Increasing frequency and magnitude of climatic extremes, such as heat waves are expected to enhance abiotic stresses on ecological communities. It has been proposed that ecological communities in disturbed habitats may be most sensitive to climatic extremes, as disturbance may reduce density and diversity of higher trophic level organisms like predators. However, there is little experimental evidence that climatic extremes indeed have stronger impact on functioning of such trophically downgraded ecosystems. Here, we experimentally examine how removal of predators from soil communities affects plant performance under periodic heat waves. We used a native plant species, and a congeneric native that is currently expanding its range because of climate change. We used soil nematode communities as the model system, as these are most abundant soil animals and their communities are trophically diverse. Predatory nematodes were manually removed from intact soil nematode communities (mainly the adults as some juveniles are impossible to manually remove) to create a trophically downgraded soil. Intact nematode communities and communities with reduced predatory nematodes were added separately to soils that were planted with either the native Centaurea jacea or the range-expanding congener Centaurea stoebe. Half the experimental units were exposed periodically to experimental heat waves of 10 °C above the control temperature. Our results show that the C: N and C: N: P ratio of plant shoots in predator-reduced soils became lower when exposed to periodic heat waves, however, only in the native plant C. jacea. The decrease in C: N ratio corresponded with increase of an herbivorous nematode in trophically intact soils of C. jacea independent of warming, whereas this relationships disappeared in warmed and predator-reduced soils. Our results accordingly highlight that periodic heat waves may affect stoichiometry of certain plant species by altering trophic interactions in predator-reduced soils.

预计诸如热浪之类的气候极端事件的频率和强度将增加对生态社区的非生物压力。有人提出，受干扰的生境中的生态群落可能对极端气候最为敏感，因为干扰可能会降低诸如食肉动物之类的较高营养级别生物的密度和多样性。但是，几乎没有实验证据表明极端气候确实对这种被营养不良的生态系统的功能产生更大的影响。在这里，我们实验性地研究了在周期性热浪下从土壤群落中去除捕食者如何影响植物的生长性能。我们使用的是本地植物物种，由于气候变化，目前正在扩大其范围。我们以土壤线虫群落为模型系统，因为它们是最丰富的土壤动物，其群落在营养上也很多样化。从完整的土壤线虫群落中手动清除掠食性线虫（主要是成虫，因为某些幼虫无法手动清除），从而造成营养不良的土壤。将完整的线虫群落和掠食性线虫减少的群落分别添加到种植了天然线虫的土壤中矢车菊（Centaurea jacea）或范围扩大的同类物矢车菊（Centaurea stoebe）。一半的实验单元定期暴露于高于控制温度10°C的实验热波中。我们的结果表明，当暴露于周期性热浪中时，被捕食者减少的土壤中的植物芽的C：N和C：N：P比变得更低，但是仅在天然植物C. jacea中。C：N比值​​的减少与在营养完好无损的C. jacea土壤中的草食性线虫的增加相对应与变暖无关，而这种关系在变暖和被捕食者减少的土壤中消失了。因此，我们的结果表明，周期性热浪可能会通过改变捕食者减少的土壤中的营养相互作用而影响某些植物的化学计量。