Résumé
Population Ecology: Population and species neighbor identity impact trait-trait relationships and plant performance 2025
Trait variation among and within plant species can affect the intensity and direction of plant-plant interactions (e.g., competition, facilitation) and influence whether species can coexist. Trait differences can result from plastic responses to plant-plant interactions and can influence trait hierarchies creating inferior and superior competitors. Trait differences between interacting species can intensify competition, strengthen competitive hierarchy or result in niche differences that help reduce competition. Consequently, it is unclear how trait values or plastic differences between individuals of the same species influence intraspecific competition, and how this impacts species coexistence, which relies upon more intense intra- rather than interspecific competition. To understand how mixtures of multiple populations (intraspecific mixtures) influence plant performance and trait outcomes in comparison to single population monocultures and interspecific mixtures, we assessed trait variation among multiple populations following plant interactions in a greenhouse common environment. We used three populations each of two forb species native to the western US in all combinations and found that four of six population pairings led to more positive interaction outcomes in intraspecific mixtures compared to monocultures. Neighbor trait differences in shoot height and root length had the largest impact on plant growth following interactions, while increasing neighbor trait differences in plant height, leaf count, and root length resulted in positive growth outcomes for both interspecific and intraspecific mixtures, and root mass fraction showed the opposite pattern. These findings show nuance in the outcomes of intraspecific interactions and that they depend on population identity and varying importance of evaluated traits.