Spatial And Temporal Variability Of Climatic Factors Over The Qinling-Daba Mountains During The Past 300 Years And Its Potential Driving Mechanisms

Under the background of global warming,climate change has a profound impact on human production activities and the change of ecological environment.As an important climatic and geographical boundary,the Qinling-Daba Mountains are of great significance to the formation of the geographical pattern of China.Additionally,this region is very sensitive to climate change and is one of the ideal places to study how regional climate change responds to global warming.In this paper,tree-ring indices in different sub-regions of the Qinling-Daba Mountains were selected to investigate the response of regional tree growth to climate change.Furthermore,mean temperature,mean maximum temperature,drought index(sc PDSI)based on tree-ring width chronology and oxygen isotope chronology were reconstructed for the past hundreds of years.This study further analyzed their spatial and temporal representativeness,periodic changes and other characteristics.And the possible forcing factors were further detected.The results provide an important reference for understanding the response of regional climate change to global change.The major conclusions are as follows:(1)The radial growth of trees in the Qinling-Daba Mountains showed differences in responding to climatic factors.5 tree-ring width chronologies in eastern section of Daba Mountains mainly responded to the winter-spring temperature.While1 chronology in western section of Daba Mountains mainly responded to the early growing season temperature.5 tree-ring width chronologies in the Qinling Mountains,showed positively correlated with precipitation and negatively correlated with temperature during the growing season,which indicates that the tree growth was mainly restricted by moisture change.In our study region,the tree species is not a controlling factor for tree growth-climate factor.(2)In order to obtain a chronology that has better representiveness for the regional tree-ring width variation characteristics,this study extracted the common signal among the chronologies that respond to winter-spring temperature change.The first principal component(PC1)has better ability to capture the climatic information at high-frequency relative to the RC.Therefore,the winter-spring(2-5)temperature was reconstructed based on PC1.This reconstruction showed good representativeness of temperature variation characteristics in a large range during the same period,and it showed good consistencies with other winter and spring temperature reconstructions based on tree-ring width reconstruction and climatic historical events recorded in the nearby region.Over the past 177 years,there were 3 warm periods,namely1841-1865,1882-1892 and 1988-2017;and there were 2 cold periods,namely1866-1881 and 1893-1979,respectively.(3)Based on the correlation analysis between radial growth and climatic factors,this study carried out a single-point reconstruction for the chronologies that response to the same climatic factors,long geographical distance and poor correlation among each other.And then the spatio-temporal differences and possible mechanisms were further discussed.The May-July mean maximum temperature reconstructions for 3 sampling sites have good spatio-temporal representative characteristics.As for the difference among the reconstructions,On the one hand,it could be attribute to the differences of "growth divergence" caused by the difference in regional response to global warming,and the differences of precipitation signals recorded in trees.On the other hand,it could be attribute to the influence of El Ni?o-Southern(ENSO)and North Atlantic Oscillation(NAO)at internanual and decadal scales,respectively.Furthermore,ENSO and NAO as the main forcing factor,caused the spatial-temporal difference in May-July precipitation inferred from tree-ring width reconstructions and instrumental data in the Qinling-Daba Mountains,and eventually in spatio-temporal difference in the May-July mean maximum temperature by the cooling effect of precipitation on the concurrent mean maximum temperature.(4)The variation of sc PDSI recosntructions based on ring-width and oxygen isotope in the Qinling-Daba Mountains also showed good spatio-temporal representative characteristics.It was found that the significant correlation between sc PDSI inferred from instrumental data and reconstructions and grid data was demarcated by approximately 110°E.Therefore,this paper first detected the possible factor that determined the consistency of the regional May-June sc PDSI.The results of spatial correlation analysis showed that the concurrent precipitation is the controlling factor for sc PDSI in the eastern and western sections of Qinling-Daba Mountains and the relationship still exists at longer time scale.Regression analysis was further used to investigate the dynamic process and proved that the spatial consistency of regional precipitation is controlled by the cyclone.In addition,the spatial difference of the May-June sc PDSI in the eastern and western sections of the Qinling-Daba Mountains was caused by the different effects of ENSO on regional May-June precipitation.The results of the unstable relationship between ENSO and regional sc PDSI well explained the phenomenon of different phase variation inferred from May-June sc PDSI reconstructions in the eastern and western sections of Qinling-Daba Mountains.When ENSO showed both significantly negatively correlated with sc PDSI in the eastern and western sections,reconstructed sc PDSI showed in-phase changes and the correlation was the most significant.When ENSO was significantly negatively correlated with sc PDSI in the western sections and positively correlated with sc PDSI in the eastern sections,sc PDSI in two sections showed out-of-phase changes.