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

本研究的目的是了解树种多样性对热带雨林功能的控制。为了实现这一目标，我们采取了一种创新的方法，我们比较了两个自然物种丰富（马来西亚）和物种贫乏（夏威夷）的热带雨林地区。工作假设是，一个更大的物种库（即区域物种多样性）可以提供更多的养分利用效率的物种减少养分的可用性，而贫困的物种库将提供较少的养分利用效率的物种减少养分的可用性，导致一个时间可变的生物量，以响应养分的可用性。为了验证这一假设，我们在每个地区建立了一个土壤时序，沿着土壤养分的有效性变化，由于土壤老化和相关的地球化学过程。我们调查了各地区主要树种的地上净初级生产力和养分利用。土坯地面净初级生产力是通过重复测量树木的次生生长加上每两周测量的凋落物速率来估计的。从每个地点的主要树木收集完整的叶片，并进行湿消化以分析叶片矿物质。在夏威夷，区域物种丰富度（即物种库）是小的，地上净初级生产力和生物量的增加，增加土壤氮的有效性对一个高峰阶段，土壤氮和磷是丰富的，然后突然下降阶段，土壤磷的有效性下降。相比之下，这种突然的变化并没有发生在马来西亚，因为更多的磷利用效率高的物种取代了低效的物种，土壤磷的可用性下降。因此，我们的工作假设被接受了。

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.