The Tree-ring Based Study On Response Of Typical Timberline Tree Species In The Qinling Mountains To Climate And Regional Temperature Reconstruction

Global warming,as the main feature of climate change,has increasing influence on human life and ecological environment,which has attracted wide attention from all over the world.The Qinling Mountains serve as an important geographical demarcation line between northern and southern China as well as the water source for the middle route of South to North Water Diversion Project.At the meantime,it is also a climate-sensitive and ecological-fragile district.Tree-ring,as an important proxy for climate change researches,is very important for ecosystem response analysis and climate change prediction in the future.In this paper,on the basis of more than 200 tree-ring cores of Larix chinensis and Abies fargesii from Mount Taibai and Niubeiliang timberline within the hinterland of Qinling Mountains,combined with the instrumental meteorological data,the NDVI data and the regional DEM data of the study area,we utilized the spatial interpolation analysis,linear regression analysis,correlation analysis,periodicity analysis and other research methods,attempt to reveal the response pattern of timberline typical species in Qinling Mountains under the background of climate change and extend the climate,vegetation history of Qinling Mountain timberline.Through this study,the following conclusions and progresses have been obtained:?1?Larix chinensis from the northern aspect of Mount Taibai is more sensitive to climate change compared with that of the southern aspect.The radial growth of Larix chinensis is mainly promoted by temperature of spring and summer,while inhibited by precipitation of corresponding period.The radial growth of Larix chinensis at Mount Taibai is mainly controlled by the early spring-summer air temperatures and precipitations of corresponding period.However,due to the environmental factors such as topography and sunlight,radial growth of the Larix chinensis in different slopes were mainly affected by different aspects of the climatic factors from different months.Residual chronology was correlated positively with mean air temperature during January to June on the northern slope,while only with that during May to June on the southern slope.The Larix chinensis' growth on both northern and southern slopes were correlated positively with the precipitation occurred in June of the previous year.However,the tree-ring width indices had a significant positive correlation with precipitation occurred in August of current year on the northern slope,while had a significant negative correlation with the mean precipitation from January to April of current year on the southern slope.Regression analysis showed that the change in tree-ring width of Larix chinensis from northern aspect was due to the variations in temperature than in precipitation in Mt.Taibai.Moreover,the tree-ring width residual chronology was more sensitive to temperature on the northern slope than on the southern slope.?2?With the rise of elevation,the response sensitivity of Larix chinensis to temperature strengthened first but then weakened,while its response sensitivity to precipitation increased gradually.The main climatic factors that affect the radial growth of Larix chinensis at different altitudes are clearly different,it is temperatures in February and June for the low altitude,temperatures in June and the precipitation in August for the middle altitude,the previous October precipitation for the high altitude.As altitude gets higher,the growth response sensitivity of Larix chinensis to temperature decreased while to precipitation increased.At the seasonal scale,the temperature has the most obvious effect on radial growth of Larix chinensis in the middle altitude,while the precipitation has the most prominent inhibitory effect on the radial growth of Taibai at high altitude.The response of lower altitude Larix chinensis to air temperature and precipitation is relatively weak.?3?The radial growth of Abies fargesii at Niubeiliang timberline is mainly controlled by the coupling effect of temperature and precipitation at the beginning of growing season in February.In February,relative higher temperature and precipitation that does not exceed a certain threshold is the ideal condition for Abies fargesii to grow.Temperature in spring and summer is the dominant factor for the radial growth of Abies fargesii.The average temperature in different periods from January to August,especially from January to February has positive effects on the growth of Abies fargesii.The mean temperature from previous October to November also had effect on matter accumulations of Abies fargesii.Precipitation is not the major limiting factor for the growth of trees,however,high precipitation occurred in January to February might slow down the organization of formation for Abies fargesii,whereas,the precipitation from February to April had positive effect on the accumulation of photosynthesis product.With regression model,the fact that the variables in the tree-ring annual width are due to a mutual and counterbalance effort of air temperature and precipitation of growth season had been proved via the analysis of typical year,the possibility that the Abies fargesii forest may migrate to higher altitude also had been forecasted.?4?With the increase of altitude,the response sensitivity of Abies fargesii to temperature increased gradually,but there is no obvious patterns in its response to precipitation