"Our results provide an important piece in the mosaic of human influence on the environment, and this began much earlier than has been previously assumed," says Dr. Mahyar Mohtadi of MARUM, Center for Marine Environmental Sciences at the University of Bremen. About 6,000 years ago humans settled down in southern China and began to grow crops. Results obtained by the research team indicate that the impact on natural vegetation began immediately. According to Mohtadi, reaching these conclusions was made possible through the comparison of data from different warm periods - for this study there were five, covering a period of about 420,000 years.
Sustained and irrevocable change in the vegetation of southern China
Comparisons of the warm periods indicated that humans changed the vegetation in a sustained and, according to Mohtadi, irrevocable way. “The pollen analysis showed a completely altered vegetation structure,” he says. Until the Holocene, about 10,000 years ago, the observed vegetation sequences were very similar: As temperatures rose at the beginning of the warm periods, tropical plants expanded and dominated as soon as the sea-surface temperature exceeded 27 degrees Celsius. Then, toward the end of the warm periods, more temperate vegetation dominated when temperatures fell below 25 degrees. In the Holocene, however, there was a disruption to this pattern, and the spread of tropical species stalled. One reason for this could have been rice cultivation by humans displacing the natural tropical vegetation. As evidence for this, the researchers were able to demonstrate the presence of charcoal. They presume that it was produced when trees were cleared by fire to make room for the cultivation of agricultural crops. "These signs of slash-and-burn, and decline of plant species that were not able to reassert themselves later, indicate that man had intervened," explains the Bremen geoscientist.
In their geoscientific investigations, the researchers also relied on data from anthropologists and archaeologists. These show when humans began to influence nature - for example, when pottery or rice pollen were found. These data are incorporated into the studies of geoscientists, explains Mohtadi, stressing once again that they are only meaningful if the vegetation patterns are compared over an extended time period, and over multiple warm periods.
Results are representative for the South China region
The basis for the study is a drill core that the team obtained with the MARUM-MeBo seabed drilling rig on an expedition with the SONNE research vessel in the South China Sea. The results of the core investigation, says Mohtadi, are representative of the entire southern Chinese region. However, the results cannot be used to draw inferences for Europe. The reason for this is the temperate climate. Compared to tropical plants, the plants in Europe are less sensitive to temperature, and therefore not as easy to displace.
Dr. Mahyar Mohtadi
Telephone: 0421-218 65660
Zhongjing Cheng, Chengyu Weng, Stephan Steinke, Mahyar Mohtadi: Anthropogenic modification of vegetated landscapes in southern China from 6,000 years ago. Nature Geoscience 2018. DOI: 10.1038/s41561-018-0250-1
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