Response Of Tree Ring Width To Climate Change And NDVI Reconstruction In Daxing'an Mountains

Daxing'an Mountains is located in the high latitude permafrost distribution area,and the forest is widely distributed in the region.Because of its strong sensitivity to climate change,many experts and scholars pay attention to and study it.In recent years,the climate tends to be dry and warm,and the forest ecosystem is bound to have a series of changes with the changes of the surrounding environment.Dry and warm may destroy the balance and stability of the forest ecosystem.As a"climate indicator"with simple acquisition and high sample resolution,tree ring can be used to reconstruct vegetation coverage and analyze the response of tree growth to climate change.In this study,larch tree ring samples were collected from Tahe and Xinlin sample plots in Daxing'an Mountains in June 2019.Combined with the monthly average temperature,minimum temperature,maximum temperature,precipitation data of Tahe and Xinlin meteorological stations from 1983 to 2018,as well as the monthly standardized evapotranspiration index(SPEI)and the seasonal surface temperature data from 2001 to2018,the tree ring sample data and precipitation data were calculated Pearson correlation coefficient of each climate factor is used to obtain the correlation,so as to study the sensitivity of tree growth to climate change;taking the vegetation coverage of Daxing'an Mountains as the research object,according to the tree ring width chronology data established by two groups of sample cores,the GIMMS data set from1983 to 2000 and the NDVI monthly data set of CAS from 2001 to 2018 are obtained,trying to explore The spatial and temporal variation of NDVI is studied,and the reconstruction sequence equation of NDVI is obtained.The results are as follows:(1)The response relationship between tree radial growth and temperature and precipitation in historical period is different in Daxing'an Mountains.The response of Tahe sample plot to monthly mean temperature is mainly in summer,and the negative response to monthly minimum temperature is mainly in summer and winter.The new forest sample plot is more sensitive to the change of monthly precipitation in December of the next year and from June to August of that year.(2)The LST of Daxing'an Mountains is always higher in the southeast and lower in the north and southwest,and the annual average value of LST varies greatly in different seasons.The relationship between tree radial growth and LST is different.The correlation between the annual table of Larch STD and the mean LST in summer in Tahe sample plot is strong(R_??=0.506),and the influence of LST on tree growth in new forest sample plot is weak.(3)The main change of SPEI in Daxing'an Mountains was that SPEI index decreased gradually in the second half of the year,showing a trend of drought.The negative response relationship between SPEI index and STD chronology of Larix gmelinii in Tahe region was reflected in the period from December of the previous year to march of the current year,and SPEI was not the main factor affecting the growth of trees in new forest plots.The negative correlation between NDVI and SPEI reached a high level in October(R_??=-0.361,R_??=-0.521).(4)The change of NDVI in Daxing'an Mountains is determined by different growth stages of local vegetation,which is usually low from November of the previous year to march of the current year and high from June to August.After September,the vegetation stopped growing and NDVI decreased again.There are differences in the response relationship between tree radial growth and NDVI in Daxing'an Mountains.Tree growth in Tahe sample area is more sensitive to the changes of NDVI in May,June and July,and the most significant is July.Based on the STD chronology,the July NDVI is reconstructed to obtain the reconstruction equation from 1959 to 2018.Using the5-year moving average to further study the NDVI curve,it is found that the reconstructed sequence has two high value stages and three low value stages.