| As an important part of terrestrial ecosystems,forest ecosystems are indispensable and important natural resources in the process of human survival and development.The Greater Khingan Mountains region is located in the northeast of China.Because of its high latitude,it has become a typical area of climate change.Larix gmelinii as the representative tree species in the Greater Khingan Mountains region,has the advantages of cold tolerance,strong cold resistance and sensitive response to climate change,which makes it an ideal tree species for dendroclimatology research.In this research,based on the basic principles and methods of dendrochronology,tree ring cores of the three altitude gradients of high,middle and low in the Greater Khingan Mountains were collected,and standard chronology of the ring width of Larix gmelinii were established to analyze whether abrupt changes and periodic changes existed in the standard chronology of the ring width of deciduous Larix gmelinii at different altitude gradients.Combined with five climate factors,namely monthly mean temperature,monthly mean maximum temperature,monthly mean minimum temperature,monthly mean precipitation and standardized precipitat in evapotranspiration index(SPEI),from 1973 to 2018 at local meteorological stations in the study area,the correlation between tree radial growth and climate factors and the dynamic changes of tree stability were analyzed.The main research conclusions are as follows:(1)In this study,the normalized chronology of ring width of Larix gmelinii was established at three sampling points with high,middle and low altitude gradients.Through the analysis of the standard chronology and chronology statistical parameters of Larix gmelinii,the signal-to-noise ratio(SNR)and expressing population signal(EPS)values of the standard chronology of tree ring width with altitude gradients were all higher.It is confirmed that the established standard chronology of ring width of Larix gmelinii has good reliability and can be used of tree ring climatology.By comparing the length and parameters of the standard chronology of tree ring width chronology of the three altitude gradients,it was found that the high altitude gradients had the longest year and the highest values of standard deviation(SD),mean sensitivity(MS),first order autocorrelation(AC1)and first principal component variance explanation(PC1).These results indicate that the high altitude gradient chronology contains more climate information than the middle and low altitude gradients,which is more suitable for dendrochronology.(2)Through the Mann-Kendall test and wavelet analysis of the three altitude gradients of Larix gmelinii,it was observed that the radial growth of Larix gmelinii in the study area did not have an obvious mutation,and the significant decrease or increase of the ring width index could be observed.The ring width index of high altitude gradient had the first main period of 64 a and the period of 42 a and 26 a.The ring width index of the middle altitude gradient has the first main period of 39 a and the change period of 23 a and 6a.The ring width index of low altitude gradient had the first main cycle of 23 a and the second main cycle of 10 a,and the fluctuation cycles of radial growth of Larix gmelinii showed a gradually transition decreasing trend.(3)By analyzing the correlation between the radial growth of Larix gmelinii with monthly and seasonal climatic factors,it was found that the radial growth of Larix gmelinii in the Greater Hinggan Mountains was affected by temperature and precipitation.Low temperature was the limiting factor for the radial growth of Larix gmelinii in the high altitude gradient,while the radial growth of Larix gmelinii in the middle and low altitude gradient was mainly restricted by precipitation and drought stress.The radial growth of Larix gmelinii with high altitude gradient was significantly positively correlated with the maximum temperature in August and May of previous year,the minimum temperature in June and August of previous year,the maximum temperature in the growing season of previous year,the maximum temperature in the month before the growing season of previous year,and the minimum temperature in the growing season of previous year and the current year.The radial growth of Larix gmelinii with middle altitude gradient was negatively correlated with the maximum temperature in September of previous year,the maximum temperature in June,July and September of previous year and the maximum temperature in the growing season and the month after the growing season,and was positively correlated with the SPEI in August and September of previous year,the precipitation in June of previous year,the SPEI,the precipitation in the growing season of previous year and the SPEI.The radial growth of Larix gmelinii with low altitude gradient was significantly negatively correlated with the average monthly maximum temperature in October of last year and after the growing season,and was significantly positively correlated with precipitation,SPEI in September of previous year,SPEI in October of previous year,precipitation and SPEI in the postgrowing season of previous year.(4)The sliding correlation analysis between the standard chronology of tree ring width and monthly and seasonal climate factors of the three different altitude gradiants showed that the relationship between the three altitude gradiants and monthly and seasonal climate factors was not stable,and all had " divergent response " The sliding correlation trends of radial growth and climate factors were also different among the three altitude gradiants,which were positively correlated with temperature and gradually increased.Precipitation was positively correlated with the trend of increasing gradually and then decreasing gradually.SPEI was positively correlated with the trend of gradual weakening,indicating that the limit of temperature on the radial growth of Larix gmelinii was gradually enhanced while the limit of precipitation and stress on the radial growth of Larix gmelinii was gradually weakened.(5)Using CRU data set to analyze the standard chronology of ring width of Larix gmelinii with three different altitude gradiations and climate factors,the spatial correlation between radial growth of Larix gmelinii with high altitude gradient and monthly mean minimum temperature in June to August of the same year was found to be moving in the northwest direction.The overall spatial correlation between radial growth of Larix gmelinii with middle altitude gradient and monthly mean maximum temperature in June to August of the same year moved westward.The overall spatial correlation between radial growth of Larix gmelinii with low altitude gradient and monthly mean maximum temperature in September to October of the same year showed a westward trend,which may be related to the activity intensity and time of East Asian summer monsoon. |
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