Vegetation and sediment characteristics in an expanding mangrove forest in New Zealand

Mangrove expansion in inlets has been widely observed in the North Island of New Zealand over recent decades. There is just one mangrove species in New Zealand, Avicennia marina var. resinifera. Our main objective was to investigate the response of mangroves to sedimentary patterns. Remote sensing and GIS was used to quantify the change in mangrove area. Vegetation and sediment characteristics were studied across seasons from December 2009 to August 2010. Comparison of digital images in 1940 and 2003 revealed that the mangrove area in our study inlet had increased by 21%. The mangroves created a rim of high fringe mangroves surrounding high-density but low height trees in the interior. The relatively low pH level and seasonally fluctuating pore water total dissolved salt (TDS) concentration reveal potentially stressful conditions in the interior mangrove zone, which may influence the forest structure in the interior.Total organic carbon (TOC), δ13Corg and δ15N along the transect indicated an increase of autochthonous organic carbon in the surface substrate landwards. The highest carbon accumulation was not observed in the fringe zone, in spite of it having the most vigorous growth of mangroves. Instead, the highest δ13C value (−23.36 ± 0.42‰, averaged over depth) and lowest C:N ratio (13.1 ± 2.0) in the fringe mangrove core reflected the preservation of allochthonous organic material in the fringe zone. Under the protection of seaward fringe mangroves, mangrove-derived carbon is retained and stored in the interior sediment.