How did the forest respond to Holocene climate drying at the forest–steppe ecotone in northern China?

A sediment sequence from Anguli Nuur Lake (41°18′–24′N, 114°20′–27′E) located at the forest–steppe ecotone in the southern Inner Mongolia Plateau of China was selected to reconstruct the Holocene history of vegetation development and to determine the primary driver of forest dynamics. The shifting of vegetation zones was reconstructed by pollen-vegetation zone discriminant functions established by contemporary distribution of surface pollen in a transect along both precipitation and vegetation gradients in northern China. Pollen indicators for human disturbance were selected by comparison with a sediment sequence from West Liao River basin which is also located in the forest–steppe ecotone but is abundant in archeological sites. Changes of coarse sand percentage show great consistency with changes of regional humidity index, suggesting a prominent role of climate in soil erosion. The discriminant functions showed that long-term replacement of the forest zone by steppe zone commenced at 2.1 ka BP. Pollen percentages also show that climate drying since 5.0 ka BP first led to replacement of broadleaf forest by pine forest due to soil coarsening. Further recession of pine forest from this region since 2.1 ka BP was primarily driven by climate deterioration, because no pollen indicator of human disturbance was recognized. The study also suggests an earlier arrival but later recession of pine forest in the study region, implying pine (Pinus tabulaeformis) migrated from the south, and migration of pine forest in the current steppe region was driven by the Pacific monsoon movement.