Carrying capacity for tree biomass of a subtropical mangrove along a river in Japan inferred from forest structural features

A subtropical mangrove along the Miyara River in Ishigaki Island, Japan was studied for evaluating the carrying capacity for biomass of the mangrove stands. The stem diameters D, tree height H, and fine roots mass were measured, while aboveground biomass (AGB) and belowground coarse root biomass (BGB_coarse) were estimated. The AGB, BGB_coarse, and fine root mass were estimated as 130, 31, and 13 Mg ha⁻¹ in the Rhizophora stylosa; 271, 94, and 11 Mg ha⁻¹ in the downstream Bruguiera gymnorrhiza; and, 228, 81, and 6.4 Mg ha⁻¹ in the upstream B. gymnorrhiza stand, respectively. The AGB and BGB_coarse in the R. stylosa stand were significantly lower than B. gymnorrhiza stands, and fine root mass was significantly higher than upstream B. gymnorrhiza stand. Significantly lower mean individual phytomass w_t specific to tree density ρ of R. stylosa stand than B. gymnorrhiza stand in the ρ − w_t relationship denoted the lower carrying capacity for AGB of R. stylosa than that of B. gymnorrhiza. The results showed that high soil pore water salinity and low pH at the downstream did not limit biomass and potential canopy height H_max of mangrove along a river gradient but AGB and BGB_coarse differed between different species at the same edaphic environment. Analysis of aboveground and belowground biomass variations between stands of two mangrove species along environmental gradients from upstream to downstream could be useful in assessing the consequences of sea level rise in relation to climate change on the evolution of blue carbon dynamics.