Pollen-detected altitudinal migration of forests during the Holocene in the mountainous forest-steppe ecotone in northern China

Distributed on the margin of the influence of the Asian monsoon, the forest-steppe ecotone in northern China is sensitive to climate change. Previous paleovegetation reconstructions in this region have led to inconsistent explanations of paleovegetation dynamics across sites. We hypothesized that the patterns observed are related to altitudinal migration of forests across mountains in this region. We combined eight lake sediment pollen records to explain how three forest types dominated by the tree genera Pinus, Quercus, and Betula changed in response to temporal and spatial monsoon changes within the forest-steppe ecotone during the Holocene. Modern pollen assemblages showed that areas with large altitude range could support more forests, which was further confirmed by the spatial patterns of late-Holocene forest distribution under dry climate. Our results confirmed that altitudinal migration of forests might exist in this region. This kind of altitudinal migration of forests under different altitudinal ranges could help to explain the inconsistent patterns observed in arboreal pollen across sediment cores as well as the non-linear response of horizontal forest distribution in relation to climate change.