Map-based Predictions of Riparian Ecotypes: Relating Climate and Hydrology to Streamside Forests in Lower Michigan

Abstract

Although it is generally agreed upon that the hydrology of river valleys plays an important role in shaping the composition and structure of riparian ecosystems, relevant hydrologic drivers are difficult to measure across broad regions and often inadequately specified in riparian studies. In this paper, we describe an empirical-statistical method using map-based models to predict the composition of riparian forests along the major rivers of Lower Michigan. Our approach, based on regional climate as well as both local and catchment-scale hydrology and physiography, accounted for between 84% and 99% of the observed classification probability for five riparian ecotypes at 94 locations. Using parameters from our models, we combined map-based estimates of groundwater flux and flood dynamics with climatic indices to extrapolate and map initial predictions of riparian character throughout much of Michigan’s Lower Peninsula. Although our ability to account for long-term variation under specific hydrologic conditions was relatively poor, we nonetheless were able to distinguish and characterize riparian conditions with an overall predictive accuracy of 84%. Although climate and hydrology are not the only determinants of riparian conditions, our results suggest that the interactions of groundwater supply and flood dynamics, as described by our spatial predictions, drive a significant portion of the spatial variation in riparian ecosystem character. The results also suggest strong variation in the relative and spatial scales of hydrologic determinants leading to specific riparian ecotypes. Models that provide insight into factors controlling diversity in riparian composition, structure, and function, also provide a context for understanding riparian contributions to in-stream habitat and water quality studies.