Reproductive biology of the monoecious clonal shrub Taxus canadensis

Strobilus production, pollination, and maturation were studied in Taxus canadensis, a procumbent clonally-spreading shrub. Plants that produced many cones had greater shoot growth; no obvious tradeoff between sexual effort and vegetative growth was observed. T. canadensis is monoecious, but plants showed considerable variation in phenotypic gender. Because related species are dioecious, we hypothesize that T. canadensis evolved from a dioecious ancestor. Monoecy may have been favored as a means of assuring pollination via selfing; the proportion of ovules pollinated was positively correlated with the number of male cones on a plant. Selfing may be at the cost of inbreeding depression; 26% of the female cones aborted, about twice the percentage of the dioecious Taxus cuspidata. Application of nutrient fertilizer did not reduce the abortion percentage. Seed and aril maturation in T. canadensis occurred over a three month period, much more gradually than in T. cuspidata. Removal of the seeds, probably by antagonistic rodents, was very rapid and thorough in T. canadensis. Asynchronous ripening and monoecy may help reduce seed predation by rodents by reducing the conspicuousness of "fruiting" displays relative to the dioecious species.