Patterns of mangrove forest disturbance and biomass removal due to small-scale harvesting in southwestern Madagascar

Abstract

Informal small-scale mangrove wood harvesting has received limited attention, though it is a widespread threat to mangroves in many parts of the tropics. We investigated wood use and the impacts of harvesting on mangrove forests in the Bay of Assassins in southwest Madagascar. We measured forest structure, composition, and harvesting across 60 vegetation plots and investigated human uses of the mangroves through Rapid Rural Appraisal techniques. We found that unlike other mangroves in the region, those in the Bay of Assassins are dominated by Ceriops tagal. Tree harvesting rates are high, with a mean of 28.7% (SD 19.4) of trees harvested per plot. This is similar to heavily harvested mangroves in other parts of the tropics. A comparison of tree versus sapling importance of the different mangrove tree species indicates that the composition of the mangrove forest is changing, with C. tagal becoming more important. Livelihood activities drive the harvesting of certain species and size classes. Mangrove wood is used mainly for the construction of traditional housing and fencing. There are also emerging uses of mangrove wood, including seaweed (Kappaphycus alvarezii) aquaculture and the production of ‘sokay’, a lime render made by burning sea shells in mangrove wood kilns and used to improve the durability of houses. Small-scale selective harvesting of mangrove wood is important for local livelihoods but may have wide-ranging impacts on forest composition and structure. Demand for mangrove wood has grown in relation to new commodity chains for marine products, demonstrating the need for integrated landscape management that considers wetland, terrestrial and marine resources together.