The "environmental heterogeneity hypothesis" predicts that variability in resources promotes species coexistence, but few experiments support this hypothesis in plant communities. A previous 15-yr test of this hypothesis in a prairie restoration experiment demonstrated a weak effect of manipulated soil resource heterogeneity on plant diversity. This response was attributed to a transient increase in richness following a post-restoration supplemental propagule addition, occasionally higher diversity under nutrient enrichment, and reduced cover of a dominant species in a subset of soil treatments. Here, we report community dynamics under continuous propagule addition in the same experiment, corresponding to 16-20 yr of restoration, in response to altered availability and heterogeneity of soil resources. We also quantified traits of newly added species to determine if heterogeneity increases the amount and variety of niches available for new species to exploit. The heterogeneous treatment contained a factorial combination of altered nutrient availability and soil depth; control plots had no manipulations. Total diversity and richness were higher in the heterogeneous treatment during this 5-yr study due to higher cover, diversity, and richness of previously established forbs, particularly in the N-enriched subplots. All new species added to the experiment exhibited unique trait spaces, but there was no evidence that heterogeneous plots contained a greater variety of new species representing a wider range of trait spaces relative to the control treatment. The richness and cover of new species was higher in N-enriched soil, but the magnitude of this response was small. Communities assembling under long-term N addition were dominated by different species among subplots receiving added N, leading to greater dispersion of communities among the heterogeneous relative to control plots. Contrary to the deterministic mechanism by which heterogeneity was expected to increase diversity (greater variability in resources for new species to exploit), higher diversity in the heterogeneous plots resulted from destabilization of formerly grass-dominated communities in N-enriched subplots. While we do not advocate increasing available soil N at large scales, we conclude that the positive effect of environmental heterogeneity on diversity can take decades to materialize and depend on development of stochastic processes in communities with strong establishment limitation.

中文翻译：

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在草地恢复期间进行20年的耕作后，土壤异质性增加了植物多样性。

“环境异质性假设”预测资源的可变性会促进物种共存，但是很少有实验支持植物群落中的这一假设。先前在草原恢复实验中对该假设进行的15年检验证明，受控土壤资源异质性对植物多样性的影响较弱。这种反应归因于恢复后补充繁殖体的添加后，丰富度暂时增加，在营养丰富下偶尔增加了多样性，并减少了土壤处理子集中的优势物种的覆盖率。在这里，我们在同一实验中报告了在连续繁殖体添加下的群落动态，对应于16-20年的恢复，以响应土壤资源的可用性和异质性变化。我们还量化了新添加物种的性状，以确定异质性是否会增加可用于新物种开发的生态位的数量和种类。异质处理包括养分利用率和土壤深度变化的因子组合。对照地块没有操纵。在此为期5年的研究中，由于先前建立的前叉（尤其是N富集的亚面）的覆盖度，多样性和丰富度较高，因此在异质处理中总多样性和丰富度更高。所有添加到实验中的新物种都表现出独特的性状空间，但是没有证据表明异质样地包含的新物种种类更多，相对于对照处理，它们代表着更广泛的性状空间。富氮土壤中新物种的丰富度和覆盖率更高，但是这种反应的程度很小。在长期添加氮的条件下聚集的社区由接受添加氮的子图样中的不同物种所主导，从而导致相对于控制样地，异构社区之间的分散程度更大。与通过异质性增加多样性（用于开发新物种的资源的更大可变性）的确定性机制相反，异质性地块中更高的多样性是由于N富集子图中原先以草为主的群落的不稳定造成的。尽管我们不主张大规模增加可用土壤氮，但我们得出结论，环境异质性对多样性的积极影响可能要花费数十年的时间才能实现，并取决于在具有强大设施局限性的社区中随机过程的发展。