Low fruit set in a dioecious tree: pollination ecology of Commiphora harveyi in South Africa

Abstract

Dioecious plant species differ in floral morphology and rewards between females and males. Pistillate flowers on female plants often lack pollen and can be less attractive to pollinators, which can have consequences for the visitation rates of the sexes. We studied the pollination ecology of the dioecious tree Commiphora harveyi in a coastal scarp forest in eastern South Africa. Floral display, visiting insect species, visitation rate and natural fruit set were recorded. Additionally, we pollinated flowers by hand to determine experimental fruit set. We found that male trees had more and larger flowers per inflorescence than female trees. Both sexes produced nectar in low amounts. During 203.5 h of observation we recorded 28 insect species visiting the flowers. No difference in mean visitation rate (0.20 visits per flower h⁻¹) was recorded between the sexes. The daily and seasonal pattern was similar between the sexes. The natural fruit set was low (3.8%) and increased significantly with hand-pollination (45.5%), an indication of pollen limitation. We compared our results with the pollination ecology of C. guillauminii in Madagascar, a dioecious tree species on an island with a depauperate pollinator fauna. This comparison revealed a similar pattern with low visitation rates, low insect diversity and low fruit set, suggesting that this pattern may be more common in dioecious tree species than previously reported in the literature.