Plant water uptake along a diversity gradient provides evidence for complementarity in hydrological niches

Biodiversity enhances a variety of ecosystem processes, and yet the underlying mechanisms through which these relationships occur remain a critical knowledge gap. Here, we used the natural abundance of stable isotopes to measure depth of water uptake in five common grassland species (Asclepias tuberosa, Lespedeza capitata, Liatris aspera, Schizachyrium scoparium, and Sorghastrum nutans) growing across an experimental grassland diversity gradient. Using this approach, we addressed the following questions: (1) Does the depth-specific provenance of water uptake differ among species and/or do interspecific differences in water source manifest with increasing community diversity? (2) Does the isotopic niche space occupied by plants change with increasing diversity? (3) Is plasticity in water uptake depth across a diversity gradient associated with functional plant responses? We found that the depth of soil water used by plants was inherently different among species when grown in monocultures. All species used less shallow soil water and more intermediate-depth soil water in mixed assemblages than in monocultures, resulting in similar interspecific differences in water source across the diversity gradient. However, plasticity in the locations of water used were positively associated with increases in plant growth in higher diversity treatments. These results indicate that plasticity in water-use may contribute to positive biodiversity-productivity relationships commonly observed in temperate grasslands.