Initial vs long-term effects of intraspecific spatial aggregation on community structure of a tallgrass prairie restoration

Background/Question/Methods

Competition plays a major role in limiting plant diversity. Since plants interact most strongly with their immediate neighbors, the spatial structure of plant communities is an important factor mediating competition. According to basic competition theory, species are more likely to coexist when populations experience relatively more intraspecific than interspecific competition, such that species suppress themselves more than others. In tallgrass prairie restorations, seeds are typically mixed and spread relatively uniformly across a site, which may encourage competitive exclusion by promoting interspecific interactions. Spatially aggregated seeding may improve biodiversity in prairie restorations by concentrating intraspecific interactions. We ask: (1) How does intraspecific aggregation affect aspects of community structure, such as prairie plant abundance, diversity, and invasibility? (2) How do these effects change from the initial years after establishment to later in community development? We address these questions in a prairie restoration experiment in eastern Kansas where initial spatial structure was manipulated. All plots were sown at constant density with the same 16 prairie species from 4 functional groups. Treatments varied only the spatial arrangement of species among cells within plots, ranging between uniform seeding and monospecifically aggregated seeding, with intermediate treatments varying the degree of functional redundancy in aggregated patches.

Results/Conclusions

Based on analysis of compositional data from the 3rd and 10th growing seasons of the experiment, intraspecific aggregation decreased the cover of sown species by the 3rd year and increased the cover of nonsown species by the 10th year. Aggregation did not significantly affect sown species richness, which was fairly high across all treatments, but did increase nonsown species richness in the 10th growing season, leading to an increase in overall richness. The suppressive effect of aggregation on sown species was greatest among those species that were most abundant overall, an effect that weakened somewhat by year 10. By year 10, a few of the less abundant sown species were positively affected by aggregation, suggesting competitive release. Based on our results, intraspecific aggregation promotes self-suppression of abundant sown species, improving their coexistence with nonsown species, and increasing richness at the cost of increased invasibility. Many of the effects of aggregation were not yet noticeable in the 3rd year. Though time provided opportunities for species to disperse and initial spatial structures to break down, multiple effects of aggregated seeding strengthened over time, suggesting initial structure can have lasting effects on the development of the community.