| With the global climate change, more and more attentions were paid to the regional climate variation and forest ecosystems response to the cureent climate changes. However, these studies were constrained by the limited number of regional meterological stations and forestry monitoring data, the short lengths of time, as well as the uneven spatial distribution of these observing stations. This limitation could be overcome by using the climate proxies (e.g. tree rings). Tree rings are not only important climatic proxy-indicator, but also are ideal carrier to study the forest ecology. Forestry in the Qilian Mountains of northwestern China play important roles in protecting regional ecological environment, regulating hydrological regime and irrigating the Hexi Corridor oasis, as well as preventing the expansion of desert. Therefore, studies of the history climate change and trees growth response to the climatic change in Qilian Mountains could provide perspective for understanding the current climate conditions and help make operational strategies about forest management. In the present dissertation, based on the tree-ring records collected from the Qilian Mountains, the studies of regional climate change and forest tree growth response to climate change were conducted from the dendroclimatology and dendroecology point, respectively. Tree-ring based past climatic variability history reconstructions were done in the western and eastern Qilian Mountains, where few tree-ring chronologies were developed previously. By using the forest process-based model (3-PG model), we tried to explain the cause and mechanism of the recent increased growth of a Qinghai spruce forest. Finally, the tree radial growth characteristics and climate-growth relationships along altitudinal gradients, among different study sites and tree species, were examined by using tree-ring records from three different mountain slopes. The main conclusions of this dissertation are summarized as follows:(1) Tree-ring based 850-year reconstruction of drought history in the western Qilian Mountains.The reconstruction suggests a relatively dry phase from the 15th century to the 18th century during the Little [ce Age (LIA), and a relatively wet period over the past two centuries. There were five relatively wet periods and four drought periods over the past 850 years (AD1161-2010). The three mega-drought periods (AD 1260s-1340s,1430s-1540s and 1640s-1740s) recorded by the tree-ring series correspond to the Wolf, Sporer and Maunder solar activity minimum periods. Results of the multi-tape method analysis and wavelet analysis further confirmed the relationship between regional hydroclimate variability and solar activity forcing at the cycles of 60-70 years and>100 years.(2) July temperature reconstruction in the eastern Qilian Mountains over the past 225-year.The reconstruction indicated five warm periods and six cold periods over the past 225-year in this region. The longest cool period occurred in AD1894-1916, while the period of AD 1990-2009 is the longest relatively warmer period. There is a dramatic increasing trend of temperature since 1900, with a slope of 0.13℃ decade-1. Comparisons of the current reconstruction with other independent tree-ring based temperature reconstructions from the northeastern Tibetan Plateau suggest similar variance patterns in these warm season temperature reconstructions regardless of the positive or negative correlation relationships between tree-ring records and temperatures. However, the positive correlation relationships based temperature reconstruction simulated the warming trend during the past one century better. The results of comparisons also indicated that the warm season temperature variances lag behind the winter temperature variability by about 20 years in this region.(3) Process-based model simulations of the increased growth of a Qinghai spruce forest in the eastern Qilian Mountains.Dendrochronological studies of tree rings in a Qinghai spruce (Picea crassifolia) forest in the eastern Qilian Mountains indicate an unprecedented 60% increase in tree-ring growth between 1980-2009 compared with any time since 1785, Over the same period, the maximum, minimum, and average temperatures all increased by nearly 2℃ during the growing season (May through September), the frequency of frost decreased 18 days, precipitation remained unchanged, while atmospheric concentrations of CO2 increased by 48 ppm. To explain how the changes in climatic variables might cause the increase in tree growth, we parameterized a process-based growth model (3-PG, Physiological Processes Predicting Growth) with values from the literature and performed a series of sensitivity tests. The results of our analysis indicated that a reduction in frost frequency during the growing season, which allows stomata to remain open, enhanced gross photosynthesis by 42%. Up to a 20% increase in PG could be attributed to rising atmospheric CO2 between 1980 and 2009. The increase in average and maximum temperatures had little direct effect on gross photosynthesis and tree growth.(4) Accessing the influences of elevation, tree species, climate change and other factors on tree-ring records.Comparing the radial growth characteristics and climate-growth relationships at four Qilian Juniper sites with different elevations in the middle of Qilian Mountains, we found that there were gently differences in the tree-ring parameters and climate-growth relationships. However, the climate-growth relationships were unstable. With an increase in temperature, the sensitivity of tree growth to temperature variation had decreased over the past few decades. The analysis of five Qinghai spruce chronologies’ statistical parameters and climatic-growth relationships along an altitudinal gradient indicated that, trees grown in the middle elevation (-3100m) suggested the highest mean sensitivity, interseries correlation and the percentage of absent rings, as well as maximum age. The tree growth patterns and growth-climate relationships at different altitudinal sites were very similar, and indicating that tree growth in this region was strongly influenced by drought conditions, and the correlation relationships were stronger at the low-frequency more than at the high-frequency variation. Accessing the relationships between tree species, elevation, climate and tree-ring growth in the eastern Qilian Mountains based on the tree-ring records of Qinghai spruce and Qilian juniper trees, we found the trees grouped according to the sampling sites. Trees from the Qinghai spruce sites showed similar growth patterns and climate-growth relationships, even though the elevation differed. Qilian juniper chronologies from the high and low elevation sites revealed different growth patterns during recent decades, which was closely related to the discrepant climate-growth relationships with temperatures and the recent warming trend. The differences in tree growth patterns between two species and elevations could be attributed to the plant physiological traits. |
