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联，作为支撑生态系统服务供给过程的核心基石，已愈发成为学界热议的主题。基于野外实验与原生野外观测的研究，得出了并不一致的生物多样性-生态系统功能格局，分别支持生态位互补（niche complementarity）假说或选择效应（selection effects）假说。本研究以南非雾带林的地上碳储量（aboveground carbon, AGC）作为生态系统功能的替代指标，通过功能多样性（functional diversity）与功能优势度（functional dominance），分析其与物种多样性的关联。我们提出两项假设：其一，物种多样性可通过功能多样性与功能优势度效应影响地上碳储量；其二，物种多样性对地上碳储量的作用强度，在功能优势度途径下要高于功能多样性途径。为评估功能优势度（即选择效应），我们计算了功能性状（木材密度、比叶面积与最大株高）的群落加权平均（community weight mean, CWM）；针对功能多样性（即互补效应），我们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（structural equation modeling）进行检验。针对第二项假设，我们首先检验了坡度、海拔等环境变量的效应，随后分别针对功能多样性、功能优势度以及二者同时存在的情况，构建线性混合效应模型（linear mixed-effects models）。研究结果显示，地上碳储量随坡度梯度呈现显著变化，在坡度更陡的样地中储量更低。即便考虑坡度效应，物种多样性（物种丰富度（species richness））与地上碳储量仍呈正相关关系。正如预期，物种多样性对地上碳储量的作用可通过功能多样性与功能优势度介导，这表明生态位互补与选择效应并非仅通过单一途径影响碳储量。然而，相较于功能优势度，功能多样性的作用强度更高。此外，最大株高的群落加权平均可强烈传递功能优势度效应，这反映出森林垂直分层（vertical stratification）在生物多样性-碳储量关联中的重要性。因此，我们认为，生长于林下层（understory layer）的乔木与物种所具备的互补性光能利用效率，亦可催生更强的互补效应。