Alpine treeline is a valuable indicator of climate change because of its sensitivity to temperature. On Pikes Peak (Southern Rocky Mountains, Colorado), tree density and elevation in the forest-tundra ecotone has increased in the last century, corresponding with a 2°C increase in regional growing-season temperature. The purpose of this study was to provide a detailed analysis of the process of treeline advancement. Spatial clustering within age classes and elevational bands was used to identify harsh environments and track the upper climatic boundary of tree establishment. Overall, clustering (Ripley’s K, p < 0.01, based on boot-strapping) was more prominent at lower elevations and for older cohorts, indicating the upward migration of the climatic boundary. However, the climatic boundary may be advancing more quickly than treeline as the moving edge changed from a clustered to a randomly dispersed distribution over time: from 1868-1940 the moving edge was clearly clustered, from 1941-1976 it showed mixed results, and from 1977-2010 it displayed a random spatial pattern. Treeline advancement also demonstrated a reach-and-fill pattern, with sudden advancement of treeline, followed by a few decades of infill at lower elevations. The reach-and-fill pattern repeated three times in the last 120 years, with exponential increases in tree density, especially in the last 40 years. The recent explosion of growth and the quickly advancing climatic boundary match temporally with a shift from an abrupt to a diffuse edge typology. To my knowledge, this is the first study that examines in detail the process of changing treeline typology of an advancing treeline.