Vertical biome shifts and climate changes since the last glacial maximum in the southeastern margin of the Tibetan plateau, Southwest China

Since the last glacial maximum (LGM), global warming has significantly influenced the intricate dynamics of vegetation on the southeastern margin of the Tibetan Plateau in southwest (SW) China. However, the responses of montane vegetation to glacial-interglacial climate changes have received less attention than those of modern alpine vegetation belts. Here, we present a continuous pollen record from the lake Caohai area of Guizhou Province and a quantitative reconstruction of the region's biome and climate changes over the past 21 ka. Our findings demonstrate that the study area was encompassed by evergreen sclerophyll Quercus forest during the LGM, which subsequently transitioned to deciduous broadleaved forest around 15.8 ka, shortly after the termination of the last glacial period. During the early Holocene, the deciduous forest underwent a transformation and gave way to evergreen broadleaved forest around 10.3 ka. The abundance of evergreen subtropical elements peaked between 9 and 5 ka, coinciding with the Holocene thermal maximum (HTM). This was followed by the return of deciduous taxa after the 4 ka in the late Holocene. However, anthropogenic impacts on local vegetation became apparent only from about 2 ka. The pollen-based biome reconstruction features clear vertical shifts that correspond to the modern vegetation belts at different elevations, with a maximum vertical movement of about 1000 m between the last glacial and the Holocene. Statistical analysis of pollen data reveals that principal components can serve as reliable climate indicators, consistent with climate simulations using the TraCE model. Both proxy and model results demonstrate that forest ecosystems undergo transformation when thermal and moisture conditions exceed bioclimatic thresholds, emphasizing the crucial role of climate boundaries in driving biome succession. Our findings unveil an extended cold and dry period during the LGM, with the lowest precipitation observed during the last glacial termination due to intense North Atlantic cooling and weak overturning circulation. The strong agreement between the transition from sclerophyll to deciduous forests and increasing temperatures suggests that deglacial warming primarily drove vertical biome shifts after the LGM in the southeastern margin of the Tibetan Plateau. Conversely, forest succession during the Holocene, which align with variations in fire intensity, were predominantly triggered by summer monsoon changes. The quantitative reconstructions of both biomes and climates presented in this study provide novel insights into temporal relationships between climate dynamics and ecosystem changes in SW China.