Sophie D. Boizard and Stephen J. Mitchell

Department of Forest Sciences, University of British Columbia,
3041-2424 Main Mall, Vancouver, BC, V6T 1Z4

Red mangrove is the dominant tree species in the intertidal zone of ecosystems on the Atlantic shores of the Caribbean and tropical western Atlantic. The propagules of this species are initially buoyant, then become negatively buoyant before rooting in a variety of substrates. Some time after establishment, these seedlings form aerial roots, leading to communities of plants with complex networks of stems and aerial roots. While established mangrove communities assist in stabilizing coastlines, seedlings are susceptible to wave, current and wind energy and this limits the habitats that they can successfully colonize. This experiment is part of a larger study of mangrove disturbance and regeneration dynamics. In this experiment, we tested seedlings growing at five sites with different substrate and canopy conditions at Turneffe Atoll, Belize. The seedlings ranged in height from 10 to 70 cm, had between one to ten pairs of leaves but had not yet formed aerial roots. We applied horizontal forces, measuring resistance to deflection in four cardinal directions and force to failure in the landward direction. Seventy-five percent of the seedlings failed at the root system and the remainder failed near the base of the stem. Larger seedlings were more likely to fail at the roots. Seedlings growing outside of mangrove overstory on coral rubble were 3.5 times more strongly anchored than those growing within the mangrove overstory on sand. Bending resistance did not vary systematically with pulling direction. It appears that seedling anchorage varies with exposure to wind and waves and is affected by overstory competition.