Repeated genetic and adaptive phenotypic divergence across tidal elevation in a foundation plant species

Microgeographic genetic divergence can create fine-scale trait variation. When such divergence occurs within foundation species, then it might impact community structure and ecosystem function, and cause other cascading ecological effects. We tested for parallel microgeographic trait and genetic divergence in  Spartina  alterniflora , a foundation species that dominates salt marshes of the US Atlantic and Gulf coasts.  Spartina  is characterized by tall-form (1-2m) plants at lower tidal elevations and short-form (<0.5m) plants at higher tidal elevations, yet whether this trait variation reflects plastic and/or genetically differentiated responses to these environmental conditions remains unclear. In the greenhouse, seedlings raised from tall-form plants grew taller than those from short-form plants, indicating a heritable difference in height. When we reciprocally transplanted seedlings back into the field for a growing season, composite fitness (survivorship and seed production) and key plant traits (plant height and biomass allocation) differed interactively across origin and transplant zones in a manner indicative of local adaptation. Further, a survey of single nucleotide polymorphisms revealed repeated, independent genetic differentiation between tall- and short-form  Spartina  at 5 of 6 tested marshes across the native range. The observed parallel, microgeographic genetic differentiation in  Spartina  likely underpins marsh health and functioning, and provides an underappreciated mechanism that might increase capacity of marshes to adapt to rising sea levels.