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Changing patterns of vegetation through Siwalik succession

The palaeobotanical record from the Neogene of Himalaya has been examined and an attempt has been made to reconstruct the vegetation patterns and throw light on palaeoclimate of the region during Siwalik time. Though the flora of the Pre-Siwalik Neogene from which the Siwalik flora evolved is poorly documented, a few palynofossils from the Kasauli and Dagshai formations indicate the existence of subtropical to temperate vegetation in the Upper reaches of the newly built Himalaya. On the contrary, a fairly rich assemblage of megafossils from the Siwalik indicates widespread tropical evergreen to moist deciduous mixed forest in the lowland sub-Himalayan zone during Middle Miocene-Pliocene The assemblage is dominated by wet evergreen dipterocarps and associated taxa, most of which are known to have entered the Indian subcontinent from southeast Asia during Miocene and subsequently spread all over and finally reached the lower slopes of sub-Himalaya. This has resulted increase in the diversity of tropical vegetation. 
The post-Pliocene orogeny of Himalaya brought great changes in the topography and climate which adversely affected the vegetation patterns of the region. The Early and Middle Siwalik tropical evergreen forest whose chief component are Anisoptera, Dipterocarpus, Hopea, Shorea (other than Shorea robusta), Polyalthia, Calophyllum, Aphanamixis, Dysoxylum, Gluta, Dracontomelum, Mangifera, Swintonia, Cynometra, Koompassia, Ormosia, Pongamia, Sindora, Duabanga, Diospyros spp., Myristica, etc. started dwindling towards the end of Middle Siwalik and subsequently disappeared from western and central sectors, though a few taxa like Mangifera, Litsea, Cinnamomum, Bauhinia, Dalbergia, Ficus, etc. continued to adjust to the new climatic conditions. Extinction of tropical evergreen taxa and further rise of Himalaya gave way to proliferation and diversification of tropical and subtropical moist deciduous to dry deciduous temperate vegetation in the lower and higher slopes respectively, as is also evidenced from the palynological record.