The alpine timberline in much of western North America is characterized by a structurally complex transition from subalpine forest to alpine tundra, the forest-tundra ecotone. Trees within the ecotone are typically arrayed across the landscape within clumps or "ribbon forests," elongated strips oriented perpendicular to the prevailing winds. This study describes the structure and reconstructs the development of two study plots, one containing ribbon forest and the other, tree clumps. These data are used to develop a theory of ribbon forest origin and development. The plots are located within a forest tundra area dominated by Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa).