Diaspore traits and inter-tidal zonation of non-viviparous mangrove species

Diaspore traits and germination of four non-viviparous mangrove species in Hong Kong, Lumnitzera racemosa (Jack.) Voigt., Heritiera littoralis (Drgand.) Ait., Excoecaria agallocha L. and Acanthus ilicifolius L., were investigated. L. racemosa fruits planted immediately after collection failed to germinate but those stored in wet condition for 35 or 50 d were successfully germinated. This suggested that L. racemosa had endogenous and morphological seed dormancy, with embryos continued to develop during the dormant period. Germination rates of L. racemosa decreased with increasing salinity and no germination was found at salinities over 25 ppt (ppt, parts per thausand). H. littoralis seeds were easily germinated if the fruit coat was artificially removed. Fruit dissection significantly shortened time for root initiation and leaf expansion. E. agallocha and A. ilicifolius seeds were also easy to germinate, initiating roots within 2 and 3 d, respectively. In terms of germination, A. ilicifolius had more tolerance to high salinity than L. racemosa. The four species exhibited three types of adaptation to unstable environments: (1) prolonged diaspore longevity as shown in L. racemosa and H. littoralis; (2) shortened rooting time as in E. agallocha and A. ilicifolius; and (3) produced sinking diaspores in L. racemosa. Diaspore buoyancy was one of the most important factors in determining inter-tidal zonation of non-viviparous mangrove species. Among the four species, L. racemosa was distributed in the most seaward zones because its diaspores were sinkers while diaspores of H. littoralis, E. agallocha and A. ilicifolius, more abundant at backshore locations, were floaters. Root initiation was also important in influencing the inter-tidal zonation of the three species whose diaspores were floaters. H. littoralis with the longest rooting time, as compared to E. agallocha and A. ilicifolius, was distributed in the most backshore zone. None of other factors including salinity of seawater, animal predation, diaspore size and seedling dimension could account for inter-tidal zonation of these species.