| Global warming has become an undoubted fact,which had a profound impact on the economic society,water resources,and ecological environment.The change of hydroclimate has become a research hotspot at present.Only based on a full understanding of the hydroclimate changes in the past long period can the change of hydroclimatic factors be revealed objectively.Tree-ring hydroclimatology has made an outstanding contribution to the study of global historical hydroclimate change so far.However,due to the continuous changes of external environmental,the tree-ring mode that containing hydroclimate signal has become more complicated and uncertain.The current research faces some bottleneck problems,For example,there was still a certain potential for hydroclimate signal mining,and the long-term evolution of hydroclimate in the large span with complex environment was not clear.Monsoon-continental transitional zone in China is one of the most sensitive regions in response to climate change,for it covers a variety of topography and climate type,and the study results of this area was representative and comprehensive.In the study,six kinds of dominant tree species were selected from seven typical regions in the transition zone to test and analyze the data of tree ring width,compare various tree ring width chronologies with different detrend method,then determine the matching detrending method for each region and construct the optimal chronology for 200～464 years.Finally,the response relationship between the optimal chronology and the change of key hydroclimate factors were revealed,the applicable reconstruction models of different reconstruction factors in different typical regions were constructed and the historical reconstruction sequences were determined,the historical evolution and spatial heterogeneity of key hydroclimatic factors in the transitional zone during 200-464 years were revealed thoroughly.The main conclusions are as follows:1.The quality of tree ring width chronology obtained by different growth trend removal methods and the degree of response to key hydroclimatic factors were significantly different.The "NEF-GA" detrending method established this time had the strongest overall applicability in the transition zone.Not only removing its own growth trend from the original sequence of tree ring width,it preserved the hydroclimatic signals to the greatest extent.Not only was the correlation between chronology and key hydroclimatic factors significantly improved(up to 60.78%),but the chronologically related parameters also reached a relatively high level.Through comparative analysis,chronologyit was finally determined that,except for the optimal chronology of the northern section of the Greater Khingan Mountains was "CEEMD" chronology,the optimal chronology of the eastern section of the Qilian Mountains is "Signal-free" chronology,the optimal chronology of the other areas were all "NEF-GA" chronology.2.The tree growth in the transition zone was closely related to the local water and heat conditions.Air temperature was the main limiting factor(the highest correlation coefficient was-0.87,p<0.001),and the limiting effect of the current year was generally stronger than that of the previous year.Runoff was the main promoting factor(0.786,p<0.001),and there was a certain accumulation and lag effect on tree growth.Precipitation(0.495,p<0.001)had a slightly weaker promoting effect.The effect of interannual temperature and runoff on tree growth was stronger than that of a single month,and the limiting effect of temperature was weakened obviously under the condition of sufficient water.March to October is the main growth season for trees in the transition zone.With the rise of temperature,runoff and precipitation,the growth of trees increases rapidly under the combined action of water and heat in the same period.The dynamic relationship between tree growth and annual mean temperature and annual runoff is more stable in the transition zone,while sliding correlation differentiation is easy to occur in the middle and low latitudes.3.The reconstructed key climatic factor of this transition zone was the annual mean temperature,and the key hydrological factor was the runoff.The SLR,MLR,MGF-LSTM and SARIMA models were different in reconstruction results,evaluation indexes of model accuracy and reconstruction factors.The reconstruction results of SLR model and MLR model were similar and stable in most areas,and the RE and CE values of the two models were both greater than 0 and closer to 1(up to 0.679 and 0.351,respectively).among them,the MLR model had higher accuracy in the reconstruction of runoff.The SARIMA model had a better reconstruction effect and was suitable in the northern part of the transition zone.While the linear model was prone to underfitting in such areas.The reconstruction effect of MGF-LSTM nonlinear reconstruction model was the worst.The RE and CE values of this model were low,and there were many anomalies in the reconstruction results.Through comparative analysis,it is finally determined that except for some high latitude areas,the rest of the region is reconstructed by linear regression model.4.The results of this reconstruction correspond well with the extreme hydrological and climatic events recorded in the local historical records,such as drought,flood and low temperature period,and the reconstruction results are reliable and usable.Throughout the annual mean temperature changes in the transition zone over the past four centuries,the complexity of the cycle is gradually increasing,and the large-scale cycle is gradually significant(>40a).The transition zone from 1560 s to 1690 s was in the low temperature period as a whole(the average value of 1690 s was between-4.23℃ and 5.25℃).From then to 1760 s,the average temperature gradually increased,and the high latitude region experienced a rapid warming rate(0.875℃/10 years),and the average value of the multiyear period reached the peak of the whole series(2.536℃).From the 18 th century,the middle,high and lowest latitude areas began to rapidly cool down to 1860s(-5.16℃～6.17℃),but some low latitude areas still showed a continuous warming trend to1910 s,and the fluctuation was drastic year by year.After the 1920 s,under the continuous effect of global warming,a significant warming trend appeared in the transition belt and continued until now,especially in an area affected by westerly winds,which not only experienced a rapid warming rate(0.648℃/10 years)but also lasted for a longer period,reaching a peak in the past three centuries(8.134℃)on average in the 1920 s.Although the high latitude area has a faster warming rate,it is not special compared with the high temperature period of 1760 s and 1910 s.It is worth noting that the warming stagnation occurred in the middle and low latitudes of the transition zone in the past half century,but not in the high latitudes.5.In the past four centuries,the cycle changes in the high and low latitudes of the transition zone are more complex,and the complexity is increasing year by year,and the mesoscale and large scale cycle development is significant(>25a).In the 17 th century,the transition zone experienced a short period of abundance,with a faster rate of abundance in the middle and low latitudes.From then to the middle of the 18 th century,runoff in the transition zone generally continued to dry up,but the average level of multi-year was generally higher,the drying rate was faster in the high latitude area(-538.2 million m3/10years),and the drying duration was longer in the low latitude area(211 years).In the past200 years,the variation of runoff in the transition zone shows a pattern of high and low latitudes in the middle latitudes,and the cycle changes become more and more complex.Since the 19 th century,the annual runoff in the middle and high latitudes has shown a trend of increasing,while that in the parts of the middle and low latitudes affected by westerlies has shown a trend of decreasing.In the early 20 th century,the annual runoff in the whole transition zone showed a significant and rapid drying trend(up to-113.8 million m3/10 years at the fastest).The drying rate in the middle and low latit udes was more significant than that in the high latitudes,and the annual runoff in most areas reached the trough value in the past four centuries.The mean value of runoff in the 2000 s to 2010 s was only 17.37% to 24.66% of that in the 17 th to 18 th centuries.This study achieved innovation in maximizing the mining of tree-ring hydroclimatic signals and revealing the change and spatial-temporal heterogeneity of key hydroclimatic factors under large-scale with complex environmental conditions.The study can enrich the research achievements of global long-term hydroclimatic change and provide references for addressing climate change,water resources,ecological environment and other issues. |
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