Morphometric and Molecular Evidence Delimit Six Species in Clematis reticulata s.l. (Ranunculaceae: Clematis subg. Viorna)

Abstract Clematis subgenus Viorna of North America currently consists of 21 species with most of the diversity found in the southeastern United States. Past taxonomic treatments of Clematis subg. Viorna have varied greatly, which has led to unclear species limits across multiple species complexes. Often recognized as a single, polymorphic species with a high degree of ecological amplitude, variation in the Clematis reticulata species complex has previously been attributed to phenotypic plasticity. A combination of herbarium specimen examination and extensive fieldwork led to the formation of seven a priori morphological groups, or hypothesized taxa, in the C. reticulata species complex based on previously used and novel morphological characters. We employ an integrative approach to species delimitation with data from morphometric studies of herbarium and common garden datasets, as well as a phylogeny inferred from ddRADseq. Of the seven hypothesized taxa, six were supported by evidence from morphology and phylogeny with corroborating biogeography and ecology. Recognition of Clematis terminalis sp. nov. renders C. subreticulata comb. nov. paraphyletic. Because of its morphological, ecological, and phenological distinctiveness, it may represent a case of peripheral speciation by isolation. Four new species are recognized: C. arenicola sp. nov., C. cumberlandensis sp. nov., C. ouachitensis sp. nov., and C. terminalis sp. nov. Viorna subreticulata is given a new combination in Clematis and the newly defined C. reticulata s.s. is re-circumscribed. An updated dichotomous key, species descriptions, ecological notes, and distribution maps are provided.