The bark and wood properties of Bruguiera gymnorrhiza and Rhizophora stylosa trees in riverine mangrove forest

Background: The mangrove species Bruguiera gymnorrhiza (L.) Lam. and Rhizophora stylosa Griff. are found in riverine mangrove forest, where the trees occur in mixed forest. They have different ecological traits and supporting buttress root systems. However, little is known about the bark and wood structural properties of the trees which are important when using biomechanical approaches to understand the ecological differences between species. Here we test the hypothesis that the structural properties of the trees are influenced by the ecology of these species in riverine mangrove forest. Methods: Plots were established in mixed forest of B. gymnorrhiza and R. stylosa in riverine mangrove forest on Iriomote Island in Okinawa, Japan. Selected trees from the two species were sampled to evaluate the bark and wood properties of the stems, branches and roots. The data were analysed and compared for the two mangrove species. Results: Bruguiera gymnorrhiza and R. stylosa have differences in their bark and wood properties. R. stylosa trees have thicker bark and higher bark density, and wood of higher density, strength, stiffness, and hardness. This applies to the stems and roots of R. stylosa trees, and supports the compressive buttresses, and exposed tidal positions of this light demanding pioneer species. Bruguiera gymnorrhiza trees have a higher proportion of wood in the stems and branches, and less in the roots, which can be attributed to the risk of canopy damage, and the presence of tensile buttresses, in this shade-tolerant gap-phase species. Conclusions: Differences in the bark and wood properties of the stems, branches and roots, were consistent with the ecological traits of B. gymnorrhiza and R. stylosa, and the structural properties of the buttress root systems. The knowledge will aid understanding of the distribution of these species in riverine mangrove forest.