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Genotypic diversity weakens competition within, but not between, plant species

Intraspecific diversity can affect the ecological function of communities. Because community function is the outcome of species interactions, understanding how intraspecific diversity affects species interactions may shed light on the mechanism by which intraspecific diversity affects communities. Competition among and within plant species may be particularly important, as the relative strengths of these interactions can influence species coexistence and community stability. We established a field experiment in a New England salt marsh to determine how plant intraspecific diversity affects both intra‐ and interspecific competition. Focal plants of two species (Spartina alterniflora and Spartina patens) were planted with S. alterniflora neighbours that were either genotypically identical to one another (monoculture) or genotypically distinct (polyculture). After two growing seasons, S. alterniflora performance was enhanced in the presence of polyculture conspecific neighbours compared to monoculture neighbours, whereas S. patens was not strongly affected by diversity of its S. alterniflora neighbours. This suggests that intraspecific competition was weakened by intraspecific diversity, potentially even shifting to facilitation, but we did not find strong evidence of intraspecific diversity impacting interspecific competition. By weakening competition within species, intraspecific diversity may promote coexistence of genotypes within that species. Synthesis. The community‐level effects of intraspecific diversity may be mediated in part by its effect on intraspecific competition/facilitation. Furthermore, stronger intraspecific competition and decreased coexistence resulting from reduced diversity may exacerbate ongoing declines in biodiversity in response to global change.