A Preliminary Study on Mangrove Forest Dieback in Kalametiya Lagoon, Southern Province of Sri Lanka.

Abstract

Sudden and progressive dieback ,,pf mangrove vegetation in Kalametiya, southern province, Sri Lanka was first obse/ved in 2015. After a preliminary investigation, we hypothesized that prolonged drought conditions occurred in 2015 may have led to mangrove tree mortality. Therefore, physicochemical parameters of soil i.e. pH, soil bulk density (SBD), conductivity, salinity, organic matter content (OMC), soil moisture content (SMC), as well as the plant species present, and their growth performances were studied using six belt transects (10 m x50 m) placed along the dead region and adjoining alive region of the mangrove forest. The results showed that only a small part ( ~2.5 ha) at the western periphery of the mangrove forest that is dominated by larger trees ( ~ 15 m in height) of Sonneratia caseoloaris were dead while rarely occurring individuals of other true mangrove species i.e. Excoecaria agallocha, and Lumnitzera racemosa as well as mangrove associated shrubs, lianas, and grass species were alive in the same region. The level of pH, conductivity, and salinity in soil were significantly higher (p<0.05) and OMC and SMC were significantly lower (p<0.05) in the dead region when compared with those in the alive region. However SBD in dead and alive regions did not show a significant difference. According to the results, it seems that negative soil water potential coupling with soil hypersalinity may be the cause of tree death. However, we argue that these two factors may not be the sole reasons for the tree mortality as individuals of mangrove associates and other true mangroves in the same area where Sonneratia trees were dead, were alive and recovery or establishment of seedlings of Sonneratia in the dead region was not observed even after the drought period. Therefore it is recommended to continue the study to find the other crucial causative factors for the forest die back.