| Over the past 100 years,global climate change has had a profound impact on forest ecosystems.The forest ecosystem of the Qilian Mountains,which located in the temperate arid and semi-arid regions,is an important part of the ecological barrier in northwest China,and plays an extremely important role in maintaining national ecological security and regional stability.Semi-arid forest ecosystems in Qilian mountains are very sensitive to climate change,but so far,there is still lack of systematic research on regional forest dynamics and its response to climate change.In this study,taking the Pinus tabulaeformis in the eastern Qilian mountains of the northwest arid region as the research object,the tree growth and forest dynamics research was carried out on the Pinus tabulaeformis forest communities at different altitudes(2000-2600m)by means of tree rings and field forest community survey methods.First,the tree-ring method was used to establish four tree-ring width chronologies of Pinus tabulaeformis at different altitudes in the eastern Qilian Mountains.Dendrochronology was used to study the radial growth characteristics and tree-ring-climate response of Pinus tabulaeformis at different altitudes,and the response relationship between the radial growth and climate change of Pinus tabulaeformis at different altitudes was clarified.Then,the community structure,spatial distribution,quantitative characteristics,tree recruitment and competition relationship at the level of the community of Pinus tabulaeformis forest were studied to analyze the stability and development trend of Pinus tabulaeformis community,in order to explore the response of forest ecosystem to the climate change and understand the impact of future climate change on tree growth,stand composition and population dynamics at the level of the entire forest community.The main conclusions of the paper are as follows:(1)The four tree-ring width chronologies of Pinus tabulaeformis at different altitudes were established in the eastern Qilian Mountains.Based on the analysis of growth characteristics and tree growth-climate response of each chronology,it was found that the tree growth characteristics and tree-climate response of Pinus tabulaeformis at different altitudes in the eastern Qilian Mountains were not significantly different in the altitude gradient.The growth characteristics are highly consistent.The radial growth of Pinus tabulaeformis trees in the study area has a high consistency,and the response patterns to climate change are basically the same.Moisture is the limiting factor for the growth of Pinus tabulaeformis in this area.(2)The results of the study on the age structure and quantity of Pinus tabulaeformis at different altitudes in the eastern Qilian Mountains showed that the population of Pinus tabulaeformis in Tianwanggou had more young trees and the population was still growing;the population of Pinus tabulaeformis in Tulugou had fewer young trees and more mature individuals.The distribution of population structure is spindle-shaped and the population of Pinus tabulaeformis presents a stable mature forest.(3)The recruitment of Pinus tabulaeformis trees in the eastern Qilian Mountains shows that the oldest trees grew before 1900 s and the trees were recruitment after 1900 s.The recruitment speed was the fastest in 1900-1939 s,,and the number of individual recruitments was large.In the two time periods,1940-1979 s and 1980 s,the recruitment speed showed a slowing trend,the recruitment of the tree decreased,and the slope position of growth decreased.But the recruitment at the lower position still continues.In addition to the influence of population density,temperature may also affects the recruitment of the population.(4)Combined with the method of mathematics and theory,the lightness radius of sample circle of Pinus tabulaeformis in this region is determined to be 7.5 m when analyzing competition.By calculating the competition index of single stands which related to distance of Pinus tabulaeformis in the plo,is found results showed that there was a power function relationship between the competition index and the DBH(diameter at breast height)of the trees,which means the larger DBH of the tree,the smaller competition index. |
