Are ectomycorrhizas more abundant than arbuscular mycorrhizas in tropical heath forests?

Tropical heath forests have accumulations of slowly decomposing organic mat ter at the soil surface. To test the hypothesis that ectomycorrhizas are mo re abundant than arbuscular mycorrhizas under such conditions, we compared the abundance of ectomycorrhizal (EcM) tree species, and the relative propo rtions of EcM and arbuscular mycorrhizal (AM) roots, in heath forest and lo wland rain forest. The mycorrhizal status of frees in two heath forest and two lowland (mixed dipterocarp) forest plots in Brunei Darussalam was determined by literature and field survey. Fine-root density, proportion of EcM roots, and fraction al colonization of EcM and AM roots were measured in monoliths from organic and mineral soil. There was no difference in the relative basal area abundance (10-41%) of Ec M trees, the proportion of EcM roots in monoliths (8-46%), or fractional co lonization (90%) of EcM roots, between the two forest types. However, fract ional colonization of AM roots was higher (54%) in heath forest than in mix ed dipterocarp forest (27%). Our data do not support the hypotheses that ectomycorrhizas are more abunda nt in, or determine the floristic composition of, tropical heath forests. ( C) New Phytologist (2001).