Temperature variability and vertical vegetation belt shifts during the last ∼50,000 yr in the Qilian Mountains (NE margin of the Tibetan Plateau, China)

Abstract

A 13.94-m-long sediment core, collected from a medium-sized lake in the Qilian Mountains (NE Tibetan Plateau, China), was analysed palynologically at 81 horizons. The interpretation of indicator taxa yielded various vertical shifts of the vegetation belts. These palaeovegetation results have been checked with lake surface pollen spectra from 8 lakes representing different altitudinal vegetation belts. Our main findings are the following: A short period of the late Marine Isotope Stage 3 (around ∼46,000 yr ago) was characterized by interglacial temperature conditions with a tree line above its present-day altitude. During the LGM, the vicinity of the lake was not covered by ice but by sparse alpine vegetation and alpine deserts, indicating that the climate was colder by ∼4–7°C than today. Markedly higher temperatures were inferred from higher arboreal pollen frequencies between ∼13,000 and ∼7,000 yr ago with a Holocene temperature optimum and a maximal Picea–Betula mixed-forest expansion between ∼9,000 and ∼7000 yr ago, when temperatures exceeded the present-day conditions by at least 1–2°C. Alpine steppes and meadows and sub-alpine shrub vegetation dominated around the lake since the middle Holocene, suggesting that vegetation and climate conditions were exceptionally stable in comparison to previous periods.

Range shifts of Potentilla fruticosa on the Qinghai-Tibetan Plateau during glacial and interglacial periods revealed by chloroplast DNA sequence variation

“…Palaeoenvironmental evidence also supports our view that the interior region may have served as a refugium for alpine species. The fossil pollen record shows that, during the mid-Holocene, when temperatures exceeded present-day temperatures by at least 1-2 1C (Yu et al, 2000), forests were located at markedly higher elevations than they are today (Herzschuh et al, 2006;Ren, 2007). This phenomenon had probably occurred during each interglacial period.…”

“…Fossil pollen records suggest that the typical vegetation of the Plateau during the last glacial period was adapted to permafrost-steppe, tundra or desert habitats (Yu et al, 2000), although the temperature was 6-9 1C lower than at present (Shi, 2002). These plant communities also developed in the interior of the Plateau during the postglacial period, when forests became widespread on the eastern Plateau (Herzschuh et al, 2006). After the forest retreat, populations may have then expanded from the interior of the Plateau towards its eastern margin.…”

Driving forces of mid-Holocene vegetation shifts on the upper Tibetan Plateau, with emphasis on changes in atmospheric CO2 concentrations

“…Declining moisture is generally interpreted as a response to monsoonal weakening as a result of low-latitude summer insolation decrease. However, vegetation records from rather moist marginal areas at lower altitudes on the Tibetan Plateau (Shen et al, 2005;Herzschuh et al, 2006a;Kramer et al, 2010a) are in line with this general trend. The continuous decline of forest since 6 cal ka BP can be most reasonably explained by a precipitation and/or temperature reduction (Herzschuh et al, 2010c).…”

“…Most sequences show an early-to mid-Holocene maximum indicating that alpine steppe had its maximum extension during that time. The fitted lines are LOESS smoothers (span ¼ 0.1) (References: Selin Lake, Sun et al (1993); Xuguo Lake and Ahung Lake, Shen (2003); Zigetang, Herzschuh et al (2006a); Koucha Lake, Herzschuh et al (2009); Yidun Lake, Shen et al (2006); Kuhai Lake, Wischnewski et al (2011)). To estimate quantitatively the annual moisture-balance (¼annual precipitation À annual potential evapotranspiration), we developed a modern pollen-climate calibration data-set based on 132 lake surface-sediments from the eastern Tibetan Plateau, covering a wide moisture-balance range (À1287 to þ1008 mm).…”

“…Zigetang Lake, Herzschuh et al, 2006b; Koucha Lake, Herzschuh et al, 2009); (2) Artemisia declines, and (3) Kobresia expands after the onset of proposed anthropo-zoogenic grazing. Evidence for Kobresia meadows being a natural element on the Tibetan Plateau is indicated by >80% Cyperaceae in lake pollen assemblages of Lateglacial (Allerød) age, indicating the widespread occurrence of Kobresia meadows on the south-eastern and northeastern Tibetan Plateau under cool and wet climate conditions (Herzschuh et al, 2006a;Kramer et al, 2010b). Still, due to the impossibility of differentiating Kobresia pollen morphologically from other Cyperaceae we cannot be absolutely sure that it was Kobresia that expanded during the Lateglacial.…”