Associations between large wood and streambed complexity in headwater streams in the western Upper Peninsula, Michigan

Habitat complexity is an important feature of fluvial ecosystems, and in forested streams large wood can indirectly and directly contribute to complexity that supports biota. In this study of six forested streams of Michigan's western Upper Peninsula we quantified large instream wood and streambed morphology analyzed as variograms to describe the complexity of the streambed. Our goal was to determine the extent to which the streambed complexity could be explained by the volume and number of instream large wood objects. Based on previous studies in other systems, we expected a positive relationship between large wood volume and streambed complexity within the channel. Across the study sites, large wood volumes (standardized by area) ranged from approximately 0.0036 m³ m⁻² to 0.029 m³ m⁻² and the sites aligned into three groups based on overall complexity in channel morphology as derived from variograms. However, variation in streambed complexity was not clearly associated with the volume of large wood present in the channels. In contrast to our hypothesis, the two sites with the highest substrate complexity did not also have the greatest wood volume. Complexity was inversely related to large wood abundance and the relationship between standardized wood volume and complexity also appeared to follow this trend. Our findings may be a result of the low stream power that likely occurs in these streams, as well as because wood in the study streams was generally smaller and in lower abundance than typically reported for other streams and rivers. We suspect that the smaller diameter wood measured in these actively managed forests of northern Michigan would have shorter lifespans (i.e., degrade more quickly) and weaker hydrological influence, and thus a less significant influence on streambed complexity.