Reproduction of Erythronium umbilicatum (Liliaceae): pollination success and pollinator effectiveness.

Erythronium umbilicatum (Liliaceae) is a common vernal herb of deciduous forests in the southeastern United States whose seed set depends on outcrossing by insects. Although only 40-60% of the ovules mature into seeds, hand-pollination experiments conducted in several populations over four years provided little evidence that reproductive success in this species was limited by pollination. Both honeybeess (Apis mellifera) and two small species of native andrenid bees effectively pollinate E. umbilicatum, as determined by measurements of seed set and counts of pollen tubes in the styles of flowers kept under cages and allowed a single visit. Full pollination of E. umbilicatum is promoted by: (1) large amounts of nectar and pollen that attract a variety of potential pollinators, (2) the small (2-3) number of visits required to fertilize all of a flower's ovules, (3) extended perianth persistence of unpollinated flowers. The latter two traits are also characteristic of other spring wildflowers and may contribute to fitness in a habitat where both physical factors and competition between plants for floral visitors can make pollinator service unpredictable. Causes of the observed deficiency in seed set in E. umbilicatum are complex and probably involve cold weather, low light levels or other unidentified physical factors, genotypes of pollen parents (i.e. quality of pollination), and diversion of resources from current year's seed set to future reproduction.