The ecology of conifers in tropical montane forests on Mount Kinabalu, Borneo

Background/Question/Methods   Tropical rain forest in Southeast Asia is usually dominated by dipterocarps and other angiosperm trees. Conifers can become dominant in marginal habitats such as montane forests and swamp and heath forests in lowland, but their ecology is poorly known. The objective of this study is to characterize the ecology of tropical conifers based on permanent plot method. We selected ten study sites along the elevational gradients (700-3100 m above sea level) on Mount Kinabalu, Borneo, the highest mountain in Southeast Asia (4095 m). These sites have contrasting substrate and soil types: zonal soil on sedimentary and granitic rocks, azonal soil on ultramafic rock, and rocky substrate of alluvial origin (probably debris flow deposit).

Results/Conclusions   In all, nine species belonging to six genera and two families occur in the plots: Agathis borneensis, A. kinabaluensis (Araucariaceae), Dacrycarpus imbricatus, D. kinabaluensis, Dacrydium gibbsiae, D. gracilis, Falcatifolium falciforme, Phyllocladus hypophyllus and Podocarpus gibbsii (Podocarpaceae). Two species are endemic to Mount Kinabalu (D. kinabaluensis and P. gibbsii). No species is restricted to ultramafic soil though most species showed biased distributions to particular soil or substrate type. Conifers are generally abundant in montane forests, but their abundance varies with substrate or soil-nutrient status and they can occur in lowland forest on ultramafic soil where soil nutrients are scarce. These patterns suggest control by substrate type and soil nutrients rather than by temperature. Conifers generally show flat or monomodal diameter distributions, which indicates that they are generally shade-intolerant and require canopy gaps for regeneration. However, some species seem to regenerate continuously in montane forests on ultramafic soil probably because of the sparse canopy structure. Analyses of growth rate and crown light conditions in a montane forest also indicated that conifers are generally fast-growing, light-demanding, pioneer-like species. Conifers generally have greater maximum tree sizes than angiosperms. Once they reach the upper canopy layer they persist for a long time due to their long life span, allowing them to regenerate in rare events of canopy disturbance. We suggest that the architectural and life-history advantages and physiological adaptations to nutrient limitation are essential for the persistence of conifers, despite their lesser shade tolerance as compared to angiosperms.