Soil nutrients and size-dependent tree dynamics of tropical lowland forests on volcanic and sedimentary substrates in Sabah, Malaysian Borneo

Soil nutrient concentrations and tree dynamics were examined in two Bornean tropical forests on contrasting geological substrates, one on volcanic and the other on sedimentary rocks. Concentrations of P extracted by the Hadley fractionation method and inorganic N of topsoils were much greater in the volcanic site than in the sedimentary site. Dipterocarps showed greater relative growth rates in the volcanic site than in the sedimentary site at the smallest size class (10 cm ≤ diameter at breast height<20 cm), but not at larger size classes. Dipterocarps in the two sites demonstrated the same values of crown position index (a surrogate of light conditions), implying that the greater tree growth in the volcanic site was associated with the greater soil nutrient availability, but not with light availability. On the other hand, relative growth rates of non-dipterocarps did not differ between the sites at all size classes despite the considerable differences in the soil nutrient concentrations, suggesting that factors other than N and P availabilities limit the growth of these trees. Contrary to growth rate, annual mortality rate was greater in the volcanic site at all size classes for all phylogenetic groups. Our results suggest that the volcanic site is characterized as high soil nutrient availability and a greater tree turnover, and that tree size is an important factor that differentiates tree growth between the two tropical forests with contrasting nutrient availabilities.