Reproductive ecology of Uapaca kirkiana(Euphorbiaceae) in Malawi, southern Africa

Abstract

The dioecious character (sex ratios and stand structures, conspecific neighbour relationships, inflorescence numbers and structure) and reproductive process (phenology, anthesis, flower visitors and fruit production) of a miombo woodland species, Uapaca kirkiana were investigated in natural populations in Malawi. The sex ratio of reproductive individuals > 10 cm diameter at breast height was about 1:1 and departures from this ratio mostly resulted from a higher proportion of the males. Spatially, the sexes were randomly distributed but gener- ally within 10 m of each other with no obvious gender differences in size. Through- out Malawi, male and female reproductive individuals flower synchronously between October-April (with a peak period in February-March). A 6: 1 male to female inflorescence sex ratio was common. Anthesis in female inflorescences lasts almost twice as long as in male inflorescences and only the latter have any odour. On male trees, inflorescences are in fascicles, whereas on female trees, they are solitary. In both cases, the infloresences are generally similar in size except for the peduncles of the male inflorescences which are longer than the female's. A range of insects visits U. kirkiana flowers but the most likely pollinators are bees (Apis mellifera), and beetles (Dothera bennigseni, Chelomenes lunata, Alcidodes denticulatus and Mitracamenta sp.). Apart from pollen from the male inflorescences, female flowers do not offer any obvious reward for flower visitors. Pollination studies revealed that assisted pollination did not significantly improve fruit production or seed quality over unpollinated or naturally pollinated inflorescences. Facultative agamospermy (apomixis) is suspected as a phenomenon requiring further atten- tion in U. kirkiana.