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

## TL;DR

This study finds that nutrient addition has a stronger impact on salt marsh cordgrass than genetic diversity within the species.

## Contribution

The study reveals that intraspecific genetic diversity has limited effects on ecological responses to nutrient addition in a foundation plant species.

## Key findings

- Nutrient addition strongly affects plant traits like belowground biomass and leaf nitrogen content.
- Intraspecific genetic diversity did not significantly influence plant responses at a local scale.
- Genetic variation among clones may still be important at larger spatial scales.

## Abstract

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.

Intraspecific genetic diversity of a foundation species did not affect plant traits at a local scale, or resilience to nutrient inputs. However, variation among clones in response to nutrient enrichment and intraspecific genetic diversity supports the idea that genetic variation may still be important at larger spatial scales.