Pollination ecology and constraints on seed set of the rare perennial Gentiana cruciata L. in The Netherlands

Pollination ecology and components of reproductive success of Gentiana cruciata, a rare species in The Netherlands, was studied in two dune populations (20 and c. 300 flowering individuals, respectively) during the summer of 1989. Gentiana cruciata was fully self-compatible, but spontaneous self-pollination resulted in severe pollen limitation: natural pollination showed lower reproductive success as compared to hand-pollinations. In addition, reproductive success in the small population was significantly lower through all pollination treatments. Natural pollination was carried out mainly by the bumblebee Bombus pascuorum visiting the flowers for nectar, which was available at all floral stages but in greatest quantities during the morning hours. The number of visitors and visits to the flowers was generally high. However, the constancy of bumblebee visitors (measured as the proportion of heterospecific pollen found on bumblebees) decreased strongly in the afternoon, when nectar standing crop was relatively low. The number of visits and visitors to the G. cruciata flowers remained still very high, but the average time spent per flower decreased significantly. We argue that, in general, the low reproductive success through natural pollination is caused by pollen limitation as a consequence of competition through heterospecific pollen (mainly of the abundantly co-flowering shrub Rubus caesius) transferred and deposited on the stigmas by the bumblebees. The overall lower reproductive potential of the small population may be the result of a combination of poor site quality, rabbit grazing, and inbreeding.