Identifying Sambucus "nigra" in eastern North America

Very nice - thank you. I assume that they may hybridize, no?

Sambucus nigra sensu stricto has apparently been reported in a few US states and Ontario, but not Illinois. I haven't really tried applying these characteristics myself yet to our local flora.

A nice summary in Applequist 2015, "A Brief Review of Recent Controversies in the Taxonomy and Nomenclature of Sambucus nigra sensu lato", in which:

Sambucus nigra subsp. nigra and S. nigra subsp. canadensis hybridize readily, with probable hybrids being described in the literature as early as 1868 (Carrière, 1868). However, meiotic abnormalities have been reported in diploid hybrids (Ourecky, 1970) and though crosses may set seed, few of the seeds germinate (Chia, 1975)....Therefore, the relative ease and success of hybridization between some subspecies of S. nigra sensu Bolli does suggest close genetic relatedness. It should also be noted that even in non-hybrid elderberries, many ovules may remain undeveloped and seed germination rates, which are always very low without stratification, can be relatively poor even with elaborate stratification regimes involving lengthy periods at specific temperatures (e.g., Conrad and McDonough, 1972; Brinkman, 1974; Clergeau, 1992; Bolli, 1994; Jinadasa, 2000; Atkinson and Atkinson, 2002). There probably are incomplete reproductive barriers between subsp. nigra and subsp. canadensis, but this does not necessarily support recognition at the species level; there are many other species in which reproduction between subspecies is impaired (e.g., Harushima et al., 2002; Nanni et al., 2004; Grundt et al., 2006; Nosrati et al., 2011).

I don't know exactly what/who Plants of the World Online is basing their taxonomy on in recognizing S. canadensis as a species, but also look at this thing o_o https://www.inaturalist.org/photos/1231608