Leaf-decomposition heterogeneity across a riverine floodplain mosaic

Abstract

Riverine floodplains are a mosaic of aquatic, semi-aquatic and terrestrial habitats. While spatially distinct, these habitats are well connected by flows of carbon and nutrients, often in the form of leaf litter, and thus the ecological processes occurring in one habitat have ramifications for others. The aim of this study was to compare leaf decomposition, a key process in riverine ecosystems, across diverse floodplain-habitat types and to assess the role of leaf-shredding detritivores and fungi. Black poplar, Populus nigra L., leaves were exposed in seven contrasting habitat types (total of 28 sites) on the floodplain of the Tagliamento River (NE-Italy). Three distinct classes of decomposition rates emerged, corresponding to sites in the river channel (fast), terrestrial sites (slow), and ponds (intermediate). In the river channel and in ponds, leaf decomposition was driven by both microbial and detritivore activity, as evidenced by differences in coarse- and fine-mesh bags, which respectively allowed and prevented access to the leaves by leaf-consuming detritivores. Additionally, we cannot rule out that decomposition in the channel was also promoted by physical abrasion and/or fragmentation. In terrestrial floodplain habitats, very little plant litter was utilized, and leaching of soluble compounds appeared to be the primary process responsible for leaf-mass loss. Our results demonstrate that the wide range of habitats of braided floodplain rivers can have diverse decomposition potentials, creating spatial variability in both the rates of decomposition and its causes. Alterations to the natural flow regime (e.g., water abstraction, or retention by dams) and morphological changes (e.g., channelization) strongly reduce habitat diversity. These impacts will likely reduce the heterogeneity in decomposition rates across floodplains of braided rivers, with unknown consequences for overall functioning of floodplain ecosystems.