European beech (Fagus sylvatica L.) forests can have a high variability in plant species richness and abundance, from monospecific stands to highly species-rich communities. To understand what causes the low plant diversity observed in some beech forests, we analyzed the drivers of plant community completeness in 155 vegetation plots. Data were collected in mature, closed-canopy beech forests in Tuscany, central Italy. Site-specific species pools were estimated based on species co-occurrences. We used Generalized Least Squares linear modeling to assess the effects of anthropogenic and environmental drivers on the community completeness of whole communities and on the set of specialist species of beech forests. We also tested the response of the total cover of the herb layer to the selected predictors and related both the predictive and response variables to species composition in a Non-metric Multidimensional Scaling ordination. The community completeness of whole communities and that of beech forest specialists were negatively affected by total beech cover and positively influenced by slope. Moreover, the community completeness of whole communities was negatively impacted by elevation and positively influenced by disturbance frequency. The cover of the herb layer decreased with increasing beech cover, elevation, and precipitation. High community completeness and high cover of the herb layer were associated with the presence of thermophilic species of mixed deciduous woods in low-elevation beech forests. Our results suggest that a low plant community completeness and a low cover of the herb layer are mainly due to the competition by beech itself when it forms pure forests in its ecological optimum. Such competition is better exerted at upper elevations and in sites with low slopes, where beech litter accumulation is a limiting factor for understory species. Such evidence suggests that species absence in mature beech forests is mainly due to natural drivers and should therefore not be considered an indicator of ecological degradation of the forest.

中文翻译：

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群落完整性的驱动因素和模式表明，托斯卡纳 Fagus sylvatica 森林自然具有较低的植物多样性


欧洲山毛榉 （Fagus sylvatica L.） 森林在植物物种丰富度和丰度方面可能具有高度可变性，从单一物种林分到物种高度丰富的群落。为了了解导致一些山毛榉林中观察到的植物多样性低的原因，我们分析了 155 个植被样地中植物群落完整性的驱动因素。数据是在意大利中部托斯卡纳的成熟、封闭的树冠山毛榉林中收集的。根据物种共现情况估计特定地点的物种库。我们使用广义最小二乘线性模型来评估人为和环境驱动因素对整个群落完整性和山毛榉林特有物种集的影响。我们还测试了草本层总覆盖率对所选预测因子的响应，并将预测变量和响应变量与非度量多维尺度排序中的物种组成相关联。整个群落和山毛榉森林专家的群落完整性受山毛榉总覆盖度的负影响，而坡度则受其正影响。此外，整个群落的群落完整性受海拔的负影响，受干扰频率的正影响。草本层的覆盖率随着山毛榉覆盖率、海拔和降水的增加而降低。高群落完整性和高草本层覆盖率与低海拔山毛榉林中混合落叶木嗜热物种的存在有关。我们的结果表明，植物群落完整性低和草本层覆盖率低主要是由于山毛榉本身在形成生态最佳纯林时的竞争。 这种竞争在高海拔和低坡度的地方表现得更好，在这些地方，山毛榉凋落物的积累是林下物种的限制因素。这些证据表明，成熟山毛榉林中物种的缺失主要是由于自然驱动因素，因此不应被视为森林生态退化的指标。