Mid-Holocene vegetation and climatic variability in tropical deciduous sal (Shorea robusta) forest of Lower Brahmaputra valley, Assam

One 2.5 m lacustrine sedimentary profile dated back to 6340 years BP from Deosila swamp under Rangjuli Reserve forest of Assam, Northeast India has been pollen analyzed for tracing past vegetation vis à vis climate variability since mid-Holocene. The pollen diagram has deciphered that during 6340 to 2970 years BP, tropical tree savannah type of vegetation grew in the region chiefly constituted of grasses interspersed with scattered trees of Salmalia, Dillenia, Emblica, Meliaceae along with sporadic presence of Artocarpus chaplasha, Symplocos, Ilex, Schima and Shorea robusta under relatively less cool and dry climatic condition with a little ameliorating trend at the upper column. The vegetation scenario implies poor growth of arboreals due to harsh abiotic dynamics causing changes in drainage system. Subsequently during 2970 to 1510 years BP, tropical mixed deciduous forest succeeded tree savannah with invasion of Shorea robusta, Lagerstroemia, Lannea, Semecarpus and Acacia under warm and humid climate. The gradual enrichment of organic soil might have been conducive for better growth of both deciduous and semi evergreen arboreals as found today. However, during 1510 to 540 years BP, the forest groves became more strengthened resulting establishment of tropical deciduous Sal forest. Steady increment of Shorea robusta along with Lannea, Lagerstroemia, Terminalia, Sapotaceae, Albizia and Adina was observed probably due to influence of active SW monsoon under increased warm and humid climatic regime. This is well substantiated by the consistent occurrence of marshy and aquatic taxa along with ferns and fungal remains. Finally during 540 years BP onward the reduced forest floristics have obviously been envisaged the change in climate which turned to warm and relatively dry probably attributable to the weak monsoon rainfall. The acceleration in human settlement during this phase is evidenced by Cerealia and by the increase of Melastoma, Ziziphus and Areca catechu implying forest clearance.