The response of the red mangrove Rhizophora mucronata Lam. to salinity and inundation in South Africa

Salinity and water level fluctuations are important factors that influence mangrove species distribution, zonation patterns and succession. Increases in salinity and prolonged inundation are predicted along the south eastern African coast associated with sea level rise due to climate change. This study investigated the response of red mangrove seedlings (Rhizophora mucronata Lam.) to these factors in controlled laboratory experiments. Seedlings were exposed to five salinity treatments (0, 8, 18, 35 and 45 PSU) and a semi-diurnal tidal cycle in an experimental tank set-up. In a separate experiment the effects of different inundation treatments: no inundation, 3, 6, 9 h tidal cycles and continuous inundation (24 h) were investigated. Both morphological and physiological responses of R. mucronata seedlings were measured. There was a decrease in growth (plant height, biomass and leaf production) with increasing salinity. Seedlings in the seawater, hypersaline and no inundation treatments showed symptoms of stress such as increased leaf necrosis (‘burn marks’). The highest seedling growth occurred in the low salinity (8 PSU) treatment, but the highest photosynthetic performance and stomatal conductance occurred in the freshwater treatment (0 PSU). The typical response of stem elongation with increasing inundation was observed in the 24 h inundation treatment. Seedlings in the no inundation treatment had significantly lower seedling height compared to the other treatments.

► Decrease in growth (plant height, biomass and leaf production) with increasing salinity.
► Seedlings in the seawater, hypersaline and no inundation treatments showed symptoms of stress.
► Highest seedling growth occurred in the low salinity (8 ppt).
► Highest photosynthetic performance and stomatal conductance occurred in the freshwater treatment (0 ppt).
► Seedlings in the no inundation treatment had significantly lower seedling height compared to the other treatments.