Through their rooting profiles and water demands, plants affect the distribution of water in the soil profile. Simultaneously, soil water content controls plant development and interactions within and between plant communities. These plant-soil water feedbacks might vary across plant communities with different rooting depths and species composition. In semiarid environments, understanding these differences will be essential to predict how ecosystems will respond to drought, which may become more frequent and severe with climate change. In this study, we tested how plant-soil water feedbacks responded to drought in two contrasting ecosystem types—grassland and shrubland—in the coastal foothills of southern California. During years 5–8 of an ongoing precipitation manipulation experiment, we measured changes in plant communities and soil moisture up to 2 m depth. We observed different water use patterns in grassland and shrubland communities with distinct plant functional types and water use strategies. Drought treatment did not affect perennial, deep-rooted shrubs because they could access deep soil water pools. However, mid-rooted shrubs were sensitive to drought and experienced decreased productivity and die-off. As a result, water content actually increased with drought at soil depths from 50–150 cm. In grassland, biomass production by annual species, including annual grasses and forbs, declined with drought, resulting in lower water uptake from the surface soil layer. An opportunistic “live fast, die young“ life strategy allowed these species to recover quickly once water availability increased. Our results show how drought interacts with plant community composition to affect the soil water balance of semiarid ecosystems, information that could be integrated into global scale models.

植物通过其根系剖面和需水量影响土壤剖面中的水分布。同时，土壤含水量控制着植物的发育以及植物群落内部和群落之间的相互作用。这些植物-土壤水反馈可能因根部深度和物种组成不同的植物群落而异。在半干旱环境中，了解这些差异对于预测生态系统如何应对干旱至关重要，随着气候变化，干旱可能变得更加频繁和严重。在这项研究中，我们测试了南加州沿海山麓两种截然不同的生态系统类型（草地和灌木丛）中植物-土壤水反馈如何应对干旱。在正在进行的降水操纵实验的第 5-8 年期间，我们测量了 2 m 深处植物群落和土壤湿度的变化。我们观察到具有不同植物功能类型和用水策略的草地和灌木丛群落的不同用水模式。干旱处理不会影响多年生、深根灌木，因为它们可以进入深层土壤水池。然而，中根灌木对干旱敏感，生产力下降，甚至死亡。结果，50-150 厘米土壤深度的含水量实际上随着干旱而增加。在草原中，一年生物种（包括一年生草和禾本科植物）的生物量产量随着干旱而下降，导致表土层吸水量减少。一旦可用水量增加，机会主义的“快活，早逝”的生活策略使这些物种能够迅速恢复。