Biogeographical and palaeoclimate appraisal of mangrove vegetation in South Asia and Southeast Asia in the post Himalayan uplift scenario

Abstract

Fossil evidence indicates that mangrove species have had their first appearance in the Late Cretaceous(70 65 Ma) and they have evolved around the Tethys seaway after the plate tectonics and the movement of the continents when Tethys Sea was fast closing as the massive continental fragments continued to shift and jostle across the globe. The oldest geological record of Acrostichum and Nypa palm offers good examples as these species prefer low-salinity requirements and have wide ecological tolerance. Such environmental and habitat preferences must have been the probable reason for their initial adaptation to mangrove habitat and a wider distribution. The open sea link must have also provided enough coastal areas for the establishment of mangrove ecosystem and subsequent spreading of the mangroves into newer areas by Early to Middle Eocene(ca 50–40 Ma). However,the mangroves started acquiring the present geographical positions towards OligoceneMiddle Miocene period. The fossil record and the modern analogues of Kandelia,Nypa and Pelliciera indicate that their biogeographical preferences have been shifted due to oceanic reorganization wherein the latter is now confined only to Atlantic East Pacific mangrove realm despite it was earlier widespread in the Tethys Sea. Although Rhizophora and Avicennia differ only at the species level and common occurrence of Acrostichum aureum,the present mangrove distribution shows the contrast in the biogeography of many genera due to geomorphic barriers and climatic changes. The Himalayan uplift and establishment of Asian summer monsoon system towards Late Neogene has also affected the coastal dynamics considerably which in turn modified the distribution of mangrove vegetation of the Indian subcontinent. The loss of sensitive habitats,disjunct distribution of a few species and local extinction of significant taxa has affected the overall biogeography of mangroves in South and SE Asia.