Floral syndrome in Amelanchier nantucketensis (Rosaceae). I. Floral density, bee activity, and characterization of andropetaly

Abstract

The clonal shrub Amelanchier nantucketensis is andropetalous; a variable number of petals as well as normal anthers both bear germinable pollen. To determine whether floral density associated with andropetaly is related to activity of native bees on flowers, we compared Amelanchier nantucketensis to Amelanchier humilis and Amelanchier stolonifera, which have sterile petals. In samples from four Maine sites, floral density in Amelanchier nantucketensis was 30.3% greater than that for Amelanchier humilis and 39.5% greater than that for Amelanchier stolonifera; in Amelanchier nantucketensis, flowers were more numerous per cubic metre and distance between flowers was shorter. There was a significant correlation across all sites and species between the number of flowers per plant and number of inflorescences visited per bee in 10 min (r = 0.711, p = 0.0001). Statistical t tests separated Amelanchier nantucketensis from both Amelanchier humilis and Amelanchier stolonifera by plant height, nearest stem, nearest flower, and number of inflorescences visited per bee during 10 min. Plant species classification in a canonical discriminant analysis was 83–93% correct with four plant variables, namely distance to nearest flower, distance to nearest inflorescence, plant width, and plant height. Our findings support optimal foraging theory in that bee visits increase with floral density in these Amelanchier species. Key words: Amelanchier, andropetaly, bees, floral density, morphology, Maine.