Floral Biology and Reproductive Phenology of Avicennia marina in South-Eastern Australia

Abstract

Flowering, pollination and reproductive phenology of the tree, Avicennia marina (Grey mangrove), were examined on the south-east coast of Australia in New South Wales. Individual flowers are protandrous and open for 2-5 days, while a flower cluster has open flowers for 2-4 weeks. About 16000 pollen grains and four ovules are produced per flower. Self-pollination of an individual flower is unlikely because of protandry, but the sequence and synchrony of flowering, together with pollinator behaviour, favour geitonogamy. Some fruit is set when cross-pollination is restricted by bagging flowers, which indicates partial self-compatibility. Subsequently, fruit abortion is higher in the bagged treatment than in those open-pollinated; this may reflect some pre-dispersal inbreeding depression. Between 4 and 41% of open-pollinated flower buds set fruit, most of which had one or rarely two seeds.

Phenologically, each reproductive stage is unimodal and the whole process from bud initiation to abscission of mature fruit is completed within a year. Initiation of floral buds, flowering, growth and abscission of fruits are almost synchronous among fecund trees at a particular latitude each year. Latitudinal differences occur consistently among populations that are separated by less than 2° of latitude; those at lower latitudes flower earlier. Flowering of individual trees varies greatly between years and many trees fail to reproduce each year, although the populations remain fecund from year to year. A flexible breeding system and regular population fecundity ensure annual propagule supply in the populations studied.