Tree-Ring Width Records Of Past Climate Variability In North Xinjiang

Twentieth-century warming and its associated climate change have caused significant social,economic and ecological impacts,and therefore has received a great attention around the world.Xinjiang is located in the interior of Eurasia and is an important part of the arid and semi-arid region of Northwest China.This area is particularly sensitive to climate change due to its special geographical location.Climate change accelerates the water cycle,intensifies the uncertainties of water resources,and could impact the construction of the Silk Road economic belt.Therefore,it is of great importance to investigate the characteristics of climate change and its potential forcing mechanisms for Xinjiang.Tree rings are of particular value in this regard because they provide past climate conditions with precise dating and annual resolution.Here,we developed a tree-ring width network to study climate changes in northern Xinjiang over the past centuries.We firstly investigated the radial growth variations and climate–growth relationships in the western Tianshan Mountains(WTSM)and the Altay Mountains(ATM),with the purpose to identify the climate signals in tree-ring widths at different elevations.Then,the high-elevation chronologies were used to reconstruct the temperature history,while the middle-to low-elevation chronologies were used to reconstruct the drought history for both of the WTSM and ATM.Finally,we used these reconstructions to investigate the characteristics of local temperature and drought variability and the linkages with global warming or large-scale drought variability.The major conclusions are as follows:(1)Based on five and six tree-ring width chronologies from the WTSM(Picea schrenkiana)and ATM(Larix sibirica),we found that elevation played a crucial role in determining tree growth and its climate response in both of the two areas,despite the tree species are different.Temperature and precipitation in different altitudes have different effects on tree growth.Tree growth at the low-elevation sites is primarily limited by moisture availability.With increasing altitude,the importance of precipitation decreased,tree growth at the high-elevation sites is mainly controlled by lower temperature.This results support the prevailing viewpoint that temperature is the key limiting factor for tree growth in the upper forest limit.(2)According to the above analysis,we used the high-elevation chronologies to reconstruct the June–August minimum temperature and June mean temperature history in the WTSM(AD 1713–2012)and ATM(AD 1513–2012),respectively.The WTSM temperature reconstruction contains considerable low-frequency variations over the past 300 years.There is a generally cooling trend from the beginning of the reconstruction to the 1920 s,from then the temperature began to increase,particularly after the 1980 s.The recent two decades were the warmest period over the entire reconstruction.The WTSM temperature reconstruction is the first tree-ring width study that records a rapid warming trend in recent century in the Tianshan Mountains,and agrees well with the global warming.Compared with the WTSM temperature reconstruction,the ATM temperature reconstruction contains more high-frequency information,but still has decadal to multi-decadal variations.The 20 th century was the warmest century over the past 500 years.However,the ATM temperature reconstruction did not have a warming trend in the 20 th century,which is different from the WTSM temperature reconstruction.The two temperature reconstructions show many differences on the low-frequency variations,but their first-order differences have a significant positive correlation.(3)We used the middle-to low-elevation chronologies to reconstruct the July–May precipitation and July–August scPDSI(self-calibrating Palmer Drought Severity Index)history in the WTSM(AD 1670–2012)and ATM(AD 1560–2011),respectively.For the WTSM precipitation reconstruction,persistent and severe dry epochs occurred in the1759–1775,1820–1831,1859–1864,and 1913–1923,and persistent wet periods were found from 1720–1724,1892–1903,1959–1869,and 1993–now.Over the past century,the WTSM precipitation reconstruction showed a rising trend,and the recent two decades were the wettest period over the past 343 years.For the ATM scPDSI reconstruction,persistent and severe dry epochs occurred in the 1573–1581,1684–1689,1811–1826,and 1977–1982,and persistent wet periods were found from 1602–1612,1621–1630,1738–1748,1795–180,1908–1912,1954–1960,and 1990–1996.The recent century was a relatively wet century,but the ATM scPDSI reconstruction did not have a rising trend in this period.(4)Combining the above precipitation and scPDSI reconstructions as well as two tree-ring width based precipitation reconstructions from the northeastern Tibetan Plateau and eastern Gansu province,we tried to investigate the large-scale drought variations over northwest China.The results showed that all of these four reconstructions generally agree well with each other in the low-frequency variations(21-year running mean).This may be because that all of them are located in the region where the climate is mainly controlled by the Northern Hemisphere westerly winds,and contain similar westerly signals.The high consistency of the average of these four reconstructions with the NAO(North Atlantic Oscillation)index support this hypothesis.