Data from: Fine-scale belowground species associations in temperate grassland

Evaluating how belowground processes contribute to plant community dynamics is hampered by limited information on the spatial structure of root communities at the spatial scale that plants interact belowground. In this study, roots were mapped to the nearest one mm and molecularly identified by species on vertical (0 – 15 cm deep) surfaces of soil blocks excavated from dry and mesic grasslands in Yellowstone National Park (YNP) to examine for the first time the spatial relationships among species at the scale that root – root interactions occur. Our results indicated YNP grassland root communities were comprised of intertwined networks of species, with average interspecific root – root distances for the majority of species within a distance (3 mm) that roots have been shown to compete for resources. We found that most species placed their roots at random; although low root numbers for many species likely led to overestimating the occurrence of random patterns. Based on theory, we expected that most of the remaining species would segregate their root systems from other species to avoid competition. Instead we found that more species aggregated than segregated from others. This study did not investigate mechanisms responsible for root patterns. However, according to a large body of evidence that roots increase the local availability water (via hydraulic redistribution), nitrogen (triggered by root exudates), and phosphorus (via community-level phosphatase production), we hypothesize that YNP species aggregate to facilitate resource uptake. Our results indicate that YNP grassland root communities are organized as closely interdigitating networks of species that can support strong interactions among many species combinations. Future root research should address the prevalence and functional consequences of species aggregation across plant communities.