Identifying emerging invasive species is a priority to implement early preventive and control actions. In terms of the number of invasive tree species, forestry represents the second largest pathway of introduction, with an invasive debt likely existing for alien conifers in Europe. In the early 1900s, a network of arboreta was established in southern Belgium to assess the wood production potential of prospective conifer and broadleaved species. Here, we use eight arboreta as natural experiments to identify alien conifers presenting invasive behavior. Through systematic sampling, we quantified the natural regeneration of alien conifers and recorded local environmental variables. For each species, regeneration density, dispersal distances, and age structure were analyzed. Generalized mixed effects models were fitted to test the effect of planted area and tree-stand type on regeneration. The environmental space occupied by regenerating alien conifers was evaluated using principal component analysis. Out of 31 planted alien species, 15 (48%) were identified in natural regeneration, of which eight (26%) exhibited important regeneration density and dispersal distances. The most invasive species were Tsuga heterophylla and Abies grandis, confirming earlier field observations. Both large planted areas and areas planted with alien conifer species increased the density of regeneration. Species that had the highest regeneration density tolerated a wide range of environmental conditions, including shaded understory, which could lead to the invasion of mature, undisturbed forests. This study showed that 17% of the studied alien conifers are potentially invasive because they show important regeneration, long-distance dispersal, and, of importance, have already produced offspring that have matured and are capable of creating new satellite populations. In conclusion, our results provide a guideline for future planting operations, recommending extreme caution when planting these species in the temperate forests of Western Europe.

中文翻译：

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Arboreta揭示了西欧温带森林中几种针叶树种的入侵潜力

识别新出现的入侵物种是实施早期预防和控制措施的优先事项。就入侵树种的数量而言，林业是第二大引进途径，欧洲外来针叶树可能存在入侵债务。1900年代初，在比利时南部建立了一个Arboreta网络，以评估潜在的针叶树和阔叶树种的木材生产潜力。在这里，我们使用八个乔木作为自然实验，以识别呈现入侵行为的外来针叶树。通过系统采样，我们量化了外来针叶树的自然再生并记录了当地的环境变量。对于每个物种，分析了再生密度，扩散距离和年龄结构。拟合了广义混合效应模型以测试种植面积和林分类型对再生的影响。使用主成分分析法评估了再生外来针叶树所占据的环境空间。在31种种植的外来物种中，有15种（占48％）可以自然再生，其中八种（占26％）表现出重要的再生密度和扩散距离。侵入性最大的物种是异花杉和大冷杉（Abies grandis），这证实了较早的现场观察。较大的种植面积和种植外来针叶树种的面积都增加了再生密度。具有最高再生密度的物种可以耐受各种环境条件，包括林木下的阴影，这可能会导致未受干扰的成熟森林的入侵。这项研究表明，研究的外来针叶树中有17％具有潜在的入侵性，因为它们显示出重要的再生，远距离散布，并且很重要的是，它们已经产生了已经成熟并能够产生新卫星种群的后代。总之，我们的结果为将来的种植作业提供了指南，建议在西欧温带森林中种植这些树种时要格外小心。