Invasive species affect community dynamics and ecosystem functions, but the mechanisms of their impacts are poorly understood. Hypotheses on invasion impact range from Superior Competitor to Novel Function, from Enemy Escape to Microbial Mediation. In this study, we examined the effects of an urban tree invader, Acer platanoides (Norway maple, NM), on a mesic deciduous forest in contrast to its native congener Acer rubrum (red maple, RM) with a split-plot design experiment. A total of 720 maple seedlings were transplanted to 72 plots under 24 trees of three canopy types. The three experimental treatments were removal of resource competition at above- and belowground and removal of leaf-litter effect. Soil moisture and nitrogen-related microbial activities were followed for each plot. We found that partial canopy removal increased canopy openness and light transmission to the forest floor, but to a greater extent under NM than under RM trees. NM seedlings were more shade tolerant than RM seedlings in height growth. During the reciprocal transplantation in the mixed forest, biomass accumulation of NM seedlings under RM trees were twice as much as under NM, while that of RM seedlings under NM trees was 23.5% less than under RM. Soil net nitrification and relative nitrification were significantly higher, but mineralization rate was much lower under NM than under RM trees, which would lead to faster N leaching and lower N availability in the soil. Plots with litter removal had significantly higher seedling mortality due to herbivory by the end of 2 years, especially for NM seedlings under NM trees. Trenching enhanced soil water availability but there was no difference among canopy types. Our results demonstrated that invasion of NM not only altered forest canopy structure but also changed herbivory rate for seedlings and N dynamics in the soils. NM seedlings were more shade tolerant under NM canopies than RM seedlings and were more protected by NM litter under NM canopies than under RM canopies. These altered biotic and abiotic environments will likely facilitate further invasion of NM in the forests, hence positive feedbacks, and make it an increasingly serious tree invader in North America.

中文翻译：

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关于北美城市树木入侵者Acer platanoides对北美森林生态系统过程影响的野外实验研究

入侵物种影响社区动态和生态系统功能，但对其影响的机制了解甚少。入侵影响的假设范围从超级竞争对手到新型功能，从敌人逃逸到微生物中介。在这项研究中，我们通过分块设计试验研究了城市树木入侵者Acer platanoides（挪威枫树，NM）对中性落叶森林与其原生同种Acer rubrum（红色枫树，RM）的影响。将总共​​720棵枫树幼苗移植到3种树冠类型的24棵树下的72个样地中。三种实验方法是消除地上和地下的资源竞争以及消除枯叶凋落的影响。每个样地都跟踪土壤水分和氮相关的微生物活性。我们发现，部分冠层清除增加了冠层的开放度和向森林地面的光传输，但在NM下比在RM树下更大。在高度生长方面，NM幼苗比RM幼苗更耐荫。在混交林互育期间，RM树下NM幼苗的生物量积累是NM下的两倍，而NM树下RM幼苗的生物量却比RM下少23.5％。土壤净硝化作用和相对硝化作用显着提高，但NM下的矿化速率远低于RM树木下的矿化速率，这将导致土壤中氮的淋失更快，氮的有效性降低。到2年末，去除草皮的地块由于食草而具有更高的幼苗死亡率，尤其是对于NM树下的NM幼苗。沟增加了土壤水分的利用率，但冠层类型之间没有差异。我们的结果表明，NM的入侵不仅改变了森林的冠层结构，而且还改变了幼苗的食草率和土壤中的N动态。NM冠层下的NM幼苗比RM幼苗更耐荫，NM冠下的NM凋落物比RM冠层更能保护NM。这些变化的生物和非生物环境将可能促进NM在森林中的进一步入侵，从而获得积极的反馈，并使之成为北美日益严重的树木入侵者。NM冠层下的NM幼苗比RM幼苗更耐荫，NM冠下的NM凋落物比RM冠层更能保护NM。这些变化的生物和非生物环境将可能促进NM在森林中的进一步入侵，从而获得积极的反馈，并使之成为北美日益严重的树木入侵者。NM冠层下的NM幼苗比RM幼苗更耐荫，并且在NM冠下的NM凋落物比在RM冠层下更受NM垫料保护。这些变化的生物和非生物环境将可能促进NM在森林中的进一步入侵，从而获得积极的反馈，并使之成为北美日益严重的树木入侵者。