Nutrient Addition Has a Stronger Effect Than Intraspecific Genetic Diversity on Critical Ecological Responses in a Salt Marsh Foundation Species

Intraspecific genetic diversity in wild populations is declining as global change intensifies. Genetic variation within populations of foundation plant species may influence ecological responses to environmental stressors, but remains poorly understood. Here, we examined how intraspecific genetic diversity in salt marsh cordgrass ( Spartina alterniflora , hereafter Spartina ) affects responses to nutrient addition. Salt marshes are often dominated by Spartina , a partially clonal foundation species that is critical to the structure and ecosystem function of salt marsh habitats. We conducted a two‐way factorial greenhouse experiment to examine plant responses to two levels of intraspecific genetic diversity and two levels of nutrient addition. We genotyped plants and estimated genetic distance and clonal identities. We measured aboveground biomass, belowground biomass, tiller production, and percent nitrogen content of leaf tissue. Nutrient addition had a strong main effect on combined plant responses, but we did not find an impact of intraspecific genetic diversity or the interaction between intraspecific genetic diversity and nutrient addition on the combined plant responses. When we examined plant responses individually, we found that nutrient addition decreased belowground biomass and increased leaf tissue nitrogen content. Nutrient addition interacted with intraspecific genetic diversity to affect tiller production, whereby nutrient addition increased tillers in low genetic diversity pots, but decreased tillers in high genetic diversity pots. Unexpectedly, neither nutrient addition nor intraspecific genetic variation affected aboveground biomass, which may be driven by divergent responses among different plant clones. For example, of the four most common clones (Multilocus Lineages), nutrient addition increased aboveground biomass in the low diversity treatment for two clones, and in the high diversity treatment for another clone. Unlike the well‐established relationship between plant interspecific diversity and ecosystem function, intraspecific genetic diversity of a foundation species did not affect plant responses at a local scale to nutrient addition.