timberline, upper limit of tree growth in mountainous regions or in high latitudes, as in the Arctic. Its location depends largely on temperature but also on soil, drainage, and other factors. The mountain timberline always would be higher near the Equator than near the poles if it were not for the abundant rainfall in equatorial mountainous regions, which lowers the air temperatures. The timberline in the central Rockies and Sierra Nevadas is around 3,500 metres (11,500 feet), whereas in the Peruvian and Ecuadorian Andes it is between 3,000 and 3,300 metres (10,000 and 11,000 feet). In much of the central and southern Rockies there is a double timberline: the usual high timberline below which there is a belt of normal tree growth; and then a low timberline below which no trees grow because of low precipitation and high temperatures.
Because the climatic zone in which the high-latitude timberline occurs is almost entirely over water in the Southern Hemisphere, this timberline exists only in the Northern Hemisphere. It crosses northern Siberia, Alaska, and Canada, and far-northern Scandinavia. Several climatic isopleths (imaginary lines connecting points of equal values for various climatic variables) have been proposed as quantitative approximations of this timberline. The Köppen–Supan line was devised by the Austrian geographer Alexander Supan (1879) for this purpose and was used by Köppen (1900) as the boundary between the tundra and tree climates in his first climatic classification; it connects points with an average temperature of 10° C (50° F) for the warmest month of the year. A similar isopleth, the Nordenskjöld line proposed by the Swedish geographer Otto Nordenskjöld (1928), is the line along which the warmest month’s average temperature is equal to (9 - 0.1k)° C, or (51.4 - 0.1k)° F, in which k is the average temperature of the coldest month.