1. It is prominently claimed that enhancing forest diversity would play a dual role of nature conservation and climate regulation. While the idea is intuitively appealing, studies show that species richness effects on above‐ground carbon (AGC) are not always positive, but instead unpredictable especially across scales and complex terrestrial systems having large‐diameter and tall‐stature trees. Previous studies have further considered structural complexity and larger trees as determinants of AGC. Yet it remains unclear what drives differential diversity–AGC relationships across vegetation types.
2. Here we test whether structural complexity and large‐sized trees play an influential role in explaining shifting diversity–AGC relationships across vegetation types, using a 22.3‐ha sampled dataset of 124 inventory plots in woodlands, gallery forests, tree/shrub savannas and mixed plantations in West Africa.
3. Natural vegetation had greater species richness and structural complexity than mixed plantations, as expected. In addition, AGC was highest in gallery forests and mixed plantations, which is consistent with favourable environmental conditions in the former and high stocking densities and presence of fast‐growing species in the latter. Significant interaction effects of species richness and vegetation on AGC revealed a vegetation‐dependent species richness–AGC relationship: consistently, we found positive species richness–AGC relationship in both mixed plantations and woodlands, and nonsignificant patterns in gallery forests and tree/shrub savanna. Furthermore, there was a vegetation‐dependent mediation of structural complexity in linking species richness to AGC, with stronger positive structural complexity effects where species richness–AGC relationships were positive, and stronger positive large‐sized trees’ effect where species richness–AGC relationships were neutral.
4. Our study provides strong evidence of vegetation‐dependent species richness–AGC relationships, which operated through differential mediation by structural complexity of the species richness and large trees’ effects. We conclude that even higher species richness in diversified ecosystems may not always relate positively with AGC, and that neutral pattern may arise possibly as a result of larger dominant individual trees imposing a slow stand dynamic flux and overruling species richness effects.

中文翻译：

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结构复杂性和大树解释了物种丰富度和碳关系在不同植被类型之间的变化

1. 众所周知，增强森林多样性将起到自然保护和气候调节的双重作用。虽然这个想法很直观，但是研究表明，物种丰富度对地上碳（AGC）的影响并不总是积极的，而是不可预测的，尤其是在具有大直径和高大树木的规模和复杂的陆地系统之间。先前的研究进一步将结构复杂性和较大的树木视为AGC的决定因素。但是，目前尚不清楚是什么因素导致了不同植被类型之间的差异多样性—AGC关系。
2. 在这里，我们使用22.3公顷的林地，长廊林，树木/灌木稀树草原和混合人工林的124个样地的抽样数据集，测试了结构复杂性和大型树木是否在解释多样性-AGC关系在不同植被类型之间的变化中起着重要作用在西非。
3. 如预期的那样，天然植被比混交林具有更大的物种丰富度和结构复杂性。此外，AGC在长廊林和人工林中最高，这与前者的良好环境条件和高放养密度以及后者的速生树种一致。物种丰富度与植被对AGC的显着交互作用揭示了植被依赖性物种丰富度与AGC的关系：一致地，我们在混合人工林和林地中都发现了物种丰富度与AGC的正相关，而在画廊森林和树木/灌木稀树草原中却没有显着的格局。此外，在将物种丰富度与AGC联系起来时，存在着一种依赖植被的结构复杂性介导，
4. 我们的研究为依赖植被的物种丰富度与AGC关系提供了有力的证据，这种关系通过物种丰富度的结构复杂性和大树的影响通过差异介导而起作用。我们得出的结论是，多样化生态系统中甚至更高的物种丰富度可能并不总是与AGC呈正相关，并且中性模式可能是由于较大的优势单株施加了缓慢的林分动态通量并否决了物种丰富度的结果。