Site-specific features affect pollination success of a gynodioecious understory shrub in a gender-specific mode1

Daphne laureola (Thymelaeaceae) is a gynodioecious, winter-flowering shrub inhabiting the understory of shady mountain forests. The small, inconspicuous yellowish-green flowers are pollinated by ectothermic insects whose activity is mainly confined to infrequent spells of sunny weather. We tested the hypotheses that (1) the solar irradiance environment of individual D. laureola plants and distance to the nearest conspecific should influence their pollination success and (2) the effect of both site-specific features on pollination success should be sex-dependent, given the difference between female and hermaphrodite flowers in conspicuousness and floral rewards. We estimated maternal pollination success (mean number of pollen tubes/pistil and proportion of unfertilized flowers), measured the distance to the nearest flowering conspecific, and evaluated the irradiance environment (percent of canopy cover) for individual D. laureola plants in 7 southeastern Spanish populations. On average, hermaphrodite flowers were pollinated more often and had more pollen tubes/pistil than females. Distance to the nearest conspecific affected individual pollination success negatively, but the effect was significant only for females. Furthermore, there existed a highly significant sex × cover interaction effect on pollination success. Among female individuals, the number of pollen tubes/pistil decreased and the proportion of unfertilized flowers increased from closed to more open sites, while hermaphrodites showed non-significant and opposite trends. The contrasting sign of the relationship between cover and pollination success for the 2 genders is expected to generate a fine-scale, within-habitat mosaic in the magnitude of the pollination advantage of hermaphrodite individuals, which would be greater in the more open, sunny locations where ectothermic pollinators should be more active. Site-specific differences in relative pollination success of sexes might contribute to the persistence of gynodioecy of D. laureola in our study region.