Land cover reconstruction in Northwest China since 6 ka BP: Preliminary application of a new strategy

Abstract

The mid-Holocene is generally regarded as a relatively warm period, and therefore reconstructing land cover change from the mid-Holocene onwards may help predict future changes under warming scenarios. In this study, we used a new strategy which combines the REVEALS model with modern analogue technique (MAT) to achieve a land cover reconstruction since 6 ka BP for Northwest (NW) China. The REVEALS results are more consistent with our general understanding of pollen-vegetation relationships in NW China than the pollen percentage data. However, although the model has potential for future applications, further quantitative testing is needed. Modern calibration sets can provide modern “analogues” for most of the fossil samples, but the numbers of modern samples from areas with a low vegetation cover are limited, which may lead to over- or underestimation of the results, despite the overall small prediction error. Our land cover results show that, since ∼6 ka BP, the vegetation cover in mountains areas was significantly greater than at lower elevations, with the threshold cover value of 45%. At lower elevations, the vegetation cover gradually decreased from east to west. Elevation differences and the distance from the sea may be the main reasons for the differences. The mountains were dominated by steppe, meadow/meadow steppe, and forest, while the lower elevations were dominated by desert and desert steppe, with generally only small changes in vegetation type, reflecting the long-term arid climatic background of the region. However, since ∼6 ka BP there were several differences in the pattern of dry-wet climate shifts across the study region. The vegetation in most areas indicates a trend towards wetter conditions, or no significant trend, except for sites G1 and G3 which show an aridification trend.