Abstract Spatial biotic positive feedback, that is, where trees improve microclimatic conditions for other trees, can influence ecotone patterns and processes. We examined the effects of positive feedback between pattern and tree establishment using computer simulation models validated against a dendrochronological reconstruction of subalpine forest advancement on Lee Ridge in Glacier National Park. We used a hybrid simulation that combines physiologically mechanistic and forest-stand models to represent multispecies interactions between spatial pattern and process, mechanistic responses of tree growth to climate, and changing environmental and vegetative conditions. Feedback was incorporated into the model as an increase in site quality due to microclimatic amelioration by upwind neighbors, primarily through snow deposition. The model was able to produce observed forest advancement rates and patterns, but did not adequately predict stand basal area or density, probably due to unrealistic mortality rates and competition for light. The results suggest that feedback is an important process controlling vegetation patterns and dynamics at treeline ecotones that may affect their sensitivity to climate change. Treeline ecotones in changing environments exhibit unique growth and mortality characteristics, as well as changes in the relative importance of endogenous and exogenous controls that must be incorporated into simulations of their dynamics.