Sex Ratios of Red Maple (Acer Rubrum) Populations in Northern Lower Michigan

Abstract

Red maple (Acer rubrum) on nine different sites in northern lower Michigan showed generally male—biased sex ratios across a wide variety of habitats and through time. These ratios were not correlated with moisture or nutrient conditions of the sites and for more mesic sites, ratios did not appear to be affected by age of the stands. The more xeric sites showed greater male bias in sex ratios with age of the stand. Within the xeric sites, the sex ratio of regularly flowering trees was more male biased than the ratio of trees at first reproduction. Although data appear to support resource allocation theory predicting more male—biased ratios in older or poorer habitats, the male—biased sex ratios did not result from measurable differences in sex expression of trees originating from sprouts vs. seeds, sex—related differences in size at first reproduction, or sex—related differential mortality, or because of patterns of sex lability. Differences in sex ratio were of minor importance in reproductive success of individuals in the xeric plots given the lack of reproduction from seed. Reproduction from seed was higher in mesic areas, but sex—related differences in life history characteristics were not evident in these areas. The similarity of ratios in a diversity of habitats and through time suggests that the ratios may reflect the underlying genetic patterns of sex expression, rather than subsequent responses to the environment. These data suggest that sex ratios alone are not sufficient to infer proximate mechanisms for biased sex ratios or sex—related differences in resource allocation.