The Ecology and Management of Wood in World Rivers

Decomposition and Nutrient Dynamics of Wood in Streams and Rivers

Robert E. Bilby


Abstract.—Wood degradation in freshwater aquatic ecosystems differs from degradation in the terrestrial environment. Oxygen concentration in the interior of wet pieces of wood is not high enough to support fungi, the primary decomposing agents of wood in terrestrial environments. The primary decomposing agents of wood in streams and rivers are bacteria and actinomycetes limited to a thin layer on the surface of the piece. As a result, microbial decomposition of wood proceeds much more slowly in water, and changes in wood properties occur only on the surface. Fragmentation is an important degradation process in streams. Flowing water or abrasion from sediment removes wood softened by microbial action, and several types of invertebrates ingest or construct burrows in the softened, outer layer. Removing the outer layer exposes new surfaces to bacterial colonization. Pieces of wood more than 1,000 years old have been identified in rivers by using dendrochronological and radioisotope techniques, although most wood degrades within 100 years after it enters a stream. Nutrient content of wood is low relative to other types of organic matter. Wood abundance in streams is often an order of magnitude greater than that of other types of organic matter, however. Thus, wood does contain a significant proportion of the nutrients associated with organic matter. Wood also affects the nutrient dynamics of streams by controlling the rate of transport of materials downstream. Wood primarily affects nutrients being transported in a particulate form. Removing wood from a 200-m section of a headwater stream in New Hampshire resulted in a sevenfold increase in the transport of sediment and particulate organic matter the next year. The increase was the combined result of mobilizing stored particulate material and increasing the efficiency with which material in the channel was routed downstream. The effect of wood on material transport influences nutrient availability in many Pacific Northwest streams by retaining salmon carcasses, which contribute significant amounts of N and P to the systems where these fish spawn and die.