Temporally changing drivers for late-Holocene vegetation changes on the northern Tibetan Plateau

Abstract

a b s t r a c t a r t i c l e i n f oFossil pollen records have been widely used as indicators of past changes in vegetation and variations in climate. The driving mechanisms behind these vegetation changes have, however, remained unclear. In order to evaluate vegetation changes that have occurred in the northern part of the Tibetan Plateau and the possible drivers behind these changes, we have applied a moving-window Redundancy Analysis (RDA) to high resolution (10-15 years) pollen and sedimentary data from Lake Kusai covering the last 3770 years. Our analyses reveal frequent fluctuations in the relative abundances of alpine steppe and alpine desert components. The sedimentary proxies (including total organic carbon content, total inorganic carbon content, and "end-member" indices from grain-size analyses) that explain statistically some of the changes in the pollen assemblage vary significantly with time, most probably reflecting multiple underlying driving processes. Climate appears to have had an important influence on vegetation changes when conditions were relatively wet and stable. However, a gradual decrease in vegetation cover was identified after 1500 cal a BP, after which the vegetation appears to have been affected more by extreme events such as dust-storms or fluvial erosion than by general climatic trends. Furthermore, pollen spectra over the last 600 years are shown by Procrustes analysis to be statistically different from those recovered from older samples, which we attribute to increased human impact that resulted in unprecedented changes to the vegetation composition. Overall, changes in vegetation and climate on the northern part of the Tibetan Plateau appear to have roughly followed the evolution of the Asian Summer Monsoon. After taking into account the highly significant millennial (1512 years) periodicity revealed by time-series analysis, the regional vegetation and climate changes also show variations that appear to match variations in the mid-latitude westerlies.

Wood Charcoal From Archaeological Sites in the Qinghai Lake Basin, Western China: Implications For Human Resource Use and Anthropogenic Environmental Change

“…After ~5,000 14 C yr BP, Asian monsoon strength declined and the level of Qinghai Lake dropped (An et al 2012;Colman et al 2007;Liu et al 2011Liu et al , 2015Yu and Zhang 2008). Arboreal pollen values decreased in the Qinghai Lake record and other Tibetan Plateau pollen records, as conifer forests retreated widely in the northeast Tibetan Plateau and elsewhere in northwest China during this time (Herzschuh et al 2010;Wang et al 2012;Zhou and Li 2012). The archaeological record from the Qinghai Lake Basin dating ~5,000-3,500 14 C yr BP is limited (Rhode et al 2104), but the few sites dating from this period contain spruce charcoal, indicating that woodlands persisted into the late Holocene after the decline of the summer monsoon and general Plateau cooling.…”

“…Among the evidence these researchers cite are (1) pollen records from lakes and wetlands that document mid-to late Holocene replacement of forests with grasslands and highlight increasingly abundant grazing indicator plants, (2) phylogeographic patterns of modern forest trees such as Qinghai spruce (Picea crassifolia) that show a more continuous distribution earlier in the Holocene (Meng et al 2007), and (3) records of charcoal from soils and archaeological sites demonstrating the presence of conifer trees where they no longer occur, leading to speculations of widespread early human-induced forest clearing by fire. Other researchers, however, conclude that this reduction of high-altitude conifer forests and woodlands was driven mainly by Holocene climate change, a weakening of monsoons, and region-wide cooling (e.g., Herzschuh 2006;Herzschuh et al 2006Herzschuh et al , 2010Kramer et al 2010;Shao et al 2010;Wang et al 2012;Zhao et al 2007a;Zhou and Li 2012).…”

Factors Influencing the Seasonal Flux of the Varved Sediments of Kusai Lake on the Northern Tibetan Plateau During the Last ∼2280 years

“…However, the climate in winter is always cold and dry with a high frequency of dust storms (Zhao and Zhou, 2002;Chen, 2017). The modern vegetation around Kusai Lake is alpine steppe dominated by drought-resistant Artemisia and Poaceae, and alpine meadow dominated by hygrophilous Cyperaceae (Wang et al, 2012;Cui et al, 2021). at a water depth of 14.5 m in September 2010 and 2015 (Figure 1B), respectively.…”

Strengthened East Asian Winter Monsoon Regulated by Insolation and Arctic Sea Ice Since the Middle Holocene

“…Various types of geological records on the EAWM changes have been well reconstructed from orbital to millennial timescales, including loess (Hao et al., 2012; Kang et al., 2018, 2020; Stevens et al., 2008; Yang & Ding, 2014), lake sediments (C. An et al., 2011; Wang et al., 2012), marine sediments (Huang et al., 2011; Yamamoto et al., 2013; Zhao et al., 2019; Zheng et al., 2014) and stalagmite (Sone et al., 2013). Since changes in EAWM have been strongly correlated with loess records from the Chinese Loess Plateau (CLP), it is commonly used as an ideal indicator to detect EAWM (Z.…”