Every spring I find swamp buttercups (Ranunculus hispidus, formerly R. septentrionalis, Ranunculaceae) growing in significant numbers in a particular wet woodland here in southeast Michigan. After floods from snow melt and spring rain recede, ephemeral pools persist for weeks and their margins seem to provide the ideal habitat for these plants.
Older literature refers to this plant as R. septentrionalis (swamp buttercup) but the USDA, Brandenburg (2010) and others treat it as a subspecies of R. hispidus (bristly buttercup). These two plants seem notably different both in morphology and growing location. R. hispidus is known for having hairs on the stems and growing in dry or mesic soil. The individual shown here (matching older descriptions for R. septentrionalis) has no hairs on the stems and is associated with wet soil. Despite these differences it seems that this is a case of ecotypes within a single species. The appearance of this plant can vary based on its growing conditions (like soil moisture, pH, or nutrient levels), but physically different individuals can still interbreed. Ecotypes are found in other members of the genus Ranunculus, including R. repens (Lynn and Waldren 2001).
Brandenburg, D.M. 2010. National Wildlife Federation Field Guide to Wildflowers of North America. Andrew Stewart Publishing, Inc., New York, NY.
Lynn, D.E. and S. Waldren. 2001. Morphological Variation in Populations of Ranunculus repens from the Limestone Lakes (Turloughs) in the West of Ireland. Annals of Botany 87:9-17.