Ecological differentiation of combined and separate sexes of Wurmbea dioica (Colchicaceae) in sympatry

The evolution and maintenance of combined vs. separate sexes in flowering, plants is influenced by both ecological and genetic factors; variation in r esources, particularly moisture availability, is thought to play a role in selection for gender dimorphism in some groups. We investigated the density , distribution, biomass allocation, and physiology of sympatric monomorphic (cosexual) and dimorphic (female and male) populations If Wurmbea dioica i n relation to soil moisture on the Darling Escarpment in southwestern Austr alia. Populations with monomorphic vs. dimorphic sexual systems segregated into wet vs. dry microsites, respectively, and biomass allocation patterns and physiological traits reflected differences in water availability, despi te similarities in total ramet biomass between the sexual systems. Unisexua ls flowered earlier at lower density, and they allocated significantly more biomass below ground to roots and corms than did cosexuals, which allocate d more biomass above ground to leaves, stems, and flowers. Females, males, and cosexuals produced similar numbers of flowers per ramet, but unisexuals . produced more ramets than cosexuals, increasing the total number of flowe rs per genet. Contrary to expectation, cosexuals had significantly higher ( more positive) leaf carbon isotope ratios and lower leaf nitrogen content t han unisexuals, suggesting that cosexuals are more water-use efficient and have lower rates of photosynthesis per unit leaf mass despite their occurre nce in wetter microsites. Cosexuals appear to adjust their stomatal behavio r to minimize water loss through transpiration while maintaining high inves tment in leaves and reproductive structures. Unisexuals apparently maximize the acquisition and storage of both water and nitrogen through increased a llocation to roots and corms and enhance the uptake of CO, by keeping stoma ta more open. These findings indicate that the two sexual systems have diff erent morphological and physiological features associated with local-scale variation in water availability.