Floristic composition and stand structure of mixed dipterocarp and heath forests in Brunei Darussalam

The stand structure and floristic composition of four 0.96-ha plots in mixed dipterocarp (MDF) and tropical heath forests in Brunei are described. All trees >5 cm diameter at breast height were mapped,identified and their diameters measured. Soil and root profiles in one MDF and one heath plot are also described. The MDF plot sampled for soil was on a sandstonederived haplic acrisol with a thick but patchy humus layer. The heath forest plot sampled was on an albic arenosol. Away from large trees, fine and coarse roots werealmost absent below 30 cm depth in the heath soil, but present in the MDF soil. Stand basal area was slightly lower in the two heath plots than the two MDF plots, whereas stem densities were not consistently different between the formations. Stand density and basal area were highest on upper land at both MDF plots and one heath plot, but werehighest in a swampy area in the other heath plot. The MDF plots had higher species richness and lower species dominance than the health plots. The MDF and heath plots were fioristically distinct. Dryobalanops aromatica contributed most to basal area in bothMDF plots. Agathis borneensis accounted for 65% of basal area and 16% of trees at one of the health plots. Gluta beccarii contributed 11 % of basal area at the other heath plot. The Euphorbiaceae and Burseraceae contributed relatively more to species richness,tree abundance and basal area in the MDF plots than in the heath plots. The Myrtaceae contributed relatively more to species richness, tree abundance and basal area in the heath plots. The Araucariaceae were unique to the heath plots, and the Clusiaceae and Sapotaceae contributed relatively more to species richness in the heath plots. The plots were compared with other MDF and heath plots in Borneo. The heath plots were typical of the coastal heath forest growing on medium-deep white sands in northwest Borneo. The MDF plots were similar to other very species-rich MDF from northwest Borneogrowing on sandstone-derived soils with moderate to high sand content.