Understanding the relationship between biodiversity and productivity can be advanced by improving metrics used to quantify biodiversity. Structural diversity, that is, variation of size and form of plant organs, is an emerging biodiversity metric. However, compared with the other biodiversity metrics, its relative importance in specific components of forest productivity, for example, recruitment of new individuals, biomass net change after accounting for mortality, is largely unknown, particularly across a large spatial scale with multiple influential gradients. To address the knowledge gap, we used USDA Forest Service Forest Inventory and Analysis (FIA) data across the southcentral USA from 2008 to 2017. We calculated forest biomass increments due to recruitment and growth and net change in biomass. Then, we quantified the effects of a range of abiotic and biotic variables on the biomass increments and net change. Our results showed that (1) Structural diversity was negatively associated with the two biomass increments and net change in biomass. The negative effects were supported by increased occurrences of insects and diseases with greater structural diversity. (2) Compared with species and functional diversity, structural diversity showed a better association with biomass increments and net change, suggested by its larger absolute values of standardized coefficients, and the effects of structural diversity were negative in contrast to species diversity. (3) The effects of structural diversity, stand age, and elevation differed between natural and planted forests that may stem from the differences in stand development and species composition between the two forest types. Together, structural diversity may represent an important dimension of biodiversity impacts on plant productivity, which could be related to the exacerbated disturbances with greater structural diversity.

中文翻译：

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与物种或功能多样性相比，结构多样性与森林生产力的相关性更好

通过改进用于量化生物多样性的指标可以促进对生物多样性和生产力之间关系的理解。结构多样性，即植物器官大小和形式的变化，是一种新兴的生物多样性指标。然而，与其他生物多样性指标相比，其在森林生产力特定组成部分（例如新个体的招募、考虑死亡率后的生物量净变化）中的相对重要性在很大程度上是未知的，特别是在具有多个影响梯度的大空间范围内。为了解决知识差距，我们使用了美国农业部林务局 2008 年至 2017 年美国中南部的森林清查和分析 (FIA) 数据。我们计算了由于补充和生长以及生物量净变化而导致的森林生物量增量。然后，我们量化了一系列非生物和生物变量对生物量增量和净变化的影响。我们的结果表明（1）结构多样性与生物量的两个增量和生物量净变化呈负相关。结构多样性更大的昆虫和疾病的发生增加也加剧了负面影响。 (2)与物种多样性和功能多样性相比，结构多样性与生物量增量和净变化具有更好的相关性，其标准化系数的绝对值较大，并且与物种多样性相比，结构多样性的影响为负。 (3)天然林和人工林之间的结构多样性、林龄和海拔的影响不同，这可能源于两种森林类型之间林分发育和物种组成的差异。总之，结构多样性可能代表生物多样性对植物生产力影响的一个重要方面，这可能与结构多样性更大的干扰加剧有关。