Abstract: Compared to pollinator limitation and inbreeding avoidance, the role of ecological factors in sexual differentiation has received less attention in sexual dimorphic plants. The effect of soil moisture and florivory on two sexual morphs in a gynodioecious orchid, Satyrium ciliatum, was investigated in seven gynodioecious (with both female and hermaphrodite individuals) and 15 hermaphroditic (with only hermaphrodite individuals) populations. Our result showed that, compared to hermaphrodites, females tended to occur in drier sites in which soil water content was consistently lower than that of hermaphrodites in all gynodioecious populations. The soil water content where hermaphrodites grew was not significantly different between gynodioecious and hermaphroditic populations. We observed that females experienced less attack by insect florivores than hermaphrodites in gynodioecious populations, and hermaphroditic populations had higher insect attack than gynodioecious populations. Our results provide evidence for females being favored in stressful sites. However, the soil moisture and degree of florivory were not correlated to female frequency among populations, suggesting that the two ecological factors have not induced strong effects or other factors that may also influence the sex ratio in the facultative apomictic orchid.

Key words: ecological factors, gynodioecious orchid, herbivory, Satyrium ciliatum, sex ratio, soil moisture.