Palynological Study On Agricultural Activities And Environmental Evolution During The Late Holocene In Heihe River Basin, NW China

The current global change is the result of concerted action of both long-term natural and human activities, therefore, more attentions have to be paid to the human influences in palynological studies in order to better recover and reconstruct the paleo-vegetation and paleo-climate. Human agricultural activity is one of the most important parts of human activities. Pollen assemblage characteristics influenced by the human activities is very complicated due to the regional differences of crops distribution. Thus, studying modern pollen assemblages is necessary, and this kind of research is rare in this area. Heihe River Basin, which is famous for agricultural development, is located in arid and semi-arid regions of northwestern China with a fragile ecological environment, and human agricultural activities are the main factors impacted on its vegetation and environmental changes. To better understand the vegetation and environmental changes as well as the history of agricultural development of the Heihe River Drainage, a total of76modern soil samples were collected from different cultivated fields, wastelands together with desert in the upper and middle reaches of Heihe River Basin, meanwhile,37profile samples were taken in Donghuishan Site, as well as38strata samples selected in Ganxu Nur. The purpose of the research is to investigate the ancient agricultural history of this area, as well as the environmental evolution.Poaceae pollen from a series of surface samples collected from several cultivated fields (wheat fields and millet fields), as well as a few non-cultivated lands (spruce land, meadow land and wetland) and archaeological site, were measured to establish the standard size to distinguish two types of Poaceae pollen:cereal type and natural one. Result of Poaceae pollen grain size measurements discovered that the grain length of wheat pollen is between37.5μm and64μm, while the pollen diameters range of Phragmites australis in wetland varied from22μm-28μm. Samples collected from archaeological site of this study, meadow land as well as spruce land of Heihe River Drainage Basin, show the same distribution character as those from millet fields, in which the pollen sizes vary from22to55μm, and its highest size frequency falls between24and 40μm, thus there are no means to distinguish the millet pollen from those wild species through diameter measuring method.The pollen assemblages of41surface soil samples collected from farmlands suggest that the crop pollen is one of the main components in the pollen assemblages with an average of45.6%and the average percentage of other grass pollen is about48.7%, as well as low frequencies of arboreal pollen and shrub pollen whose contents are1.6%and4.3%separately. When different crops are planted, the abundances of the crop type pollen are different. The average percentage of Brassica campestris pollen is85.8%in the rape fields, while the other herbs except crop pollen types are dominant in those farmlands planted with corn, wheat and mixed crops. The relationship between farmland pollen assemblages and altitude indicates that the Cruciferae pollen mainly appears in a height which is higher than2000m and its percentage peaks at2800m. The Cereal-type of Poaceae (>37.5μm) pollen mainly appears below the height of2000m, which is in accordance with the distribution altitude of the oasis agriculture. Therefore, it could be considered as an important indicator of human agricultural activities.The pollen assemblages of the wastelands, compared with those from cultivated fields, are dominated with herbs pollen, and the shrubs pollen content increase but the percentages of the agricultural Poaceae pollen decrease sharply, meanwhile, the pollen concentration is relatively high. In the desert, with rare human activities impacted, the pollen types is monotonous, mainly consist of Chenopodiaceae, Ephedra and Nitraria pollen. The abundance of cereal pollen in the desert is extremely rare.Comparing the pollen characteristics of the farmlands, wasteland and desert can make contribution to the identification of meaningful and rewarding information on human activities indicated by pollen. The percentages of cereal-type Poaceae in cultivated fields, wasteland and desert decreased gradually which indicate that the cereal-type Poaceae pollen have a relatively lower transmission capacity and it can be identified as an important indicator of agricultural activities when there are a certain amount of those kinds of pollen in the strata. Besides, the pollen concentrations have some relationship with the human activities, and the agricultural activities could make the pollen concentration lower. The lower pollen concentration of the cultivated fields is the results of the agricultural policies such as fertilizer and weeding, which also caused lower frequencies of arboreal and shrubs pollen as well as herbs pollen except the crops in the farmlands. The variations of the W/D ratio (ratio between pollen percentage of wet herbs and dry herbs) in the low plain (corridor region) could indicate the influences of human activities on the local environment.The pollen records of two cultural strata from Donghuishan site have23.2%of wheat pollen on average suggest that the wheat were cultivated in the site during the past3000years, which coincide with the results of flotation that the wheat grains content are12.2%on average. Besides, the pollen assemblages of Donghuishan site are similar with those of modern wheat fields, but different from the natural desert and wastelands in this area, indicate that the crop pollen responds to the agricultural activities and the pollen analysis method could be used to trace early agricultural activity in those areas without evidence of grains’remains, especially for wheat cultivation during Neolithic period in northwestern China.The pollen assemblages of lake sediments in Gaxun Nur show that, between870-1350AD, the pollen concentration and A/C ratio are relatively low, and the percentages of xerophytic plant pollen like Nitraria and Ephedra are higher, all of these suggest that the environment of this region is moderate wet and dry, as well as the vegetation cover is lower and dominate with desert vegetation types. The appearance of cereal Poaceae pollen indicated agricultural activities during this period, which is consistent with the agricultural activities of Hexi Corridor and Ejina Banner in Western Xia and Yuan Dynasty documented in history materials. From1350AD to1770AD, which is the Ming and Qing Dynasties of China, this phase corresponds to the global meaningful Little Ice Age. During this period, the pollen concentration and A/C ratio increase and have higher value, which indicate a more humid environment as well as a well developed vegetation condition. From1770AD to present, the pollen concentration and A/C ratio decreased to a relative low value, together with the increase of Chenopodiaceae, Ephedra and many those kinds of dry pollen. These results suggest that the environment became extremely dry and the desert vegetation predominate in this area. The appearance of less amount of crops pollen might indicate the agricultural activities of Hexi Corridor in Ming and Qing Dynasty.