The Ecology of Sex Expression in Red Buckeye

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Abstract
I examined the reproductive biology of the andromonoecious shrub, red buckeye (Aesculus pavia, Hippocastanaceae) in order to describe the distribution of the two flower types in space and time, and to explain these patterns in an ecological context. Bisexual flowers were rare relative to male flowers, comprising typically <2% of the total flowers at woodland sites and 5—40% at field sites. Light availability was apparently the most important environmental factor influencing among—plant differences in ratios of flower types and production of mature fruit, though variability of the former may also have had a large genetic component. Within inflorescence, bisexual flowers were most common basally, especially near the central inflorescence axis, and on the side of the inflorescence away from the branch bearing the inflorescence. Predator and pollinator behavior, and requirements for fruit support were potentially important in determining this pattern. Bisexual flowers tended to occur in those inflorescences receiving the most light, and having the most subtending leaves and greatest yearly twig elongation. Abundance of bisexual flowers through time showed (1) a strong negative correlation with intensity of predation on fruits produced from flowers borne at these times, (2) a negative correlation with intensity of predation on flowers, and (3) a positive correlation with probability of pollinator visitation. I suggest that spatial and temporal distributions of flower types are often selected for by patterns of resource availability and behavior of predators, pollinators, or dispersers, and that selection on monoecious species will generally favor individuals in which sex expression and fruit production respond to the same environmental factors.