along the gradient.With the rise of elevation,the sensitivity of tree growth to climate factors are higher than that of the low elevation,however,the consistency of the series and the amount of the climate information showed the tendency of first decreased and then increased.In general,the arboreal radial growth is more sensitive to temperature than to precipitation,tree-ring width indices' correlation coefficients with temperature is increasing with the rise of elevation,of which sensitive period is from February to August in current year;Whereas,based on different altitudes and habitats,the chronology of high elevation presents a significant positive correlation with the air temperature of August in current year,while the chronology of middle elevation presents a significant positive correlation with the average precipitation from March to April in current year.Under the influence of the lagging effect of temperature in previous year,the chronology of low elevation presents a significant positive correlation with the average precipitation from January to February in current year.The contribution of each factor to the low,medium and high altitude model was evaluated by regression analysis,and the comprehensive effect of multiple climatic factors on the growth of trees was visually reflected.The variation trend of contrast significantly factor revealed that climate change could promote growth season of Abies fargesii in advance,but growth dynamic of trees varies at different elevations.The results above showed that at the timberline of the study area,Abies fargesii growth from different altitudes may had different response mechanisms to global change.?5?Tree-ring width-climate relationship between the two typical timberline species in Qinling mountains exist significant differences.The dominant climatic factors of Abies fargesii radial growth are mainly during January-February,but it is March to June for Larix chinensis.With the increase of altitude,the effect of temperature on the Larix chinensis radial growth has gradually weakened,while it is opposite for Abies fargesii.Spring and summer are the critical periods for the two typical tree species to grow,during which both temperature and precipitation are the main factors that influence tree ring growth.However,the factors that controlling tree-ring width are not exactly the same.On the whole,the growth of Abies fargesii is mainly affected by the temperature in winter,while the growth of Larix chinensis is mainly impact by the temperatures during early growth season.With the increase of altitude,the response sensitivity of Abies fargesii to temperature has rised,while the response sensitivity of Larix chinensis to temperature first increased but then decreased.Gradually,temperature that has significant promoting effect on higher elevational Abies fargesii is increasing,while precipitation that has inhibitory effect on the higher elevational Larix chinensis is decreasing,thus,this warming and drying trend may force timberline to climb higher.?6?The March-June temperature at Mount Taibai timberline has been reconstructed for the past 144 years,which revealed that the Larix chinensis timberline experienced 3 cool and 3 warm periods during 1870-2013.Climate growth response analysis revealed that the air temperature,particularly from March to June,was the principle factor that limited the radial growth of L.chinensis.By using dendrochronology techniques,we obtained a reconstruction of the March to June mean air temperature for the timberline of Mount Taibai over the period from 1870 to 2013.The reconstruction captured 57.2%of the temperature variance?55.5%after the degree of freedom is adjusted?after removing the tree-ring width values of 1997 and 1998 during the calibration periods.On the decadal scale,the pattern of cool-warm fluctuations exhibited frequent repetitive features.,On average,the cold periods?16 years?always persist longer than the warm periods?10.8 years?.The years 1870—1881,1903—1918,and 1977—1996 were colder periods,whereas the years 1882—1892,1919—1929,1950—960,and 1997—2013 were relatively warmer.The temperature variations indicated three quasi—periodic changes on scales of 22—31a,18—22a and 10—13a,which suggested that the Mount Taibai region may be associated with a huge—scale climate drive and solar activity.Our reconstruction exhibited good consistency with local historical records and reconstruction results from similar climatic regions for the same intervals.?7?The July NDVI during recent 146 years at Mount Taibai timberline has been reconstructed,which revealed 5 tense periods and 6 sparse periods from AD 1868-2013,which is closely related to the local dry and wet conditions.On the basis of the remarkable positive correlation between the July NDVI and tree-ring width,we reconstructed the annual maximum NDVI for Mt.Taibai over AD 1868—2013.The trans-function accounts for 52.8%of the observed NDVI variance during the period 1991—2013.5 dense vegetation coverage periods occurred from AD 1885₁896,1922—1932,1953—1964,1974—1980 and 2001—2007,while 6 sparse vegetation coverage periods occurred from AD 1868—1885,1907—1921,1933—1937,1948—1952,1965—1968 and 1981—2001.The NDVI reconstruction reasonably and positively correlated with the north-central China precipitation while negatively with Dryness/Wetness Index at decadal time scales.The spectral analysis indicates the existence of 2—3a,2—4a,5—7a and 7—11a cycles,which may potentially reflect the links between the local NDVI evolution and larger-scale climate drives.