Interactive effects of salinity and light on the growth and survivorship of mangrove Heritiera littoralis seedlings

Salinity and light are two important environmental factors that interactively influence mangrove performance. Heritiera littoralis, a mangrove associate widely distributed in East Africa and southeast Asia, has been threatened by habitat loss throughout its range. Understanding the growth response of H. littoralis to salinity and light gradients is essential for its conservation, especially in the context of sea level increases. Here we investigated the interactive effects of salinity and light on the growth and survivorship of H. littoralis seedlings by conducting a greenhouse experiment with two-phase light × salinity treatments. The results showed that salinity and shading exerted negative effects on H. littoralis survivorship and growth in terms of seedling height increases, leaf number increases and total biomass. H. littoralis displayed higher sensitivity to salinity than to light, and the response to light depends on the salinity level. Furthermore, salinity exerted prolonged effects on the H. littoralis response to subsequent light and salinity conditions. Relative to seedlings with no previous salinity exposure, the seedlings with salinity exposure were less responsive to increases in subsequent light availability and allocated more biomass to below vs. aboveground parts, resulting in lower biomass. Together, these results indicate that H. littoralis growth is strongly influenced by salinity. To improve H. littoralis conservation, salinity effects interacting with light should be considered when restoring disturbed habitats for H. littoralis.