Temperate forest understorey vegetation poses an excellent study system to investigate whether increases in resource availability lead to an increase in plant species richness. Most sunlight is absorbed by the species-poor tree canopy, making the much more species-rich understorey species inhabit a severely resource-limited habitat. Additionally, the heterogeneity of light availability, resulting from management-moderated tree composition and age structure, may contribute to species coexistence. One would therefore expect that the diversity in the herb layer correlates positively with either the overall light availability, or the light heterogeneity, depending on whether resource availability or heterogeneity are more important drivers of diversity. To test this idea, we assessed variability of light conditions in 75 forest plots across three ecoregions with four different methods. We correlated these data with vegetation relevés and found light availability to be strongly positively correlated with understorey plant species richness, as well as with understorey cover. Light variability (assessed with two approaches) within plots was positively correlated with transmittance, but did not improve the relationship further, suggesting that the main driver of species richness in this system is the overall resource availability. Two of the three beech-dominated regions exhibited near-identical effects of light transmittance, while the third, featuring pine alongside beech and thus with the longest gradient of transmittance and lowest species richness, displayed a weaker light response. While site conditions are certainly responsible for the trees selected by foresters, for the resulting forest structure, and for the differences in plant species pools, our results suggest that light transmittance is a strong mediating factor of understorey plant species richness.

中文翻译：

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在温带林下层中，植物物种的丰富度随光照的增加而增加，但不随变异性增加。

温带森林下层植被为研究资源可利用性增加是否导致植物物种丰富度增加提供了极好的研究系统。物种贫乏的树冠吸收了大多数阳光，使物种丰富得多的下层树种栖息于资源严重受限的栖息地。此外，由于管理适度的树木组成和年龄结构而导致的光利用率的异质性可能有助于物种共存。因此，人们希望草药层中的多样性与总体光利用率或光异质性呈正相关，这取决于资源的可获得性还是异质性是多样性的更重要驱动因素。为了验证这个想法，我们用四种不同的方法评估了三个生态区中75个森林地的光照条件的变化。我们将这些数据与植被相关性相关联，发现光的利用率与下层植物物种的丰富度以及下层植被的覆盖度密切相关。地块内的光变异性（用两种方法评估）与透射率呈正相关，但并没有进一步改善这种关系，这表明该系统中物种丰富度的主要驱动因素是整体资源的可用性。在三个以山毛榉为主的区域中，有两个显示出几乎相同的透光率效果，而第三个以松树和山毛榉为特点，因此具有最长的透射率梯度和最低的物种丰富度，显示出较弱的光响应。