Clarifying the limits and geography of Limp and Ottawa Mannagrass (Glyceria laxa and G. ×ottawensis; Poaceae)

Taxonomic uncertainty is an important challenge to biodiversity conservation, given that accurate status assessment and management decisions depends on sound species-level taxonomy and accurate distribution range maps. Glyceria laxa (Scribn.) Scribn. provides an excellent example. This wetland grass is rare in eastern Canada, but debates on its taxonomic status and a lack of clarity in discriminating characters led to its neglect in ecological surveys. Historically, it was treated either as a full species, as a variety or synonym of G. canadensis, or as a hybrid, and was confused with the sterile hybrid G. ×ottawensis. Our objectives were to clarify the taxonomic status of these entities and reassess their distribution range in Canada. We conducted a comprehensive review of herbarium specimens from eastern Canada, combined with a morphometric analysis of eight Glyceria taxa in sects. Hydropoa and Striatae. Multivariate analyses separated all taxa with excellent classification accuracy. No Ontario or Québec specimens previously identified as G. laxa were correctly determined; they represented either G. ×ottawensis or G. canadensis s.str. Our results support the recognition of G. laxa as a distinct species restricted to the Maritime provinces in Canada. We provide an updated identification key and discuss implications for regional floristics and conservation.