The pollination ecology of two nectarless Cimicifuga sp. (Ranunculaceae) in North America

The pollination ecology of Cimicifuga rubifolia Kearney and C. elata Nutt., endemic nectarless species in Tennessee and the Pacific Northwest, respectively, was studied. Both species are pollinated by bumblebee workers, which run over the flowers pressing their bodies against them. Large syrphids act as copollinators. Extremely high pollen/ovule ratios are explained as optimal sex allocation in response to pollen-tolling by the pollen-collecting pollinators. Facultative geitonogamy in the self-compatible C. elata seems to be evolved in response to very low pollinator density in the habitat. Because of the pollinator behavior, geitonogamy virtyally always precedes xenogamy. C. rubifolia encounters higher pollinator visit frequency than C. elata , and self-incompatibility prevents selfing. Since it is nectarless, C. rubifolia is inferior in competion for bumblebees with most other entomogamous plants. It appears to have a commensalistic relationship with its constant associates, Impatiens pallida (Balsaminaceae) and/or Polymnia canadensis (Asteraceae), which attract large numbers of bumblebees. C. rubifolia is pollinated by bumblebees encountering them when scouting around the nectar-flower stand. This conclusion is based on: 1) Negative correlation for distance between nectar-flowers and Cimicifuga and the fruit set of the latter, 2) non-skewed flowering period, 3) extremely extended flowering on both the ramet and population levels, only limited by flowering period of nectar-flower associates and by pollinator abundance, and 4) fruit set rates independent of visual display size, and also of flower density in the habitat (i.e., the pollination rate was directly proportional to the density).