Depending on the taxonomic treatment, Glehnia is either monotypic or ditypic (containing one or two species). In the recently-published Jepson Flora, a single species with two subspecies is recognized, while the Flora of China recognizes two species. However it is treated, though, there is agreement that there is a distinction between the taxon that grows along the coast of western USA and Canada (from California to Alaska) and the taxon that grows along the coast of China, Taiwan, Japan, Korea, and Russia.
I presented yesterday on “Plants and Landscapes of Haida Gwaii” to the UBC Friends of the Garden, featuring photographs from my trips in 2013 and 2018. Haida Gwaii is sometimes described as the “Galapagos of the North”, due in part to a number of endemic and rare taxa. Although not globally rare, American glehnia or American silvertop is blue-listed in British Columbia (a taxon of special concern). That said, it is perhaps the most readily found of any “rare plant” on Haida Gwaii–just head to a sandy beach and look in the dunes.
In Jenny Cohen’s 2010 thesis, “Paleoethnobotany of Kilgii Gwaay: a 10,700 year old Ancestral Haida Archaeological Wet Site”, Glehnia littoralis is noted as “not ethnobotanically recorded” (presumably among the Haida Nation, although I’ve not found any references to its use by any other indigenous group in North America). Conversely, both the Flora of China and Wikipedia make mention of Glehnia littoralis being a Chinese herbal remedy for the treatment of coughs.