Preliminary studies of altitudinal zonation and nitrogen supply on Bukit Belalong, Brunei Darussalam

Abstract

Forest structure is briefly described from plots at 200 m, 500 m and 850 m on Bukit Belalong (913 m). Evergreen lowland rain forest is found at 200 m and 500 m with tall lower montane rain forest at 850 m. Automatic weather stations at the base and the summit suggest that the summit receives 20% more rainfall and mean temperatures there are 3.9 °C lower. A cloud cap does not usually form and mean daily totals of solar radiation are similar at the base and summit. Total small litterfall was measured for one year and the following values (t ha⁻¹ yr⁻¹) were recorded: 10.56 (200 m), 10.48 (500 m), and 8.26 (850 m). Leaf litterfall values (t ha⁻¹ yr⁻¹) were 7.88 (200 m), 7.89 (500 m) and 6.02 (850 m). Nitrogen concentrations (mg g⁻¹) in total Iitterfall were 14.5 (200 m), 12.7 (500 m), 12.7 (850 m) and in leaf litterfall 11.4 (200 m), 10.6 (500 m) and 11.3 (850 m). Total litterfall and leaf litterfall were significantly lower at the highest altitude (p<0.05), but differences in nitrogen concentrations were not significant. The following soil nitrification rates (kg ha⁻¹ 21 d⁻¹) were measured: 4.65 (200 m), 2.60 (500 m) and 3.72 (850 m), with no significant differences between altitudes. A bioassay using rice (Oryza sativa L.) was carried out near the base and summit of Bukit Belalong using soils from both altitudes. There was a dramatic response to altitude, a lesser one to soil type and only a slight response to nitrogen fertilisation. It is concluded that nitrogen is not more limiting in the montane forest compared with the lowland forest and that forest differences with altitude are most likely to be related to changes in temperature.