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

Abstract

Chemical, physical, morphological and mineralogical propenies of soils in the toposequence of Gunung Gadut tropical rain forest, West Sumatra, were determined.The study area has an amual rainfal of mere than 5,OOO mm, 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 wel1 as geologic conditions.Soils in four perrnanent plots for study of forest ecology and flora were exarnined in detail.The Pinang Pinang plot is in a foothill Dipterocarp forest on a gentle hill top with a partly narrow and partty broad ridge at 550m altitude.Soils were relatively young Typic Dystropepts devetoped from Quaternary andesite.Although the soils show a prominent red color and strong acidity, adequate nitrogen and base status make them relatively fertile, Clay mherals were kaoliri and 2: 1 type vermiculite.The Gajabuih plot is also in a fbothill Dipterocarp forest on a nomhwestern slope near a ridge with moderately steep to undulating relief at 550m altitude.Soils were Oxic Dystropepts or Orthoxic Tropudults developed from Perrnian shale and phyllite, which had yellowish brown coLor and a well-developed structure.Although levels of nitrogen and avalable 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 hi" oak forest on a broad and undulating ridge at 1,1oo m altitucle.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 rnouritain oak forest on a wide plateau at 1,600 m altitude.Seils were extremely infertite Tropaquods developed from Quaternary volcanic ashes.A thick organic horizon was underlain by bleached and extremely leached subhorizons.The CIN ratios were very high.Clay mherals were allophane and imogolite.Chemical fertMty, especially avalable calciurn, correlated well with tree heights.The upper limits of tree height were estirnated at 119 m for the Pinang Pinang plot, 109 m for the Gajabuih plot, 51 m for the Airsirah plot, and 38 m for the G, Gadut plot, whereas the measured rnaximum heights were respectivety 59 m, 61 m, 34 m, and 20 m.