Accurate estimates for biomass pools and fluxes are the key for assessing the potential of vegetation to counteract anthropogenic carbon emissions. However, such data are laborious to retrieve and still missing for many ecosystems. In this study, I used the harvest method to estimate above- and below-ground biomass, and litter traps and litterbags to measure litterfall and decomposition of the New Zealand mangrove Avicennia marina var. australasica at Mangawhai Harbour, northern New Zealand from April 2013 to March 2014.

Total above-ground biomass of A. marina was estimated at 2.69 - 8.88 kg m-2 and below-ground biomass at 11.62 - 14.7 kg m-2. The root-shoot ratio at this site was 1.73. Fine roots contributed most to the biomass stock with almost 50 % of the below-ground and 37 % of the total biomass. Woody biomass made up 32 %, coarse roots 27 %, leaf biomass 3 %, and pneumatophores and seedlings 1 % of the total biomass. More than 70 % of the root mass was located within 40 cm below the surface.

Annual litterfall production of A. marina was estimated as 331.94±69.8 - 476.63±66.1 g m-2 y-1, with leaf litter accounting for 66.6 % - 82.3 %. Litterfall production positively correlated with temperature but it did not show any relationship with monthly maximum wind speed nor total rainfall. Leaf litter of A. marina at Mangawhai harbour decomposed rapidly during the first 37-39 days with 50-60 % of the initial dry weight lost. A further ~15-25 % was lost during the following 40 days, after which biomass loss became even slower. Litter decomposition was 96.5 % complete after 332 days with the average (linear) rate of decomposition of -3.99 % per day.

From my findings, New Zealand mangroves store a total of 0.2 - 1.1 Mt carbon (C) above-ground and 1.06 - 1.72 Mt C below-ground. This, together with the high carbon density (6 - 9.9kg C m-2), implies that New Zealand mangroves are carbon-rich ecosystems.