The Sequence Of Vegetation Change By Pollen Records Of Xifeng Area During The Holocene

From field investigation, we found the Xifeng profile in the middle part of the loess plateau, which developed in Holocene, appears the continuous sediment and consists of 1 paleosol layer, 2 loess bed and 2 transitional layers. Pollen-anaysis is ofen used in the research of pale-vegetaion and pale-environment. The high resolving-ratio pollen analysis in Xifeng profile reveals that the coniferous forest and deciduous broad-leaved forest grew in the region of Xifeng during the Holocene Megathermal. The coniferous forest was mainly composed of Pinus, and the deciduous forest was made up of Betulaceae Ailanthus and Anacardiaceae. According to pollen analysis, the vegetation succession during the Holocene was reconstructed:(1) In the forepart of the Holocen, the sequence shows the equation of the arboreal pollen concentration with the herb pollen concentration. The vegetation was a forest-steppe which contained some coniferous tree. During 6.4-3.1 kaB.P. (TL age), the period of Holocene Megathermal, pedological process was the strongest, and the precipitation increased remarkably, so was the temperature. The vegetation was a forest which comprised of coniferous trees and deciduous broad-leaved trees. In the terminal Holocen, the content of arboreal trees descended evidently and the concentration of the herb rose too much. In this period, the vegetation was a sparse forest-steppe which mainly contained Artemisia and Polygonaceae.(2) There are "3 wave crest and 2 trough" in the sequence of the Holocene Megathermal. These record the fluctuation of the climate's descension, namely 5.2-4.9 kaB.P. (approximately 5.3kaB.P.) and 5.8- 5.6 kaB.P. degressive climate affair. It needs to point out that the concentration of the herb was greatly low in this period, and Artemisia merely appears about 1 grain/gram.(3) The Pinus Betulaceae Ailanthus Anacardiaceae and Gramineae descend so quickly that the concentration of them is around 1 grain/gram in the beginning of the terminal Holocen. It also record the degressive climate affair during about 3.1 -2.5 ka B.P..