A. Randall Hughes

Plant genotypic diversity can influence population‐ and community‐level processes, yet we have limited understanding of how these effects vary across environmental gradients that are ubiquitous in nature. We conducted a 2‐year field experiment manipulating plant ( Spartina alterniflora ) genotypic diversity across a natural stress gradient in tidal elevation, both with and without the addition of nutrients. Spartina diversity increased stem production, but the magnitude of this effect was reduced at both the most stressful and most benign endpoints of the combined elevation and nutrient gradient, consistent with recent species diversity studies. Complementarity among individuals likely underpins the observed benefit of Spartina diversity. Spartina diversity also affected the associated marsh community, with higher consumer ( Littoraria irrorata ) abundance in more diverse plots, owing to both greater Spartina density and increased variation in Spartina traits. Synthesis . The positive effects of Spartina diversity on population‐ and community‐level responses under most environmental conditions highlights the ecological importance of plant genotypic diversity for the maintenance of function across the marsh landscape.