Growth and carbon stocks in four mangrove species planted on a former charcoal concession site in Ranong, Thailand

Abstract

The ending of mangrove concessions for charcoal production in 1998 gave new impetus to mangrove conservation and rehabilitation in Thailand, including the designation of the Ranong Biosphere Reserve (RBR) to protect Thailand’s largest single mangrove ecosystem. Four of the dominant tree species in the RBR were planted as seedlings in single species blocks on a former concession site in 1994: Rhizophora apiculata (Ra) and R. mucronata (Rm); and 1995: Bruguiera cylindrica (Bc) and Ceriops tagal (Ct). Tree growth and natural recruitment of seedlings and saplings were recorded in 100 m2 sampled quadrats in each species block in 1999, 2008, 2019 and 2023. All four species exceeded 10 m mean height by 2019 (range 12.1 ± 3.8 m to 19.6 ± 2.3 m), while mean DBH was 7.5 ± 3.4 cm to 9.1 ± 6.9 cm. There was evidence of self-thinning mortality by 2023, especially in the Ra and Bc blocks. Some illegal cutting of Bc trees between 2019 and 2023 further impacted the growth performance of this species, which exhibited a compensatory strategy of self-planting many seedlings. The height and DBH of the four planted species in 2023 were still less than in a mature Rhizophora- and Ceriops-dominated conservation forest area (mean tree height 17.4 ± 6.7 m; DBH 15.1 ± 7.3 cm). However, soil organic carbon (SOC) was high and not significantly different between the monoculture species blocks (525 ± 107 Mg C ha-1 to 743 ± 31 Mg C ha-1) and the conservation forest (616 ± 91 Mg C ha-1). SOC accounted for 74%-90% of the total ecosystem C, which was 650 to 829 Mg C ha-1 in the planted species blocks and 828 Mg C ha-1 in the conservation forest. The estimates for plant, soil and ecosystem C are compared with those reported from other natural and planted mangrove sites in Southeast Asia, especially along the Andaman Sea coast. The findings confirm the importance of conserving the few remaining areas of near-primary mangrove forest in this region.