Coexistence and reproductive isolation of two sympatric, deciduous forest species of Erythronium (Liliaceae)
Erythronium americanum and E. umbilicatum are spring wildflowers of deciduous forests in the eastern United States whose ranges overlap in North Carolina and Virginia. E. umbilicatum is a diploid progenitor of E. americanum, an allotetraploid. Because of the potential to form sterile triploid hybrids where they occur sympatrically, these species are especially susceptible to competition for pollination. The goal of this study was to determine how they coexist and maintain reproductive isolation at a site in Piedmont North Carolina where they co-occur. Specifically, I tested the hypothesis that these congeners were more clearly partitioning the pollination resource than other, less closely related members of the spring wildflower community, which typically share pollinators extensively and have highly overlapping flowering times. Interspecific crosses in the greenhouse were highly fertile, yet natural hybridization was rare or non-existent, as indicated by chromosome counts of suspected hybrids. The main floral visitors were the same two species of solitary, andrenid bee. Habitat overlap was extensive; although E. umbilicatum was more abundant on upland slopes while E. americanum preferred floodplains, the two species grew intermingled throughout the site. Notably, however, there was almost no overlap in flowering phenology. E. americanum started blooming only after E. umbilicatum had nearly finished and the few remaining open flowers had been depleted of pollen. This same pattern was observed in both years with unusually early and unusually late springs. Such complete phenological separation is unusual in the spring wildflower community and contributes not only to the current co-existence of these species of Erythronium but may also have aided the establishment of E. americanum after its hybrid origin.
