REGIONAL PATTERNS OF TREE AGE STRUCTURE IN NORTHEASTERN QAIDAM BASIN: CLIMATIC INFLUENCES: A CASE STUDY BY USING METHODS OF ECOLOGY AND DENDROCHRONOLOGY
In this study, a sufficient number samples of Qilian Juniper (Sabina przewalskii Kom.) were collected using quadrats for analyzing the response of the forest age structure to climatic change. The sampling site is along a steep slope of Errige,Ela Mountain,which is a mountain range of Kunlun-Buerhanda Mountains located at the eastern margin of the Qaidam Basin. Tree cores were dated following standard dendrochronological method. For those samples that failed to intercept the pith,the method of initial radial growth was employed to estimate the number of missing rings. Then a mean age to coring height was added to obtain the final tree age. The distribution pattern of the tree ages along the forest slope suggests : (1) An abundance of trees were established around 1200A.D. This is a very prominent feature which is especially obvious in the central section with elevation from 3900m to 4110m; (2) A lack of tree establishments from 1601 A.D. to 1650A.D. with a peak in tree death in the upper section from 4100m to 4210m. The oldest stands at different elevations show that trees might regenerate at lower elevations at the same time when most individuals established. The effects of climate variations on tree age structure were analyzed for the past 1000 years with tree-ring records-derived climatic time series, including the Wulan mean temperature from prior September to current April (T94) and the Delingha total precipitation from prior July to current June (P76). The strongest association between tree regeneration and temperature variation is on 40-year scales with pearson correlation coefficient r = 0. 464 (P> = 0. 030). During the period of 1120A.D. to 1560A.D.,T94 is usually above the average temperature of the past (-7. 34℃),and warm conditions are believed to result in enhanced establishment and survival. When temperature fell below the average, especially in the next 50 years after 1560A.D., less regenerated stands as well as a higher mortality can be observed. The influence of precipitation variation on tree age structure is less clear based on the correlation analysis, although more trees died almost in every transition period when climate shifted from wet to dry conditions. Analyses indicate that warm climate conditions were not only favorable for tree regeneration,but also allow tree establishment at lower elevations. In contrast, the relentless cold and dry climate conditions were inhospitable to trees.