/ Evidence for a Species-Level Distinction of Two Co-occurring Asters: Aster Puniceus L. And Aster Firmus Nees
ï~~20 THE MICHIGAN BOTANIST Vol. 38 older nomenclature, and since the new work cited above has not been widely utilized to date, we will refer to the more traditional names in this paper. Aster puniceus (sensu lato) is distinguished from other species of Aster by several characters: achenes with 3-5(6) ribs; leaves clasping, often strongly auriculate; principal cauline leaves sparsely toothed, gradually tapered toward the base; stem variably hispid; and phyllaries glabrous, eglandular and long-acuminate or attenuate. When first named, this taxon was not described as exhibiting significant morphological variation. However, as variation has increasingly been recognized, two main subtaxa within Aster puniceus have been delineated. One, A. puniceus (sensu stricto) has densely hispid stems, purple ray flowers, and a widely spreading capitulescence. The other taxon (which has been variously named) tends to be less hispid, with white ray flowers and a more leafy, crowded capitulescence. Wiegand (1924) recognized the two types and segregated the smooth-stemmed Aster as a separate species, Aster lucidulus (Gray) Wiegand. However, this taxon had previously been named Aster firmus Nees; therefore, A. firmus should be the correct name for this taxon when segregated at the rank of species (Jones 1980b). Yet, the greatest confusion involving these plants is not which species name is most appropriate for the smooth-stemmed Aster, but whether or not this taxon truly deserves species-level rank. Many taxonomists include both plants within Aster puniceus, recognizing the swamp aster as A. puniceus var. puniceus, and the smooth-stemmed aster as A. puniceus var. firmus (Nees) Torrey & A. Gray (Jones 1989, Semple et al. 1983, Semple & Heard 1987; Voss 1996). At least one author (Jones 1984) has separated the taxa at the subspecies level. Others recognize two species (Gleason 1952; Gleason & Cronquist 1991; Jones 1980a, 1980b; Shinners 1941, 1946; Wiegand 1924), while still others apparently make no distinction below the species level (Britton & Brown 1913; Chmielewski 1987; Semple 1980a, 1980b; Van Faasen 1971, Van Faasen & Sterk 1973). Authors who do not recognize these two asters as separate species generally appeal to the presence of overlapping characters, suggesting this as evidence of intergradation or even complete lack of discontinuity (Jones 1989; Voss 1996). We evaluated many of these overlapping traits as well as some infrequently cited and novel characters from field and herbarium specimens to ascertain whether morphological discontinuity exists between these asters. Although we did not examine the type specimens, our designations are based upon and consistent with descriptions by Gleason (1952), Gleason & Cronquist (1991), Jones (1980a), Shinners (1941, 1946), Voss (1996), and Wiegand (1924). We collected data from both herbarium and field specimens. While it is likely many of the herbarium specimens had come from sites where the two taxa do not co-occur, our field observations were done exclusively at sites where both asters are found in coexisting populations. We predicted that if the taxa are able to cross, introgression would most likely occur at these field sites where the plants exist in proximity (often within one meter of each other). If plants with intermediate traits were found at these locations, we would conclude that these are two varieties of one species. By contrast, if these asters clearly retain their distinctness while living in such proximity, we would conclude that no gene flow is occurring and a species-level designation is warranted (Wagner & Wagner 1983).
Top of page Top of page