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 productio...