Floristic zonation of tree communities on wet tropical mountains revisited

The altitudes of the ecotones between the main vegetation-zones up mountains of the humid tropics are reviewed, with emphasis on floristics and those areas with which the author is personally familiar. A detailed comparison is made of the zonation on Mt Kinabalu (c. 6 degreesN) and the Himalaya of Bhutan (c. 27degreesN), and is supported by information from other parts of SE Asia, and to a decreasing extent South America and Africa. The relationship between forest-types based on structure and physiognomy and forest-types based on floristics is complicated. In SE Asia lower montane forests may be dominated by dipterocarps or an 'oak-laurel' assemblage. The dominant leaf size in both is notophyll, but the oak-laurel assemblage shifts to the nucrophyll size-class in China where the transition from tropical lower montane to warm temperate lowland forest begins. The opposite trend is seen where the lower montane flora of Papua New Guinea corresponds to the lowland flora in NE Australia. There are only modest changes between the Equator and the Tropic in the attitudes at which major transitions occur. It is concluded that soil changes, mediated only in part by temperature and associated climatic factors, may play a more direct and diverse role than has previously been indicated. The soil factors emphasized are the increase in organic matter content in lower montane soils as opposed to lowland, accompanied by a change from termite-dominance to earthworm-dominance, and the frequent occurrence of a dense moss layer and mor humus in upper montane soils. The case is made particularly by reference to east Asian mountains, which have been the most studied.