The taxonomic diversity of the weed flora and seedbank in agricultural fields with a history of applying glyphosate-resistant (GR) crops is influenced by geographical regions, GR genetic traits, and years, and community assembly is shaped by environmental filtering and biotic drivers alone or interactively. Phylogenetic diversity as an intrinsic biodiversity component illustrates the evolutionary history of species in a community, which could build bridges between evolutionary potential and ecosystem functioning, and is a promising way to explain the role of species interactions and biogeographic history in community structure and composition. However, the effects of assembly drivers on phylogenetic diversity and structure of the soil seedbank in crop fields are largely unknown. The objective of this study was to investigate how the influence of geography and soybean cropping systems affect phylogenetic diversity of the weed seedbank over time. A large, field-scale study was conducted across 5 plant hardiness zones across the mid-western and southeastern coastal United States over 6 years. Weed species emerging from soil seedbank samples were identified and counted, and used to construct a phylogenetic tree including 234 species. Mixed effects models were constructed to evaluate regions, cropping systems, and years on four phylogenetic metrics.
Results/Conclusions
Plant hardiness zone (Wald’s-χ2=200.00, P<0.001) and its interaction with year (Wald’s-χ2=28.35, P<0.001) and with cropping system (Wald’s-χ2=15.23, P=0.018) had highly significant effects on phylogenetic redundancy. The test on abundance weighted Faith’s phylogenetic diversity showed a highly significant effect of plant hardiness zone (Wald’s-χ2=35.71, P<0.001) and its interaction with cropping system (Wald’s-χ2=23.96, P=0.001). Meanwhile, cropping system and plant hardiness zone individually showed a highly significant effect on the standardized effect size of mean pairwise distances (sesMPD, cropping system, Wald’s-χ2=13.95, P=0.001; plant hardiness zone, Wald’s-χ2=23.69, P<0.001). However, when seed bank abundance of species was considered for testing the effects on phylogenetic divergence, all the variables showed no significance. These results showed that the weed seedbank in plant hardiness zones 7 and 8 showed higher phylogenetic divergence and lower phylogenetic richness than in plant hardiness zones 4, 5, and 6. The cropping system with a rotation between two GR crops exhibited a more over-dispersed phylogenetic pattern than either a single continuous GR crop or a rotation between a GR crop and a non-GR crop. This study provides a valuable evolutionarily based characterization of community assembly in an agricultural ecosystem and highlights the value of studying the soil seedbank in community ecology.