HETEROSPECIFIC POLLEN TRANSFER BETWEEN SYMPATRIC SPECIES IN A MIDSUCCESSIONAL OLD-FIELD COMMUNITY

The cumulative (season-long) incidence of heterospecific pollen transf er (HPT) was examined using nine sympatric species in a midsuccessiona l old field. Inflorescences were collected weekly during the flowering season, and the proportion of foreign pollen/stigma was recorded. Flo wering phenologies of sympatric species and ovule and seed counts of s tudy species were also recorded. Heterospecific pollen was detected on some stigmas of each species. Medicago sativa (Fabaceae) received the most foreign pollen; in some cases, all of the grains on a stigma wer e heterospecific. Lotus corniculatus (Fabaceae) received the least amo unt of foreign pollen; the incidence of heterospecific pollen was near zero in most cases. The mean and range of foreign pollen received var ied by as much as an order of magnitude between species. The six speci es with zygomorphic flowers, all Fabaceae, received more heterospecifi c pollen than the three species with actinomorphic flowers, Potentilla recta and P. simplex (Rosaceae) and Ranunculus acris (Ranunculaceae). This probably reflects a bias because our data were analyzed on a cum ulative basis and the Fabaceae had longer flowering phenologies. HPT w as not correlated with the species' relative abundance within the comm unity. Proportion of foreign pollen received varied temporally within species, and this variation generally was not related to phenology of any sympatric taxa or the species' own phenology. Pollen grain diamete r was positively related to levels of foreign pollen received by speci es. This might be caused by poor adhesion of large pollen grains to sm all stigmatic papillae or if generalist pollinators carrying large amo unts of heterospecific pollen visit the large-grained species and spec ialists with little foreign pollen visit the small-grained species. Th e large proportions of heterospecific pollen on stigmas of many specie s indicate that HPT occurs frequently in the community we studied and the implications may include reduced seed set because of occlusion by foreign grains. As yet, however, it is unclear how important a factor HPT is in mediating pollen limitation of reproductive success.