Spatio-temporal dynamics of two alpine treeline ecotones and ecological characteristics of their dominant species at the eastern margin of Qinghai-Xizang Plateau
(2.University of Chinese Academy of Sciences, Beijing, China 101408)
(3.Key Laboratory of Mountain Ecological Restoration and Bioresource Utilization, Chinese Academy of Sciences, Chengdu, China 610041)
(4.Central Department of Environmental Science, Tribhuvan University, Kathmandu, Nepal)
(5.Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Xishuangbanna, Yunnan, China 666303)
(6.Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China 100101)
(7.International Center for Integrated Mountain Development, Kathmandu, Nepal)
(8.Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China 730000)
【Abstract】 Aims Temperature limit is the main cause of alpine treeline formation. Therefore, it is important to understand the response mechanisms of alpine treeline as well as their tree species under the global climate change. The present study focused on the spatio-temporal dynamics of treeline and ecological characteristics of the tree species in two treeline ecotones. Methods Two vertical belt-transect plots were established in each treeline ecotone of Zheduo Mountain and Jianziwan Mountain of the eastern Qinghai-Xizang Plateau. Top and bottom of each transect were lain between species line and forest line, respectively. Detailed information of each tree species, including species name, latitude, longitude, height, age, base diameter, and coordinates, was recorded accordingly. Important findings The temperatures of the two research areas have increased during the past 58 years. The precipitation has decreased slightly in both Zheduo Mountain and Jianziwan Mountain. The age structure of Abies fabri from Zheduo Mountain and A. squamata from Jianziwan Mountain showed a reversed “J” shape curve and a bimodal shape, respectively. Within the two transects, due to the limitation of seed diffusion, the dominate species showed aggregated distributions at the small scale. At the large scale, A. fabri was aggregated at Zheduo Mountain, while A. squanmata of the Jianziwan Mountain was randomly distributed due to the impact of surrounding environmental factors. Both tree height and base diameter decreased with the increase of altitude. The fir trees (Abies spp.) at the upper part of the treeline ecotone presented an allometric growth, whose height growth was faster than that of base growth, while the relationships between height growth and base growth were isometric at mid and lower parts of the treeline ecotone. Compared with those 10 years ago, there was no significant change at the position of treeline and tree species line of Zheduo Mountain and Jianziwan Mountain, neither of the tree density in Jianziwan Mountain. However, the number of trees in Zheduo Mountain increased by about 25%. Compared with those 20 years ago, the tree species lines of Zheduo Mountain and Jianziwan Mountain shifted upwards by 50 m and 30 m, respectively. Besides, their treeline positions increased by 75 m and 40 m, respectively. Furthermore, the number of trees also increased significantly by 220% and 100%, respectively. Therefore, the treeline and its constructive species are mainly affected by temperature at the large spatio-temporal scale, while influenced by temperature and ambient environment at the small spatio-temporal scale.
【Keywords】 treeline; spatio-temporal dynamics; age structure; allometry; altitude gradient;
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