Zonobiome of the Temperate-Nemoral Climate

Abstract

A temperate climatic zone with a marked but not too prolonged cold season occurs only in the Northern Hemisphere; apart from certain mountainous districts in the southern Andes and in New Zealand, it is absent from the Southern Hemisphere. The phenomenon of facultative leaf-shedding has already been met within the tropics. There the leaves are shed only when the water balance is disturbed by a lenghty period of drought, and this leaf loss decreases water losses (p. 75). Leaf-shedding in the temperate zone, however, is an adaptation to the cold season. It is not facultative, but obligatory, occurring even if the trees grow in a greenhouse where they are protected from the cold of winter. The factor responsible for setting off the change in color of the leaves in autumn, even before the first frost occurs, is unknown. It may partly be due to the decrease in day length. A remarkable fact is that the various species of trees turn color within a very short period of time in central Europe, according to the phenological calendar, between the 10th and 20th of October, with no sharp distinction between west and east or between west and east or between lower or higher situations in the mountains. (Trees near street lamps remain green longer.) An evergreen broad leaf is neither resistant to cold nor to winter drought, that is to say, to sustained temperatures below freezing.