Phenological Trends with Latitude in the Mangrove Tree Avicennia Marina

(1) Leaf fall and reproductive phenology of Avicennia marina assessed during 1982-83 using litter fall collections from twenty-five sites in Australia, Papua New Guinea and New Zealand revealed major trends with latitude. Flowering shifted from November- December in northern tropical sites, to May-June in southern temperate sites. Periods between flowering and fruiting increased from two to three months in tropical sites to ten months in southernmost sites. Leaf fall was more variable with unimodal annual peaks in temperate sites and often multimodal patterns in the tropics. (2) Correlative evaluation of simple models suggested that initiation of the reproductive cycle occurred when daylength exceeded 12 h, followed by rates of development to flowering and fruit maturation given by a linear function of mean daily air temperature. This explained up to 92% of variance in total cycle duration and timing. Temperature affected reproductive development by increasing growth rates by a factor of two or three for each 10 'C rise. The model was tested using independent data to predict timing of reproductive events in respective sites. (3) Appearance of new leaves on canopy shoots in situ near Townsville, northern Queensland, had seasonal highs followed by peak falls a month later, and leaf longevity was estimated to be around thirteen months. Timing of new leaf appearance and leaf fall were comparable with observations from other studies and sites, geographically overlapping with the present litter-fall sites in Australasia. These had seasonal peaks, chiefly related to either the reproductive cycle during initiation of inflorescence development or independent growth. Respective heights of these peaks for particular sites appeared to depend on environmental factors of temperature and moisture. Thus, inflorescence leaf appearance was predominant in low-moisture and low-temperature sites during early summer, and independent leaves appeared mostly in wetter tropical sites during winter months. (4) Distributional limits of A. marina in higher latitudes coincide with trends towards zero reproductive success (notably seen in flowering). This was apparently brought about by a convergence of phenological events within the shorter growth period of summer in these latitudes.