Structure and composition of riparian forests in an old-growth northern hardwood–hemlock watershed

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We examined riparian areas in an old-growth northern hardwood watershed of the Porcupine Mountains, Upper Michigan, focusing on how stream valley geomorphology and associated factors influence the structure and composition of both the overstory and ground-flora vegetation. We collected data from 92 transects stratified among three different geomorphic settings characteristic of portions of the northern Lake States, including a low-gradient, bedrock controlled geomorphic type, a high-gradient bedrock controlled geomorphic type, and a clay lake plain geomorphic type. Individual transects extended from the stream channel to at least 40m into the adjacent uplands, with transects arrayed roughly perpendicular to streamflow. Along each transect we collected: (1) overstory basal area, species composition, and canopy closure data on a series of 100m2 plots; (2) ground-flora species composition on a series of 1m2 plots; and (3) physiographic data (e.g., distance from and elevation above the stream, landform) collected across each transect. Overall, the structure and composition of overstory and ground-flora vegetation is ordered along a complex environmental gradient running from the stream channel into the adjacent uplands. Additionally, our multivariate analyses suggest that the structure and composition of the overstory and ground-flora is related to specific valley floor landforms, as well as distance from and elevation above the bankfull stream channel. Conifer basal area increases across the valley floor landforms (floodplains, terraces, and hillslopes or valley walls) in all three geomorphic settings, while hardwood basal area remains relatively constant across these same landforms. Hardwood basal area, however, dominates the adjacent upland forest ecosystems in all but the clay lake plain geomorphic type which is characterized by fine-textured soils. In all three geomorphic settings, the near stream environment often lacks a significant overstory component, and these fluvial surfaces are often dominated by a mixture of woody shrub and herbaceous species. These results suggest that changes in vegetation are associated with the proximity of the stream, and these patterns are influenced strongly by large-scale landscape factors such as stream valley geomorphology and associated disturbances. Additionally, our results provide much-needed baseline information on the vegetation patterns and dynamics of riparian forests in the northern Lake States, and suggest that riparian areas may require different management strategies than the adjacent upland forest ecosystems.