Ecological Studies on Gunung Silam, A Small Ultrabasic Mountain in Sabah, Malaysia. I. Environment, Forest Structure and Floristics

Abstract

(1) Gunung (Mount) Silam, Sabah is an ultrabasic coastal mountain with primary rain forest from about 200 m to its summit at 884 m. (2) The mean annual rainfall (1971-83) at the base of the mountain was 2011 mm. During two eight-week expeditions in July-September of 1983 and 1984, there was frequently a cloud cap on the mountain. The summit often received less precipitation, had a higher mean windspeed, and received less solar radiation than the base. The lapse rate was relatively low (0.48.degree.C per 100 m). (3) Soil analyses were made at a range of altitudes. The surface horizons were slightly acidic, had a high Mg/Ca quotient and substantial concentrations of nickel. The deeper horizons has much higher Mg/Ca quotients and sometimes very high nickel concentrations. On soils at 790 m and above mor humus had developed. (4) Ten plots (of 0.04-0.4 ha) at altitudes from 280 m to 870 m on a main ridge of the mountain were studied in detail. All trees .gtoreq. 10 cm dbh were enumrated, and structural features of the trees and smaller plants were quantified. Despite the unusual soil chemistry, the forests at lower altitude had a large stature and resembled lowland evergreen rain forest on other substrata. All forest with rich in tree species, ranging from nineteen species in the 0.04-ha plot at 870 m to 104 species in the 0.4-ha plot at 480 m. (5) There was a dramatic change in species composition between 610 m where the Dipterocarpaceae wiere abundant and 770 m where they did not occur. The forest at 770 m and above was stunted and the mountain shows a classic ''Massenerhebung effect" (i.e. the compression of forest zones on a small mountain compared with a large one). Stunting of forests on ultrabasic rocks elsewhere may often have involved the ''Massenerhebung effect'' rather than a soil toxicity. The causes of the stunted forests at higher altitude on Gunung Silam remain unexplained but do not seem to include soil toxicity. (6) Only two tree species found on Gunung Silam, Borneodendron anaegmaticum (Euphorbiaceae) and Buchaniana arborescens (Anacardiaceae) are apparently endemic to ultrabasic substrata. The reasons for the paucity of endemic species compared with their abundance on the well-studied island of New Caledonia are discussed.