This study relates climatic parameters and altitudinal position of the ecotone between forest and alpine orobiomes in southern Siberia. Particular emphasis is laid on the upper forest boundary, or alpine timberline, which marks the lower altitudinal limit of the alpine ecotone. The most indicative climatic parameters, i.e. those which are less variable at the upper forest boundary are, in order of importance: a) duration of the growing season with temperatures exceeding 0 °C, and + 5 °C, b) potential evaporation, c) duration of the sinow-covering period, and, d) mean July air temperature. Less informative parameters are: a) length of the period with temperatures exceeding 10 °C. b) sum of temperatures exceeding 0 °C, 5 °C, and, especially, 10 °C. In semiarid areas, the altitude of the alpine ecotone depends both on heat balance and on dryness, so that the upper forest boundary can be higher on north- than on south-facing slopes. Compared with Arctic timberline, the alpine forest boundary has a longer growing season, and, accordingly, higher temperature totals, but somehow lower mean July temperatures.
