Translocating Panicum virgatum L.: Performance, Community Impact and Competitive Outcome

Abstract

This dissertation elucidates the effects that selection history and foundation-number have on switchgrass vigor and competitiveness, and plant community diversity and performance, so as to help inform germplasm-sourcing decisions for grassland reconstructions. Chapter 1 examines the effects that selection history had on switchgrass stand density, persistence, and aboveground biomass, and tested whether multi-foundation strains performed consistently across four sites. Cultivar and ecotype biomass did not differ and while cultivars exceeded ecotypes in stand density, cultivar persistence was lower due to self-thinning. Strains’ geographic origin was related positively to stand density and negatively to biomass across evaluation sites. I found no evidence that multi-foundation strains performed more consistently across sites than did single-foundation strains. Collectively, these results suggest that cultivars may colonize adjacent areas through strong propagule pressure exerted by dense stands but colonization would not necessarily result in extreme dominance. Switchgrass establishment and persistence vary through the interaction of geographic origin and reconstruction location. Chapter 2 examines the effects of switchgrass selection history and foundation-number on plant community diversity and switchgrass performance in experimental prairie plots that harbored cultivars or ecotypes. The effects of switchgrass characteristics on community metrics were variable and generally small. Cultivars exceeded ecotypes in establishment, third-year stand density, and biomass but effects were modest and site-contingent. My results suggest that modestly stronger performance by cultivars does not translate into adverse effects on plant diversity. Under strong competition from weeds, switchgrass does not persist. Switchgrass is unlikely to naturalize in reconstruction-adjacent areas that are heavily vegetated. Chapter 3 evaluates competitive response and effect of juvenile switchgrass cultivars, ecotypes, and wild collections when grown with heterospecifics commonly found in reconstructed grasslands. Height and biomass were generally lowest in wild collections and similar for ecotypes and cultivars. However, belowground biomass in cultivars exceeded that of the other groups. All switchgrass groups were smallest and allocated the most to aboveground tissues when grown with an annual forb. Cultivar biomass was greatest when grown with a perennial C3 grass. I found little evidence of variation among switchgrass groups in competitive effect; however, competitor belowground biomass was lowest when grown with cultivars. History of selection in switchgrass contributes to intraspecific differences in competitive outcomes and should be taken into account when sourcing germplasm for reconstructed grasslands. Collectively, these chapters suggest that switchgrass is unlikely to become overly dominant within reconstructed grasslands or to pose an invasive threat to adjacent vegetative areas. Artificial selection has increased cultivar performance relative to ecotypes and wild collections but the differences are contingent on site conditions and competition regime. Discussion of the risks posed by translocated switchgrass populations should shift away from invasiveness and focus on the impacts of gene-flow between translocated and wild populations.