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Alpine regions on the Tibetan Plateau are sensitive to climate change, however, little is known about their long-term hydroclimate variability due to short and few instrumental records. Alpine shrub provides a rare opportunity to reconstruct past high-resolution hydroclimate conditions. Here, we used living and dead juniper shrubs (Juniperus pingii var. wilsonii) to develop a 537-year ring width chronology nearby the Nam Co Lake (4,725 m a.s.l.). The chronology explained 46% of variance in May-June Standardized Moisture Anomaly Index (SZI) in the period from 1970 to 2010. We reconstructed May-June SZI since 1605. The reconstructed SZI matched well with several tree-ring based moisture or drought reconstructions for the Tibetan Plateau. In particular, two long dry May-June periods of 1637-1683 and 1708-1785 during the Little Ice Age (LIA) implied that long-term cold temperature tends to slow down hydrological cycle in the margin areas of the Indian summer monsoon.

Plain Language Summary As the Third Pole, the Tibetan Plateau is a core region for global change studies given that it is warming at twice the global average rate. The central Tibetan Plateau with an elevation above 4,500 m a.s.l. is characterized by extremely cold and dry treeless environment. However, very few alpine shrub species there provide a rare opportunity to retrieve changes of alpine ecosystem by shrub-ring analysis. In spite of their small basal diameters (mean of 12 cm), we established a 537-year standard shrub ring chronology by crossdating living and dead Wilson juniper shrubs sampled above 4,740 m a.s.l. on the central Tibetan Plateau. It is, to date, one of the world's longest shrub-ring chronologies. Shrub-ring chronology from 1605 to 2010 was then used to reconstruct mean May-June drought severity (Standardized Moisture Anomaly Index). Two long-term dry spring periods (1637-1683 and 1708-1785) occurred during the Little Ice Age (LIA), implying that cold temperature slows down hydrological cycle. This study highlights the importance of alpine juniper shrubs in understanding hydrological cycle in dry, continental alpine treeless areas.