Patterns of fruit production in the subdioecious plant Astilbe biternata (Saxifragaceae)

1 Patterns of fruit production in the subdioecious plant Astilbe biternata were used to test whether sex ratio is correlated with relative fruit produ ction by females and males; whether males that produce large inflorescences also produce proportionally more fruit; whether male fruit production is c orrelated with habitat type; and whether pollen limitation could affect fru iting. 2 Inflorescence length, a good predictor for flower production, was surveye d along with percentage fruit production and sex ratio in 22 populations fr om throughout the species range, including eight populations where plants w ere growing in both roadside and understorey habitats. In one population, t he percentage fruit production of males and females was assessed in relatio n to the distance from to the nearest pollen source. 3 There was significant variation in sex ratios among populations, but this was not correlated with relative (female:male) total fruit production. Fru iting males produced smaller inflorescences than either females or non-frui ting males, and within fruiting males, individuals with large inflorescence s produced proportionally fewer fruits than those with small inflorescences . The proportion of males that produced fruit was higher in roadside than u nderstorey habitats whereas all females produced fruit in both habitats. Fe male, but not male, percentage fruit production decreased with distance fro m the nearest potential pollen source. 4 Allocation patterns to fruiting differed between males and females. Perce ntage fruit production declined as inflorescence size increased for both ma les and females. Astilbe biternata males with a genetic propensity to produ ce fruit might produce smaller inflorescences, or fruit production might be determined by resource availability within the inflorescence after the inf lorescence is produced.