Résumé
Evaluating restoration practices such as admixture provenancing or mixing of multiple source populations for greater intraspecific variation in plant restorations helps to understand the utility of the practice in different scenarios. We do not know whether any population mixtures in admixture provenancing will deliver positive outcomes when interacting. To understand how population and species' identity influence plant performance and trait outcomes in mixtures, we assessed trait variation among populations of two forb species native to the Western United States (Dieteria canescens and Heterotheca villosa) following pairwise competition in pots with single-population monocultures, two-population mixtures, and two-species mixtures with D. canescens as a focal plant in a greenhouse common environment. Plants were harvested after 7 weeks and height, leaf count, root length, and root mass fraction (RMF) traits were measured, as well as whole plant biomass. We found two-population mixtures and single-population monocultures to have similar biomass while some two-species mixtures differed from respective single-population monocultures. Neighbor trait differences in shoot height and root length best predicted plant growth following interactions, while increasing neighbor trait differences resulted in positive growth outcomes for all but one trait by treatment interaction (R-2 = 0.17, p < 0.001). In this single case, plant growth decreased with increasing differences in RMF for the two-species mixture treatment, surprisingly. These findings show nuance in the outcomes of interactions within and between populations and species: they are highly dependent on population identity and traits measured, which are aspects that must be considered when evaluating restoration materials and strategies.