Leaf litter decomposition in Guinean savannah streams

Abstract

ABSTRACTLow-order streams of Upper Guinea receive substantial leaf litter inputs and are characterized by intermittent flow. The ecosystem functioning of such streams flowing in savannah, a common tropical vegetation and the dominant biome in West Africa, remains largely overlooked. We hypothesized litter decomposition in such streams to be (1) fast due to the expectedly high nutrient content of tropical litter, and (2) mostly driven by microorganisms due to the widely revealed paucity of tropical macroinvertebrate consumers, and even exacerbated in these systems by water temporariness. Decomposition rates of 2 common riparian tree species, Alchornea cordifolia and Pterocarpus santalinoides, in 2 streams ranged from 0.0287 to 0.0649 d−1. The process was clearly governed by microbial decomposers, as shown by the exceptionally low mass loss discrepancy between fine and coarse mesh bags, a lack of shredders, and high leaf-associated fungal biomass (measured as ergosterol: up to 0.92 and 0.37 mg g−1 litter of ash-free dry mass for Alchornea and Pterocarpus, respectively), together with moderate conidial production rate. In such savannah streams, the absence of leaf-shredding invertebrates seems to be compensated by a high microbial activity, providing an ultimate case for the reported preponderance of microbial decomposers over invertebrate decomposers at low latitudes. In contrast with temperate temporary streams, the processing of leaf litter from riparian trees, characterized by high nitrogen and phosphorus content, is sufficiently fast to be unaffected by flow intermittency.