Latitudinal variation in root neighborhood β-diversity and its role in belowground community assembly

Understanding the geographic patterns of root neighborhood β-diversity is critical for ecology, biogeography, and conservation biology. However, the mechanisms driving belowground β-diversity in response to root functional traits and phylogenetic structure remain unexplored in grassland plant communities. We analyzed eight root functional traits for 110 herbaceous species from 675 root clusters across 207 soil samples along a latitudinal gradient in the northeastern Tibetan Plateau, China. Probabilistic hypervolumes were used to estimate root trait space, while mean pairwise distance (D_pw) and mean nearest neighbor distance (D_nn) quantified species' ecological differences. β-diversity was partitioned into spatial and environmental components to infer the roles of abiotic filtering, dispersal limitation, and biotic interactions. Our results indicate that 71.5 % of root functional traits were clustered, suggesting abiotic filtering as the primary driver of root aggregation. Root neighborhood functional and phylogenetic D_pw exhibited a distance decay pattern, reflecting abiotic filtering or dispersal limitation. In contrast, root functional D_nn showed an inverse distance decay pattern, suggesting strong abiotic filtering effects among species with distinct functional traits. Soil factors explained more variation than geographic distance, and the combined effects of spatial and environmental variables had the strongest influence on phylogeny. These findings highlight the potential for trait differentiation even in environmentally harsh ecosystems, with the relative influence of abiotic filtering and biotic interactions shifting along a latitudinal gradient. Our findings emphasize the necessity of integrating belowground trait diversity into grassland conservation and restoration strategies. Specifically, maintaining environmental heterogeneity could enhance underground biodiversity and ecological resilience, which is critical under escalating global anthropogenic pressures. Understanding these belowground assembly processes will improve predictions of ecosystem responses and support effective grassland biodiversity management under future environmental scenarios.