Résumé
Combined variations of insolation and ice sheet led to major global climatic and ecosystem changes between the Last Glacial Maximum (LGM) and the Holocene. However, little is known about effects of these changes on the alternating dominance of deciduous and evergreen vegetation in East Asia, particularly in the regions now dominated by evergreen broadleaved forest (EBLF) that forms a vast ecotonal zone between tropical and temperate belts. In this study, we have used two representative records in center of the EBLF distribution, associated with broader regional pollen data from eastern China, to reconstruct spatiotemporal biome changes over the last 22 ka. The results showed a dominance of deciduous broadleaved forest at the LGM in most mountainous regions of the subtropical zone, and the EBLF began to migrate and expand into the low latitude subtropical areas during the last deglaciation, particularly at ca.11.3 ka after the Younger Dryas. However, major biome replacement from deciduous to evergreen forest across eastern China did not occur until 8.1 ka, indicating an earlier forest replacement at sites in low latitude regions as compared to sites located further north or at higher mountains. Our evidence agrees well with quantitative climate reconstruction and simulations at local and regional scales, showing that forest transformed when thermal conditions reached relevant thresholds. Both simulated and reconstructed paleotemperatures are consistent with the early-to-middle Holocene being a key period when climatic thresholds (particularly winter and/or annual temperatures) gradually approached then exceeded bioclimatic constraints on EBLF recolonization. By contrast, the precipitation in most part of mountainous subtropical southeast China may not be a key constraint for evergreen/deciduous forest transformation since the last deglaciation. During the late Holocene, the reconstructed biome has shifted unstably, probably due to interactive effects of several regional-scale factors, including seasonal climate biases and major fluctuations in effective moisture associated with weakening of summer monsoon before the enhanced human interventions of the last 3000 years.