Abstract
The reproductive capacities of dioecious plant species may be limited by severe pollen limitation and narrow seed shadows for the two reasons. First, they are unable to self‐pollinate, and seed production occurs only with pollinator movement from males to females. Second, only 50% of the individuals in populations contribute to seed production. Despite these handicaps, dioecious plants maintain their populations in plant communities with cooccurring cosexual plants, and no substantial difference in population growth rates has been found between dioecious and cosexual plants. Hence, dioecious plants are thought to mitigate these disadvantages by adopting ecological traits, such as insect pollination, animal‐dispersed fleshy fruits, and precocious flowering. We studied the relationship between flowering and plant size in 30 woody species with different sex expressions, leaf habits, fruit types, and maximum plant sizes. The study site was located in an evergreen broad‐leaved forest on the island of Honshu, Japan. A phylogenetic linear regression model showed that dioecious species tended to mature at smaller sizes than did cosexual taxa. At the population level, given equal plant densities and reproductive efforts, the precocity of dioecious plants could serve as one of the factors that mitigate the limitations of pollen and seed‐shadow handicaps by increasing the density of reproductive individuals in the population. At the individual level, smaller size of onset of flowering may play a role in enhancing reproductive success over a lifetime by increasing reproductive opportunities. We discussed the possible effect of the relationship between precocity and some ecological traits of dioecious plants, such as small flowers pollinated by unspecialized insects, fleshy fruit dispersed by animals, and their preferential occurrence in the tropics and in island habitats. The universality of precocity among dioecious plants should be investigated in diverse plant communities. Such studies will increase our understanding of the evolution of plant breeding systems.
Highlights
Populations of dioecious plant species may grow more slowly than those of cooccurring cosexual taxa for two reasons
The ability to colonize uninhabited sites is relatively limited in dioecious species because only 50% of the individuals in a population contribute to seed production (Heilbuth, Ilves, & Otto, 2001; Vamosi, Zhang, & Wilson, 2007)
Wilson and Harder (2003) examined competition for space between dioecious and cosexual species using mathematical models. They showed that separation of the sexes increased the variance in the fertilization probability and seed production among plants, and the reproductive uncertainty reduced the mean recruitment of the dioecious species
Summary
Populations of dioecious plant species may grow more slowly than those of cooccurring cosexual taxa for two reasons. To resolve this problem, Bruijning et al (2017) first analyzed field demographic data covering the entire life cycles of trees in tropical moist forests in Panama They showed that the disadvantages of dioecy are largely negated in long-lived plants mainly by the larger number of seeds produced by dioecious females compared with cosexual species. We examined the relationships of plant size at the onset of flowering with (1) sex expression (dioecious vs cosexual); (2) ecological traits examined in previous studies, i.e., leaf habit (deciduous vs evergreen), fruit type (fleshy vs dry), and growth form (tree vs shrub); and (3) phylogenetic constraints
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