Native community diversity has historically been thought to play a central role in ecosystem processes such as invasion resistance and productivity by increasing interspecific trait variation. Trait variation also occurs at the intraspecific level, where phenotypic and/or genotypic diversity could confer community invasion resistance and increased productivity via the same mechanisms. Genotypic variation within a dominant species may account for overall phenotypic trait variation within a given community, and thus may play a central role in determining invasion resistance and productivity. Thus, understanding the relationship between all three levels of diversity - genotypic variation, phenotypic variation, and community diversity - is critical to determining what governs ecosystem processes. We conducted our study in 66 1-m² plots located in intact tallgrass prairie. To understand how multiple levels of diversity may have differing impacts on productivity and invasion, for each plot, we measured the genotypic and corresponding phenotypic variation of dominant C₄ grass individuals (Andropogon gerardii) and plant community diversity. Aboveground biomass was collected for productivity. Establishment and persistence of added invasive grass seeds (A. bladhii) was monitored to measure invasion resistance. Finally, because resource availability has been shown to affect productivity and invasion resistance, we measured nitrogen, water and light availability.

Results/Conclusions We used multiple regression analyses to examine relationships between resource availability, genotypic and phenotypic variation of A. gerardii, community diversity on aboveground productivity and invasion at the plot level. We found that productivity was significantly related to early-season light availability and genotypic variation of A. gerardii. Early-season soil moisture content and end of season individual biomass of the dominant species were found to be significantly related to both early season establishment and late season persistence of the invasive grass. Genotype variation of A. gerardii was also found to be significantly related to late season persistence of the invasive grass. These results suggest that both resource availability and genotypic variation within a dominant species are important drivers in productivity and invasion resistance. Furthermore, we found evidence that genotypic and phenotypic variation are positively correlated (height r=0.28178, p=0.023; total leaf area r=0.26487, p=0.0330) which suggests that phenotypic trait variation in a dominant species could be one key mechanism driving productivity and invasion resistance. Finally, community diversity was not found to be significantly related to invasion or productivity, suggesting that it is population level diversity of the dominant species that plays an important role in the ecosystem processes governing the tallgrass prairie.