Abstract Low-input grasslands depend on the presence of grasses and legumes for the provision of nitrogen and a sufficient amount and quality of herbage for livestock. Increasing variability in the amount and distribution of rainfall, associated with climate change, is a major factor affecting production from permanent grassland and particularly grass-clover swards. Previous work has shown that introduction of deep-rooting dicotyledonous forbs into grass-legume mixtures increases functional trait diversity (FTD), which facilitates the resistance and resilience of grassland to drought. It also increases species diversity and functional group diversity (FGD). However, the role of species diversity at a given level of FGD and associated FTD for the resistance and resilience of low-input grassland towards drought remains unclear. This knowledge is required to design forage mixtures adapted to drier climatic conditions. We conducted a drought-stress experiment with 16 different sown sward types, grown in cylindrical mesocosms, combining three levels of species diversity (one, three, five) and three levels of FGD (one, two, three) all being associated with FTD in terms of phenology, rooting depth and nitrogen fixation to determine whether maximum levels of species diversity or FGD promote adaptation to drought. The study was conducted over three drought-stress cycles in an open-sided greenhouse where the climate followed ambient natural conditions. Over the duration of 685 days, we found no clear species diversity effect but a distinct species identity effect on biomass production and agronomic water use efficiency. A positive species identity effect depended largely on white clover presence and on the FGD within mixtures. The best performing mixtures for resistance and resilience combined FGD and FTD in a complementary way, irrespective of whether the swards contained three or five species, as FGD and FTD were already maximized within the best-performing three-species swards. Increasing complementarity of traits by species grown in carefully designed mixtures is one measure to achieve diversity effects and facilitate future sustainable grassland production. We propose to exploit the benefits of species identity and functional group diversity in designing future mixtures to cope with drought.

中文翻译：

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耐旱性由物种身份和功能群多样性决定，而不是由多物种区域内的物种多样性决定

摘要 低投入草原依赖于草类和豆类的存在以提供氮，以及为牲畜提供​​足够数量和质量的牧草。与气候变化相关的降雨量和分布的可变性越来越大，是影响永久性草原特别是三叶草草地生产的主要因素。先前的工作表明，将根深蒂固的双子叶杂草引入草豆科植物混合物会增加功能性状多样性 (FTD)，从而促进草原对干旱的抵抗力和恢复力。它还增加了物种多样性和功能群多样性 (FGD)。然而，在给定 FGD 和相关 FTD 水平下，物种多样性对低投入草地对干旱的抵抗力和恢复力的作用仍不清楚。需要这些知识来设计适应较干燥气候条件的饲料混合物。我们对 16 种不同的播种草进行了干旱胁迫实验，这些草在圆柱形中层中生长，结合了三个级别的物种多样性（一、三、五）和三个级别的 FGD（一、二、三），所有这些都与 FTD 相关。物候、生根深度和固氮的术语，以确定最大水平的物种多样性或 FGD 是否促进对干旱的适应。该研究是在气候遵循周围自然条件的开放式温室中进行的三个干旱胁迫循环。在 685 天的持续时间内，我们没有发现明显的物种多样性影响，但对生物量生产和农艺用水效率有明显的物种同一性影响。积极的物种身份效应在很大程度上取决于白三叶草的存在和混合物中的 FGD。抗性和弹性方面表现最好的混合物以互补的方式结合了 FGD 和 FTD，无论草丛是包含三个还是五个物种，因为 FGD 和 FTD 在表现最佳的三个物种草丛中已经最大化。在精心设计的混合物中种植的物种增加性状的互补性是实现多样性效应和促进未来可持续草原生产的一项措施。我们建议在设计未来混合物以应对干旱时利用物种特性和功能群多样性的好处。无论草丛包含三个还是五个物种，因为 FGD 和 FTD 在表现最佳的三个物种草丛中已经最大化。在精心设计的混合物中种植的物种增加性状的互补性是实现多样性效应和促进未来可持续草原生产的一项措施。我们建议在设计未来混合物以应对干旱时利用物种特性和功能群多样性的好处。无论草丛包含三个还是五个物种，因为 FGD 和 FTD 在表现最佳的三个物种草丛中已经最大化。在精心设计的混合物中种植的物种增加性状的互补性是实现多样性效应和促进未来可持续草原生产的一项措施。我们建议在设计未来混合物以应对干旱时利用物种特性和功能群多样性的好处。