Stand dynamics and aboveground net primary productivity of a mature subtropical mangrove forest on Ishigaki Island, south-western Japan

Abstract

There are insufficient data regarding forest productivity of mature mangroves near the northern limit of mangrove distribution in subtropical East Asia. We conducted the present study to determine the stand dynamics and net primary production (NPP) of a mature mangrove forest on Ishigaki Island in south-western Japan over three successive years, using a large permanent plot. The two mangrove species present were Bruguiera gymnorrhiza and Rhizophora stylosa. The aboveground biomass in the plot was high, despite the high latitude (24 ∘29′N) of the site, gradually increasing from 158.4 Mg ha−1 to 164.6 Mg ha−1 over the three-year study period. This increase was attributable to an increase in the biomass of B. gymnorrhiza. The biomass of R. stylosa, however, decreased during the same period because of a threefold-higher mortality rate and a lower relative growth rate of this species compared with B. gymnorrhiza. The mangrove forest was in the late successional stage, following a pioneer R. stylosa forest that was widely distributed throughout the study plot as standing dead trees and logs. The aboveground NPP of the mangrove forest was 10.66 ± 1.46 Mg ha−1 y−1, partitioned into 3.10 ± 0.51 Mg ha−1 y−1 as woody NPP (net increase in aboveground woody parts, SI), and 7.56 ± 0.99 Mg ha−1 y−1as foliage NPP (litter production, including foliage and reproductive organs, Ln). The mature mangrove forest had a relatively low SI/Ln ratio (0.41 ± 0.03), although litter fall production was within the range previously recorded for mangroves. The lower woody NPP in the mature mangroves was due to the exclusion of R. stylosa once the stand was in a late successional stage dominated by B. gymnorrhiza.