Global warming has strong effects on life history strategies of plants located in the cold regiones such as alpine treeline ecotone. More studies have paid attentions on the shift of leaf traits (e.g. photosynthesis and leafing intensity). However, the trade-off different-age leaves between leaf size and leafing intensity of evergreen plants within treeline ecotone in the future warming scenario is still limited, which is important to understand life history strategies of evergreen plants. In this article, in situ open-top chambers were used to simulate warming effect on A. faxoniana seedlings in the treeline ecotone of Wanglang National Natural Reserve, western Sichuan Province. Our objectives were to test whether the negative and isometric relationship between leaf size (individual leaf biomass and individual leaf area) and leafing intensity (based on stem biomass and stem volume) could be found in different-age stems(current-year, 1 a; 2 a and 3 a), and explore the effects of simulated warming on the relationship between leaf size and leafing intensity. Standardized major axis estimation method was used to examine the scaling relationship between leaf size and leafing intensity within stems at different ages. The results showed that allometric relationships were different when leaf/stem size was expressed as different traits. Constantly, there were negative and isometric scaling relationships between individual leaf biomass and leafing intensity in stems of different ages. The intercept with common slope decreased with increasing stem ages. Long-term warming did not shift the negative and isometric scaling relationships between individual leaf biomass and leafing intensity. Allometric constants of stems at different ages responded differently to long-term warming. Under the same individual leaf biomass, current-year stems within the open-top chambers had lower leafing intensity, in exchange for an increase in the total number of leaves, so that current-year stems had greater plasticity, hence it could adapt to global warming and grew better in the treeline ecotone. Although the effect of simulated warming would decrease with increasing stem ages, the whole plant would still accumulate more biomass. These findings suggested that A. faxoniana seedlings could adapt to global warming by coordinating leaf size and leafing intensity in different-age stems, and provided the evidence for the acceleration of tree growth with climate waming, as changes in plasticity of stems allowed plants to accumulate more biomass.