Ecological Observations on the Rain Forest of Mount Dulit, Sarawak. Part I

Abstract

Evergreen rain forest is the natural climax vegetation of Mount Dulit in NW. Borneo. The virgin forest belongs to 3 main types: mixed forest on heavy clay and loam below 1150 m., heath forest on sandy soils below 1150 m., and montane rain forest or moss forest above 1150 m. The mixed forest is composed of: (1) trees averaging 110 ft., mostly Dipterocarpaceae; (2) trees about 60 ft., representing numerous dicot families and rarely including a few palms; (3) trees about 25 ft., comprising small individuals of spp. forming the higher stories and undergrowth trees among which members of the Anonaceae are important; (4) a layer about 4 m. high comprising young trees. many genera of palms and a variety of dicot shrubs; and (5) a patchy ground flora about 1-2 m. high of tree seedlings, ferns, and herbs of many spp. belonging chiefly to the Begoniaceae, Melastomaceae, Gesneriaceae, Araceae, Zingiberaceae, and Cyperaceae. Many of the bigger trees have plank buttresses. Climbers are abundant and include spp. of Ficus and other dicots, palms, ferns such as Trichomanes sp., and 2 spp. of Gnetum. Epiphytes are much less abundant than in S. Amer. rain forests. The society of shade epiphytes is composed largely of ferns and bryophytes. The society of sun epiphytes consists of herbs and shrubs of which Orchidaceae and Gesneriaceae are most numerous and semi-parasitic Loranthaceae are abundant. Mycetanthe of. lowii was the only parasitic flowering plant found besides the green epiphytic Loranthaceae. A society of small colorless saprophytes includes Epirixanthes elon-gata (Polygalaceae), Sciaphila sp. (Triuridaceae), Gym-nosiphon aphyllum, Thismia sp., and Burmannia sp. (Burmanniaceae). The stratification of this forest is similar to that of evergreen Dipterocarp forests in other parts of the Malayan region and to the mixed forest of Moraballi Creek, British Guiana. In the heath forest the 1st and 2nd stories of the mixed forest are represented by a single uneven layer about 25-35 m. high. Buttressing of trees is slight but stilt-rooted trees are common. Characteristic trees include Agathis sp., Tris-tania sp., and Casuarina sumatrana. Undergrowth is dense, important woody components being Podocarpus nerii-jolius, Polygala venenox, Euthemis leucarpa, and Pan-danus sp. Herbaceous ground flora consists of orchids (Plocoglottis lowii and Coelogync peltaslas) and ferns (Lindsay a borneensis, Syngramme lobbiana, Polypodium metteidanum). Saprophytes are scarce, the commonest being the liliaceous Petrosavia stellaris. Lianes are rare but small undergrowth climbers, particularly Nepenthes, are common. Epiphytes grow at much lower levels than in mixed forest; the sun epiphyte society descends so low that the shade epiphyte society is very limited. Although general appearance of heath forest is unlike that of moss forest, the 2 are much alike, the resemblances depending on similarity of soil. Montane spp. accordingly descend to lower levels in heath forest than in mixed forest. There is a close resemblance in structure and floristie composition between Bornean heath forest and the Wallaba forest of British Guiana although the spp. composing; the 2 forests are totally different. The resemblance is attributed to similarity of soil and climate.