PLANT DIVERSITY CONTRIBUTIONS OF RIPARIAN AREAS IN WATERSHEDS OF THE NORTHERN LAKE STATES, USA

Abstract

In most forested watersheds, riparian areas constitute a small proportion of the total land area, yet their contributions to overall plant diversity can be significant. However, little information is available on which portion of riparian areas (defined as functional ecotones comprising all fluvial landforms, including floodplains, terraces, and connecting hillslopes) contribute the most to plant species richness and at what scale these contributions are most evident. In order to better understand the contributions riparian areas provide to plant species richness in forested watersheds, we examined herbaceous ground‐flora richness and similarity from 56 reaches in eight different valley types (defined as unique combinations of stream order, surficial geology, and stream‐valley constraint) across the northern Lake States, USA. We analyzed these data at two scales: (1) at the individual reach scale; and (2) at the watershed scale by pooling individual reaches by valley type. At the reach scale and regardless of valley type, there is not significant (<50%) species similarity among landforms; however, levels of species richness are not significantly different among the floodplain, terrace–slope complex landforms (e.g., terraces, connecting hillslopes), and adjacent uplands. This suggests that individual reaches (representing an individual riparian area) may not provide significant contributions to overall plant species richness. However, when individual reaches are pooled by valley type, the floodplains are almost always more species rich than the terrace–slope complex and upland landforms, suggesting that the environmental heterogeneity associated with a variety of individual reaches can be responsible for greater species richness provided by riparian areas at the watershed level. Our results also suggest that floodplains are not the only riparian landform that adds significantly to the overall plant diversity of watersheds, especially in areas with broad, unconstrained valleys. In these systems, the terrace–slope complex landforms comprising a series of broad fluvial terraces and connecting slopes have significantly higher species richness than the adjacent uplands. Consequently, it is apparent that, to maintain plant diversity at both local and regional levels, efforts should focus on all fluvial landforms of the riparian area, not just their floodplains. This is particularly relevant when management of riparian areas uses fixed‐width riparian management zones (RMZs). In our study areas, 33 m‐wide RMZs (the recommended width in each of our study states) often failed to include fluvial landforms beyond the floodplain. The consequence is that areas of increased species richness and/or unique plant assemblages in the watershed may receive inadequate protection during forest‐management operations.