Towards an unknown fate: The floating behaviour of recently abscised propagules from wide ranging Rhizophoraceae mangrove species

Abstract

The persistence of a mangrove ecosystem depends on the potential of its propagules to disperse. In case of the Rhizophoraceae mangrove species, propagules are elongated cylinder-shaped seedlings that have an initial ability to float upon abscission from the parent tree. During the hydrochoric dispersal period, propagule traits (e.g. volume and density) change over time, which in parallel influences propagule buoyancy behaviour. Recently abscised, mature propagules of three rhizophoracean mangrove species (Bruguiera gymnorrhiza (L.) Sav., Ceriops tagal (Perr.) C.B. Rob., Rhizophora mucronata Lam.) were submitted to a three-month floating experiment to investigate the timing and pattern in their buoyancy behaviour linked to changes in propagule density. Anatomical analyses of the aerenchymatous tissues complemented the floatation data. Initial propagule density (between 0.96 (C. tagal) and 0.99gcm−3 (B. gymnorrhiza and R. mucronata)) was on slightly lower than that of seawater and increased over time in all species but at dissimilar rates. Intra-individual density increased from the plumule- towards the radicle end, and corresponded with a decrease in proportion of intercellular surface area per unit area. The interplay between propagule traits and surface water conditions determine the fate of each propagule and its capacity to escape the local forest and reach open water. The combination of techniques used and hence combination of data obtained in this study, contribute to an increased understanding of mangrove forest dynamics: the potential and the limits therein to expand the species range or to replenish existing populations with new recruits.