Evaluating Carbon Stocks in a Tropical Urban Mangrove Forest, Suva, Fiji

Abstract

Fiji's volcanic islands are home to the third largest mangrove ecosystem in the South Pacific. These hardy trees that inhabit the land-sea interface provide important ecosystem services, including coastal protection from storm surges, water regulation and nutrient cycling, and are an important source of food, fuel, and medicine. Mangroves are also very efficient at sequestering and storing large quantities of carbon. This study combined field observation methods with spectral information from satellite imagery to estimate carbon stocks in the mangrove forests along the Veisari River Delta, near Suva, in southeastern Viti Levu, Fiji. The Veisari mangrove forest was dominated by a single species, the bruguiera gymnorrhiza, with smaller numbers of rhizophora. spp. Despite species uniformity, there was a significant difference in forest structure and disturbance levels in the two transects measured for this study. The young forest in Transect 1 had higher tree density with smaller trees (average DBH of 5.2 cm) and an estimated biomass of 79.3 Mg C/ha, while the mature, older trees of Transect 2, with lower tree density and higher average DBH (23.2 cm) recorded 323.7 Mg C/ha in above and belowground biomass. The overall estimated carbon content of biomass for this site was 120 Mg C/ha. The satellite image of the Veisari River revealed high average NDVI values (0.92-0.94) in both transects (and in the surrounding deltaic forests), indicating an almost closed canopy cover across all plots. The comparison of field data and NDVI analysis for this site did not produce a strong correlative relationship to derive a model for predicting biomass in the Veisari mangrove ecosystem. Further research with larger sample sizes would provide a more comprehensive assessment of biophysical attributes and how forest age impacts biomass estimates for this forest. The inclusion of soil carbon measurements would provide a more complete picture of the carbon storage capacity of this site. The results from this study suggest that urban mangrove ecosystems can be extraordinarily resilient and highly efficient at sequestering and storing carbon, especially old growth forests with minimal disturbance and harvesting.