POPULATION BIOLOGY OF CHAMAELIRIUM LUTEUM, A DIOECIOUS LILY. I. SPATIAL DISTRIBUTIONS OF MALES AND FEMALES.

Abstract

The present series of papers considers the population biology of the dioecious herb Chamaelirium luteum (L.) Gray (Liliaceae) with special reference to the impact of sexual dimorphism on spatial distribution, resource allocation, life history, and sex ratios. This species is a perennial forest floor herb found typically on slopes of mesic hardwood forests. Its range extends from southwestern Massachusetts, as far west as Illinois and throughout the southeastern United States into northern Florida (Fernald, 1950; Radford et al., 1968). The plant consists of a basal rosette which does not undergo vegetative reproduction. It flowers from late April through May. The infloresence is produced from the center of the rosette and is an upright spike with leaves along 2/3 to 3/4 of its length and flowers on the remaining apical portion. There is a marked morphological sexual dimorphism: females have tall persistent inflorescences with many leaves whereas males have shorter inflorescences with fewer leaves. The male inflorescence withers away in late spring shortly after flowering, but the female inflorescence persists until the fall when the fruits mature. In many animal groups sexual dimorphism has been demonstrated in feeding behavior (Morse, 1968; Jackson, 1970), habitat choice (Feduccia and Slaughter, 1974), and, in some extreme cases, in geographical range (Bartholomew, 1970). Sexual dimorphism has also been documented in many dioecious plant species (see Lloyd and Webb, 1977, for recent review); and, since plants are nonmotile, it seems likely that ecological consequences of sexual dimorphism should be reflected in the relative distribution patterns of male and female plants. Studies into distribution of the sexes in dioecious plant species have either looked at nearest neighbor effects (Bawa and Opler, 1977; Melampy and Howe, 1977), or have examined the distribution of males and females along environmental gradients (Davey and Gibson, 1917; Harris, 1968; Dzhaparidge, 1969; Rathore, 1969; Freeman et al., 1976; Brockman and Bocquet, 1978; Opler and Bawa, 1978; Grant and Mitton, 1979). None of these studies used mapping techniques to follow the histories of individual plants in order to analyze in detail the influence of sexual dimorphism on the ecology and evolution of dioecious plant populations. In this paper, I address the problem of spatial patterns in the distribution of males and females in four populations of the markedly sexually dimorphic C. luteum with regard to the following questions. What is the scale of pattern in populations of this species? What factors might bring about differences in the distributions of the two sexes? Are the spatial distributions of males and females influenced by differences in microhabitats? What are the evolutionary consequences of differences in the distribution of males and females in a plant population?