MATING SYSTEM, LIFE-HISTORY, AND REPRODUCTION IN CANADA THISTLE (CIRSIUM-ARVENSE ASTERACEAE)

Canada thistle (Cirsium arvense) (L.) Scop. is an almost perfectly dio ecious, perennial plant that can express strong vegetative reproductio n by means of its extensive root system. We explore some of the conseq uences of this type of reproductive strategy on the plant's pollinatio n success, its ability to allocate resources to individual achenes, it s ability to abort excess achenes, and on how pollen limitation affect s the primary sex ratio of its offspring. Seed set in females is const rained by the availability of pollen. Clumps of female thistle isolate d from males by at least 50 m set far fewer achenes per head than fema les that are interspersed with males. Even when such interspersion occ urs, 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 a chenes that abort is not affected by the availability of pollen. Plant s partially compensate for pollen limitation by maintaining stigmas in a receptive stage longer when pollination is sparse. Primary sex rati o is not affected by the availability of pollen; both high- and low-po llen availability treatments produced highly female-biased sex ratios.