Mangrove trees survive partial sediment burial by developing new roots and adapting their root, branch and stem anatomy

Abstract

Mangrove forests are generally sites of sediment deposition at the coast facilitated by the unique root structure of the trees, thus serving to help the system keep pace with rising sea level. However, at high levels it can cause anoxia in sediments and consequently tree mortality. This study evaluates the morphological and anatomical response of bark and roots of three mangrove tree species (Avicennia marina, Ceriops tagal and Rhizophora mucronata), commonly found along the Kenya coast and around the Indian Ocean, to partial burial by sediment. This was done through simulation of natural and rapid sedimentation in an experiment involving 15, 30 and 45 cm burial levels. Partial sediment burial resulted in an increase in root density which also aided survival particularly in C. tagal, where 5 of the 17 buried trees that did not form new roots in the 45 cm treatment died. Air lacunae in the root cortex became larger in pneumatophores following burial, while the ray parenchyma and cylinder of secondary xylem showed increased widths in cable roots of C. tagal. There was also an induction of the phellogen which produced more outer tissue in the buried section of stems in all three studied species (two fold increase in A. marina and C. tagal and fourfold in R. mucronata). The results suggest that the observed morpho-anatomical adaptations could lead to enhanced performance or recovery of biological processes in the burial-affected trees.