Drought Study In The Central And Western Yunnan By Tree-ring Proxies

Drought disaster has significant influence in economy?agriculture?social activity and ecology environment,as it is the most widely and serious natural disaster.Drought is the imbalance between water supply and demand,it is also stand out in semi-humid and humid area.There are frequent drought disasters in Southwest area,especially Yunnan province,southwest border of China.The drought disasters of Yunnan area are seasonal and regional because of its complex landform and the influence of monsoon system.At the background of global warming,it's much meaningful to study the characteristics and trends of drought in this area.Since the meteorological data is too short to study the characteristics of drought in the long time scale,so the tree ring proxies with high resolution are considered to work it out.The spatial distribution of dendroclimatology studies is quite uneven,mainly focus on the northwest of Yunnan,but the rest of Yunnan is the study vacancy.We choose the joint part of the Tibetan plateau and the Yunnan-Guizhou plateau as the study area,which located in the Central and Western Yunnan.Firstly,In the Wuliang Mountains,which close to the Yunnan-Guizhou plateau,we developed the first tree ring width chronology of this area and then reconstructed the Standardized Precipitation and Evapotranspiration Index(SPEI)in drought season(November to April)of the past 140 years,the characteristics of drought history and its connection with large scale circulation system are analyzed;Secondly,According to the frequent intra-annual density fluctuations(IADFs)(also called false ring,double ring and so on)of tree ring structure in the Wuliang Mountains,the first IADFs chronology of Yunnan was developed,and the environment trigger factors and climate application of the IADFs were further researched.Lastly,based on two Tsuga dumosa chronologies which response to climate change similarly,we reconstructed the spring drought history of the Yunling Mountains,which are near to the Tibetan Plateau,after that,we compared our drought reconstruction with other tree ring based hydroclimate reconstructions.The mainly conclusions are summarized as follows:The reconstruction of drought history in the Wuliang Mountains reveals that AD 1888-1908 and 1948-1965 are the continuous drought periods,while AD1950s-1960 s is the most severe drought period during the past 140 years;AD1876-1887 is the continuous pluvial periods.AD1976-1997 has the maximum droughts and pluvial fluctuations in intensity and extent,actually this strengthen in climate variability may begin from the 1960 s,all the extreme drought and pluvial years occur in the second half of 20 th century.Recently decades,the climate variability of Central Yunnan shows an evident uptrend and the extreme climate events are increase in frequency and intensity.This phenomenon may be related to the rapid global warming.The hydroclimate of the Central Yunnan in drought season is influenced by the intensity of southern branch tough,as for the larger scale circulation,it is connected with North Atlantic Oscillation and Arctic Oscillation,the influence of AO is more significant,the westerly system has great impacts on study area.Intra-annual density fluctuations(IADFs)of tree ring in the Wuliang Mountains mainly appears in earlywood,The climate pattern of higher temperature in April?May and June,fewer precipitation in May and superior precipitation in July is much advantage in the formation of IADFs,high temperature and rainy climate in May with the addition of rainy July plays the key role of IADFs formation.The drought stress in earlier stage of growing season especially May slows down even stops the cambial activity,thus forms the latewood-like cells with greater density,In the middle stage of growing season,i.e.July,appropriate climate makes the cambial activity restart,earlywood growing continues,so there is a density fluctuation in a ring.The amount of precipitation in May indicates the start time of rainy season,For the reason of drought stress in May is important for IADFs formation,the frequency of IADFs may be a great proxy for timing the rainy season onset.According to the responses of the two Tsuga dumosa width chronologies and a Pinus yunnanensis width chronology to the climate change,we know that the available moist is the limiting factor for the radial growth of trees in the Yunling Mountains,we averaged the two Tsuga dumosa chronologies that have a similar response to climate,based on this composite chronology,the Palmer Drought Severity Index in April to May is reconstructed.AD 1856-1865 and 1874-1894 are the continuous pluvial periods,AD1953-1962 and 2005 to today are the continuous drought periods,all the pluvial periods occur in 20 th century,while all the drought periods occur after 1950 s,climate of this region is drying generally.In line with the climate characteristics of central Yunnan in recent decades,the droughts and pluvial variability of the Yunling area also increase in frequency and intensity since 1950 s,and the extreme climate events occur frequently,this coherence may indicates that the widely warming increases climate variability among the seasons and regions.Comparisons of our drought reconstruction and other tree ring based hydroclimate reconstructions show that the coherence of hydroclimate fluctuations is strengthen between our study area and surrounding regions after 1960 s,but their absolute condition is different even opposite from the end of 20 th century to today,it may implies that there is a difference in response to climate change between the southern and northern Hengduan Mountains.