Forest structure of a subtropical mangrove along a river inferred from potential tree height and biomass

The hypothesis was tested that potential tree height and biomass in mangroves decrease downstream with the tidal gradient along the Okukubi River in Okinawa Island, Japan. The mangrove stands consisted of Bruguiera gymnorrhiza (L.) Lamk. and Kandelia obovata (S., L.) Yong (Rhizophoraceae). Four sites were selected considering the distance from the mouth of the river. Soil salinity increased downstream, while soil total nitrogen content decreased. The soil redox potential did not vary along the river. Maximum gross photosynthesis and tree height for each species decreased downstream. The potential tree height (Hmax) inferred from the stem diameter (D0.1)–tree height (H  ) relationship (1/H=1/aD0.1h+1/Hmax;a, h, coefficient) in each species decreased downstream. The tree density (ρ  )–mean tree size (D0.12H) relationships (D0.12H=Kρ−α;K, α  , coefficient) determined for four sites revealed that the mean tree size at any given tree density decreased downstream, which indicates the decrease of potential biomass. Furthermore, an index for biomass (D0.12H ρ=K) was homogeneous within a site regardless of tree density, i.e. the value of α at each site did not differ significantly from 1.0 (p > 0.05). The decreases in potential tree height and biomass may be partially ascribed to the stressful environments at the downstream sites characterized by high salinity (>2.6%) and nitrogen-poor soils (<0.25 ppt) in our study area.