DISPERSAL OF GRAY MANGROVE (AVICENNIA-MARINA) PROPAGULES IN SOUTHEASTERN AUSTRALIA

Dispersal in Avicennia marina was related to attributes of the propagules and their response to environmental conditions. Viability of propagules and periods of their dispersal were measured in field experiments. Propagules extended their hypocotyl and developed root primordia within a few weeks, hence the phase of obligate dispersal is about 1 week. However, propagules remain viable, either exposed by tides or totally submerged, for up to 5 months during enforced dispersal. The fates of propagules were followed in two types of tidal creek over three tidal cycles. Newly released propagules are mostly moved on the initial flood tide and strand at the high tide mark less than 500 m from the point of their release. Buoyancy of the propagules, tides and currents mainly determine patterns of seedlings within undisturbed mangrove stands. Observations of propagules washed onto beaches show that most strand within 1 km of an estuary, and very few propagules were observed to disperse more than 10 km. Dispersal between populations is likely to be a rare event in southeastern Australia because of low numbers of floating propagules, the discontinuous distribution of habitat and the hazards of recruitment. This has two consequences: first, it may restrict gene flow among populations; second, recovery from mass mortality within an estuary will be slow.