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Biodiversity has attracted much attention recently due to its important relationships with ecosystem function under various global warming scenarios. However, an understanding of biodiversity mechanisms requires study over long time scales. Three high-resolution pollen records of Zoige Basin in the eastern Tibetan Plateau are used to reconstruct changes in vegetation diversity during the Holocene, allowing the mechanisms that drove the dynamic to be quantitatively explored. Rarefaction and Hill's indices are used to estimate the diversity richness and evenness based on pollen data. The results show that changes in palynological richness can be divided into five stages: an abnormal change from 10,500 to 9,000 cal bp, an obvious increase from 9,000 to 6,500 cal bp, a decreasing trend from 6,500 to 4,000 cal bp, an increasing trend after 4,000 cal bp until 1,500 cal bp, and a highly fluctuating stage from 1,500 cal bp to the present. Palynological evenness is relatively stable throughout the Holocene except for during a briefly elevated period from ca. 4,000 to 1,500 cal bp. The result of Boosted Regression Tree analysis indicates that climate is the main driving factor and the effect of temperature is stronger than that of precipitation in the study region. However, during ca. 4,000–1,500 cal bp, palynological diversity is primarily affected by vegetation structure, as shown by an increase in palynological evenness, which can in turn be explained by the climate threshold theory. This research provides a long-term, high-resolution reconstruction of palynological diversity which could be used to infer vegetation diversity change in the ecologically sensitive Tibetan Plateau. The results imply that vegetation diversity in the region may increase under global warming if human impacts are not considered. [ABSTRACT FROM AUTHOR]

Temperature profiles provided by the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) global positioning system (GPS) satellite constellation were used to study an eight-year series (2007 to 2015) of gravity wave (GW) potential energy in the stratosphere (18-30 km) around the Tibetan Plateau (TP). We found that with increasing altitude, the GW potential energy ( E ) values in the stratosphere caused by convection decreases. The importance of GWs that are stimulated by topography is enhanced in this area. In the TP, which was considered to lack strong topographical GW activity, clear activity existed in the spring and winter of all studied years. Based on the latitudinal zone of the TP, the distribution of GW potential energy is highly consistent with the elevation of the local topography. The activities of topographical GWs are strongly filtered as they propagate upward to the area of zero speed wind. The analysis indicates that in the TP, clear orographic GW excitation exists and propagates upward to the upper stratosphere, where it is greatly influenced by the wind. [ABSTRACT FROM AUTHOR]

Terrestrial palaeo-temperature data are of great value in improving our understanding of past climate and they provide a basis for evaluating climate simulations. Such data are, however, poorly constrained for long time-scales. In addition to the scarcity of high-quality continuous time-series, finding proxies with a clear response to past temperature changes and developing appropriate reconstruction methods are major challenges. We present a new and robust method – Locally-weighted Weighted-average partial least squares (LW-WAPLS) to reconstruct quantitative temperature changes based on a high-resolution 1.74-Ma pollen record from the Zoige Basin on the eastern Tibetan Plateau, where the vegetation today is mainly controlled by temperature. The reconstructed mean annual (MAT) and warmest month (MTWM) temperatures reveal a general cooling trend with two major shifts at ~1.54 and 0.62 Ma BP, and regular glacial-interglacial variability ranging from ~ − 4 to 2 °C and from 8 to 16 °C, respectively. They indicate ~4–5 °C (MAT) and ~ 5–6 °C (MTWM) magnitudes of glacial-interglacial temperatures. Both statistical and ecological evaluations validate the reliability of the reconstructions. The reconstructions provide important insights into the spatial aspects of long-term terrestrial temperature change. LW-WAPLS shows advantages over both the traditional modern analogue technique and non-linear transfer-function methodologies such as WAPLS for reconstructing the broad-scale climate changes for the Zoige Basin, by combining the strength of both methods. The LW-WAPLS approach potentially provides a robust tool to develop pollen-based climate reconstructions over long time-scales. • LW-WAPLS approach is a powerful tool for the quantitative reconstructions of climate on glacial-interglacial scales. • Temperature reconstructions of the eastern Tibetan Plateau reveal the stepwise cooling trend with two major shifts at 1.54 and 0.62 Ma BP and with ~4-5oC (MAT) and 5-6oC (MTWM) magnitude. • Our long-term temperature reconstruction fills a data gap in mid-latitude terrestrial regions • The results provide valuable data for model simulations to elucidate the climate mechanisms [ABSTRACT FROM AUTHOR]

Existing pollen records reveal that a shift from tree-rich to herb-dominated vegetation occurred on the eastern Tibetan Plateau (TP) during the mid-to-late Holocene, which gave rise to the modern vegetation pattern. However, given the complex climatic regulation of vegetation growth, the main climatic controls on arboreal vegetation in the eastern TP still need to be explored. In this study, a 387.5 cm sediment core covering the last 12.6 cal ka was recovered from Emu Co on the eastern TP. Based on pollen analysis in conjunction with the modern analogue technique, the role of various climate factors influencing past vegetation changes was modelled. Our results show that, during 12.6–9.5 cal ka BP, the study area experienced an arid and cold climate, with a sparse vegetation cover, as evidenced by the high abundance of herbaceous pollen and low total pollen concentration. A cold-tolerant deciduous broadleaved arboreal component, dominated by Betula , began to expand at ca. 9.5 cal ka BP, replacing herbaceous vegetation habitat, suggesting a gradual warming and wetting trend. Controlled mainly by rising temperatures, evergreen coniferous tree components develop from 8.7 to 5 cal ka BP, with Picea pollen reaching its maximum abundance. After 5 cal ka BP, the herbaceous vegetation re-expands under the combined pressure of lower temperatures and reduced precipitation. Declining arboreal pollen abundance in the mid-to-late Holocene is prevalent in the available fossil pollen records, but the main drivers have regional peculiarities: in the east-central region of the TP near Emu Co, lower temperatures may have played a more important role in the retreat of trees than reduced precipitation. Conversely, arboreal retreat and vegetation change in the north-east and south-east regions may be related to the weakening of the Asian summer monsoon circulation and seasonal climate changes caused by the East Asian winter monsoon and Indian summer monsoon, respectively. • A Holocene pollen record from the forest/meadow ecotone on the Tibetan Plateau. • Arboreal pollen reduces strongly at ca. 5 cal ka BP on the eastern Tibetan Plateau. • The importance of temperature and precipitation for arboreal decline varies by region. [ABSTRACT FROM AUTHOR]