Population source affects competitive response and effect in a C₄ grass (Panicum virgatum)

Grassland restoration success depends on the development of plant communities that accord with restoration goals. Intraspecific variation in competitiveness may affect community development. For some grassland species, germplasm can be obtained from sources ranging from wild collections to selectively bred cultivars. The extent to which population source affects competitive outcomes in restoration projects is unclear. We addressed this knowledge gap in a glasshouse experiment comparing competitive response and effect among three sources of switchgrass (Panicum virgatum) that are available for restoration: selectively bred cultivars, commercial ecotypes (commercially produced but not deliberately selected), and wild collections. Two strains per source type were grown with four associates chosen to encompass varied functional groups: conspecifics, Bromus inermis, Cirsium arvense, and Solanum ptycanthum. Switchgrass competitive response was evaluated for survival, height, biomass, and shoot:root biomass ratio; competitive effect was assessed as associate survival, height, biomass, and shoot:root ratio. Competitive responses of cultivars and commercial ecotypes were broadly similar, although cultivar biomass exceeded both that of ecotypes and wild collections, and ecotypes had the highest shoot:root ratio. Wild collections were most negatively affected by competition. The shoot:root ratios of all sources were highest when grown with S. ptycanthum, indicating that competitive responses were plastic; plasticity in fitness-related traits can contribute to persistence in variable environments. Cultivars exerted negative effects on B. inermis. Secondary analyses indicated that all switchgrass sources were most inhibited by the annual S. ptycanthum. To summarize, population source affected multiple aspects of switchgrass competitive ability, when grown against functionally varied associates.