Research articleGenotypic interactions limit growth and stimulate flowering in a salt marsh foundation plant species

- We examine how intraspecific interactions may structure populations of a foundational plant.
- We monitor the interactions of plant genotypes using morphology and DNA genotyping.
- Intraspecific interactions among genotypes have mutually limiting effects on growth.
- Intraspecific interactions among genotypes have stimulatory effects on flowering.

Ecologists have a growing appreciation for the influence of intraspecific, or genotypic, diversity on ecosystem functioning in species depauperate systems such as salt marshes, with intraspecific interactions serving as one potential driver of such diversity. We examined the role of intraspecific interactions, both competitive and facilitative, in structuring populations of Spartina alterniflora, a foundational salt marsh plant known to influence community structure and function throughout its native distribution in eastern North America. We conducted a transplantation experiment at a created site in southwestern Louisiana, USA, by growing five S. alterniflora genotypes, or clones, for two years in all possible combinations, and monitored their interactions using morphological measurements and molecular marker genotyping. Using stem densities as a proxy for above-ground biomass, we found support for our hypothesis that interactions have a mutually limiting effect on growth, although we were unable to find statistical support for competitive asymmetries between specific clone pairs, which would have explained earlier observations of declining genotypic diversity in aging marshes. Using proportions of stems flowering as a proxy for reproductive effort, we also found support for our hypothesis that interactions can have a stimulatory effect on flowering, which could promote genotypic diversity through the recruitment of outcrossed seedlings in a species known for severe inbreeding depression. Reproductive interactions were at times mutually inhibitory or stimulatory, as evidenced by positive correlations of flowering responses between clones. To our knowledge, ours is one of the first studies to evaluate the outcome of one-on-one interactions among genotypes of a foundational clonal plant.