Establishing mixed-species tree plantings and fencing them to protect seedlings from herbivory is a valuable strategy for reconnecting forest fragments separated by agropastoral lands. However, fencing may provide exotic plants with the escape from herbivory required to invade the understory of planted communities. Here we take advantage of such a situation to ask how the identity of planted species and the resulting canopy cover influenced invasion success by the Asian swordtail fern (Nephrolepis brownii Desv. Nephrolepidaceae) in a 13-year-old tropical restoration experiment. Through a seed addition experiment, we also evaluated the effects the ferns had on recruiting seedlings. We found that the invasion was most acute in the unplanted control plots where canopy cover was consistently scarce. Frond density correlated negatively with canopy cover, though most of the variance in the model is explained by the design of our experiment (r2m = .161, r2c = .460). Between planting treatments that differed in the dispersal mode of the planted trees, the wind-dispersed treatment had higher fern density and longer fronds than the animal-dispersed treatment. The animal-dispersed treatment had the highest recruiting species richness, which was negatively correlated with fern density (r2 = .748). The seed addition experiment confirmed that mortality rates increased where frond density was higher (F1,41 = 7.159, p = .011) and germination rates were lowered for the smaller-seeded species (F1,42 = 13.2, p = .002). To prevent recalcitrant understory layers from establishing in plantings in the future, we recommend: (1) establishing larger plantings or expanding existing ones to minimize edge effects (particularly light filtration), (2) supplementing young plantings with additional seedlings to prevent canopy gaps from forming, and (3) planting an assemblage of species that cover the full forest strata and have consistently full tree-canopies.

中文翻译：

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封闭的热带恢复实验中顽固的林下蕨类植物层的影响

建立混合物种的树木种植园并围起来保护幼苗免受草食是重新连接由农牧业土地分隔的森林碎片的一种有价值的策略。但是，篱笆可能会为外来植物提供摆脱草食的入侵，而这些食草入侵了种植社区的底层。在这里，我们利用这种情况来询问种植物种的身份以及由此产生的树冠覆盖如何在13年的热带恢复实验中影响亚洲剑尾蕨（Nephrolepis brownii Desv。Nephrolepidaceae）的入侵成功。通过种子添加实验，我们还评估了蕨类植物对招募幼苗的影响。我们发现，在始终缺乏树冠覆盖的未种植对照地块中，入侵最为严重。叶面密度与冠层覆盖率呈负相关，尽管模型中的大多数变化都由我们的实验设计解释（r2m = .161，r2c = .460）。在植树的扩散方式不同的植树处理之间，风分散处理的蕨类密度更高，叶长比动物分散处理更长。动物分散处理具有最高的募集物种丰富度，与蕨类密度呈负相关（r2 = .748）。种子添加实验证实，叶密度较高时，死亡率增加（F1,41 = 7.159，p = .011），种子较小的种子的发芽率降低（F1,42 = 13.2，p = .002）。为了防止将来在种植物中建立顽强的林下层，我们建议：