The effects of salinity on growth and survival of mangrove seedlings changes with age

Six wide-ranging mangrove species, Rhizophora apiculata, R. mucronata, Avicennia marina, A. officinalis, Bruguiera gymnorrhiza, and B. sexangula, were selected to study the growth and survival of seedlings under three contrasting salinity treatments over a 30-week period: low (3-5psu), moderate (15-17psu) and high (33-36psu). Seedlings grown under high salinity exhibited significantly lower performance (p<0.05) in survival rates, cumulative shoot height, mean growth rates, mean total leaf area, and mean dry weight, compared to those under low and moderate salinity regimes. The low salinity treatment provided the best conditions for initial establishment and growth of the seedlings of all species until 15-20 weeks of age. However, the same seedlings showed better performance under moderate salinity after 15-20 weeks of age (shift in optimal salinity), implying that adaptation to salt and physiological needs of mangrove seedlings varies with age. These results have practical implications of use in raising up mangrove nurse species for planting since it indicates that seedlings should get low salinity water until four to five months of age and then moderately saline water, in order to achieve maximum growth and survival.