Structure, growth, and sustainability of mangrove forests of mainland Tanzania

Abstract

Mangrove forests provide a range of ecosystem services, which are important for the well-being of humankind. Hence, they need to be managed sustainably. Datasets on forest structure, relationships between structural parameters as well as growth and yield are essential inputs in the sustainable management planning of mangrove forests. This study assessed the structure of mangrove forests and determined the relationships between various forest structural parameters. Furthermore, the study determined tree volume Mean Annual Increment (MAI) and evaluated the sustainability of tree removal and the effects of tree removal on forest stocking. The study was carried out in mangrove forests of mainland Tanzania and employed cross-sectional and longitudinal forest inventory datasets. The analysis was carried out using quantitative statistical techniques. Eight mangrove tree species namely Avicennia marina, Bruguiera gymnorhiza, Ceriops tagal, Heritiera littoralis, Lumnitzera racemose, Rhizophora mucronata, Sonneratia alba, and Xylocarpus granatum were recorded in both Rufiji and Kilwa. Community-level Shannon-Wiener Index (h’) was 1.54 and 1.38 in Rufiji and Kilwa respectively. Avicennia marina, C. tagal, and R. mucronata were the three species with a higher Important Value Index (IVI) at both sites. Tree volume (V), aboveground carbon (AGC), belowground carbon (BGC), and the total carbon (TC) densities were estimated at 109.1 ± 10.8 m3 ha−1; 44.3 ± 4.2 Mg C ha−1; 29.3 ± 2.4 Mg C ha−1 and 73.5 ± 6.1 Mg C ha−1 respectively in Rufiji. Similarly, V, AGC, BGC, and TC were 100.2 ± 10.4 m3 ha−1; 42.5 ± 4.1 Mg C ha−1; 40.2 ± 3.8 Mg C ha−1 and 82.7 ± 7.1 Mg C ha−1 respectively in Kilwa. The results also showed mixed patterns on the relationships between forest structural parameters. An outstanding and positive relationship was observed between AGC and basal area in both Rufiji and Kilwa. Tree volume MAI is estimated at 13.8 m3 ha−1 year−1 for mangrove forests of mainland Tanzania. However, to avoid over-harvesting, this study recommends an overall conservative tree volume MAI of 6 m3 ha−1 year−1 to be used in harvest planning of species demonstrating active regeneration and recruitment. The observed tree removals in the study sites were sustainable since they were less than the conservative tree volume MAI recommended in this study. Further, tree, carbon, and tree volume densities were higher in the stratum that experienced tree removal than in the stratum that never experienced tree removal. Accordingly, management of mangrove forests and other forest types in Tanzania and beyond should be enhanced using stand structure, growth, and yield information.