Vegetation history, monsoonal fluctuations and anthropogenic impact during the last 2330 years from Loktak Lake (Ramsar site), Manipur, north-east India: a pollen based study

Abstract

ABSTRACTPalaeoclimatic information is still sparse for South-east Asia, despite the fact that this region contains numerous lakes and wetlands that may hold potential sedimentary archives. Pollen analysis of sediment samples from a 100-cm-deep sedimentary core from Loktak Lake, the biggest freshwater lake in Manipur state, north-east India, has provided insight into the changing vegetation and climate of the region during the Late Holocene. The palyno-investigation has revealed that between 2330 and 1460 cal yr BP (LL-I zone), an open vegetation consisting mostly of grasses and heathland taxa, namely Xanthium, Artemisia, Asteraceae and Amaranthaceae, with scattered tree elements, viz. Fabaceae, Terminalia, Schleichera, Syzygium, Elaeocarpus, Meliaceae and Sapotaceae, existed near the lake under a warm and relatively dry climate. Between 1460 and 560 cal yr BP (LL-II zone), the consolidation of core forest constituents was noticed, with relative enhancement of major arboreal taxa along with marshy and aquatic taxa such as Cyperaceae, Polygonum and Lemna during 1460 to 1150 cal yr BP (LL-IIa subzone), indicating a humid climate attributable to a strong south-west monsoon. Evidence of agricultural practices showed the presence of cereal and other cultural pollen taxa. Later, a more humid climate persisted during the period 1150 to 560 cal yr BP (LL-IIb subzone) as indicated by an exceptional increase in a major arboreal group (Syzygium-Schleichera-Lagerstroemia-Dipterocarpaceae-Moraceae), suggesting the consolidation of vegetation around the lake under warm and increasingly humid climate conditions, and this occurs during the Medieval Warm Period (MWP). From 560 cal yr BP to the present (LL-III zone), a rise in cereal and heathland taxa indicates a relatively less humid climate; reduction in south-west monsoon precipitation is invoked to explain this which is supported by a steep fall in woodland, consistent with an anthropogenic influence.