Growth and development of mangrove forests overlying smothered coral reefs, Sulawesi and Sumatra, Indonesia

Abstract

Smothering of many fringing coral reefs in Indonesia has led to mangrove colonization of the overlying soils in a process of human-induced succession. On the islands of Sulawesi and Sumatra, we measured some structural and functional attributes of 4 mangrove forests that have colonized such soil drapes. The forests varied extensively in stem density, above-ground biomass, and in leaf area index. We estimated accumulation rates of above-ground forest biomass of 1.5 to 8.1 t C ha -1 yr -1 , similar to other mangroves. The ratio of greenhouse C and N emissions from soils averaged 35 compared with an average soil C:N ratio of 38. Soil ammonification was sufficient to meet forest N demand. A value of 1 to 4% of soil C metabolism was involved in dissolving carbonate, with sulfate reduction being the major decomposition pathway. Rapid rates of N 2 loss were measured, but there was no detectable N 2 O release. Rates of nitrogen fixation on the soil surface ranged from 97 to 1648 mol N m -2 d -1 , but higher rates (860 to 4316 mol N m -2 d -1 ) were measured on microbial mats covering pneumatophores, suggesting N 2 fixation on above-ground tree parts may balance soil N 2 losses. Rates of forest biomass accumulation related exponentially to rates of soil C and N metabolism, reflecting a close relationship between tree growth and nutrient availability as mediated by microbes in these shallow soils. These patterns are similar to those in other mangrove forests, and illustrate that human-induced change has resulted in a clear shift from reef-dominated to mangrovedominated habitats and functional characteristics in parts of coastal Indonesia.