Ecological response of a high-elevation peatland to late Holocene hydroclimate change on the southeastern Tibetan Plateau

Abstract

High-elevation regions around the world have experienced pronounced climate change, resulting in a multitude of responses in terrestrial ecosystems. Here we used multi-proxy data—including dry bulk density, loss on ignition, plant macrofossils, diatoms, stable oxygen isotopes (δ¹⁸O_bulk) and C/N ratios—to understand the response of peatland vegetation and carbon accumulation to hydroclimate changes from two peat cores of a bamboo-sedge-Sphagnum peatland (27.621°N, 98.590°E; 3037 m asl) on the southern Hengduan Mountains at the southeastern edge of the Tibetan Plateau. The cores were dated by AMS ¹⁴C radiocarbon dates with a basal age of 4500 cal yr BP. The peatland was formed on an abandoned floodplain, as evidenced by the gravel and biotite at the base, likely in response to the weakening South Asian summer monsoon (SASM) in the late Holocene. From 4500 to 1300 cal yr BP, the gradual increase in the δ¹⁸O_bulk values—reflecting less intense convection in monsoon source regions and locally at time—indicates a continued weakening SASM and drying trend during the late Holocene. Furthermore, the low apparent carbon accumulation rate (aCAR) of 4 g C m⁻² yr⁻¹ during that time is likely due to the great decomposition of dominant bamboo-derived organic matter. The higher δ¹⁸O_bulk value at 1300-800 cal yr BP—possibly corresponding to the “Medieval Climate Anomaly” (MCA)—than the Little Ice Age (LIA) at 800-200 cal yr BP indicates a drier MCA, supported also by high charcoal content, low diatom abundance and high C/N ratios at our sites. The large difference of 8‰ in the δ¹⁸O_bulk values between the MCA and LIA suggests that local hydrological processes might have amplified the effect of monsoon-mediated hydroclimate on δ¹⁸O variations. The higher aCAR during the MCA at 15 and 28 g C m⁻² yr⁻¹ for two cores than during the LIA at 13 and 18 g C m⁻² yr⁻¹ suggests that the relatively dry climate during the MCA promotes plant production and peat accumulation in this terrestrialized, still-wet and then sedge-dominated peatland. The increase of Sphagnum and other mosses and high δ¹⁸O_bulk values during the last 70 years reveal a dry local environment that is also confirmed by the decreasing precipitation and weakening SASM in instrumental climate records. Our results indicate that the development and carbon accumulation of this river-side peatland are sensitive to the changing monsoon strengths and hydroclimate, with strong modulations and amplifications by local hydrology and water sources on carbon accumulation.