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.