East Asian summer monsoon and topography co-determine the Holocene migration of forest-steppe ecotone in northern China

Forest-steppe ecotones are generally regarded as very sensitive to climate change. However, it is still unclear whether they can be used to track past climate changes due to the combined effects of climate forcing and topographic factors. We first explored shifts of the whole forest-steppe ecotone in northern China during the Holocene by collecting 383 topsoil pollen samples to establish discriminant functions representative of forest, forest-steppe ecotone, and temperate steppe. The discriminant functions were applied to 39 fossil pollen sites to reconstruct the range of the forest-steppe ecotone during the Holocene. Our results showed that the shift of the forest-steppe ecotone exhibited a generally consistent trend with the intensity of the EASM, which was characterized by southward retreat during the early Holocene from 12,000 to 8000 cal. yr BP, northward expansion during the middle Holocene from 8000 to 4000 cal. yr BP, and southward retreat during the late Holocene from 4000 to 0 cal. yr BP, suggesting a dominant role of precipitation provided by the EASM. However, some sites in mountainous regions still indicated forest group membership within the ecotone, implying possible vertical forest migration. We stress that EASM and topography co-determine the shift of the forest-steppe ecotone in northern China, and mountainous terrain differences also benefit the vertical forest migrations when threatened by the dry climate. Our study implies that future climate change may cause three-dimensional changes in vegetation, which should be considered in climate change mitigation.