Drought Reconstruction Over The Past200Years In Southern Gansu As Inferred From Tree Rings

Climate change has a huge impact on the ecological balance and even human society, which also has a great scientific value and reality socio-economic value to study. As accuracy dating, high resolution and widely geographical distribution, tree ring has became a ideal proxy date to climate change study.The southern part of Gansu province located in eastern of Tibet Plateau, which belongs to the transition area between the Qinghai-Tibet Plateau and the Loess Plateau, and is the transition zone between the arid and semi-arid as well. Extreme drought events have serious impact on regional economic and social development. We carried out the dendroclimatology research in this region, which is important that understanding the global change and making up for lack of tree ring studies.4sites Pinus tabulaeformis (Chinese pine)(tree/cores,123/248) samples are taken in the Diebu Lazikou in the Gannan Tibetan autonomous. Standard dendrochronology techniques were used to process the sampled tree cores. After air drying and sanding, all samples were cross-dated and measured, and the chronology were developed by the program ARSTAN using conservation methods to fit a negative exponential curve or a straight line of any slope. We get rid of one chronology that is impacted by the human activities and deforestation by analyzing. Finally, we developed one chronology (1798-2011) with segments average methods from the rest of three sample sites tree cores. Correlation analyses were carried out between the three different chronologies (STD, RES and ARS) and the observed precipitation and air temperature records of the nearest Mingxian meteorological station. The results showed that all these chronologies (STD, RES and ARS) positively responded to the precipitation, but negatively responded to temperature.We used the bilinear interpolation method calculate the PDSI grid point around the four interpolated in the study area. The correlation analysis between the STD chronology and the interpolation results showed that the STD chronology are significantly correlated with the PDSI (c3c7), and the value is0.704(p<0.01). According to the results, we established the conversion equation; the reconstruction explains49.5%of the actual PDSI variance during the period from1951to2005, and past a series of stability testing. The result shows eight distinct wet years(≥M+1.17SD, M:mean of reconstruction value; SD: standard deviation, the same below)(1804-1805,1814-1818and1937, respectively) can be identified of Lazikou region over the past214years in our reconstruction, which account for3.7%of the whole reconstruction interval. And nine distinct drought years (1798,1901,1997-2002and2011, respectively) were found in our reconstruction, which account for4.2%of the whole interval,1996-2002is the most extremely drought period. These extreme drought/wet periods were also evidenced by the historical documents.The spatial remote correlations between PDSI and SST shows the PDSI of study region significantly negative correlated (p<0.1) with SSTs of the eastern equatorial Pacific Ocean. Multi-taper method (MTM) spectral analysis indicates the existence of some significant peaks at2-3years,7year and40year in the reconstruction. The2-3years and7year cycles fall within the range of ENSO variability, and the40year cycle correspond to the Bruckner cycle, suggesting the impacts of East Asian summer monsoon (EASM) on droughts in the study region. The southern part of Gansu province tends to be dry as the ENSO is intense, which is consistent with the decline of the EASM.Comparisons of this PDSI reconstruction with other tree-ring studies in surrounding areas and the spatial interpolation using the kriging method suggest that the drought in1930s was more serious for the north of the study region, and it killed tens of thousands of lives. While the drought in1996-2002was particularly significant, with Lzk and Guiqing Mountains as the center and it was gradually reduce to around. Although the extent of the1996-2002drought was severer than that of1930s, there was little casualty in1996-2002drought event. That is because the social productivity has been greatly improved as the rapid development of economic society, the government and the individuals have sufficient capacity to take effective measures to reduce the hazards of the drought, flood and other extreme disasters, and people’s productions and lives can be saved from deadly threat.