An Interpretation of Latitudinal Patterns of Forest Limits in South and East Asian Mountains

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Abstract
On the mountains in humid East and South Asia, arboreal life-forms at the forest limit depend on their latitudes relative to c. 20.degree.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. These contrasting patterns relate to temperature conditions both at the forest limit and at the upper limit of evergreen broad-leaved forests. At the forest limit the annual sum of monthly mean temperatures above 5.degree.C is 12-15.degree.C months; this is equivalent to a mean temperature of 10.degree.C in the warmest month on temperature mountains, and to annual and monthly mean temperatures of 6-77%C in tropical mountains because of the non-seasonal pattern of temperatures in the tropics. On the other hand, the upper limit of the evergreen broad-leaved forests corresponds to a sum of monthly mean temperatures below 5.degree.C of 10.degree.C months, or a mean temperature of -1.degree.C during the coldest month. In latitudes south of c. 20.degree.N, evergreen broad-leaves trees reach the forest limit determined by a temperature sum of 12-15.degree.C .cntdot. months, because the mean temperature of the coldest month exceeds -1.degree.C at the forest limit. However, north of c. 20.degree.N, winter temperature at the forest limit falls below the mean of -1.degree.C during the coldest month, and evergreen broad-leaves trees are therefore confined to lower altitudes. The other two forest types, temperate deciduous forests and boreal coniferous forests, which can tolerate winter coldness below -1.degree.C, develop above the zone of evergreen broad-leaved forest by exploiting the summer temperature sum. Boreal forests also reach the limited determined by the temperature sum during summer, and tolerate severe winter cold. Accordingly, the pattern of altitudinal zonation in the high mountain of humid East and South Asia differs north and south of c. 20.degree.N, mainly due to the seasonality of temperature. The zonation north of c. 20.degree.N is of the temperature type, with a sequence of tropical, nemoral and boreal floristic elements; with increasing latitude, first the tropical and then the nemoral zone drop out. From 20.degree.N to the equator, there is tropical zonation type in which evergreen broad-leaved trees of tropical affinity prevail to the forest limit. This mainly latitudinal boundary of vegetation pattern corresponds to the boundary between the Holarctic and Palaeotropic floristic kingdoms. Climatically this boundary corresponds to a 10.degree.C annual range of monthly tempertures: this annual temperature range corresponds to the northern limit of the evergreen broad-leaved forests at the forest limit.