Conclusion

In summary, through the novel application of molecular sexing of all individuals in a large forest plot, our study revealed an ontogenetic shift in the sex ratio of a dioecious tree population for the first time, and shed light on the factors driving this change. A higher cost of reproduction for females likely leads to an increasingly male-biased sex ratio in later life stages, and Y-chromosome degeneration may account for the pronounced female-biased sex ratio at early stages. The opposite sex ratios of seedlings and large trees were in accordance to the expectations of sex allocation theory. The difference between OSR and true population sex ratio supported previous results based on the sexing of flowering individuals that males invest less in reproduction, and so can flower at smaller sizes, more frequently, and may grow faster than females. Further studies of sex ratio in a wider variety of dioecious tree species using sex-specific markers are needed to obtain more comprehensive knowledge for sex ratio patterns and process of dioecious trees, and to improve our understanding of the ecology and evolution of breeding systems in plants.