Candidate perennial bioenergy crops such as switchgrass (Panicum virgatum) often have numerous cultivars that differ in morphological and physiological traits correlated with productivity. Despite interest among ecologists in the role of species complementarity for enhancing ecosystem services, including productivity, it is not known if trait differences in switchgrass cultivars affect productivity. We compared aboveground biomass of four cultivars of switchgrass in monocultures and all possible mixtures in a common garden experiment to assess how complementarity might affect switchgrass yield and weed suppression ability. Four cultivars that differ in traits – Alamo, Dacotah, Trailblazer and Cave-in Rock – were planted 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 in June 2016. A single ramet was planted in each corner of a 0.5m2 plot, with eight replicates per treatment. Plots were watered following planting and weeded occasionally the first year to encourage establishment. Thereafter the plots were not weeded so the capacity to reduce weed competitors could be assessed. Above-ground biomass was harvested in August 2017 and 2018 and yield of switchgrass individuals and weeds was determined.
Results/Conclusions
In both years, Dacotah monocultures produced the least biomass. Cave-in-Rock monocultures and select bi-cultures including Cave-in-Rock produced the most. Yield of the four-cultivar polyculture exceeded the production of the lowest-performing monocultures and some bi-cultures with Dacotah in at least one year, but was equivalent to all others. There was no difference in the treatments’ suppression of weeds in 2017. But in 2018 Cave-in-Rock monocultures (the most productive treatment) and Alamo-Cave-in-Rock bi-cultures had significantly lower weed biomass compared to Alamo-Dacotah bi-cultures. Individuals in the four-cultivar polyculture had lower weed biomass in their vicinity than those with only Alamo or Trailblazer as their neighbors, suggesting that complementarity of cultivars could enhance weed suppression. Despite differences in traits that affect aboveground production in monoculture plantings, our results suggest that complementarity among cultivars is unlikely to enhance productivity in switchgrass mixtures. However, complementarity may reduce the invasion of weeds in wet years, especially when high-yielding cultivars are used. Interestingly, the inclusion of low-performing cultivars in a mixture reduces the ability of high-performing cultivars to suppress weeds, suggesting that the lack of evidence for complementarity in yield may be due to competition from weeds. Ongoing work in these plots will determine if removing weeds affects the potential for complementarity to increase production in switchgrass mixtures.