Soils in the Toposequence of the Gunung Gadut Tropical Rain Forest, West Sumatra

Abstract

Chemical, physical, morphological and mineralogical properties of soils in the toposequence of Gunung Gadut tropical rain forest, West Sumatra, were determined. The study area has an annual rainfall of more than 5, 000mm, among the highest in the world, and no real dry season. No water deficit in the soil was found throughout the year. A reconnaissance soil survey revealed a close relation between the distribution of soil types and the topography as well as geologic conditions. Soils in four permanent plots for study of forest ecology and flora were examined in detail. The Pinang Pinang plot is in a foothill Dipterocarp forest on a gentle hill top with a partly narrow and partly broad ridge at 550m altitude. Soils were relatively young Typic Dystropepts developed from Quaternary andesite. Although the soils show a prominent red color and strong acidity, adequate nitrogen and base status make them relatively fertile. Clay minerals were kaolin and 2 : 1 type vermiculite. The Gajabuih plot is also in a foothill Dipterocarp forest on a northwestern slope near a ridge with moderately steep to undulating relief at 550m altitude. Soils were Oxic Dystropepts or Orthoxic Tropudults developed from Permian shale and phyllite, which had yellowish brown color and a well-developed structure. Although levels of nitrogen and available calcium seemed to be adequate, Mg was relatively poor in comparison with Ca. Clay minerals were predominantly kaolin and gibbsite. The Airsirah plot is in a hill oak forest on a broad and undulating ridge at 1, 100m altitude. Soils were highly leached and strongly acid Tropudults developed from Quaternary volcanic ash. Soil fertility was low. Clay minerals were spheroidal halloysites. The G. Gadut plot is in a mossy mountain oak forest on a wide plateau at 1, 600m altitude. Soils were extremely infertile Tropaquods developed from Quaternary volcanic ashes. A thick organic horizon was underlain by bleached and extremely leached subhorizons. The C/N ratios were very high. Clay minerals were allophane and imogolite. Chemical fertility, especially available calcium, correlated well with tree heights. The upper limits of tree height were estimated at 119m for the Pinang Pinang plot, 109m for the Gajabuih plot, 51m for the Airsirah plot, and 38m for the G. Gadut plot, whereas the measured maximum heights were respectively 59m, 61m, 34m, and 20m.