The mechanistic basis of habitat specialisation in dipterocarps
The aims of this thesis were to determine resource availabilities in the distinct floristic associations of a tropical lowland rain forest, and establish the mechanisms that partition dipterocarp species in relation to environmental variation. The Sepilok Forest Reserve, Malaysia supports three floristic associations: alluvial, sandstone and kerangas forest. Soil chemical analysis and nutrient flux via litter production and decomposition indicated that nutrient availability increased from kerangas to sandstone and from sandstone to alluvial forest. Forest gaps received more light that understorey environments, and the alluvial forest had lower availability and constancy of irradiance than the sandstone forest. A field-based reciprocal transplant experiment using seedlings of sandstone and alluvial specialists and one non-native species and the study of naturally established dipterocarp trees indicated that sandstone species exhibited traits a strategy of nutrient conservation, whilst alluvial specialists had traits characteristic of high growth rate strategy and yet, in low light, some alluvial species exhibit traits that confer shade tolerance. Seedlings of sandstone species maintained smaller leaves with lower concentrations of N and P, and in gaps, lost leaves more slowly than alluvial specialists. In contrast, seedlings of alluvial species maintained higher foliar concentrations of N and P and higher rates of photosynthesis and stomatal conductance than the sandstone specialists but, in understorey environments, had lower leaf loss rates than sandstone specialists. A pot-based experiment revealed that seedlings of alluvial and sandstone specialists changed biomass allocation, in response to differential irradiance and nutrient availability, to maximise capture, and reduce loss, of resources most limiting in their native habitats.