Riparian plant communities differ from upslope communities at regional but not local scales in species richness, and at both scales in species’ identities

Background/Question/Methods

Riparian (or riverbank) plant communities are often thought to be more diverse than the surrounding landscape, and may contribute to regional plant biodiversity.  Riparian communities may also harbor more non-native species.  Riparian community composition is expected to reflect the composition of the surrounding landscape, due both to shared regional environmental conditions and a shared species pool. However, riparian communities may also have unique characteristics caused by both biotic and abiotic influences, including the scouring of banks and vegetation by flowing water, propagule transport, flood events, and nutrient and debris deposition. Many studies of riparian plant communities have been carried out on large streams and rivers, although most streams are small, and the greatest total riparian area is associated with small streams. I surveyed the species composition of riparian and upslope plant communities along three small streams in the Upper Hudson watershed (NY, USA) to determine if riparian plant communities were more diverse, richer in non-native species, or contained a unique assemblage of plant species when compared to upslope locations. I also performed a broader regional field survey along 50 small streams throughout the Upper Hudson watershed to understand patterns of regional conditions in influencing species richness along riverbanks.

Results/Conclusions

I found that neither diversity nor abundance of native or non-native species differ at local scales between riverbanks and the surrounding landscape, in contrast to previous studies.  However, species identity differs both among streams and between river edge and upslope sites. When pooled across all sites, riverbank locations contained a greater total of individual species than did upslope sites.  This suggests that riparian zones of small streams may contribute to regional biodiversity by maintaining unique suites of particular species and by maintaining a greater number of species at the regional scale, rather than by maintaining especially high species richness at the local scale. Regional species richness in riparian zones does not appear to be driven by variation in bioclimatic variables such as precipitation or temperature, nor does species richness vary notably among land cover types (e.g. developed, forest or wetland areas). Additional work will focus on identifying characteristic functional plant traits of species assemblages in river edge versus upland habitats, as well as investigating smaller-scale variation in abiotic conditions that may influence plant distribution and community composition.