CLIMATIC DEPENDENCE OF THE ECOTONE BETWEEN ALPINE AND FOREST OROBIOMES IN SOUTHERN SIBERIA

This study relates climatic parameters and altitudinal position of the ecotone between forest and alpine orobiomes in southern Siberia. Part icular emphasis is laid on the upper forest boundary, or alpine timber line, which marks the lower altitudinal limit of the alpine ecotone. T he most indicative climatic parameters, i.e. those which are less vari able at the upper forest boundary are, in order of importance: a) dura tion of the growing season with temperatures exceeding 0 degrees C, an d +5 degrees C, b) potential evaporation, c) duration of the snow-cove ring period, and, d) mean July air temperature. Less informative param eters are: a) length of the period with temperatures exceeding 10 degr ees C, b) sum of temperatures exceeding 0 degrees C, 5 degrees C, and, especially, 10 degrees C. In semiarid areas, the altitude of the alpi ne ecotone depends both on heat balance and on dryness, so that the up per forest boundary can be higher on north- than on south-facing slope s. Compared with Arctic timberline, the alpine forest boundary has a l onger growing season, and, accordingly, higher temperature totals, but somehow lower mean July temperatures.