Vegetation evolution and its driving mechanism on the Chinese Loess Plateau over the past 34 kyr

The evolution of vegetation types (herbaceous vs. woody; C₃ vs. C₄ plants) on the Chinese Loess Plateau (CLP) is highly sensitive to regional climate variability and closely linked to changes in solar radiation and the intensity of the East Asian Summer Monsoon (EASM). However, drivers of vegetation change on the CLP, including the timing and nature of human activities as an ecological influence remain subjects of ongoing debate. Here, we reconstruct vegetation evolution over the past ~34 kyr BP using biomarker evidence from a lacustrine sedimentary sequence from Tianjiao Lake in the southern CLP. Multiple proxies, including total organic carbon (TOC), carbon isotopic composition of bulk organic matter (δ¹³C_toc), n-alkane distribution indices (e.g. C₃₃/(C₂₇+C₃₃)), and carbon isotopes of n-alkanes (δ¹³Cₐₗₖ), are used to infer changes in plant functional types and photosynthetic pathways.

The results indicate that vegetation throughout the last 34 kyr consisted of a mixture of C₃ and C₄ plants, with C₃ herbs remaining dominant. Between 34 and 15 kyr BP, weaker solar radiation and reduced monsoon intensity produced relatively cold and dry conditions, limiting biomass production and suppressing C₄ plant abundance. After ~15 kyr BP, intensified solar radiation and monsoon strength led to warmer and more humid conditions, promoting vegetation expansion and an increased contribution of C₄ plants, although C₃ herbs continued to dominate. From ~2.7 kyr BP onward, signals of human activity increasingly overprint climatic controls, indicating that anthropogenic disturbance became the primary driver of vegetation change on the CLP.

These findings demonstrate the effectiveness of n-alkane biomarkers for reconstructing long-term vegetation dynamics on the Loess Plateau and highlight the increasing role of human activities in reshaping plant communities during the late Holocene. The results provide important context for understanding ecosystem responses to future climate change and anthropogenic pressures in this environmentally sensitive region.