Allometric models to estimate above-ground biomass and carbon stocks in Rhizophora apiculata tropical managed mangrove forests (Southern Viet Nam)

Mangrove forests can fix and store high quantities of carbon both in their soil and in their biomass, the latter peaking in the equatorial regions and decreasing with latitude. In Vietnam, more than 80% of the mangroves develop either in the Mekong Delta or in the Can Gio Estuary, which are characterized by tropical monsoon climates. Most of these mangroves were planted and are dominated by Rhizophora apiculata Blume. The main objectives of this study were to determine forest structure, above-ground biomass and carbon conversion factors for each tree component in order to obtain allometric equations and to derive carbon stocks for managed Rhizophora stands of different ages developing within this context. Thirty-six trees, having a diameter at breast height (DBH) ranging from 7.0 to 36.2 an, from a planted mangrove forest were harvested in Ca Mau (Mekong Delta) to determine allometric equations. In addition, thirteen plots were established in both Ca Mau and Can Gio mangrove forests, to determine above-ground carbon densities. We proposed the following specific allometric equation to estimate total above-ground biomass (kg) for managed R. apiculata mangrove stands in Southern Vietnam: W-Total = 0.38363 * DBH2.2348 (R-2 = 0.976, SE = 1.17, F = 1401, P < 0.001). The total above-ground biomass ranged from 135.4 to 523.6 Mg ha(-1) depending on forest age and tree density. Consequently, and taking into account a carbon conversion factor of 44.09%, carbon stocks in the above-ground biomass of R. apiculata mangrove forests in Southern Vietnam ranged from 59.7 to 230.9 Mg C ha(-1). The mean carbon partitioning in the tree biomass was: 77.11% for trunks, 11.87% for branches, 8.66% for prop roots, and 2.36% for leaves. However, this distribution, as well as annual height increments and biomass increase rates, also varied with forest age and tree densities. We suggested that tree density reduction through thinning activities allowed easier tree development, resulting in an increased biomass with enhanced allocation to branches and above-ground prop roots for the stability of the trees. Using the specific allometric equation and specific carbon conversion factor reduced the uncertainty in the estimation of above-ground biomass and carbon stocks.