As traditionally defined, this widespread eastern species is variable in morphology and ecology; see also notes under the potential segregate, A. carolinianum. A. rubrum in its broad sense may be mostly hexaploid or octoploid (2n = 78 or 104), but with additional cytological complexity (Santamour 1965). Meiosis can fail in octoploids, producing a range of pollen sizes including diploids; trees with septaploid and aneuploid condition have also been reported. Moreover, there are occasional hybrids (sometimes aneuploid) with the tetraploid saccharinum (2n = 52), and there is apparent local introgression of these two species in more northern regions (T. Reznicek, pers. comm.). Santamour (1965) reported: "Neither euploid or aneuploid interspecific hybrids show meiotic irregularities associated with hybridity, although there may be a precocious separation of the silver maple chromosomes at late metaphase I." Hybrids in Ky. are suggested by colls. from JEFF (KY, DHL) and MONT (JC at Jeffersonville). Cultivars of rubrum-saccharinum hybrids (X freemanii) are now widely promoted by some nurseries. In Ky. typical rubrum is locally abundant but largely restircted to non-calcareous soils. Moreover, it is curiously rare to absent in some regions that have a long history of disturbance from burining (e.g. Land-Between-the-Lakes) or browsing (e.g. most of the Bluegrass). Repeated fire may reduce rubrum relative to oaks (Quercus) and hickories (Carya). The potential role of browsing by native mammals or livestock remains uncertain, partly since palatabilities and potential toxcities vary through the seasons, perhaps with more browsing by deer on rubrum during the winter (Burns & Honkala 1990, Adams 1998, Averill et al. 2016). Beaver appear to avoid rubrum sensu lato (Müller-Schwarze 1994). Horses can develop hemolytic anemia and related blood disorders after feeding on leaves, apparently due to gallic acid and a pyranone; similar problems can develop with saccharinum or saccharum but to a lesser degree (Wu et al. 2016; and citations).