Viviparous mangrove propagules of Ceriops tagal and Rhizophora mucronata, where both Rhizophoraceae show different dispersal and establishment strategies

For plants to reproduce successfully, it is crucial that their seeds be adapted to survive the environmental conditions in which they disperse and establish. The buoyant dispersal units (propagules) of viviparous mangrove species seem perfect adaptations for dispersal and establishment within the mangrove environment. However, much remains unknown about the structural changes mangrove propagules undergo between abscission from the parent tree and establishment. Mature propagules of two viviparous mangrove species (Rhizophora mucronata and Ceriops tagal) were submitted to experimental conditions in order to test: (i) how substrate influences propagule mass and volume during dispersal; (ii) if stranding of propagules on solid soil triggers root development and thus establishment. Our results showed that dehydration stimulates root formation and propagule establishment and that the establishment phase is faster at lower rather than higher salinity. Furthermore, it was found that the larger propagules of R. mucronata were less vulnerable to dehydration than those of C. tagal, that their root growth started later and that, once initiated, their roots grew faster. This indicated that Rhizophora propagules are better suited for long distance dispersal than those of Ceriops and that Rhizophora has an advantage for establishment in the lower part of the intertidal zone, where inundation is more frequent and propagules need to anchor more rapidly. This study therefore points out that two co-living species of the same family have different dispersal and establishment strategies, thereby contributing to the understanding of their local and global distribution and of the species-specific dynamics within mangrove forests.