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Plant succession is regulated by a combination of abiotic and biotic factors. However, previous studies of biotic drivers have focused overwhelmingly on direct pairwise species interactions, ignoring the likely prevalent higher-order interactions (HOIs) in natural systems. Climate also plays a significant role in determining successional dynamics with both direct effects and indirect effects via altered biotic interactions. Here we explored the relative effects of direct species interactions, HOIs, climate, and their interactions on population dynamics of herbaceous plants during 50 years of post-agricultural secondary succession and tested whether the inclusion of HOIs and climate data improved forecasts of population dynamics. Direct intraspecific interactions were competitive and prevalent across the 90 herbaceous plants examined, while direct interspecific interactions only affected populations of 29% species. HOIs, mainly arose from intraspecific HOIs of conspecifics, were mostly positive and thus largely mitigated the competitive effects of direct intraspecific interactions. Species with lower peak cover experienced stronger intraspecific competition and positive intraspecific HOIs of conspecifics. Direct interspecific interactions had neutral or facilitative effects on species with lower peak cover, and tended to have competitive effects on species with higher peak cover. Climate simultaneously influenced population dynamics both directly and indirectly via altered species interactions. Forecast performance was significantly improved with the inclusion of HOIs or climate for about half and one-third of species, respectively. Our study emphasizes the importance of HOIs, which largely mitigated direct competitive effects on population dynamics of herbaceous plants during succession. Teasing apart HOIs from direct species interactions substantially refined our understanding of successional dynamics of herbaceous plants and improved the accuracy of forecasting population dynamics during succession in a changing world.

Despite the importance of fine roots for the acquisition of soil resources such as nitrogen and water, the study of linkages between traits and both population and community dynamics remains focused on aboveground traits. We address this gap by investigating associations between belowground traits and metrics of species dynamics. Our analysis included 85 species from a long‐term data set on the transition from old field to forest in eastern North America (the Buell‐Small Succession Study) and the new Fine‐Root Ecology Database. Given the prominent roles of life form (woody vs. non‐woody) and species origin (native vs. exotic) in defining functional relationships, we also assessed whether traits or their relationships with species dynamics differed for these groups. Species that reached their peak abundance early in succession had fine‐root traits corresponding to resource acquisitive strategies (i.e., they were thinner, less dense, and had higher nitrogen concentrations) while species that peaked progressively later had increasingly conservative strategies. In addition to having more acquisitive root traits than native species, exotics diverged from the above successional trend, having consistently thinner fine roots regardless of the community context. Species with more acquisitive fine‐root morphologies typically had faster rates of abundance increase and achieved their maximal rates in fewer years. Decreasing soil nutrient availability and increasing belowground competition may become increasingly strong filters in successional communities, acting on root traits to promote a transition from acquisitive to conservative foraging. However, disturbances that increase light and soil resource availability at local scales may allow acquisitive species, especially invasive exotics, to continue colonizing late into the community transition to forest.