The high-altitude limit of forests, commonly referred to as the treeline, timberline, or forest line, represents one of the most obvious vegetation boundaries. In most cases, however, the transition from the uppermost closed montane forest to the treeless alpine vegetation is not a line, but an ecotone characterized by stand fragmentation and stuntedness. Owing to its high altitude and complex ecological dynamics, the alpine treeline ecotone is very sensitive to climate changes. This ecotone has accordingly been extensively studied, and a growing body of evidence has already revealed that treelines are moving upslope in the Swedish Scandes, North America, and Eurasia. Although there is some photographic evidence of a rapid treeline advance in the Hengduan Mountains on the southeastern Tibetan Plateau, its dynamics have not been studied in detail. To model how alpine forests will adapt under the predicted temperature increase, a thorough understanding of alpine treeline dynamics is essential. In this study, we chose Gongga Mountain (29°20'-30°20'N, 101°30'-102°15'E, 7556 m asl), a typical mountain in the Hengduan Mountains, southwest China, as our study area. Six rectangular plots (each 3000 m², 30 m × 100 m) were established within the natural alpine treeline ecotone on shady and sunny slopes of the Yajiageng area, on the eastern slope of Gongga Mountain. On the basis of a detailed study of the age and distribution of treeline trees (Abies fabri) in the alpine treeline ecotone, the spatial-temporal dynamics of the A. fabri population were analyzed over a 50-year period. The results suggested that the six study plots showed a similar pattern of regeneration dynamics, characterized by a gradually increased recruitment in the last 100 years and an abrupt increase in the last 50 years. However, the position of the treeline has moved only slightly and insignificantly upslope, despite the apparent warming on the east slope of Gongga Mountain. Moreover, we found that there were some differences in the spatial-temporal dynamics of the A. fabri population between the sunny and shady slope. The altitude of the timberline and treeline on the shady slope was (3770.4±6.6) m and (3771±7.7) m, respectively, whereas that on the sunny slope was (3617.9±10.0) m and (3635.7±7.8) m. The altitude of the timberline and treeline on the shady slope was significantly higher than that on sunny slope (152.5 m and 135.8 m for timberline and treeline, respectively). In addition, we also discovered that the growth rate of A. fabri on the shady slope was higher than that on the sunny slope during the early stage of tree growth (tree age below 110 years), but lower during the later stage of growth. Our results showed that the thermal limitation hypothesis alone could not explain the present treeline pattern in the Yajiageng area. In addition to temperature, other environmental factors might also affect the formation of the treeline in Yajiageng. Hence, in the case of non-climatic climax treelines, such as those in our study area, both climate-driven model projections of future treeline positions and the use of the treeline position for bioclimatic monitoring should be used with caution.