Landscape hierarchies influence riparian ground-flora communities in Wisconsin, USA

Abstract

We examined the distribution of ground-flora species (herbaceous and woody species <1 m tall) across riparian areas of northeastern Wisconsin in an effort to understand how hierarchical landscape properties, such as the physiographic system (ground moraine and outwash plain), valley system (constrained and unconstrained), and valley floor landforms influence distribution patterns of ground-flora species and functional plant guilds across riparian areas of small streams and rivers in a glacial landscape. A total of 162 species were recorded on 417 (1 m 2) plots stratified by four different valley types that reflect the dominant physiographic system and valley system (constrained ground moraine, constrained outwash plain, unconstrained ground moraine, unconstrained outwash plain) and transverse geomorphic structure (valley floor landforms including floodplains, terraces, slopes, and adjacent uplands). Although distribution patterns of individual ground-flora species are highly variable among the four valley types, canonical correspondence analyses (CCA) of individual valley types indicate that ground-flora vegetation is related strongly to hierarchical landscape properties, including valley type and the transverse geomorphic structure of the stream valley. Vegetation ultimately reflects the influence of hydrogeomorphic processes that shape valley floor landforms. Constrained valley types tend to be characterized by diverse floodplain ground-flora communities dominated by graminoids and pteridophytes, while the terraces and slope plant communities are comprised of facultative upland and obligate upland perennial forb and woody species whose distribution appears to be a function of topographic features, such as aspect. However, ground-flora communities of unconstrained valley types appear to be responding not only to changes associated with the transverse geomorphic structure of the riparian ecotone, but also to more localized changes in environmental conditions associated with flooding and hydrologic regime. As a result, floodplain, terrace, and slope ground-flora communities tend to be diverse, dominated by a mixture of obligate wetland, facultative wetland, and facultative species. Thus, variation in riparian plant community characteristics can be explained using a nested, hierarchical landscape framework to organize and group different riparian settings based on the underlying geomorphic processes shaping stream valleys. Based on these results, we suggest that riparian management zones (RMZs) designed to maintain riparian function should be variable in width rather than fixed, encompassing variation in valley floor landforms and valley walls, regardless of the physiographic system.