Fine-scale spatial variability in organic carbon in a temperate mangrove forest: Implications for estimating carbon stocks in blue carbon ecosystems

Abstract

Mangrove ecosystems have the potential to store large amounts of carbon but detailed studies on the fine scale spatial variability of biomass components and sediment organic carbon (SOC) and their relationships are lacking. Here we investigated the fine-scale spatial variation of SOC, aboveground and belowground biomass using systematic grid sampling in a temperate Avicennia marina ((Forssk.) Vierh) forest in Australia. We estimated aboveground biomass in a 9 × 9 sampling grid within a 0.16 ha plot and measured SOC, fine root biomass and necromass down to 1 m at 81 sampling points in an A. marina forest in Western Port Bay near Melbourne, Australia. We detected large differences in all measured carbon pools. SOC increased from low to high intertidal position but showed no consistent trend with depth. Aboveground biomass carbon was highly variable (mean 84 ± 56 Mg C ha−1), and showed no consistent trends with intertidal position. The average total organic carbon stock at the study site was 207 ± 80 Mg C ha−1. Aboveground biomass contributed 41% and coarse roots around 6% to the total carbon content of the forest. The bulk of organic carbon in the ecosystem was SOC (53%) and necromass was an important contributor to SOC. Aboveground biomass was a poor predictor of all belowground carbon pools. Our data indicate that 43 sediment cores would be required to determine mean SOC content at the level required by international carbon accounting standards. The high spatial variability of all carbon stocks in the system indicates that current sampling strategies for SOC need to be revised in order to improve predictions of the amount of carbon stored in blue carbon ecosystems.