Increasing nitrogen deposition and more frequent drought events are likely to change plant interactions in natural grasslands. Both factors may also influence the interactions between hemiparasitic plants, regarded as keystone species in many grasslands, and their host species. We grew a combination of three suitable hosts, a grass, a forb and a legume, with and without the hemiparasite Rhinanthus alectorolophus at three levels of nitrogen (N) and two levels of water availability in a factorial design. Biomass of the hemiparasite and host community increased with N level and was reduced by drought to a similar degree. Larger plants in fertilised pots started to wilt earlier, and the presence of a hemiparasite further increased drought sensitivity. The hemiparasite strongly reduced biomass of the host community and overall productivity, and affected the competitive balance among host plants because it particularly reduced biomass of the dominant grass. These effects were the opposite of those of high N. The hemiparasite increased the root mass fraction of the hosts at all levels of N and water availability, indicating that the effect of the hemiparasite on the hosts was mainly due to loss of belowground resources. Our results indicate that hemiparasites will not always respond more strongly to increased N availability and drought than autotrophic plants, and that hemiparasites can have similarly strong effects on grassland communities as soil fertility and drought. By preferentially attacking dominant species the hemiparasites might alleviate the negative effects of nutrient enrichment on grassland diversity.

中文翻译：

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干旱和氮水平对三种寄主结合的根寄生半生Rhinanthus alectorolophus相互作用的影响。

越来越多的氮沉降和更频繁的干旱事件可能会改变天然草原上的植物相互作用。这两个因素也可能影响半寄生植物（在许多草原中被认为是主要的树种）与其寄主物种之间的相互作用。在因子设计中，我们将三种合适的宿主（草，草和豆科植物）组合在一起，并在半水平的氮（N）和两个水平的水利用率下添加和不添加半寄生性Rhinanthus alectorolophus。半寄生虫和寄主群落的生物量随着氮水平的增加而增加，并因干旱而减少到相似的程度。受精盆中较大的植物开始枯萎，而且半寄生虫的存在进一步增加了干旱敏感性。半寄生虫极大地减少了寄主社区的生物量和整体生产力，并影响了寄主植物之间的竞争平衡，因为它尤其减少了优势草的生物量。这些效应与高氮相反。在所有水平的氮和水供应下，半寄生虫均增加了寄主的根质量分数，这表明半寄生虫对寄主的影响主要是由于地下资源的损失。我们的结果表明，半寄生虫并不总是比自养植物对氮素利用率和干旱的反应更强烈，并且半寄生虫对草原群落的土壤肥力和干旱具有类似的强大影响。通过优先攻击优势种，半寄生虫可减轻养分富集对草地多样性的负面影响。