Modern pollen deposition in relation to vegetation and climate of Balpakram valley, Meghalaya, northeast India: Implications for Indo-Burma palaeoecological contexts

摘要

Balpakram valley, the sacred grove of the Garo tribe in Meghalaya, is world famous for its tropical unique biodiversity, physiography, seismically active zone, and proximity to Cherrapunjee. The area was not previously approached for palynological study due to its remoteness, impenetrable forest and wildlife. A modern pollen database of 120 surface samples from Balpakram valley indicates a good correlation of modern pollen, extant vegetation, and climate. Five pollen zones (forest types) have been recorded based on the regional pollen marker taxa corresponding to the ratio of evergreen and deciduous forest elements (＞30% signifies the forest type). The present vegetation is similar to the Indo-Burmese vegetation except for a unique Duabanga-Schima-Nepenthes assemblage due to high monsoonal activity with a strong perennial river and streamlet system. The evergreen woody taxa in abundance along with Den-dropthoe, Piperaceae and Impatiens in the assemblage are significant and suggestive of dense forest with high monsoonal activity. The abundance of Poaceae along with an Artemisia-Chenopodiaceae assemblage shows a savanna forest type, attributed to seasonal dryness and a poor water system in a restricted region. The absence of Dipterocarpaceae pollen, a characteristic taxon of the Indo-Malayan forest, is indicative of a regional effect. The poor representation of bamboo pollen may be attributed due to low pollen production (one-time flowering) and preservation. Pollen of Nepenthes khasiana in sediment signifies high precipitation, and is also indicative of this high pollen producer with good preservation status. No medicinal plants, specifically Costus speciosus and Swertia chirata, are observed despite their luxuriant growth in the region, which may be due to poor pollen production. Abundance of Cyathea (tree fern) along with Lycopodium and Pyrrosia signifies high rainfall and humid climate. The presence of coprophilous fungi including Sordaria and Cercophora is indicative of wildlife habitation in the area. The unique forest vegetation of Balpakram valley is mainly controlled by annual precipitation with perennial rivers and streamlets, moisture availability, and altitudinal variation in the region. This study will be helpful to generate palaeoecological data for surrounding ecozones including eastern Himalaya and Indo-Malayan regions, to trace migratory routes for flora and fauna in the Indo-Burma ranges.