Holocene vegetation and climate changes from fossil pollen records in arid and semi-arid China

Abstract

Abstract Holocene variability of the summer monsoons has been documented in a variety of proxy records in east and southwest China. However, its influence on regional climate in other parts of China is still poorly understood, especially further inland in north China and northwest China. Here we review fossil pollen records available from arid and semi-arid areas of China (including Inner Mongolia, the northwestern Loess Plateau, the northern Tibetan Plateau, and Xinjiang) to document regional patterns of Holocene vegetation and climate change, and to understand the large-scale controls of these changes. Pollen records from the four regions reveal different vegetation and wet–dry changes during the Holocene. With the exception of the westernmost sites, vegetation at most sites in Inner Mongolia switches between forest, forest steppe, and typical steppe. There is a dry climate after ~6 ka following early to Early – Mid Holocene maximum moisture conditions. At western sites, the climate was dry in the Early Holocene, wet in the Middle Holocene and dry again in the Late Holocene. Vegetation in the northwestern Loess Plateau switches between desert steppe, steppe, and steppe forest, with corridor forests often occurring in loess valleys during the steppe forest period. These changes indicate wet–dry oscillations, from an initial dry climate to a wet Middle Holocene and then back to a dry climate in the Late Holocene. In the northern Tibetan Plateau, vegetation is characterized by steppe, desert steppe, or desert. However, the Qinghai Lake area was dominated by tree pollen during the Early- and Mid-Holocene, indicating a wet climate, until a drying trend started after 6–4.5 ka. In Xinjiang, pollen assemblages show changes between desert, desert steppe or steppe during the Holocene, with a wet period occurring briefly during the early Mid-Holocene at most sites. The highest moisture interval during the Holocene (the so-called Holocene climate optimum) occurred in the Early- to Mid-Holocene in eastern Inner Mongolia, but apparently occurred later during the Mid-Holocene at sites in the west (northern Tibetan Plateau and Xinjiang). The complex climate patterns during the Holocene in arid and semi-arid China suggest regional climate responses to large-scale climate forcing was controlled by interactions of competing factors including the monsoons, westerlies and topography. The decline of forest in the eastern Tibetan Plateau, the Loess Plateau valleys and Inner Mongolia during the Late Holocene may have been caused by human activity.