Pollen limitation in a northern population of Hepatica acutiloba

Pollen and (or) pollinator limitation may be most likely to occur in spring wildflowers or populations of species near their distribution limits. From 1990 to 1992, we examined a 2-ha population of a perennial spring wildflower, Hepatica acutiloba (Ranunculaceae), at the northern distribution limit of the species. To distinguish between the often confounded effects of pollen and resource limitation, we applied partial and whole-plant treatments. Ramets were open-pollinated, cross-pollinated, open- and cross-pollinated (50% of flowers were open-pollinated, and 50% were hand-pollinated), or self-pollinated. Although there was year to year variation, plants with cross-pollinated flowers produced significantly more viable seeds and heavier seeds than plants with self- or open-pollinated flowers. Comparisons between seed numbers and masses from complete and partial treatments suggested that there was no reallocation of resources among flowers on a plant. From 1990 to 1992, a minimum of 95% of all visits to a subset of ramets of H. acutiloba were from Asclera ruficollis (Coleoptera: Oedemeridae), i.e., virtually the only insect active during flowering. Mark–recapture data for A. ruficollis indicated that even after 6–48 h, 22–40% of the individuals remained within 10 m of the same ramet of H. acutiloba. Observational data indicated that less than 50% of all flowering ramets of H. acutiloba may be visited by A. ruficollis. We conclude that pollen limitation, mediated by reliance on one sedentary pollinator, was the main cause of low seed production in open-pollinated flowers of H. acutiloba. Key words: Hepatica acutiloba, pollen limitation, pollinator limitation, distribution limit, Asclera ruficollis, Oedemeridae.