Diversity–biomass relationships (DBRs) often vary with spatial scale in terrestrial ecosystems, but the mechanisms driving these scale-dependent patterns remain unclear, especially for highly heterogeneous forest ecosystems. This study explores how mutualistic associations between trees and different mycorrhizal fungi, i.e., arbuscular mycorrhizal (AM) vs. ectomycorrhizal (EM) association, modulate scale-dependent DBRs. We hypothesized that in soil-heterogeneous forests with a mixture of AM and EM tree species, (i) AM and EM tree species would respond in contrasting ways (i.e., positively vs. negatively, respectively) to increasing soil fertility, (ii) AM tree dominance would contribute to higher tree diversity and EM tree dominance to greater standing biomass, and that as a result (iii) mycorrhizal associations would exert an overall negative effect on DBRs across spatial scales. To empirically test these hypotheses, we collected detailed tree distribution and soil information (e.g., nitrogen, phosphorus, organic matter, pH) from seven temperate and subtropical AM–EM mixed forest megaplots (16–50 ha). Using a spatial codispersion null model and structural equation modeling, we identified the relationships among AM or EM tree dominance, soil fertility, tree species diversity, and biomass and, thus, DBRs across 0.01- to 1-ha scales. We found the first evidence overall supporting the three aforementioned hypotheses in these AM–EM mixed forests: (i) In most forests, with increasing soil fertility, tree communities changed from EM-dominated to AM-dominated; (ii) increasing AM tree dominance had an overall positive effect on tree diversity and a negative effect on biomass, even after controlling for soil fertility and number of trees. Together, (iii) the changes in mycorrhizal dominance along soil fertility gradients weakened the positive DBR observed at 0.01- to 0.04-ha scales in nearly all forests and drove negative DBRs at 0.25- to 1-ha scales in four out of seven forests. Hence, this study highlights a soil-related mycorrhizal dominance mechanism that could partly explain why, in many natural forests, biodiversity–ecosystem functioning (BEF) relationships shift from positive to negative with increasing spatial scale.

中文翻译：

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规模相关的多样性-生物量关系可以由树菌根关联和土壤肥力驱动

多样性-生物量关系 (DBR) 通常随陆地生态系统的空间尺度而变化，但驱动这些尺度相关模式的机制仍不清楚，尤其是对于高度异质的森林生态系统。本研究探讨了树木与不同菌根真菌之间的互惠关联，即丛枝菌根 (AM) 与外生菌根 (EM) 关联如何调节依赖于尺度的 DBR。我们假设在混有 AM 和 EM 树种的土壤异质森林中，(i) AM 和 EM 树种将以相反的方式（即分别为积极与消极）对增加土壤肥力做出反应，(ii) AM树木优势将有助于提高树木多样性，而 EM 树木优势将有助于提高常设生物量，因此 (iii) 菌根联合会在空间尺度上对 DBR 产生总体负面影响。为了对这些假设进行实证检验，我们从七个温带和亚热带 AM-EM 混交林大地块（16-50 公顷）中收集了详细的树木分布和土壤信息（例如，氮、磷、有机质、pH 值）。使用空间共分散零模型和结构方程模型，我们确定了 AM 或 EM 树优势、土壤肥力、树种多样性和生物量之间的关系，因此确定了 0.01 到 1 公顷尺度的 DBR。我们在这些 AM-EM 混交林中发现了全面支持上述三个假设的第一个证据：（i）在大多数森林中，随着土壤肥力的增加，树木群落从 EM 主导变为 AM 主导；(ii) 即使在控制了土壤肥力和树木数量之后，增加 AM 树优势对树木多样性也有总体积极影响，对生物量有负面影响。总之，(iii) 菌根优势沿土壤肥力梯度的变化削弱了几乎所有森林中在 0.01 至 0.04 公顷范围内观察到的正 DBR，并在七分之四的森林中推动了 0.25 至 1 公顷范围内的负 DBR。因此，这项研究强调了一种与土壤相关的菌根优势机制，该机制可以部分解释为什么在许多天然森林中，生物多样性-生态系统功能 (BEF) 关系随着空间尺度的增加从正向负向转变。(iii) 菌根优势沿土壤肥力梯度的变化削弱了几乎所有森林中在 0.01 至 0.04 公顷范围内观察到的正 DBR，并在七分之四的森林中驱动了 0.25 至 1 公顷范围内的负 DBR。因此，这项研究强调了一种与土壤相关的菌根优势机制，该机制可以部分解释为什么在许多天然森林中，生物多样性-生态系统功能 (BEF) 关系随着空间尺度的增加从正向负向转变。(iii) 菌根优势沿土壤肥力梯度的变化削弱了几乎所有森林中在 0.01 至 0.04 公顷范围内观察到的正 DBR，并在七分之四的森林中驱动了 0.25 至 1 公顷范围内的负 DBR。因此，这项研究强调了一种与土壤相关的菌根优势机制，该机制可以部分解释为什么在许多天然森林中，生物多样性-生态系统功能 (BEF) 关系随着空间尺度的增加从正向负向转变。