A Dendroecological Study In The Northeastern Tibetan Plateau

Since the short record of the environment and climate factors?it is difficult to understand thelong-term mechanism of the climate change. Hence?vairous different paleo-proxies become themost important method of past climate research to overcome this limited shortness. Tree-ring datahas been widely used to reconstruct the past climate change (e.g., temperature, precipitation andPDSI), due to their high resolution, easy to sample, wide spatial distribution and the possibility ofusing simple linear modes of tree growth-climate relationships between tree ring data and climatevariability. The relationships between the tree ring and the climate factors are the fundamentalbase and the prerequisite of the dendroclimatology. The rapid global temperature increasing hascaused a lot of environmental issues and climate extremes during the last half of20th century. Atthe same time, tree line made a response to the rapid temperature increasing during the end of the20th century. More and more investigations found that the trees growing in high latitude and highaltitude of northern hemisphere happened to the divergence of the relationship between the treeradial growth and climate factors. Specially tree radial growth responded to temperature, and theylost their sensitivity to temperature recently; the relationship between tree growth and climatefactors are changing and unstable over the time, and they response to climate depend on therfequency. What is more, some kind of tree species growth responses to the climate changeschanging with the different tree ages.Tree ring data in China, particularly in the Tibetan Plateauhas an indispensable role for study of global change. In order to explore the regional tree ring datahow to response to climate change in the Tibet Plateau area, we select the Qilian Mountains andthe A'nyemaqen Mountains which are sensitive to the climate change as research area in this study.In total, we used29sample site including Qilian juniper^Z'/)/^prezewalskii Kom.)?Chinese^m^(Pinus tabulaeformis Carr) and Picea wilsomi, basing on this dataset, we discussed therelationship between the tree growth and climate factors during past50years, and we analyses thetree line dynamics respond to the climate.Three different tree species tree ring width chronologies from the A'nyemaqen Mountainsand the eastern part of Qilian mountains were analyze age-dependent growth-climate responserelationships. More than120years old Chinese pine are more sensitive to the climate factors thanthe less than70yeas old trees. More than170years old Picea wilsonii trees are correlated wellwith the climate factors, especially correlated well with the PDSI significantly. More than500years old Qilian juniper trees are correlated well with the temperature and precipitation. The oldtrees are more sensitive to the climate change than the young trees. Our results showed that the impact of tree age on tree radial growth is relatively weak, tree age maybe appropriate to considerwhen establishing the chronology, but it is not necessary as an important factors. Overalllong-lived trees are still ideal materials for dendroclimatic reconstruction.We selected6Qilian juniper sample sites and1Chinese pine site rfom east to west in QilianMountains, and discussed the relationships between the tree radial growth and the main climatefactors during the50years. The results showed that the correlation relationships between everysingle tree and climate factors are unstable and changing over time. Moving correlation resultsshowed that the consistency between the every single tree and temperature is significantly reduced,and there is clear response divergence phenomenon, and their relationships appeared a clear twodifferent pattern: positive and negative trend. P. tabulaeformis tree radial growth responses toclimate either full time or moving correlation analysis results are relatively stable, and no responsedivergence. The trees within every sampling sites were divided into two groups by the clusteranalysis based on the correlations between the every single tree radial growth and the last threedecades of the last century. The correlations between the sub-chronologies and the climate factorsshow that there appears divergence phenomenon between them in the past50years. And thedivergence phenomenon was mainly including the STD chronology of the low-rfequency signal,and there is almost the divergence in the including high-rfequency signal RES chronology.In order to study the difference between the upper and the lower tree radial growthresponse to the climate change, we selected10sample sites rfom upper tree limit and the lowertree limit on the A'nyemaqen Mountains. The results show that the tree radial growth are mainlylimited by the moisture in the lower tree limit, and the relationships between the upper tree radialgrowth and climate factors are complicated. The tree ring is not related to climate factors at theMQ and HBS tree limit sites. The tree radial growth is limited by the August temperature and theMay precipitation at the DQ upper tree limit. Tree radial growth are mainly limited by thetemperature at the YYC upper tree limit and precipitation at the HHB upper tree limit.Tree-line ecotones are strongly climatically limited and serve as potential monitors ofclimate change. We employed annual growth increment from tree-rings, tree density and tree agestructure data derived rfom tree-line sites in Qilian Mountains and A'nyemaqen Mountains,northeastern Tibetan Plateau, to detect the response of tree growth and populations dynamics toclimate change. The age structure analysis results showed that the J. przewalskii forest growingappeared at the end of14th century on YYC slope, at the end of15th century on HBS slope and atthe end of18th century on HHB slope. The appearance of trees at the upper treeline limit dates todifferent periods. There was one recruitment peak during the period1800-1900on three slopes and upper limit sites. Tree regeneration and treeline dynamics coincide well with air temperature.High temperature favors tree growth and is associated with increased tree density at tree-line, andan advance in tree-line position. Significantly positive correlations were found between ring-widthand mean monthly air temperature. Tree recruitment began to increase rapidly at all the sites atferthe Little Ice Age. The number of trees established coincides with temperature changes. Thewarming trend atfer the Little Ice Age favors increase of tree density and an advance of tree-line.The majority of trees established during the period, which coincides well with the rapid radialgrowth of the tree.