Ecological interactions differ for switchgrass cultivars and wild populations

Background/Question/Methods

There is growing interest in using switchgrass for bioenergy and bioproduct applications in the United States. Breeding programs are developing and releasing switchgrass germplasm with improved agronomic traits to support bioindustrial feedstock requirements. These agronomically valuable traits (i.e. enhanced growth rate and increased yield) are also shared with many highly invasive plants, and this has provoked concern that bioenergy grasses pose a risk of invasion. However, we lack basic knowledge to evaluate such risks. To inform risk assessment, we address the following questions: 1) How does establishment and growth of switchgrass vary in relation to seed source (cultivar versus remnant populations) and seed density in recently restored and remnant prairie? 2) How does switchgrass seed source and seed density influence the plant community structure of a recently restored prairie? 3) Does herbivory by resident leaf-chewing insects differ among switchgrass seed sources and with propagule pressure? We address these questions using separate factorial experiments: switchgrass seed addition (5 densities) in a recently restored prairie and introduction of switchgrass seeds and seedlings into an established prairie at Cedar Creek Ecosystem Science Reserve (East Bethel, MN). 

Results/Conclusions

We observed significant differences between switchgrass cultivars and remnant switchgrass populations in their establishment in recently restored prairie. Initial seedling emergence and end-of-season cover of cultivars surpassed that of remnant populations by over 200-300%. The magnitude of the effect varied among cultivars. Establishment of advanced cultivars ‘EG-2101’, ‘WS12L-IL’, and ‘Trailblazer’ was 200-600% greater than remnant populations whereas establishment of ‘Summer’ was similar to remnant populations. The difference in establishment between cultivar and remnant switchgrass was much larger at higher seed densities. Further, high seeding densities of some switchgrass cultivars led to increased leaf area index suggesting the rapid formation of a switchgrass canopy in restored prairie. Switchgrass seed addition to established prairie, however, resulted in extremely low end-of-season switchgrass density despite a disturbance treatment before seeding. Low establishment of cultivars may have been caused by poor soil quality and insect herbivory. Frequency of insect herbivory on planted seedlings was 180-300% greater for most cultivars as compared to remnant switchgrass. These data suggest that emergent properties of switchgrass cultivars contribute to substantial differences in early establishment and interactions with resident insect herbivores relative to remnant populations of switchgrass.