Vegetation and climate dynamics in Khyber Pakhtunkhwa (NW Pakistan), inferred from the pollen record of the Kabal Valley in Swat District during the last 3300 years

摘要

We present a pollen-based palaeoenvironmental reconstruction of the past 3300 years in the Kabal Valley of Swat District in the Hindu Kush mountains of Khyber Pakhtunkhwa Province, north-western Pakistan. We studied the pollen record from 38 samples taken from a 150 cm long radiocarbon-dated sediment core in order to analyse the vegetation history of the area. Only the upper 76 cm of the core, with 20 samples recording the last 3300 years, had sufficiently preserved pollen. Conifers such as Pinus, Picea, Abies, Cedrus and Taxus , and herbs belonging to Poaceae, Cyperaceae and Amaranthaceae were found consistently throughout the period, at varying abundance. The vegetation reconstruction revealed that Cyperaceae and Poaceae dominated the conifers from 3300 to 300 cal yr BP. The decrease in herbaceous vegetation (mainly Poaceae) from 2400 to 1500 cal yr BP, and its increase from 1500 to 1200 cal yr BP, indicate contraction followed by expansion of grassland in the Kabal Valley of Swat, pointing to corresponding dry-cool and wet-warm periods. Herbs were abundant in most samples from 900 to 300 cal yr BP. This change from conifer forest to open grassland can be attributed to the more pronounced impact of widespread deforestation, agricultural activity and a drier summer climate. Evergreen trees and shrubs such as Oleaceae, Myrtaceae, Moraceae species, Juglans and Dodonaea dominated and were constant from 2400 cal yr BP to the present. Conifers such as Pinus , Taxus , Picea , Abies and Cedrus were frequent in the study area from 300 cal yr BP to the present. Today these conifers occur mostly in mixed coniferous forests at higher elevation in the alpine area.