The vegetation over ultramafic rocks in the tropical far east

Abstract

The tropical Far East has many outcrops of ultramafic rock including very large areas in Sulawesi and New Caledonia. The outcrops occur under many different climates and give rise to soils of a very wide range of chemical compositions. The vegetation of the ultramafic soils is poorly known but is very varied. At one extreme are stunted grassland and shrublands, at the other, species-rich large-stature rainforests. In general the causes of this variation remain unexplained. The best investigated area by far is in New Caledonia. Most of the ultramafic soils in that country are covered by a more or less bushy evergreen sclerophyllous vegetation, whilst about 10% of the soils bear evergreen rainforests. There are a very large number of species restricted to ultramafic soils in both these vegetation types. The vegetation overlying the ultramafic soils in New Caledonia is not necessarily lower nor more sparse than that on different substrates nearby. Foliar analyses have shown that a high proportion of the species on New Caledonian ultramafic soils have an unusual chemical composition. About 21% of the species had more than 1,000 µg g-1 manganese, 5% of the species had more than 1,000 µg g-1 nickel, including nine species with more than 10,000 µg g-1 nickel. Detailed information for vegetation on ultramafic soils elsewhere in the tropical Far East is available only for Gunung Silam, a small mountain in Sabah. Malaysia. The vegetation of Gunung Silam is species-rich and ranges from large-stature lowland evergreen rainforest to small-stature lower montane forest. Forest stature there seems to be determined more by altitudinal effects (which might include hydrological effects) rather than by any soil toxicities. In contrast to New Caledonia only two tree species on Gunung Silam are restricted to ultramafic soils. A further contrast is provided by studies of leaf chemical composition since Gunung Silam has only five species reported to have more than 1.000 µg g-1 manganese and only one with more than 1,000 µg g-1 nickel. Very little experimental work has been made on the plants of ultramafic soils of the tropical Far East.