Forest canopy structure is closely related to species diversity, crown packing efficiency, and ecological processes, while influencing ecosystem functions. However, most existing structural diversity indices only consider internal heterogeneity within the canopy but often neglect the heterogeneity of the external canopy. While, the external canopy heterogeneity can increase the exchange surface area between canopy and external atmosphere, capture more light, which is closely linked to tree growth and ecosystem functions. Comprehensively and accurately estimating canopy structural diversity is crucial in understanding the underlying mechanisms of productivity change. Therefore, in this study, we apply a topographic index – canopy surface rugosity (CSR) – to capture canopy external heterogeneity, and use crown complementary index (CCI) and other canopy structural variables to quantify canopy internal heterogeneity. We find that CSR varies significantly among quadrats (20 ​m ​× ​20 ​m) and it decreases with species diversity and wood net primary productivity (NPPwood). Species diversity increases NPPwood directly and indirectly through CSR and CCI. Functional diversity increases NPPwood through CSR and CCI. Community weighted mean of light demand decreases with NPPwood through CCI. The mediating effect of CSR on the relationship between species (functional) diversity and NPPwood is greater than the mediating effect of CCI. We find soil fertility positive effects on NPPwood mainly through species richness and CSR. Conversely, water availability affects NPPwood by a direct positive effect and indirect negative effects through species richness, CSR and CCI. Overall, our CSR and CCI provide strong support for a space-based niche partitioning mechanism regulating the relationship between species diversity and NPPwood. Additionally, CSR is sensitive to gap dynamics, suggesting mechanistic relationships between local disturbance, species diversity, and NPPwood.

中文翻译：

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亚热带森林树冠外、内结构异质性对多样性与生产力关系的控制


森林冠层结构与物种多样性、树冠包装效率和生态过程密切相关，同时影响生态系统功能。然而，现有的结构多样性指数大多只考虑了冠层内部的非均质性，而往往忽略了外部冠层的非均质性。同时，外部冠层异质性可以增加冠层与外部大气之间的交换表面积，捕获更多的光，这与树木生长和生态系统功能密切相关。全面准确地估计冠层结构多样性对于理解生产力变化的潜在机制至关重要。因此，在本研究中，我们应用地形指数——冠层表面粗糙度 （CSR） ——来捕捉冠层外部异质性，并使用冠互补指数 （CCI） 和其他冠层结构变量来量化冠层内部异质性。我们发现 CSR 在样方 （20 m × 20 m） 之间差异很大，并且随着物种多样性和木网初级生产力 （NPPwood） 的增加而降低。物种多样性通过 CSR 和 CCI 直接或间接地增加了 NPPwood。功能多样性通过 CSR 和 CCI 增加 NPPwood。通过 CCI ，NPPwood 的光照需求社区加权均值降低。CSR对物种（功能）多样性与NPPwood之间关系的中介作用大于CCI的中介作用。我们发现土壤肥力对 NPPwood 的积极影响主要是通过物种丰富度和 CSR。相反，水的可用性通过物种丰富度、CSR 和 CCI 对 NPP 木材产生直接的积极影响和间接的负面影响。 总体而言，我们的 CSR 和 CCI 为调节物种多样性与 NPPwood 之间关系的基于空间的生态位划分机制提供了强有力的支持。此外，CSR 对间隙动力学很敏感，这表明局部干扰、物种多样性和 NPPwood 之间存在机制关系。