Sex allocation in Carex (Cyperaceae): effects of light, water, and nutrients

Sedges of the genus Carex L. are mostly wind-pollinated, self-compatible, monoecious herbs in which female flowers are receptive before pollen is released (protogyny). One hypothesis was examined for the adaptive importance of the monoecious sexual system, namely that having two flower types permits flexibility in allocation of resources to male and female reproductive functions in the face of changing environmental conditions. Fourteen greenhouse experiments were conducted on a total of six Carex species in which I manipulated one or more of three environmental variables: nutrients, water, and light. These variables had small effects on the proportion of female flowers, though the effects were significant for at least one variable in 8 of the 14 experiments. High nutrient and light levels were generally associated with increased femaleness, while high water levels were associated with increased maleness. Increased female allocation in response to enhanced resource availability has been demonstrated in many species and is thought to reflect the greater potential fitness gains via the female function than via the male function when resources are plentiful. The opposite effects of water are anomalous and contrary to results in the literature. Because the effects of environmental manipulations on sex allocation are modest, the evolution and maintenance of monoecy in sedges probably reflect advantages other than flexible sex expression, at least in part. Selection for avoidance of self-pollination may have been particularly important in favoring the combination of protogyny and monoecy in Carex.