Sex Ratio Variation in Populations of a Dioecious Desert Perennial, Simmondsia Chinensis

The dioecious wind-pollinated shrub Simmondsia chinensis is common to much of the Sonoran Desert of Mexico, Arizona, and California. A survey of sexual expression over several years at two widely-separated sites indicates that males slightly but consistently outnumber females on the whole; that sex ratios vary along moisture gradients with male bias tending to be strongest in the driest sites; and that sexual expression is independent of plant size and not labile from year to year. These results are compared with those from other studies of Simmondsia sex ratios, and to the predictions of theories, including adaptive theories, of sexual expression. I conclude that surveys alone cannot distinguish among alternative causal hypotheses, and that experiments and demographic studies will be necessary in general to elucidate variation in sexual expression of plants.