cm (±1.8) seaward; whereas, propagules emplaced into plots with a discrete succulent-grass ecotone dispersed slightly further, a mean 21.0 cm (±3.2) seaward. Our data demonstrate that A. germinans propagules are infrequently dispersed in the landward direction (an average of only 17.6% (±2.9) of propagules per plot dispersed more than 5 cm landward). These findings indicate that, at our study site, mangrove encroachment may be limited by dispersal processes.