Channel, Riparian and Catchment Features as Predictors of Wood Abundance in Low Gradient, Agricultural Streams

Abstract

Wood is an important component of small to medium streams in forested regions, but has been little studied in agricultural areas. Although wood habitat has been shown to be an important factor controlling macroinvertebrate biodiversity in agricultural regions of the Midwestern U.S., there is little information on how much wood is available and what factors control its abundance and distribution. The goals of this study were to: 1) characterize the abundance, size, and distribution of wood in low gradient streams in a predominantly agricultural region, and 2) quantify the relative influence of reach- and catchment-scale factors on the abundance and distribution of wood in these streams. Standing stocks of wood were quantified in 49 stream reaches in the Saginaw Basin of central Michigan, USA. An array of stream channel, riparian zone, and catchment features were quantified. Multiple regressions were conducted to predict standing stocks from explanatory variables at three spatial scales. Features at the local scale (e.g., bank-full width, % open canopy) had a large influence on the density and size of accumulations, and a moderate influence on wood abundance. In contrast, riparian and catchment features including riparian vegetation type, link number, % urban land use in the catchment, and topographic heterogeneity exerted greater control over wood abundance and the mean size of wood accumulations. The differences in the factors predicting wood standing stocks versus accumulation density are probably related to the presence of structures that entrain wood into accumulations. In contrast, wood standing stocks reflect current and past land use practices, as well as underlying processes (e.g., hydrologic regime) controlled by landforms. Patterns in wood standing stock and distribution differ from those observed in high gradient regions, and low gradient streams in forested regions. This has important implications for ecosystem processes and management of headwater streams in agricultural regions.