Niche complementarity is often used to explain positive biodiversity-productivity relationships, yet interspecific differences in resource use are seldom measured in experimentally manipulated plant communities. Here, we evaluated hydrological niche space for common grassland species growing within the ‘BigBio’ Biodiversity Experiment at the Cedar Creek LTER in central Minnesota, USA. Specifically, we addressed the following questions: (1) Does hydrological niche complementarity exist in a diverse grassland? (2) If so, does this result from inherent species differences in water uptake or from plastic responses to changes in biodiversity? (3) Do water uptake patterns impact plant performance (e.g., transpiration, water potential)? We measured leaf physiology and depth of water use on two C4 grasses (Sorghastrum nutans and Schizachyrium scoparium) and three C3 forbs (Asclepias tuberosa, Liatris aspera, and Lespedeza capitata) in plots containing 1, 2, 4, and 16 grassland species. Water was extracted from plant tissue and soil cores using a cryogenic vacuum distillation method and analyzed for water isotope (δD and δ18O) signatures. Isotope data were then used in a Bayesian isotope mixing-model to determine the depth of water uptake for each species growing across the diversity gradient.
Results/Conclusions
We found that the depth of water utilized by plants varied across species and through time. The C4 grasses, S. nutans and S. scoparium, used a greater proportion of shallow soil water than the C3 forbs, and these differences were most pronounced during the middle of the growing season. We also found that both C4 grasses and C3 forbs utilized different proportions of shallow and deep soil water depending on species diversity. For instance, A. tuberosa utilized 40 % shallow water when grown as a monoculture and less than 10% shallow water when grown with 16 species. However, these differences in water uptake did not correspond with differences in leaf physiology. These results indicate that hydrological niche complementarity does occur in diverse grasslands and results as plants shift their reliance on water sources with changes in the surrounding plant community. Future studies should further investigate physiological mechanisms underpinning the positive biodiversity-productivity phenomenon.