Reproductive Processes Do Not Constrain the Western Range Limit of Gelsemium sempervirens (Gelsemiaceae)

Range limits are often hypothesized to arise from reduced reproductive success at distributional margins, yet direct tests integrating pollination and post-pollination processes remain uncommon. Whether reproductive failure constrains the distylous Gelsemium sempervirens at its western range edge in eastern Texas was investigated by quantifying flowering phenology, floral visitation, pollinator effectiveness, and seed fate over two flowering seasons. Flowering timing differed markedly between years due to freeze events, but flowering effort and morph synchrony remained high. Although multiple floral visitors were recorded, fruit set was overwhelmingly associated with the southeastern blueberry bee (Habropoda laboriosa), which dominated visitation and remained active throughout the flowering period. No evidence of autonomous self-pollination or breakdown of functional distyly was detected. Seed set in unattacked fruits was high and comparable to values reported from central-range populations. In contrast, post-pollination seed loss due to cryptic fruit herbivory substantially reduced seed survival, though herbivory patterns did not differ qualitatively from those documented elsewhere in the species’ range. Together, these results indicate that reproductive failure does not explain the abrupt western range limit of G. sempervirens and instead suggest that ecological transitions associated with the forest–prairie ecotone, rather than pollination or early seed development, may play a more important role in shaping the species’ distribution.