What drives carbon stocks in a mangrove forest? The role of stand structure, species diversity and functional traits

Mangrove forests provide a variety of ecosystem services, and among them, the ability to sequester large quantities of below-ground carbon reservoirs is considered the most critical service for mitigating climate change. Therefore, most mangrove studies are highly concerned with estimating ecosystem carbon stocks, while only a few studies have focused on the factors driving these carbon stocks. Thus, we examined the role of stand structure, species diversity, and functional traits on above-ground, below-ground, and total carbon stocks using data from 28 sample plots from the low salinity zone of the Sundarbans mangrove forest of Bangladesh. We also aimed to understand the distribution patterns of carbon stocks among different components of the ecosystem, such as above-ground, below-ground soil, and roots. The study results revealed that tree height, diameter at breast height (DBH), and basal area were highly correlated with both above-ground (AGC) and below-ground carbon (BGC, incuding soil carbon at 0-50 cm depth) stocks. Multiple regression models indicated that tree height and basal area were two significant positive predictors for AGC, BGC and total carbon stocks (TCS) (p < 0.05). Species richness and community-weighted mean wood density were significant positive predictors for BGC and TCS. In contrast, Simpson diversity and community-weighted mean specific leaf area negatively influenced BGC. Furthermore, we observed that the above-ground tree carbon (AGTC = 80.4 +/- 32.0 Mg ha(-1)) was significantly higher than below-ground soil carbon to 50 cm depth (BGSC-50 cm = 41.0 +/- 5.4 Mg ha(-1)), followed by below-ground root carbon (BGRC = 37.1 +/- 10.1 Mg ha(-1)), pneumatophore (Pneu_C = 28.8 +/- 18.8 Mg ha(-1)), and downed wood (DW = 0.03 +/- 0.02 Mg ha(-1)) (p < 0.05). In terms of specific species contribution, Heritiera fomes contributed the most to AGTC and BGRC, followed by other species such as Avicennia officinalis > Excoecaria agallocha > Sonneratia apetala > Xylocarpus mekongensis > Bruguiera sexangula. Our findings indicate that maintaining dominant trees (H. fomes) and a diverse stand can be an effective way to enhance carbon stocks in a mangrove ecosystem.