Causes and Consequences of Seasonal Variation in Pollen Limitation of Seed Production in Blandfordia grandiflora (Liliaceae)

Abstract

Natural seed set was consistently pollen-limited for 5 yr in the perennial herb Blandfordia grandiflora. The extent of pollen limitation varied within seasons, depending on when individual plants lowered. In all years, percent seed set was greatest in January (36-41%) and least in March (12-14%). Seed set of hand-pollinated fruits did not differ within lowering seasons (46-52%), indicating that pollination factors caused natural seed set to vary. Variation in natural seed set was positively correlated with frequency of visits to lowers by nectarivorous birds, but not their abundance in the area. Bird abundance increased during the lowering season, but floral visits decreased. B. grandiflora lowers received about 2 visits in December, 3 in January, fewer than 1 in February and none in March. In February and March, B. grandiflora plants coflowered and competed for bird pollinators with Banksia serrata. In these months, birds preferentially foraged at Banksia serrata inflorescences which produced about 10-fold more nectar per 24 h. The consistently greater natural seed set in January suggests that optimizing selection would favour plants that lower in this month