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Data syntheses
- East Asian Monsoon and paleoclimatic data analysis:a vegetation point of view

An interesting story has been depicted by Ren and Beug (2002) in the northern half of China (north of Yangtze river) for the whole Holocene.

Forests generally expanded in the early Holocene times, reaching their maximum at 6 or 4 ka BP, with a maximum in Central China, and then re- gressed during the late Holocene. An exception was found for northeast China where the maximum development of for- est occurred during the last 4000 or 2000 years. They con- cluded that, if the EAM enhancement seems to be responsi- ble of the forest expansion at the beginning of the Holocene, disturbance by human activities may be responsible of the forest decline after 6 ka BP.

This picture was completed by the study of Yu et al. (1998) who analysed the vegetation variations at the biome level for the whole China, but restricted at the 6 ka BP pe- riod. In eastern China at 6 ka BP, forest shifted northwards, with broadleaved evergreen forest extended about 300 km and temperate deciduous forest about 500­600 km beyond their present northern limit. In northwestern China, the area of desert and steppe vegetation was reduced as compared to present. They concluded that these shifts were likely a re- sponse to enhanced Asian monsoon.

Lake levels data are less susceptible to be influenced by human disturbances. Yu et al. (2003) proposed a story of the lake levels since the Last Glacial Maximum (LGM, about 21 ka BP). This compilation showed LGM conditions much drier than today in eastern China but somewhat wetter in western China. These east-west differential patterns of cli- mate conditions were completely different from the modern dry-wet conditions with a north-south opposition. During the Holocene, at the mid-Holocene, both regions were wetter than present. Modern dry conditions returned after 5­4 ka BP depending on the region. Then if humans played a role in the forest decline in the Late Holocene, they simply accentu- ated a climatic trend. Atmopheric general circulation models (AGCM) coupled with land surface process model showed that the dry conditions in eastern China resulted from less summer precipitation due to the Pacific Subtropical High oc- cupying eastern China and the decline in the summer mon- soon.

More at north, in Mongolia, Tarasov et al. (1999) recon- structed, from pollen, warmer and wetter at 6 ka BP condi- tions for the northern part of the country, in agreement with higher lake levels. In the central part of the country, warmer and drier conditions prevailed (inferred from pollen, no lake data being available). But these dry conditions are likely due to more evapotranspiration and not necessarily to less precip- itation.

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