A comparison of the soil water, nutrient status, and litterfall characteristics of tropical heath and mixed-dipterocarp forest sites in Brunei

Two of the main hypotheses to explain the distribution and special characteristics of tropical heath forest are nutrient and water limitation. A study was undertaken to investigate both factors on two sites under tropical heath forest (Badas Forest Reserve) and mixed-dipterocarp forest (Andulau Forest Reserve) in Brunei. Soil water potentials were monitored at depths of 20, 50, and 90 cm over wet and dry periods for five months at each site. The results showed the mixed-dipterocarp forest site to be drier at 50 cm depth compared to the tropical heath forest site. There was no significant difference in water potentials between sites at 20 or 90 cm. Nutrient concentrations in the soil solution were monitored at the same depths over a seven-month period at the same sites. A 12-month litterfall study was also undertaken to monitor nutrient returns from the canopy at each site. The results of both studies suggest that the tropical heath forest site is poorer in nitrogen, but richer in calcium, than the mixed-dipterocarp forest site. The results for phosphorus are less clear, but do not suggest that its limitation is a significant factor at the tropical heath forest site compared to the mixed-dipterocarp forest site. Phosphorus and magnesium concentrations in the soil solution showed a strong positive correlation with sliding 30-day rainfall totals at both sites.