In support of the prediction of the enemy release hypothesis regarding a growth-defense trade-off, invasive alien plants often exhibit greater growth and lower anti-herbivory defenses than native plants. However, it remains unclear how nutrient enrichment of invaded habitats may influence competitive interactions between invasive alien and co-occurring native plants, as well as production of anti-herbivore defense compounds, growth-promoting hormones, and defense-regulating hormones by the two groups of plants. Here, we tested whether: (i) nutrient enrichment causes invasive alien plants to produce greater biomass and lower concentrations of the defense compounds flavonoids and tannins than native plants; and (ii) invasive alien plants produce lower concentrations of a defense-regulating hormone jasmonic acid (JA) and higher concentrations of a growth-promoting hormone gibberellic acid (GA3). In a greenhouse experiment, we grew five congeneric pairs of invasive alien and native plant species under two levels each of nutrient enrichment (low vs. high), simulated herbivory (leaf clipping vs. no-clipping), and competition (alone vs. competition) in 2.5-L pots. In the absence of competition, high-nutrient treatment induced a greater increase in total biomass of invasive alien species than that of native species, whereas the reverse was true under competition as native species benefitted more from nutrient enrichment than invasive alien species. Moreover, high-nutrient treatment caused a greater increase in total biomass of invasive alien species than that of native species in the presence of simulated herbivory. Competition induced higher production of flavonoids and tannins. Simulated herbivory induced higher flavonoid expression in invasive alien plants under low-nutrient than high-nutrient treatments. However, flavonoid concentrations of native plants did not change under nutrient enrichment and simulated herbivory treatments. Invasive alien plants produced higher concentrations of GA3 than native plants. Taken together, these results suggest that impact of nutrient enrichment on growth of invasive alien and co-occurring native plants may depend on the level of competition that they experience. Moreover, invasive alien plants might adjust their flavonoid-based defense more efficiently than native plants in response to variation in soil nutrient availability and herbivory pressure. Our findings suggest that large-scale efforts to reduce nutrient enrichment of invaded habitats may help to control future invasiveness of target alien plant species.
天敌逃逸假说预测:由于在入侵地逃逸了部分天敌，外来入侵植物往往比本地植物生长更快，但是其对食草动物防御能力更低。然而，养分富集如何影响外来入侵植物与本地植物在入侵地的竞争能力和对食草动物的防御能力、以及如何影响调控其生长与防御的相关激素含量仍需进一步探究并予以明确。因此，本文拟验证以下科学问题:(i)与本地植物相比，养分富集是否会促使外来入侵植物积累更多的生物量和产生更低的防御物质(类黄酮和单宁)；(ii)与本地植物相比，外来入侵植物是否会产生较少的用于调控植物防御的激素茉莉酸，以及较多的用于调控生长的激素赤霉素。通过温室控制实验，向5对同属配对的外来入侵植物与本地植物进行养分添加(低vs. 高)，模拟虫食(剪叶vs. 不剪叶)，竞争(单种vs. 竞争)，及其交互作用处理。在无竞争条件下，养分添加对外来入侵植物总生物量的增加效应强于本地植物，而在竞争条件下呈现出相反的结果:即本地植物比外来入侵植物从养分富集中受益更多。此外，在模拟虫食条件下，养分添加对外来入侵植物的总生物量促进作用强于本地植物。在竞争条件下，植物产生了更多的类黄酮和单宁。与高养分水平相比，低养分水平下的模拟虫食处理对外来入侵植物类黄酮含量的促进效应更强。然而，养分添加和模拟虫食处理均未改变本地植物的类黄酮含量。另外，外来入侵植物的赤霉素含量高于本地植物。综上，这些结果表明养分富集对外来入侵植物和本地植物生长的影响取决于他们的竞争水平。此外，在养分富集和虫食处理下，外来入侵植物可能比本地植物更高效的调节防御组分类黄酮的含量变化。因此，大范围的降低入侵地的养分富集可能有助于控制外来植物的入侵。.
Keywords: competition; exotic; invasion ecology; nutrient; phytohormone; secondary metabolites.
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