Palaeo-palynology of late Quaternary peat deposit from Lower Bengal Basin, India: A palaoecological approach

Palynological analysis was conducted in view of past ecological reconstruction trends of Calcutta, the Lower Bengal Basin (West Bengal), India. Floristic compositions were derived from peat samples collected from Garia, located at the southern extremity of the city. Garia, being one of the oldest settlements in this region, is situated by the ‘Tolly Nullah’ connecting Calcutta to the Vidhyadhari River at the east. Bore hole samples were radiocarbon dated and three successive peat layers of 7000 (+170) y. B.P. to 2650 (+150) y. B.P. were analyzed for their pollen composition. Pollen data indicates the existance of typical swampy vegetation of mangrove plant community in and around Calcutta, which are presently confined to the Sundarbans forest, the further south of the studied area. The lower most peat layer (ca. 7000–6500 y. B.P.) reveals that the vegetational sequence was comparatively uniform and mainly comprised of Heritiera, Rhizophora, Sonneratia, Excoecaria, Phoenix paludosa and grasses, implying insignificant fluctuations in climatic conditions. The middle layer indicates a mixed vegetational type of fern spores, mangroves and some terrestrial plants (e.g. Acanthus, Cynometra, Suaeda, and grasses). The lower part of the middle peat layer (ca. 6450–6100 y. B.P.) consisted of marshy habitat as evidenced by fern spores, whereas their absence in the upper part implies relatively dry environmental conditions. The upper peat layer (ca. 2600 y. B.P.) is dominated by grass, pollen grains terrestrial habitats (Typha, Cassia, Eucalyptus, Desmodium, Salicornia, and Rhamnaceae) and some of typical mangroves (Heritiera and Excoecaria). Low amounts of fern spores indicating Lower Bengal change towards the terrestrial habitats. The pollen/spore assemblages in the three successive peat layers indicate the presence of typical mangrove vegetation, of the type currently observed in the Sundarbans region. Frequent sea water inundation mixed with fresh water flow from the northern upstream at the low lying topography of Basin seems to have led to the formation of peat by deposition rather than by drifting. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Palaeoecology of the Lower Bengal Basin, Calcutta, India, during the Late Quaternary.

Black peat samples deposited in between sandy clay layers were collected from an uncovered profile at Rajarhat, 24 Paraganas (N), 20 km extreme North East of the city of Kolkata, (22°34'N, 88°30' E) West Bengal. The profile resulted from an excavation made during the construction of a multistory building. Palynological investigations of the peat samples revealed a large deposits of pollen grains of mangrove species such as , Heritiera Excoecaria Rhizophora Sonneratia Avicennia Bruguiera, Nypa Barringtonia Phoenix paludosa, , , , , , , Aegiceras etc. Pollen grains of grasses, some freshwater plants, as well as fern spores (such as Acrostichum, Pteris, Pteridium Polypodium and ) were also recovered in large quantities. All were found from a peat layer formed at a depth of about 5.35m to 6.14m from the surface. The fossil pollen assemblage found indicates the prevalence of a past typical mangrove vegetation, which is presently observed in the Sundarbans region (a mangrove swamps, about 85km south of greater Kolkata). The present study reflects a low lying landscape frequently transgressed by sea water and getting supply of fresh water from rivers. Ecological and edaphic factors such as increase of salinity, submergence, reclamation, flood etc. could have been responsible for any vegetation fluctuations.

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Quaternary Vegetation History of the Lower Bengal Basin (India) Inferred from Palaeo-palynological Analysis

Nandy(Datta), P., M. Ghose, R. Spooner-Hart and S. Das. 2006. Is Heritiera fomes Buch. Ham. less adapted to the present conditions of the Indian Sundarbans compared to other mangrove species?Ann. Trop. Res. 28(2):76-91. The Sundarbans forest forms one of the largest mangrove vegetation in the Bengal Basin. The name indicates dominance of the tree species Heritiera fomes Buch. Ham., locally known as 'Sundari' because of its elegance. Explosive demographic pressure and indiscriminate exploitation of natural resources caused imbalance in this ecosystem. Moreover, reduced fresh water influx and increased sedimentation appeared to be an increasing problem for navigation that in turn, led to elevated soil salinity. In the Indian part of this forest, scarcity of Heritiera fomes can be attributed to these manual and environmental adversities. In comparison to the other predominating species in Sundarbans, H. fomes is less equipped in its leaf anatomical adaptations and water-use management. Considerably thin cuticle and high stomatal frequency encourage water efflux; whereas, poorly developed water storage tissue and low frequency of terminal tracheioids weaken the water retention mechanism. This inefficiency is reflected well by the transpiration rate and stomatal conductance that continuously increase even beyond 1600 μmol m-2s-1. In H. fomes, assimilation potential declines beyond 1250 μmol m-2s-1 PAR, which unlike the other taxa, can be attributed to the poorly developed palisade tissue and low nitrogen content in leaf. The optimum leaf temperature for photosynthesis is 30oC that is considerably lower than that in the others. Osmotic potential (O) of root and leaf is not low enough to facilitate water uptake from the extremely saline substrate; absence of free proline coupled with low Na+ content explains the loss of turgidity in Heritiera fomes Buch. Ham. of the Indian Sundarbans 77 leaf cells. Reduced leaf N+ may interfere the metabolic processes and hinder synthesis of the compatible solutes to some extent thus renders the species incompatible to the soil and microclimate of western Sundarbans. On the verge of its extinction, these observations can help to chase up suitable strategies to restore the environment for H. fomes instead of unplanned plantation and wasting the propagules of this rare species. Correspondence:S. Das Present Address: Agricultural and Ecological Research Unit, Indian Statistical Institute, 203, B.T.Road, Calcutta 700 108, India Email:sauren@isical.ac.in

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P Nandy Datta

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Is heritiera fomes Buch. Ham. less adapted to the present conditions of the Indian Sundarbans compared to other mangrove species