Establishment and Biomass Allocation of Black and Red Mangroves: Response to Propagule Flotation Duration and Seedling Light Availability

Global climate change is driving the expansion of mangroves into salt-marsh habitats around the world, and the ability of mangroves to displace salt marsh is attributable to a combination of factors. Early life history is critical to initial establishment; the mangroves' hydrochorous propagules allow for long-distance dispersal, and plasticity allows for establishment in varying environmental conditions. To examine differences in propagule flotation duration and the influence of light availability on seedling establishment and growth, the growth dynamics of Avicennia germinans (black mangrove) and Rhizophora mangle (red mangrove) seedlings were documented in a manipulative laboratory experiment. Propagules were collected on the Atlantic coast of Florida and were grown under two light levels (sun and shade) after being floated in full-strength seawater for varying lengths of time (0, 1, 2, 3, and 4 weeks). Seedling establishment was best after 1 week of flotation, and seedling productivity decreased the longer the propagule floated. Total biomass (g) and root: shoot ratio were significantly higher in the sun than in the shade for both species, whereas seedlings in the shade were significantly taller, with larger and more nitrogen-rich leaves. Total biomass was shaped by a two-way interaction between light and flotation duration; seedlings grew more in the sun, and growth decreased over time. This study suggests that mangroves will do best after short distance-dispersal events and that the plastic nature of mangrove biomass partitioning is highly advantageous to establishment in the salt marsh because of the varying canopy conditions that seedlings may encounter.