Tree-ring-based Droughts Variability In Western Tianshan Mountains,Central Asia

Climate change is currently an issue of scientific challenge and political debate.Global warming has altered precipitation and the hydrologic cycle in various regions.Past global climate changes had strong regional expression.Especially the change of precipitation is strong locality,therefore,it is important to understand the regional droughts and pluvials change.Central Asia is a typical inland arid area,located on the upper reaches of westerly.There have fragile ecological environment,response sensitive to climate change.The Tianshan Mountains are known as the “water tower”,and are the largest and most important mountain system in the arid region of Central Asia,which has produced many civilizations of the central Asian.It is vital to understand the fact of past droughts variability.The length of instrumental meteorological observation data is limited,and thus,long-term climate change research has been restricted.Dendroclimatology is an important method for examining pre-instrumental climate change.The tree-ring of Schrenk spruce(Picea schrenkiana Fisch.et Mey)growing in arid and semi-arid mountainous environments is sensitive to climate and is very suitable for climate reconstruction.However,numerous climate reconstructions are concentrated in China.This dissertation is aimed at the weak areas and regions of the dendroclimatology in Tianshan Mountains.First of all,the intra-annual radial growth regularity of Schrenk spruce and its response to daily meteorological factors were revealed based on the dendrometer method.Then,many tree-ring samples were collected,and tree-ring width chronologies were established from western Tianshan Mountains,central Asian.The drought variability in Issyk Lake Basin of Kyrgyzstan,southern Kazakhstan and Dzungarian Alatau of the eastern Kazakhstan were reconstructed and analyzed,respectively.The regional differences and possible causes of the past drought variability in the western Tianshan Mountains were discussed.Finally,the main law of the past drought variability in the Tianshan Mountains is suggested.The main conclusions of this dissertation are summarized as follows: 1.The response of Schrenk spruce radial growth to climate1)Intra-annual radial growth response to daily meteorological factorsThe critical growing season of Schrenk spruce is from late May to late July and that the rapid growth stage is from mid-June to early July.Meanwhile,in the growing season,changes in the radial growth of Schrenk spruce were negatively correlated with daily temperature,evaporation,sunshine hours and vapor pressure deficit(VPD),and were positively correlated with precipitation and relative humidity(RH).The moisture plays a major role in the growth of Schrenk spruce.We suggest that precipitation between late May and late June is a limiting factor for radial growth of Schrenk spruce on the northern slopes of the Tianshan Mountains.2)Tree-ring width response to climateThe precipitation from previous July to current June has an important effect on the radial growth in Tianshan Mountains.The moisture before the growth season and the early growing season are the main limiting factor of the tree-ring width at the lower treeline in the Tianshan Mountains.3)Tree-ring width response to snow coverThere are significant positive correlation between radial tree growth and snow cover parameters(maximum snow depth,precipitation in winter,duration of snow cover ?10 cm and duration of snow cover ?1 cm).The maximum snow depth might be an important limiting factor affecting the radial growth of Schrenk spruce.2.The centennial drought variability in the western Tianshan MountainsIn general,the centennial drought variability in the Tianshan Mountains is more consistent: In the 1900 s,there was slow wetting,then a conversion to dry in 1910 s and it was the driest decade,the driest year occurred in 1917.Drought decreased slowly in the 1920s-1930 s,increased in the 1940 s,weting in the 1950s-1960 s and drying in 1970 s.Then,from 1980s-2000 s,the climate of the Tianshan Mountains began the longer and more significant wetting period of the past century.In recent decades,the trend of drought change in the Tianshan Mountains has been obviously different.The climate in Issyk lake basin has experienced the longest,most significant and rapid wetting period of the past century.But the northern slope of the Tianshan Mountains has volatility aridized since 2000,while the eastern Tianshan Mountains have continued to dry since 1940.The widespread extreme drought years of the Tianshan Mountains in the past century were 1879,1917 and 1945.The extreme drought year in 1879 corresponds to the continuous severe drought in 1876-1879 of the widespread northern China.The extreme drought year in 1917 and 1945 were commonly found in the reconstruction of the Tianshan Mountains and Xinjiang,and were recorded in the historical documents.The centennial drought variability of the western Tianshan Mountains in central Asia had significant short cycles of the 2-7-yr and 11-yr.Meanwhile,there also had long cycles about 17-yr and 30-yr.The drought variability may have been controlled by North Atlantic Oscillation(NAO)in the past century,and may have been associated with large-scale oscillation in the climate system.But,the drought variability of the eastern Tianshan Mountains may have been affected by El Ni?o-Southern Oscillation(ENSO).