Interactive effects of salinity and irradiance on growth: implications for mangrove forest structure along salinity gradients

Early growth of Ceriops australis and C. decandra was studied in response to salinity and irradiance under laboratory conditions. These results provided a basis for interpretation of growth patterns during seedling establishment by seven species of Rhizophoraceae with variation in irradiance under natural low and high salinity regimes in a tropical mangrove forest. Survival declined with decrease in irradiance, except where very low salinities apparently induced sensitivity to high irradiance in vulnerable species. Survival in understorey shade was lower in the high than low salinity environment. However, these apparent effects of salinity were eliminated by reducing below-ground interactions with adult trees. Interspecific differences in initial propagule mass had no effect on survival, but were correlated with differences in average shoot mass after 1 year, emphasising the importance of juvenile reserves to early growth. There were interspecific differences in the salinity regime in which seedlings grew best, but shoot growth declined with decreasing irradiance. In most species, shoot mass of plants grown under natural canopy shade was less than initial propagule mass and was little affected by salinity. Salinity had greater effects on shoot growth under high irradiance than under natural canopy shade, implying effects of salinity on the capacity to use resources in growth. Each species exhibited a different pattern of shoot growth relative to initial propagule mass in response to salinity and irradiance. These patterns were consistent with, but do not fully explain, the distribution of the Rhizophoraceae along a natural salinity gradient in the Adelaide River system.