Species composition, biomass, and net primary productivity of mangrove forest in Okukubi River, Okinawa Island, Japan

Species composition and productivity of mangrove forest at Okukubi River, Okinawa Island, Japan, were examined during a period from February 2010 to January 2013. Species composition, above and below-ground biomass changes, and litterfall production were measured. Bruguieragymnorrhiza has maintained its dominance (93.1%, relative dominance) of the stand. Mean mortality of trees was 16.4%, due to frequent attack of typhoons creating tree fall and hitting of other trees. The total above and below-ground biomass of the stand was 162.7 and 125.1Mgha−1, respectively. Among the total aboveground biomass of the trees, 7.2% was allocated to leaf, 69.2% to stem, and 10.1% to branch. In case of species wise contribution of biomass allocation, B. gymnorrhiza showed the highest score and R. stylosa the lowest. Mean annual litterfall was 11.8Mgha−1yr−1, with the maximum litterfall in summer and minimum in winter. The mean aboveground biomass increment and aboveground net primary productivity was 8.5 and 28.6Mgha−1yr−1, respectively. Total net primary productivity was estimated to be 42.5Mgha−1yr−1. This study revealed that mangrove forest with similar height and diameter produced different biomass production with different basal area. In the present study, the root biomass was large and the mean ratio of above/belowground biomass was estimated to be 1.3. It is emphasized that mangrove forests growing at the northern limit of their biogeographical distribution showed high biomass and net primary production indicating its ecological significance.