The potential of mature Sonneratia apetala plantations to enhance carbon stocks in the Zhanjiang Mangrove National Nature Reserve

Non-native Sonneratia apetala plantations have been widely recommended as part of China's mangrove reforestation and afforestation plans, partly due to their rapid carbon accumulation in biomass and sediments. Nonetheless, reviews of the available data estimating carbon dynamics in the S. apetala plantations reveal a lack of monitoring data concerning carbon dynamics for the mature planted mangroves. This study sought to quantify how biomass and SOC stocks of the mature S. apetala plantations varied with mangrove growth between 2012 (aged 19 years old) and 2019 (aged 26 years old) using a continuous monitoring method. The results show that total biomass, as well as tree height and stem diameter at breast height (DBH), increased significantly between 2012 and 2019, while tree densities of both mangrove canopy and understory layers decreased. However, there was no significant difference in SOC content/stocks between 2012 and 2019. This result about SOC change may be partly attributable to the relatively shorter monitoring time span and the slower accumulation rate of SOC in comparison with biomass. In addition, fine root biomass was positively correlated with SOC content and regulated the vertical distribution of SOC content. Regression analysis for China's S. apetala plantations found that biomass, tree height and DBH, and SOC stocks all increased with forest age. Moreover, SOC stocks were significantly positively correlated with biomass. This suggests that mangrove afforestation and reforestation can offer carbon-sink potential, but this should be confirmed by longer-term monitoring to further evaluate the SOC accumulation of S. apetala plantations over time.