Introduction: Treeline ecotone complexity might function as a barrier to altitudinal migration of Pinus hartwegii Lindl. Objectives: P. h ar t we g ii forest structure was characterized through its treeline ecotone at Nevado de Toluca; in addition, possible changes in altitudinal limits over the last 25 years were determined. Materials and methods: Height and diameter at breast height (DBH) of each tree were determined, as well as natural forest regeneration and tree density. Altitudinal limits were determined on satellite images from 1989 and 2014 using the moving split window method. Results and discussion: Tree density fell between 8 and 70 % across altitudinal gradient. Altitudinal limits (3,980-4,130 m) did not show significant differences ( P = 0.07) between two evaluated years. The P. h ar t we g ii ecotone displays a diffuse form, regulated by growth limitations. This implies that the forest might modify its altitudinal distribution based on environmental changes, according to its ecological amplitude and how fast new individuals can become established at higher altitudes. Conclusion: Long-term monitoring is important to know if P . hartwegii is actually capable of migrating altitudinally as a result of increased environmental temperature.