LATE QUATERNARY VEGETATION AND CLIMATIC HISTORY OF EASTERN NEPAL

The late Quaternary vegetation and climatic history at high altitude i n eastern Nepal was studied through stratigraphy, radiocarbon dating a nd pollen analysis. The Thulo Pokhari lake (27 degrees 41' N, 87 degre es 43' E, ca. 3980 m a.s.l.) is just above the subalpine Abies spectab ilis-Betula utilis forest zone and surrounded by Rhododendron scrub. T he climate in the studied area is very humid under the influence of th e monsoon. Silty sediments suggesting lake conditions prevailed at the lower part of the sediment samples, whereas the upper part consisted mainly of herbaceous peat indicating a mire condition. The pollen reco rd extends back to ca. 11000 yr BP. Although the absence of major chan ges between the pollen assemblages of the Pleistocene and those in the Holocene was attributed to the situation of the sampling plot above t he forest limit, the results suggest the following history. (I) The ab undance of Pinus pollen around 11000 yr BP represents expansion of the area unoccupied by late successional trees and/or a drier condition. Significant occurrence of Picea pollen is noteworthy, since this genus is absent in eastern Nepal at present in spite of its wide distributi on throughout the neighbouring areas of the Himalayas. (2) The increas e in abundance of Quercus pollen and the decline in Pinus pollen in th e latest Pleistocene represent climatic amelioration favourable to lat e successional mesophytes. (3) The climate was most moist and mild in the SP-III, although the absolute age of the pollen zone is not distin ct. The percentages of broadleaved trees such as Alnus, Betula, Carpin us and Corylus were increased at the expense of Quercus and coniferous pollen types. Sedimentation was interrupted by lake level decline and /or glacial advance during the period from ca. 11000-1600 yr BP. (4) T he increase in abundance of coniferous pollen types such as Pinus, Abi es and Tsuga indicates that the climate became drier and cooler, and t he present subalpine forest zone composed of A. spectabilis, T. dumosa and Betula utilis was established around 1600 yr sp. The formation of bog or alpine meadow vegetation during the period was shown by the C- 14 dates of the peat and the increase in Rosaceae and herbaceous polle n. (5) The increase in abundance of Ericaceae and Alnus pollen in the SP-V indicates that the present alpine Rhododendron scrub zone has bee n established since ca. 940 yr BP as a result of human influence on th e original vegetation. This interpretation is supported by the decline in the proportion of arboreal pollen. Since the age of the initiation of human influence varies with the altitude and region, further study will be necessary to determine the accurate age.