Self-thinning process, dynamics of aboveground biomass, and stand structure in overcrowded mangrove Kandelia obovata stand

Abstract

Stand structure and aboveground biomass dynamics of overcrowded Kandelia obovata forest under self-thinning were investigated at Manko Wetland. Self-thinning process was accompanied by changes in aboveground biomass dynamics and stand structure. Aboveground biomass was not completely constant as the stand grew because death and growth of trees occurred at the same time. However, the values of Spearman’s rank correlation coefficient of w significantly differed from zero (p<0.01). Therefore, aboveground biomass of overcrowded K. obovata stands was constantly changing as the stands continued to grow, showing that the death of trees was not random (only small trees died). Most of the frequency distributions b1 of tree height H was J-shaped indicating that the variation of H became small with stand growth, and finally all trees reached almost a similar height. The b1-values of aboveground biomass w and stem diameter at H/10 of tree height D¯0.1H were positive indicating that most of the frequency distributions of w and D0.1H were L-shaped. On the other hand, SD of D0.1H was constant, which means that stem diameter did not vary that much during the study period. Stem diameter and tree height showed an independent dynamic for each other. Size variation among individuals was very high, but tended to be reduced as the stand grew. Mean total aboveground biomass of the studied stands ranged from 68.8∼113.2 Mg ha−1 yr−1. The biomass production of the mangrove forest near the northern limit of its biogeographical distribution in Okinawa Island was higher comparing with others mangrove forests, and was influenced by size inequality of the individuals in the stands. Self-thinning process acts as a regulatory mechanism that is necessary for an overcrowded stand to assure sufficient seed for the next generation and enhance biomass.