Mating system,life-history,and reproduction in Canada thistle (Cirsium arvense; Asteraceae)

Canada thistle (Cirsium arvense) (L.) Scop, is an almost perfectly dioecious, perennial plant that can express strong vegetative reproduction by means of its extensive root system. We explore some of the consequences of this type of reproductive strategy on the plant's pollination success, its ability to allocate resources to individual achenes, its ability to abort excess achenes, and on how pollen limitation affects the primary sex ratio of its offspring. Seed set in females is constrained by the availability of pollen. Clumps of female thistle isolated from males by at least 50 m set far fewer achenes per head than females that are interspersed with males. Even when such interspersion occurs, distance to nearest effective pollen donor correlates negatively with fertilization. Achene mass is significantly higher in plants that set fewer seed due to pollen limitation; however, the proportion of achenes that abort is not affected by the availability of pollen. Plants partially compensate for pollen limitation by maintaining stigmas in a receptive stage longer when pollination is sparse. Primary sex ratio is not affected by the availability of pollen; both high- and low-pollen availability treatments produced highly female-biased sex ratios.