LAUSR

Mangrove ecosystems store large quantities of organic carbon for long periods of time. This study explores organic carbon stock change through the first comparative study of radiometric analysis and repeat field measures over a multi-decadal period in a mangrove system. Examining one tall gallery forest of Avicennia marina , and an adjacent interior scrub mangrove of mixed Avicennia marina and Aegiceras corniculatum , radiometric analysis estimated a soil organic carbon accumulation rate of 4.3 ± 0.6 Mg C ha −1  y −1 in the tall gallery forest and 2.2 ± 0.5 Mg C ha −1  y −1 in a stunted mangrove. Repeat measures of root carbon separated by 30 years estimated an increase of 5.06 Mg C ha −1  y −1 in the tall forest and 6.63 Mg C ha −1  y −1 in the stunted forest—suggesting an underestimate of carbon accumulation by radiometric dating of 15% and 67% in the tall and stunted forest, respectively. A higher carbon stock in the interior forest was attributed to root mass increase, associated with landward mangrove encroachment. Extrapolated to the entire region of NSW we estimate that mangrove encroachment has contributed at least about 1.8 Tg C sequestration over the 70 years for which this has been observed in New South Wales, Australia.