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Litterfall production and decay, root biomass, and sediment accretion dynamics were investigated from restored 'planted' (R5, R8, R15, and R30 stands) and recolonized stands (C5, C12, and C20 stands) to investigate patterns in primary productivity, belowground biomass, and sediment accretion dynamics. Litterfall data was collected using litter traps over 12 months, while decay kinetics was investigated using a litterbag experiment. Root biomass and sediment accretion data were collected using makeshift acrylic corers. Litterfall production increased as stands aged, and tended to stabilize as it matured in restored (R8: 10.05 Mg/ha/yr; R30: 6.1 Mg/ha/yr) and recolonized stands (C5: 18.75 Mg/ha/yr; C20: 9.05 Mg/ha/yr). Leaf litter decay rates (K/d) showed no pattern with stand age, although the recolonized stands (range: 0.059–0.113 K/d) had lower decay rates compared to the restored (range: 0.073–0.123 K/d) and natural stands (range: 0.064–0.123 K/d). Root biomass declined with age in restored stands (R5: 67.16 Mg/ha, R30: 49.67 Mg/ha), but increased in recolonized stands (C5: 5.41 Mg/ha, C20: 19.50 Mg/ha). Very high rates of sediment accretion were found in younger restored (R5: 10.1 cm/yr) and recolonized stands (C5: 8.1 cm/yr) than mature stands (R30: 6.3 cm/yr; C20: 4.3 cm/yr). Our results showed disparities of patterns in mangrove vegetation growth in recolonized stands and huge potential contribution on mangrove productivity when these areas are effectively restored. • Litterfall dynamics, root biomass, and sediment accretion were investigated in restored and recolonized mangroves. • Litterfall production increased as stands aged and tended to stabilize as it matured in restored and recolonized stands. • High rates of sediment accretion were found in younger restored and recolonized stands than the mature stands. • Recolonized stands showed varying recovery potentials in mangrove vegetation growth post-fishpond abandonment. • Recolonized mangroves can contribute to mangrove productivity when these areas are effectively restored. [ABSTRACT FROM AUTHOR]

Mangrove forests play an important role in climate change adaptation and mitigation by maintaining coastline elevations relative to sea level rise, protecting coastal infrastructure from storm damage, and storing substantial quantities of carbon (C) in live and detrital pools. Determining the efficacy of mangroves in achieving climate goals can be complicated by difficulty in quantifying C inputs (i.e., differentiating newer inputs from younger trees from older residual C pools), and mitigation assessments rarely consider potential offsets to CO2 storage by methane (CH4) production in mangrove sediments. The establishment of non‐native Rhizophora mangle along Hawaiian coastlines over the last century offers an opportunity to examine the role mangroves play in climate mitigation and adaptation both globally and locally as novel ecosystems. We quantified total ecosystem C storage, sedimentation, accretion, sediment organic C burial and CH4 emissions from ~70 year old R. mangle stands and adjacent uninvaded mudflats. Ecosystem C stocks of mangrove stands exceeded mudflats by 434 ± 33 Mg C/ha, and mangrove establishment increased average coastal accretion by 460%. Sediment organic C burial increased 10‐fold (to 4.5 Mg C ha−1 year−1), double the global mean for old growth mangrove forests, suggesting that C accumulation from younger trees may occur faster than previously thought, with implications for mangrove restoration. Simulations indicate that increased CH4 emissions from sediments offset ecosystem CO2 storage by only 2%–4%, equivalent to 30–60 Mg CO2‐eq/ha over mangrove lifetime (100 year sustained global warming potential). Results highlight the importance of mangroves as novel systems that can rapidly accumulate C, have a net positive atmospheric greenhouse gas removal effect, and support shoreline accretion rates that outpace current sea level rise. Sequestration potential of novel mangrove forests should be taken into account when considering their removal or management, especially in the context of climate mitigation goals. [ABSTRACT FROM AUTHOR]

• Natural regeneration (NR) offers an excellent model system to examine carbon sequestration potential. • Mean TEC stocks ranged from 232 ± 69 to 296 ± 49 Mg C/ha for NR3 and NR25 respectively. • Average C sequestration potential in the study site was 7.36 ± 1.88 Mg C/ha yr−1. • The Younger stands (NR3) holding six times higher total C sequestration rates than older mangroves (NR25). Naturally regenerated (NR) mangroves offer an excellent passive approach for climate change mitigation and adaptation under UN decade of restoration program. However, the support of NR mangroves on total ecosystem carbon (TEC) stocks and sequestration potential remained unclear across forest stand development process that can significantly vary spatially and temporally due to regeneration age. Using the 'space for time' approach, this study quantified the TEC stocks along the Merlimau - Kuala Sebatu fringing mangrove habitat expansion chrono-sequence that comprised of 3, 6, 12, 18, 25 years age and intact stands. We also estimated the carbon (C) sequestration potential of each stand-age by comparing with un-vegetated mudflats as control. We collected aboveground tree, belowground root, downed woody debris, litterfall and soil core up to 2 m for TEC quantification and C sequestration. Mean TEC stocks ranged from 232 ± 69 to 296 ± 49 Mg C/ha for NR3 and NR25 respectively at the study area. TEC stocks from different aged NR and intact mangroves were not significantly different from each other and therefore did not affect by age. Aboveground trees C and downed wood C pools were significantly affected by stand-age. In contrast, sediment C stocks (representing up to 80 %) remained generic over many years, driving the constant of TEC stocks in this study. Equally affected by age, the NR mangroves sequestered total C of 2.28 to 17.38 (mean ± SE: 7.36 ± 1.88 Mg C/ha yr−1), with younger stands (NR3) holding almost six times higher total C sequestration rates than older mangroves (NR25). Our study suggests that highly suspended sediments driving from upstream, especially in the younger mangrove stands at a lower elevation, supports higher bulk density and C accumulation rate in the sediment as they expand seaward. This study proves that NR mangroves comprised primarily of younger mangrove stands can be a C sequestrator powerhouse that promises nature-based climate solutions through avoidance and management. Though, future research is needed to understand how better management and conservation practices can affect or towards increasing trend of TEC especially sediment carbon stock that holds almost 80 % of total TEC. [ABSTRACT FROM AUTHOR]