On account of the low temperatures and short summers, trees do not grow at high altitudes. The region above the timber line is known as the ‘Alpine zone’ in all parts of the world. It is, in many ways, similar to the Arctic regions discussed in the previous chapter. There are certain differences, however. At high altitudes, atmospheric pressure is reduced. Photoperiod is related to latitude rather than to altitude and, at high elevations, the atmosphere is thin. Consequently, insolation is powerful on mountain tops, in contrast to Arctic snowlands where the rays of the sun are strongly filtered as they pass obliquely through the atmosphere.