Soil Moisture Regulates Hydrologic Niche Overlap in Mixed Plant Ecosystems: A Global Synthesis

Abstract Mixed plant ecosystems (MPEs) play a crucial role in climate‐smart ecosystems by optimizing resource use and enhancing sustainability. However, ongoing climate change may exacerbate hydrologic niche overlap in MPEs by altering water availability, which could enhance interspecific water competition and undermine the sustainability of these systems. Despite its importance, the variation and dominant factors influencing hydrologic niche overlap remain understudied in MPEs. Here, we synthesized 2,152 observations from 65 studies worldwide which analyzed hydrologic niche overlap using stable water isotope techniques, covering various plant mixtures, including woody‐herbaceous, woody‐woody, and herbaceous‐herbaceous combinations. Our results show that herbaceous plants primarily rely on shallow soil water (0–40 cm), while woody plants in drylands seasonally switch between shallow and deep soil water sources. As a result, woody‐herbaceous systems exhibit the lowest niche overlap index (NOI), while herbaceous‐herbaceous systems show the highest. Given these rooting patterns, soil moisture emerges as the primary factor regulating NOI across all systems. Specifically, in woody‐herbaceous systems, NOI increases with both shallow and deep soil water content, whereas in woody‐woody and herbaceous‐herbaceous systems, NOI decreases as available soil moisture increases. This study reveals the impact of plant mixture on water use, deepens the understanding of hydrologic niche differentiation, and emphasizes the importance of designing MPEs to enhance their adaptability and sustainability.