Insects facilitate wind pollination in pollen-limited Crateva adansonii (Capparaceae)

Low fruit-set in obligately outbreeding plant species is attributed to a variety of reasons that can be ascertained from reproductive studies. In the present work, the causes of poor natural fruit-set in Crateva adansonii DC. were investigated. Floral biology, the role of wind and insects in pollination and the breeding system of the species were studied in two natural populations for three consecutive seasons (2006–08). The flowers exhibited traits conducive to a mixture of wind and insect pollination (ambophily). Although a variety of insects visited the flowers, they were ineffective in pollinating. Nevertheless, active foraging by the honeybees (Apis dorsata, A. mellifera and A. cerana indica) facilitated enhanced pollen dispersal in the air and resulted in indirect pollination by wind. Airborne pollen grains pollinated the plants only up to 10 m. Fruit-set from open pollination was comparable to wind-pollinated flowers. Supplemental pollination treatments established the occurrence of strong self-incompatibility (SI) (index of SI = 0.14). Spontaneous autogamy was prevented by pronounced herkogamy. Low natural fecundity in C. adansonii is due to pollination failure, pollen limitation (pollen limitation index = 0.98) and the sparse distribution of the conspecifics; partial SI may partly ensure reproductive assurance through geitonogamy. In the absence of a pollinator wind appears to act as a secondary mode of pollination.