Aboveground Carbon Stocks in Rapidly Expanding Mangroves in New Zealand: Regional Assessment and Economic Valuation of Blue Carbon

Blue carbon is increasingly being considered in global carbon budgets; however, there is poor understanding of how carbon stocks vary within mangrove forests with heterogeneous growth forms and in areas where mangroves are showing rapid changes in distribution. Here, field measurements and LiDAR data were used to estimate aboveground carbon stocks and economic benefits of Avicennia marina in the Auckland Region, New Zealand, where the mangrove area has increased at an annual rate of 3.2% since 1940. Multiple allometric equations were explored to determine best fits for different growth forms (tall and dwarf mangroves) that minimised uncertainty in conversions of tree size to aboveground carbon stock estimates. Across the Auckland Region, the mean total aboveground carbon stock (including trees, seedling, pneumatophores and woody debris) was 40.2 Mg C ha−1 with over 75% contained in tree biomass. Total aboveground carbon stocks varied considerably between mangrove growth form (tall and dwarf mangroves) and hydrodynamic conditions (estuarine, riverine and delta mangroves). Total aboveground carbon storage across the Auckland Region amounted to 384,451 Mg C with an economic value of US$ 70 million. This study demonstrates that LiDAR data can be used to produce reliable and high-resolution tree carbon stock estimates in estuaries characterised by spatially variable distributions of dwarf mangroves. These estimates are critical for the incorporation of mangroves into national blue carbon accounting schemes.