Climatological Study On Tree-ring Stable Carbon And Oxygen Isotopes Over East Part Of The Chinese Loess Plateau

Climate change is related to human survival and development,and the global climate is warming under the influence of human activities.Atmospheric warming makes the frequency of extreme events higher and higher,and North China and the Loess Plateau have become high-risk areas of drought.China’s Loess Plateau is located in arid and semi-arid areas,which is the main catchment and sand source area of the Yellow River.The impact of climate anomalies on the ecological,agricultural and social development in local and even the lower reaches of the Yellow River is often very huge.The climate of the Loess Plateau is largely regulated by the East Asian Monsoon with great temporal and spatial variability.Only the instrumental data with short time span are often insufficient to understand its change law.Tree-ring stable oxygen isotope(¹⁸O)is one of the important climate proxy,which can provide long enough information for the past climate change research,and retain low-frequency climate signals to a large extent.Therefore,we established tree-ring ¹⁸O chronologies using-cellulose of Chinese pines（Pinus tabulaeformis Carr.）at Huanglong of Shaanxi and Beiwudang Mountain of Shanxi in the east of the Loess Plateau(referred to as¹⁸O_HLand ¹⁸O_BWDrespectively).We analyzed its response to climate factors and discussed the temporal and spatial variation of tree-ring ¹⁸O by combining other tree-ring ¹⁸O sequences over the whole Loess Plateau.At the same time,2 tree-ring¹⁸O chronologies are used to reconstruct the variation history of their main climate impact factors.Combined with the results and other tree-ring climate reconstruction sequences in the whole Loess Plateau region,the climate change characteristics of the Loess Plateau and its regional climate driving factors are discussed.Vegetation on the Loess Plateau plays an important ecological role in soil and water conservation and sediment control of the Yellow River.Therefore,this paper established tree-ring¹³C chronologie of Chinese pine in Beiwudang Mountain(hereinafter referred to as ¹³C_BWD),and it is used to calculate the physiological indexes such as intercellular CO₂concentration（c_i）and intrinsic water use efficiency（i WUE）.Finally,the results are combined with several tree-ring indexes such as basal area growth increment（BAI）and ¹⁸O to comprehensively understand its physiological status,and discuss the potential of vegetation in the Loess Plateau to deal with drought risk.This paper finally obtains the following results:1.The main climate factors controlling the change of ¹⁸O_HLand ¹⁸O_BWDare May–September（1951–2019,r=-0.66,p<0.0001）and July–August average Relative Humidity（1961–2013,r=-0.70,p<0.0001）,respectively.The temporal and spatial variations of tree-ring ¹⁸O in the whole Loess Plateau are also greatly affected by climatic dry-wet conditions:in terms of temporal changes,tree-ring ¹⁸O sequences from different places reflect the changes of Relative Humidity or precipitation,and the correlation between each sequence is significant and their change trends are consistent.It can be seen that tree-ring ¹⁸O of the Loess Plateau has the potential of regional grid climate reconstruction by ignoring the differences of tree species and terrain.In terms of spatial variation,the spatial distribution of ¹⁸O average value of Chinese pine in the Loess Plateau is relatively consistent with the spatial distribution of precipitation,showing the distribution characteristics of high in the northwest and low in the southeast.The tree-ring ¹⁸O of Chinese pine mainly reflects the evaporation enrichment degree of leaves in different places on the Loess Plateau.2.Using ¹⁸O_HLto reconstruct the change history of average Relative Humidity from May to September in Huanglong,Shaanxi Province from 1810 to 2019(hereinafter referred to as RH₅₉),and using ¹⁸O_BWDto reconstruct the average Relative Humidity from July to August in Beiwudang Mountain,Shanxi Province from 1765 to 2013(hereinafter referred to as RH₇₈).Looking at the whole Loess Plateau region,the tree-ring based hydro-climate reconstructions show significant correlations between each other,and they all show that 1877,1900 and 1928–1929are extreme drought years.Each reconstruction sequence has an obvious downward trend in recent decades,indicating that the Loess Plateau has experienced a general drying trend in recent decades under a series of changes such as the climate warming and the weakening of East Asian monsoon.In terms of climate driving factors,RH₅₉and RH₇₈can reflect the change of East Asian summer monsoon,and its representative climate dry-wet conditions mainly affected by ENSO（El Ni（?）o-Southern Oscillation）in high frequency and PDO（Pacific Decadal Oscillation）in low frequency.3.The main climate impact factor of ¹³C_BWDis the Relative Humidity in April–August（1961–2013,r=0.77,p<0.0001）.By analyzing the change of c_i,it can be seen that the main influencing factor of ¹³C_BWDchange in other periods is the dry-wet condition.In addition,BAI and i WUE are significantly negatively correlated at high frequency.It can be seen that the main influencing factor of local Chinese pine growth variations at high frequency is dry-wet conditions.BAI and i WUE showed an upward trend from 1850 to 1950.It can be seen that the growth of local Chinese pine was positively affected by atmospheric CO₂fertilization,and the improvement of i WUE mainly came from the improvement of photosynthesis;After about 1950,i WUE increased more rapidly and BAI decreased,indicating that drought restriction reduced stomatal conductance of trees,its negative effect on tree’s growth exceeded the positive effect of CO₂fertilization,and the increase of i WUE was mainly due to the combined effect of the increase of photosynthesis and the decrease of stomatal conductance.Finally,¹⁸O_BWDand ¹³C_BWDhave continuous and significant negative correlation.It can be seen that stomatal conductance is more affected by environmental factors than photosynthetic intensity.To sum up,the physiological indexes of Chinese pine samples in Beiwudang Mountain,Shanxi Province are dominated by the dry-wet climate condition.Under the drought force in recent years,the Chinese pine adopted the growth strategy of reducing stomatal conductance to improve water use efficiency to enhance their potential of dealling with the risk of drought.