Ecological studies on rain forests at three altitudes on Bukit Belalong, Brunei

Abstract

Altitudinal zonation of rain forests was investigated on Bukit Belalong (913m), Brunei. Mean annual rainfall was 4100mm at 45m and 5500mm at 913m. Mean annual temperatureswere 25.7°C (45m) and 21.8°C (913m). Three 0.25ha plots were set up at each of three altitudes. At 200m and 500m there was evergreen lowland rain forest and at 850m there was lower montane rain forest (LMRF). The Dipterocarpaceae had the highest proportion of basal area throughout, but their importance declined in the LMRF where the Fagaceae, Myrtaceae and Lauraceae were increasingly important. Soils in the LMRF were more organic and had higher concentrations of total nitrogen and phosphorus and the soils from 500m were the most acid and least base saturated. Rates of nitrogen mineralisation and soil concentrations of inorganic nitrogen did not differ significantly among altitudes. The rates (t ha-1 yr-1) of total small litterfall and leaf litterfall were significantly lower in the LMRF (10.6 and 7.9 at 200m; 10.5 and 7.9 at 500m; 8.3 and 6.0 at 850m). Litterfall nutrient concentrations were similar among altitudes, but smaller quantities of litterfall nutrients were cycled at 850m. The mass (t ha-1) of the small litter layer was similar throughout (5.2 at 200m; 6.1 at 500m; 5.2 at 850m) but leaf litter kL values were lower at 850m (2.4 at 200m; 2.4 at 500m; 2.0 at 850m). Fine root (<5mm) mass (t ha-1) in the top 100cm of soil was 8.3 (200m); 12.0 (500m); 10.6 (850m). Rates (t ha-1 yr-1) of fine root growth (estimated by ingrowth bags) were 0.9 (200m); 2.2 (500m); 0.5 (850m). A bioassay experiment using rice was made at 30m and 913m. Nutrients were more limiting in the montane soil, but climate was of overriding importance for rice growth. It seems that the LMRF is not nutrient limited and the lower temperatures at 850m are the primary cause of the change in species composition and reduction of stature there.