AN INTERPRETATION OF LATITUDINAL PATTERNS OF FOREST LIMITS IN SOUTH AND EAST-ASIAN MOUNTAINS

On the mountains in humid E and S Asia, arboreal life-forms at the forest limit depend on their latitude relative to c20°N: to the north there are mostly evergreen needle-leaved trees with some deciduous broad-leaved trees, while to the south, evergreen broad-leaved trees prevail. At the forest limit the annual sum of monthly mean temperatures >5°C is 12-15°C.months, equivalent to a mean temperature of 10°C in the warmest month on temperate mountains, and to annual and monthly mean temperatures of 6-7°C in tropical mountains because of the non-seasonal pattern of temperatures in the tropics. The upper limit of the evergreen broad-leaved forests corresponds to a sum of monthly mean temperatures <5°C of -10°C.months, or a mean temperature of -1°C during the coldest month. In latitudes south of 20°N, evergreen broad-leaved trees reach the forest limit determined by a temperature sum of 12-15°C.months, because the mean temperature of the coldest month exceeds -1°C even at the forest limit. However, north of 20°N, winter temperature at the forest limit falls below the mean of -1°C during the coldest month, and evergreen broad-leaved trees are therefore confined to lower altitudes. Temperate deciduous and boreal coniferous forests, which can tolerate winter coldness below -1°C, develop above the zone of evergreen broad-leaved forests by exploiting the summer temperature sum. Boreal forests also reach the limit determined by the temperature sum during summer, and tolerate severe winter cold. Accordingly, the pattern of altitudinal zonation in the high mountain of humid and S Asia differs N and S of 20°N, mainly due to the seasonality of temperature. The zonation north of 20°N is of the temperate type, with a sequence of tropical, nemoral zone drop out. From 20°N to the equator, there is a tropical zonation type in which evergreen broad-leaved trees of tropical affinity prevail to the forest limit. -from Author