Data from: A native plant competitor mediates the impact of above- and belowground damage on an invasive tree

Plant competition may mediate the impacts of herbivory on invasive plant species through effects on plant growth and defense. This may predictably depend on whether herbivory occurs above- or belowground and on relative plant competitive ability. We simulated the potential impact of above- or belowground damage by biocontrol agents on the growth of a woody invader (Chinese tallow tree, Triadica sebifera) through artificial herbivory, with or without competition with a native grass, little bluestem (Schizachyrium scoparium). We measured two defense responses of Triadica through quantifying constitutive and induced extrafloral nectar production and tolerance of above- and belowground damage (root and shoot biomass regrowth). We examined genetic variation in plant growth and defense across native (China) and invasive (US) Triadica populations. Without competition, aboveground damage had a greater impact than belowground damage on Triadica performance, whereas with competition, above- and belowground damage impacted Triadica similarly. Whole plant tolerance to damage belowground was negatively associated with tolerance to grass competitors indicating tradeoffs in the ability to tolerate herbivory versus compete. Competition reduced investment in defensive extrafloral nectar (EFN) production. Aboveground damage inhibited rather than induced EFN production while belowground plant damage did not impact aboveground nectar production. We found some support for the evolution of increased competitive ability hypothesis for invasive plants as US plants were larger than native China plants and were more plastic in their response to biotic stressors than China plants (they altered their root to shoot ratios dependent on herbivory and competition treatments). Our results indicate that habitat type and the presence of competitors may be a larger determinant of herbivory impact than feeding mode and suggest that integrated pest management strategies including competitive dynamics of recipient communities should be incorporated into biological control agent evaluation at earlier stages.

植物竞争可通过调控植物生长与防御过程，介导植食作用对入侵植物的影响。该效应可预测地取决于植食作用发生于地上还是地下，以及植物的相对竞争能力。本研究通过人工模拟植食作用，探究了生物防治天敌造成的地上或地下损伤对木本入侵种乌桕（Triadica sebifera）生长的影响，实验设置了与本土草种小须芒草（Schizachyrium scoparium）共存及无共存两种竞争处理。我们通过量化组成型与诱导型花外蜜（extrafloral nectar, EFN）产量，以及对地上、地下损伤（即地上部与地下根系的生物量恢复能力）的耐受性，测定了乌桕的两种防御响应。我们检测了源自本土分布区（中国）与入侵分布区（美国）的乌桕种群在生长与防御性状上的遗传变异。在无竞争条件下，地上损伤对乌桕生长表现的影响显著大于地下损伤；而在有竞争条件下，地上与地下损伤对乌桕的影响程度相近。整株植物对地下损伤的耐受性与对草类竞争者的耐受性呈负相关，表明植物在耐受植食作用与竞争能力之间存在适应性权衡。竞争会降低植物在防御性花外蜜生产上的资源投入。地上损伤会抑制而非诱导花外蜜的产生，而地下植株损伤不会影响地上花外蜜的产量。我们的研究结果为入侵植物的增强竞争能力进化假说（evolution of increased competitive ability hypothesis）提供了部分支持：美国种群的植株体型大于中国种群，且对生物胁迫的响应可塑性更强——它们会根据植食作用与竞争处理调整根冠比。本研究结果表明，生境类型与竞争者的存在，可能比植食者的取食模式更能决定植食作用的影响强度；同时建议在早期阶段开展生物防治天敌评估时，应将接收群落的竞争动态纳入有害生物综合治理（integrated pest management）策略框架中。