A subtropical mangrove along the Miyara River in Ishigaki Island, Japan was studied for evaluating the carrying capacity for biomass of the monospecific stands. Rhizophora stylosa and Bruguiera gymnorrhiza were dominant in the downstream area whereas B. gymnorrhiza in the upstream. The stem diameter D, stem height H, fine roots mass were measured and, aboveground biomass AGB, belowground coarse root biomass BGBcoarse were estimated. The AGB, BGBcoarse and fine root mass were estimated as 128.46 Mg ha-1, 31.01 Mg ha-1 and 12.75 Mg ha-1 in the R. stylosa; 269.82 Mg ha-1, 93.68 Mg ha-1 and 11.13 Mg ha-1 in the downstream B. gymnorrhiza; and, 227.94 Mg ha-1, 81.05 Mg ha-1 and 6.35 Mg ha-1 in the upstream B. gymnorrhiza plots, respectively. The AGB did not differ among the plots, meanwhile BGBcoarse was significantly lower and fine root mass was significantly higher in the R. stylosa plots than in the downstream B. gymnorrhiza plots. Significantly lower mean individual phytomass wt specific to tree density  of R. stylosa plots than B. gymnorrhiza plots in the  – wt relationship was found, which denoted the lower carrying capacity for AGB of R. stylosa than that of B. gymnorrhiza. The results rejected our hypothesis that the stressful edaphic conditions, such as high soil salinity and low pH at the downstream, limit biomass and potential canopy height Hmax of mangrove along a river gradient but partly supported another hypothesis that biomass and Hmax differs between different mangrove species at the same edaphic environment.

Mangrove forests with complex root systems contribute to increased coastal protection through drag effects. Previous flume studies proposed a predictive model of drag in Rhizophora mangrove forests based on quadratic drag law. However, its general applicability on mangrove forests in the field has not been tested. To fill this knowledge gap, this study quantified drag in a 17-year-old planted Rhizophora mangrove forest using a comprehensive measurement of hydrodynamics and vegetation morphology. The vegetation projected area density, a, showed an approximate exponential increase towards the bed, mainly due to root branching. This vertical variation led to enhanced vegetation drag per unit water volume relative to velocity with decreasing water depth. Alternatively, the drag per vegetation projected area solely depended on the square of velocity, indicating association with the quadratic drag law. The derived drag coefficient (CD) was 1.0 ± 0.2 for tide-driven currents, consistent with previous flume studies. By using the mean value of derived CD (1.0), it was confirmed that the quadratic drag model expresses well the field-measured drag. We also presented a method for predicting a value for a, another unknown parameter in the drag model, using an empirical Rhizophora root model, and confirmed a successful prediction of a and drag. Therefore, the drag in a Rhizophora mangrove forest can be accurately predicted only using the input parameters of the Rhizophora root model – stem diameter and tree density. This provides insights into effectively implementing the drag model in hydrodynamic models for better representation of mangroves’ coastal protection function.