Abstract: Competitive effects on biomass and vegetative reproduction can be especially strong when plants are young and establishing their clonal network of ramets. Invasive clonal plants may mitigate the effects of competition better than native clonal plants. We evaluated the competitive effects on the propagule supply and the number and biomass of tillers and rhizomes of invasive (Bromus inermis) and native (Pascopyrum smithii) perennial grass seedlings. Competition reduced aboveground biomass and the number of tillers for both species in most competitive scenarios, except for B. inermis under inter-specific competition. While intra-specific competition had similar reductions for each species in the number of belowground propagules (buds, juvenile tillers and rhizomes) that produce the next generation of ramets, inter-specific competition did not impact belowground propagule production of the invasive species but negatively impacted the native species. Belowground rhizome production and lateral spread of both species were less affected by competition enabling plants to successfully establish belowground clonal networks that will potentially foster physiological integration and future clonal plant success. Invasion by B. inermis may be facilitated by clonal traits, such as its ability to maintain larger tiller and propagule production than the native when under competition. Plant invasions involving clonal species would benefit from considering the role of clonal traits in their success.

Key words: Bud bank, Bromus inermis, Invasion, Pascopyrum smithii, Northern Great Plains