青藏高原东缘两处高山树线交错带时空动态及其建群种的生态学特征

Spatio-temporal dynamics of two alpine treeline ecotones and ecological characteristics of their dominate species at the eastern margin of Qinghai-Xizang Plateau

热量匮乏是高山树线的主要成因,在全球变暖趋势下对高山树线及其建群种的生态学过程及特征的研究具有重要意义。该文以青藏高原东缘的折多山和剪子弯山两处高山树线(海拔分别为4 265 m和4 425 m)作为研究对象,通过设置垂直样带,同时结合区域温度、降水的长时间序列分析,探究两处树线的时空动态过程,并明确了建群种冷杉(Abies spp.)的生态学特征。结果表明:1)折多山和剪子弯山区域的气温在过去58年均存在显著的上升趋势(分别上升了0.72和0.91℃),而折多山和剪子弯山区域降水均存在微弱的降低趋势。2)折多山的峨眉冷杉(A. fabri)龄级结构呈反J形,剪子弯山的鳞皮冷杉(A. squamata)龄级结构呈双峰形,二者种群结构均相对稳定。3)在小尺度上,种子扩散限制使得两处树线的冷杉聚集分布。在大尺度上,折多山峨眉冷杉亦呈聚集分布,而剪子弯山鳞皮冷杉受生长环境以及种内或种间关系的影响呈随机分布。4)两处样地建群树种的树高和基径均随海拔升高而降低,位于树线交错带上部的冷杉均呈现树高生长大于径向生长的异速生长关系,而位于样地中、下部位的冷杉大部分呈等速生长关系。5)相比10年前,折多山和剪子弯山的树线及树种线位置均无明显变化,剪子弯山鳞皮冷杉种群的树木密度亦无明显变化,而折多山的树木个体数提高了约25%;相比20年前,折多山和剪子弯山的树种线分别上移了50和30 m,树线位置分别升高了75和40 m,树木个体数亦明显增加,分别提高了约220%和100%。树线及其建群种在较大时空尺度上主要受热量的控制,而在较小时空尺度上受温度及生长环境共同作用的影响。

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 the Zheduo Mountain and Jianziwan Mountain of the eastern Qinghai-Xizang Plateau.Top and bottom of each transects were lain between species line and forest line,respectively.Detailed information of each tree species treeline,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 the Zheduo Mountain and Jianziwan Mountain.The age structure of Abies fabri from the Zheduo Mountain and A.squamata from the 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 the 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 elevation.The fir trees(Abies spp.)at the upper part of the treeline ecotone presented an allometric growth,whose height growth rate was higher than that of base growth,while the relationships between height growth and base growth were isometric at almost mid and lower part of the treeline ecotone.Compared with 10 years ago,there was no significant change at the position of treeline and tree species line of the Zheduo Mountain and the Jianziwan Mountain,neither of the tree density in the Jianziwan Mountain.However,the number of trees in the Zheduo Mountain increased by about 25%.Compared with 20 years ago,tree species lines of the Zheduo Mountain and Jianziwan Mountain were shifted upwards by 50 and 30 m,respectively.Besides,their treeline positions were increased by 75 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.