Data from: Fungal symbiont effects on dune plant diversity depend on precipitation

1. Historically, mutualisms have been considered to be less important than antagonisms in affecting the composition of ecological communities. In plant communities, beneficial microbes may feature as keystone mutualists in structuring community composition. Understanding the direction and magnitude of mutualist effects at the community scale may be critical for making accurate predictions on plant responses to climate change, particularly for mutualists that ameliorate climate-induced stressors. Such mitigation could shift outcomes between mutualist-enhanced species diversity and mutualist-reduced diversity, depending on whether a mutualist accelerates habitat modification or competitive exclusion by its partner species. 2. Here, we tested the relative importance and interactive effects of altered precipitation and symbiosis between an epichloid fungal endophyte and a dominant grass species for dune plant communities along the Great Lakes, USA. In 2010, we imposed field manipulations of endophyte presence in the foundation dune grass Ammophila breviligulata in combination with rain-out shelters and rainfall additions. We monitored natural rates of colonization by new plant individuals over three years. 3. Under the current precipitation regime, endophyte symbiosis in A. breviligulata reduced colonizing plant diversity, species richness, and evenness. This effect depended on the amount of precipitation, with the symbiosis having weaker effects on plant diversity under both augmented and reduced rainfall treatments. 4. Despite the overall negative effect of endophyte symbiosis on plant diversity, plant responses to the endophyte were species-specific. A federally threatened forb, Cirsium pitcheri, increased in abundance when the symbiosis was present, regardless of precipitation regime. Endophyte symbiosis in A. breviligulata caused minor reductions in the abundance of other grass species; however, augmented precipitation benefited other grasses. 5. Synthesis. We show that microbial mutualisms can have strong effects on community structure in a native ecosystem and that the amount of precipitation has the potential to alter how these keystone species interactions affect community composition. Predictions on future plant community structure, for both restored and native dunes, can be improved by accounting for the presence of fungal symbionts in the foundation plant species.1. 过往研究普遍认为，在调控生态群落组成的过程中，互利共生作用（mutualisms）的重要性弱于种间拮抗作用（antagonisms）。在植物群落中，有益微生物可作为关键互利共生种（keystone mutualists）参与群落结构的构建。明晰群落尺度下互利共生效应的方向与强度，对于精准预测植物对气候变化的响应至关重要，尤其针对那些能够缓解气候诱导胁迫的共生伙伴。此类胁迫缓解效应可能会在“共生促进物种多样性”与“共生降低物种多样性”两种结局之间产生偏移，具体取决于该互利共生菌是否会加速其宿主的生境改造，或是加速伴生物种的竞争排除过程。 2. 本研究以美国五大湖（Great Lakes）沿岸的沙丘植物群落（dune plant communities）为研究对象，探究了降水格局改变与香柱菌属真菌内生菌（epichloid fungal endophyte）、优势禾草物种之间的共生关系的相对重要性及其交互效应。2010年，我们对建群沙丘草短颖沙茅（Ammophila breviligulata）的内生菌定殖状态开展了野外操控实验，即设置内生菌存在与不存在两组处理，并结合防雨棚（rain-out shelters）与增雨处理（rainfall additions）调控降水量。随后连续三年监测了新植物个体的自然定居速率。 3. 在当前降水格局下，短颖沙茅体内的内生菌共生作用会降低定居植物多样性（colonizing plant diversity）、物种丰富度（species richness）与物种均匀度（evenness）。该效应随降水量的变化而改变：在增雨与减雨处理下，共生作用对植物多样性的影响均有所减弱。 4. 尽管内生菌共生作用整体上对植物多样性产生了负面影响，但不同植物物种对内生菌的响应存在特异性。其中一种受美国联邦政府列为受威胁的杂类草（forb）——皮彻蓟（Cirsium pitcheri），在存在内生菌共生的情况下种群丰度有所提升，且该效应不受降水格局的影响。短颖沙茅体内的内生菌共生作用仅会轻微降低其他禾草类物种的丰度；而增雨处理则对其他禾草类物种的生长更为有利。 5. 综合分析与结论。本研究表明，微生物互利共生作用可对本土生态系统的群落结构产生显著调控效应，且降水量可通过改变这类关键物种种间互作的方式，进而影响群落组成。未来针对修复沙丘与原生沙丘的植物群落结构预测，可通过纳入建群植物物种中的真菌共生菌信息，提升预测精度。