Differentiated climate-driven Holocene biome migration in western and eastern China as mediated by topography

There are large differences in terrain from west to east in mainland China. However, the relative influence of topography and specific climate systems, such as the East Asian monsoon, on past biome migration is poorly understood, which is problematic for the prediction of future biome dynamics. To the best of our knowledge, this study is the first to comprehensively determine whether topography has mediated climate-driven Holocene biome migration in China. We compiled 69 pollen sequences in mainland China to reconstruct Holocene biome migration using a standard biomization approach of biome assignment according to pollen by plant functional type. Our results show that western China, with its east-west mountain chains, experienced a differentiated forest migration pattern, while middle and eastern China, with no east-west mountain chains, had similar responses, with a directional forest migration pattern in terms of horizontal and vertical belts. Geographical barriers might have acted as refugia in western China and facilitated forest survival and spread at the biome level, characterized by a faster forest migration rate than that experienced in middle and eastern China. This type of continental-scale biome migration could also balance fragmentation in species range-shifts. When taking middle and eastern China as a whole, we found that the water-bearing East Asian summer monsoon significantly influenced the forest northern boundary in the horizontal belt and the forest upper boundary in the vertical belt. Interactions between the mid-latitude westerlies and the low-latitude summer monsoon had a complex role in determining the position of the forest northern boundary in the East Asian monsoon-margin region, resulting in a millennial-scale unstable boundary with fluctuating variations. This study highlights the importance of topography mediation when investigating climate-driven Holocene biome migration in western and eastern China.