Carbon stocks and productivity of mangrove forests in Tanzania

Abstract

Mangroves offer a number of ecosystem goods and services, including carbon (C) storage. As a carbon pool, mangroves could be a source of CO₂ emissions as a result of human activities such as deforestation and forest degradation. Conversely, mangroves may act as a CO₂ sink through biomass accumulation. This study aimed to determine carbon stocks, harvest removals and productivity of mangrove forests of mainland Tanzania. Nine species were recorded in mainland Tanzania, among them Avicennia marina (Forssk.) Vierh., Rhizophora mucronata Lam. (31%) and Ceriops tagal (Perr.) C.B.Rob. (20%) were dominant. The aboveground, dead wood, belowground and total carbon were 33.5 ± 5.8 Mg C ha⁻¹, 1.2 ± 1.1 (2% of total carbon), 30.0 ± 4.5 Mg C ha⁻¹ (46% of total carbon) and 64.7 ± 8.4 Mg C ha⁻¹ at 95% confidence level, respectively. Carbon harvest removals accounted for loss of about 4% of standing total carbon stocks annually. Results on the productivity of mangrove forests (using data from permanent sample plots monitored for four years [1995–1998]) showed an overall carbon increment of 5.6 Mg C ha⁻¹ y⁻¹ (aboveground carbon), 4.1 C ha⁻¹ y⁻¹ (belowground carbon) and 9.7 C ha⁻¹ y⁻¹ (total carbon) at 23%, 32% and 27% levels of uncertainty, respectively. Both natural death and tree cutting/harvest removals resulted in significant decline of annual carbon productivity. Findings from this study demonstrate that mangroves store large quantities of carbon and are more productive than other dominant forest formations in southern Africa. Both their deforestation and forest degradation, therefore, is likely to contribute to large quantities of emission and loss of carbon sink functionality. Therefore, mangroves need to be managed sustainably.

Keywords: above- and belowground carbon stocks, carbon increment, carbon  sequestration, dbh–stump diameter relationship, harvest removals (forest degradation), REDD+, species composition