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.