Switchgrass (Panicum virgatum), a C4 grass native to North America, is an important potential bioenergy feedstock that exhibits considerable variation in morphological and physiological traits correlated with productivity. A number of cultivars of switchgrass have been developed to enhance its productivity and sustainability as a bioenergy feedstock in different habitats. Mixed stands of species that differ in traits may result in higher productivity and enhancement of other ecosystem services due to complementarity. However, it is not known if mixed stands of switchgrass cultivars will differ in productivity from monocultures. To evaluate this, we grew four cultivars of switchgrass – Alamo, Dacotah, Trailblazer and Cave-in Rock – in monocultures, all two-cultivar bi-cultures, and a four-cultivar polyculture in a common garden at the W.K. Kellogg Biological Station in southwest Michigan. A single ramet was planted in each corner of a 0.5m2 plot, with eight replicates per treatment, that were randomly assigned to 88 plots in summer 2016. Switchgrass and weed biomass along with several traits likely to influence performance of switchgrass were measured annually from 2017-19. In 2019, half of the plots were randomly assigned to a weed-removal treatment to assess the role of weed suppression on complementarity in switchgrass biomass production.
Results/Conclusions
In all years, Dacotah monocultures produced the least biomass. Cave-in-Rock monocultures and bi-cultures that included Cave-in-Rock produced the most biomass; in no year did the four-cultivar polyculture outperform the most productive monoculture. However, the polyculture increased in productivity over time and was among the least variable. Steep declines in Alamo biomass were largely offset by increases in Cave-in-Rock. We observed a strong competitive hierarchy among the cultivars in mixtures; Cave-in-Rock was frequently over-represented, whereas Alamo and Dacotah were under-represented. Aboveground switchgrass production was negatively correlated with weed biomass across all treatments in 2017 and 2018, suggesting that weed suppression (or the presence of weeds) was an important determinant of switchgrass production in monocultures and mixtures. Weed removal increased light availability, but had little effect on switchgrass production at the plot level. Alamo production was affected by weeds, but the magnitude depended on treatment. Overall, despite differences in traits and performance, it does not appear that complementarity among cultivars will have large impacts on switchgrass yield. Removal of weeds also did not result in higher yield or complementarity in yield, indicating that these weeds, primarily C3 species, were not having strong competitive effect on production of perennial C4 grasses.