Climate-tree Growth Interactions And Reconstruction Of Winter-spring Temperature In Middle And Lower Reaches Of Yangtze River

As Global warming has been the truth, the frequence of extreme events incesases sharply. The radial growth of trees is not much sensitive in southeastern of China because of its humid climate. So the researchs about dendroclimatology is still deficient. Over the recent years, increasingly more attention has been paid on the dendroclimatology studies in the middle and lower reaches of Yangtze River. Currently, most existed studies mainly focused on the linear climate-growth relationships and the past climate reconstruction in single site. Information on the nonlinear climate-growth interactions, the nonlinear trend of climate variability and its driving forces are rarely reported. In this paper, we use the newly-developed and preexisting tree-ring records to explore the nonlinear climate-growth interactions. Based on the instrumental climate dataset, the complexity and the nonlinear tendency of regional temperature fluctuations are also investigated. Finally, the tree-ring based winter-spring temperature reconstruction is conducted to reveal the related driving forces of extremely cold events across time and space.Combined with the existing tree-ring chronologies, the tree-ring records in Shennongjia(Abies fargesii Franch)AD1843-2013、AD1834-2013, Huping Mountain(Pinus massoniana) AD 1830-2013 and Zhangjiajie(Pinus massoniana) AD1883-2013over the middle and lower reaches of Yangtze River are used to examine the nonlinear climate-growth relationships by using the boosting regression tree model (BRT). The results exhibit that temperature in February, March and April seems to be the dominant climatic limitations for regional tree growth. The nonlinear response has a logistic function which is consistent with the principle of the ecological amplitude. The results also reveal that the accumulation of the organic matter in winter can "compensate" tree growth in springtime and the former tends to be more important for the information of tree rings.Before conducting the ring-based winter-spring temperature reconstruction, it is vital importance to check the temperature variations over the instrumental periods. We investigate the complexity and the nonlinear trends of the regional winter-sping temperature variability by using Higuchi Fractal Dimension (HFD) and Ensemble Empirical Mode Decomposition (EEMD). We find that the disparity of the complexity of temperature records exists in different regions. The HFD values of temperature variations in coastal areas are higher than that in inland areas, the reason of which may be related to the more frequent ocean-atmosphere interactions in coastal regions. The EEMD analysis reveals that the regional winter-spring temperature variability basically maintained a downward trend before 1980. Thereafter, the temperature in more and more regions started to increase remarkably and the entire region are experiencing a warming climate. In addition, we find that the warming rates in high-latitude area were faster than low-latitude area which may associate with the location of the Hadley circulation sinking tributaries.Finally, a new robust tree-ring chronology is developed from Abies fargesii Franch wood at two sites in the Shennongjia Mountains over the western part of Central China. This chronology exhibits significant (at 0.01 level) positive correlations with temperature in February, March, April and June. The highest correlation (0.64) is observed between tree rings and February-June temperature, suggesting that tree growth anomalies tend to manifest winter-spring temperature signals back to 1842. In total, six relatively coldperiods (i.e.1848-1859,1869-1874,1888-1900, 1927-1932,1938-1973 and 1979-1994) are found in the chronlogy, which is in good agreement with historical documents nearby in the eastern parts of Central China. The decadal-scale winter-spring temperature variations cohere with that of the Qinling Mountains over the warm-temperature China before 1870s and the South/Southeast China over the subtropical China after 1920s. Comparing the tendence of the three chronologies, we find that the temperature in Shennongjia has a earlier time and a greater increase to become warm than that in South/Southeast. This may be relative to its high elevation and low heat capacity. The composite analysis of 500hPa geopotential height fields reveals that cold interval is triggered by obvious high ridges over the Ural Mountain.