As global climate change alters patterns of selective environmental pressures, ecotypically structured variation in functional traits can be beneficial at the population level by allowing species sufficient variability to accommodate local conditions. However, it is unclear what factors drive this trait variation or how trait variation might scale up to the ecosystem level with consequences for vital ecosystem processes such as carbon sequestration. Whether heritable local trait variation is driven by adaptation or genetic drift, it is heavily influenced by gene flow. Reproductive isolation due to flowering phenology could be a functional mechanism that enforces population genetic structure and heritable trait variation by altering rates of gene flow. Coastal marsh ecosystems provide a valuable system to explore the role of heritable trait variation on higher level processes by studying the variation in functional traits of coastal plants that contribute to marsh sediment accretion, a process that enables coastal marshes to keep pace with sea level rise and relates to carbon sequestration. In a common garden study, we explored the extent of morphological and reproductive trait variation in a foundational coastal sedge, Schoenoplectus americanus, from four marsh sites and determined whether reproductive isolation could be enforcing genetic structure. We did not find strong effect sizes on any of the ecosystem traits measured, however, there were strong effects for flowering synchrony which could suggest a mechanism for local genetic differentiation. Overall, trait variation between marshes do not seem to be producing phenotypic differences that have a strong impact on ecosystem function. Further investigation should be done with more samples and include environmental covariates to clearly determine what drives heritable trait variation in coastal marsh ecosystems.