Data from: Tree species diversity promotes aboveground carbon storage through functional diversity and functional dominance

The relationship between biodiversity and ecosystem function has increasingly been debated as the cornerstone of the processes behind ecosystem services delivery. Experimental and natural field-based studies have come up with nonconsistent patterns of biodiversity–ecosystem function, supporting either niche complementarity or selection effects hypothesis. Here, we used aboveground carbon (AGC) storage as proxy for ecosystem function in a South African mistbelt forest, and analyzed its relationship with species diversity, through functional diversity and functional dominance. We hypothesized that (1) diversity influences AGC through functional diversity and functional dominance effects; and (2) effects of diversity on AGC would be greater for functional dominance than for functional diversity. Community weight mean (CWM) of functional traits (wood density, specific leaf area, and maximum plant height) were calculated to assess functional dominance (selection effects). As for functional diversity (complementarity effects), multitrait functional diversity indices were computed. The first hypothesis was tested using structural equation modeling. For the second hypothesis, effects of environmental variables such as slope and altitude were tested first, and separate linear mixed-effects models were fitted afterward for functional diversity, functional dominance, and both. Results showed that AGC varied significantly along the slope gradient, with lower values at steeper sites. Species diversity (richness) had positive relationship with AGC, even when slope effects were considered. As predicted, diversity effects on AGC were mediated through functional diversity and functional dominance, suggesting that both the niche complementarity and the selection effects are not exclusively affecting carbon storage. However, the effects were greater for functional diversity than for functional dominance. Furthermore, functional dominance effects were strongly transmitted by CWM of maximum plant height, reflecting the importance of forest vertical stratification for diversity–carbon relationship. We therefore argue for stronger complementary effects that would be induced also by complementary light-use efficiency of tree and species growing in the understory layer.

生物多样性与生态系统功能的关系，作为支撑生态系统服务供给过程的核心基石，已成为日益受到学界关注的热点议题。基于野外自然样地与控制实验的相关研究，得出了关于生物多样性-生态系统功能关系的不一致结论，分别支持生态位互补效应假说或选择效应假说。本研究以南非雾带林为研究对象，以地上碳储量（aboveground carbon, AGC）作为生态系统功能的替代指标，并通过功能多样性与功能优势度两个维度，分析其与物种多样性的关联。本研究提出两项假设：（1）物种多样性通过功能多样性与功能优势度效应间接影响地上碳储量；（2）相较于功能多样性，功能优势度对地上碳储量的多样性调控效应更强。研究通过计算功能性状（木材密度、比叶面积、最大株高）的群落加权平均值（community weight mean, CWM）来表征功能优势度（即选择效应）；针对功能多样性（即互补效应），则计算了多性状功能多样性指数。第一项假设通过结构方程模型进行验证；针对第二项假设，研究首先检验了坡度、海拔等环境变量的调控效应，随后分别针对功能多样性、功能优势度以及二者共同作用，构建独立的线性混合效应模型。研究结果显示，地上碳储量随坡度梯度呈现显著变化，坡度越陡的样地其地上碳储量越低。即便在控制坡度效应的前提下，物种多样性（物种丰富度）与地上碳储量仍呈现显著正相关关系。如研究预期所示，物种多样性对地上碳储量的调控效应确实通过功能多样性与功能优势度两条路径实现，这表明生态位互补效应与选择效应并非单独作用于碳储量的调控过程。但相较而言，功能多样性对地上碳储量的调控效应要强于功能优势度。此外，功能优势度的调控效应主要通过最大株高的群落加权平均值实现，这反映出森林垂直分层在生物多样性-碳储量关系中的重要作用。因此，本研究认为，林下植被与乔木树种互补的光能利用效率，同样能够诱导产生更强的生态位互补效应。