Niche differentiation along multiple functional-trait dimensions contributes to high local diversity of Euphorbiaceae in a tropical tree assemblage

Understanding the mechanisms that drive community assembly in species-rich tropical forest remains a fundamental challenge in ecology. Here, we integrated multivariate functional trait dimensions, phylogeny, and metabolomics to test fundamental predictions concerning the role of differentiation with respect to abiotic and biotic niche axes in the maintenance of high local diversity of woody plants in the Euphorbiaceae.

We measured 40 functional traits related to resource acquisition, photosynthetic capacity, hydraulic efficiency, and secondary-metabolite profiles generated using untargeted metabolomics in all 26 Euphorbiaceae species in a 20-ha forest dynamics plot in tropical southwestern China. We examined the correlation structure of 40 traits using a trait networking approach. We coupled these traits with variation in soil nutrients, light environment, soil water content and herbivore pressure within the plot to assess niche differentiation in space. We compared phylogenetic signal among multivariate trait dimensions and secondary metabolites to assess niche differentiation in evolutionary time.

Network analysis revealed that a small number of traits with high network centrality reflected variation in ecological strategy among the Euphorbiaceae. Using these high-centrality traits, we observed significant functional turnover along environmental gradients defined by light, soil moisture, soil nutrients and leaf herbivory, respectively. Most resource utilization traits showed significant phylogenetic signal, whereas almost all defensive traits lacked phylogenetic signal, including species similarity with respect to plant secondary metabolites.

Synthesis. Our results suggest that resource-utilization traits and the habitat associations play a significant role in the niche segregation of co-occurring woody plants in the Euphorbiaceae. Secondary metabolites, however, may enhance diversity at a finer spatial scale by allowing closely related species with similar functional traits to partition biotic niche space within shared habitats in tropical rainforest.

解析物种丰富的热带森林群落构建的驱动机制，始终是生态学领域的核心挑战之一。本研究整合多维度功能性状、系统发育与代谢组学数据，旨在检验关于非生物与生物生态位轴分化作用的核心预测，以探究其在维持大戟科（Euphorbiaceae）木本植物本地高物种多样性中的功能。

本研究在中国西南热带地区一处20公顷的森林动态样地中，针对26种大戟科植物，测定了40项与资源获取、光合能力、水分传导效率相关的功能性状，以及采用非靶向代谢组学（untargeted metabolomics）检测得到的次生代谢产物谱。我们采用性状网络分析法解析了40项功能性状的关联结构；将这些性状与样地内土壤养分、光照环境、土壤含水量及植食压力的空间变异相结合，以评估空间维度上的生态位分化；同时比较多维度功能性状与次生代谢产物的系统发育信号，以此探究进化时间尺度上的生态位分化。

性状网络分析结果显示，少数具有高网络中心性的性状能够反映大戟科植物间的生态策略差异。基于这些高中心性性状，我们发现分别沿光照、土壤湿度、土壤养分及叶片植食压力构成的环境梯度，均存在显著的功能周转现象。多数资源利用类功能性状表现出显著的系统发育信号，而几乎所有防御类性状均无系统发育信号，包括与植物次生代谢产物相关的物种相似性模式。

研究综合结论：本研究结果表明，资源利用类功能性状与生境关联，在大戟科同域共存木本植物的生态位分隔中发挥了重要作用。然而，次生代谢产物或许可通过帮助具有相似功能性状的近缘物种，在热带雨林共享生境中划分生物生态位空间，从而在更精细的空间尺度上维持物种多样性。