Investigation of the relationships between tree-specdes diversity and the functions of tropical rain forests

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The objectives of this study are to understand the control of the functions of tropical rain forests by tree-species diversity. In order to achieve this goal, we took an innovative approach in that we compared two naturally species-rich (Malaysia) versus species impoverished (Hawaii) tropical rain-forest regions. The working hypothesis was that a greater species pool (i.e. regional species diversity) could provide more nutrient-use efficient species to reduced nutrient availability while a impoverished species pool would provide lesser nutrient-use efficient species to reduced nutrient availability leading to a temporally variable biomass in response to nutrient availability. To test this working hypothesis, we set up one soil chronosequence in each region, along which soil nutrient availability changed due to soil aging and associated geochemical processes. We investigated the above-ground net primary productivity and nutrient-use of the major tree species in each region. The adove-ground net primary productivity was estimated by repeated measures of secondary growth of trees plus litter-fall rate which was measured bi-weekly. The intact leaves were collected from the major trees in each site and wet-digested to analyze foliar minerals. In Hawaii where regional species richness (i.e. species pool) was small, above-ground net primary productivity and biomass increased with increasing soil nitrogen availability towards a peak stage where soil nitrogen and phosphorus were abundant, followed by an abrupt decline phase where soil phosphorus availability declined. By contrast, such an abrupt change did not occur in Malaysia because more phosphorus-use efficient species replaced inefficient species with declining soil-phosphorus availability. Thus our working hypothesis was accepted.