Climate Change Over The Past 400 Years In Northwestern China As Inferred From Tree Rings

Drought variability of northwestern (NW) China is of serious concern due to its significant impacts on local ecosystems and human populations. Tree rings were used in this thesis to reconstruct the spatiotemporal features of the drought variability over the past 400 years. The reconstructions enable scientists and policy-makers to place the recent drought events into a longer context of climate change to evaluate its severity.For these purposes, the author first discussed the "start-fitting" and "end-fitting" problems on the standardization of tree rings. The earlier one indicates that the determinisitive fitting curves (e.g. negative exponential or Hugershoff curves) tends be biased close to the pith. The latter one suggests that any changes in tree growth occurred in a given period are able to change the entire deterministive fitting curve, leading to biased tree-ring indices. It is also suggested to testing the homogeneity of tree rings and climate associations prior to the establishment of a tree-ring chronology.Investigations on tree growth and climate-growth relationships along altitudinal gradients at Xinglong Moutain, Tulugou and Moushigou indicated that tree rings showed negative correlations with temperature and postive correlations with precipitation and higher correlations with drought indices, indicating a drought-stressed pattern. It is, however, not the case for the high-altitude sites over at Anyemaqen and central Qilian Mountain where tree growth shows positive correlations with temperature. Tree rings at the uppermost site at Xinglong Mountain showed negative correlations with Palmer drought severity indices (PDSI). A middle-altitude at Mount Emei showed decrease in drought association due to the acidophilous induced "swamping-type effect".A comprehensive investigation on the spatiotemporal characteristics of tree growth and climate-growth associations was performed on 64 chronologies in NW China. These tree-ring chronologies were classified into 9 groups. The common signal strength between chronologies tends to increase in severe dry or wet epochs for most of the groups. The climate-growth relationships are classified into 8 clusters based on their tree-growth features. Most of the tree rings are close related to drought conditions in growing seasons of current and previous years. The features of climate-growth relationships are associated with to the tree species and climate gradients.Tree rings at Xinglong Mountain and Guiqing Mountain were used to reconstruct the regional PDSI from 1794 to 2003 and for the period 1618-2005, which explain 40% and 49% of the total variance, respectively. The thesis compared 6 drought reconstructions of central high Asia and 7 reconstructions in monsoonal Asia. A PDSI reconstruction of 568 years for Hengduan Mountain was presented at the Hengduan Mountain in monsoonal Asia. Drought variations of the western and eastern parts of the central High Asia show different patterns and drought histories of the southern and northern portions of the monsoonal Asia show divergent patterns.The author used a tree-ring network of 64 chronologies in NW China and vicinity to reconstruct the leading drought variability of the western and eastern parts of central High Asia, including NW China, since 1587 and 1660. Meanwhile, the thesis conducted point-by-point precipitation reconstructions for the 26 stations in NW China. Based on the reconstructed precipitation dataset, singular value decomposition method was employed to study the key linkages between precipitation of NW China and sea surface temperatures in Pacific and Indian oceans.