Flowers adaptively face down-slope in 10 forest-floor herbs

Abstract

1. An animal-pollinated plant living on a slope should orientate its flowers down-slope towards the more open space if by doing so it receives more pollinator visits and thereby achieves increased reproductive success. 2. We measured flower orientation relative to slope direction on individuals of 10 species of forest-floor herbs in cool temperate forests in Japan. For one of these species, Erythronium japonicum, we also manipulated flower orientation to test experimentally for its effects on both male and female reproductive function. 3. In all 10 species, flowers were preferentially orientated down-slope. This pattern was more pronounced in plants growing on steeper slopes. 4. Our manipulative field experiment in Erythronium japonicum demonstrated that pollen dispatch was highest in flowers orientated down-slope. Additionally, flowers orientated up-slope may have achieved a lower seed set on steep slopes. 5. We conclude that down-slope orientation of flowers was a general phenomenon among the species that we studied, and that this behaviour was adaptive in enhancing plant fitness through pollination. © 2006 The Authors.