Pollination-related functions of decorative sterile flowers of nine Japanese Hydrangea species (Hydrangeaceae)

Most species of Hydrangea (Hydrangeaceae) have a flower arrangement in which several large sterile decorative flowers surround a dense cluster of small fertile flowers. These decorative flowers have been hypothesized to attract pollinators, but this has yet to be tested for this genus. To identify whether the decorative flowers have functions related to pollination success, we carried out field manipulation experiments in which we clipped or mounted decorative flowers of nine Hydrangea species, eight with and one without inherent decorative flowers, and recorded insect visits to the intact and treated inflorescences. The results showed that for most of the studied species, the presence of decorative flowers increased the frequency of visits and weakly increased the frequency of visits per unit area to inflorescences. Decorative flowers appeared to act as landing-sites for three species for which flowers represented a high proportion of the inflorescence surface area (i.e., H. scandens, H. luteovenosa, and H. macrophylla). The results suggest that the main function of decorative flowers of the studied Hydrangea species is to boost attractiveness to pollinators by increasing the inflorescence surface area, whereas the landing-site function was evident only in certain species.