Phytolith assemblages of grasses from the Sunderbans, India and their implications for the reconstruction of deltaic environments

摘要

Grasses are among the most abundant, diverse and ecologically important group of plants; their number and diversity make them a significant part of the ecosystem. The characteristic presence of opaline silica in grasses makes them a valuable tool in environmental reconstruction over other groups of vascular plants. Fifty grass taxa belonging to four different subfamilies viz., Panicoideae, Chloridoideae, Ehrhartoideae and Bambusoideae of family Poaceae were collected for phytolith analysis from different eco-vegetational zones viz., swampy mangrove or intertidal mangrove zone, tidal mangrove zone, true mangrove decline zone, zone for colonization of non-littoral species and xerophytic non-mangrove species and dry evergreen forest zone along the coastal regions of the Indian part of the Sunderbans. Predominant phytolith assemblages recovered from these grass taxa might help in discriminating different deltaic sub-environments. It was observed that the dominant grasses of the true mangrove decline zone and the zone for colonization of non-littoral species produced abundant lobate and cross morphotypes. On the other hand, a predominance of flat towers followed by short saddles, three-horned towers, two-horned towers and keeled rondels were noticed in the members of Chloridoideae growing in the xerophytic non-mangrove and dry-evergreen forest zone along riverbanks. In contrast, chloridoid members adapted to the tidal mangrove ridge forest produced spool/horned towers in the highest frequencies followed by ellipsoid rondels and two-horned towers. Grasses exclusively growing on salt marshes of the swampy intertidal mangrove zone showed a dominance of ellipsoid rondel morphotypes followed by flat tower and two-horned towers. The observations were corroborated by the results of principal components analysis (PCA) on modern phytolith data. The result may serve as a basis for the future interpretation of depositional environments of fossil phytolith assemblages recovered from deltaic regions along a salinity gradient.