Environmental and Genetic Influences on Species Composition and Belowground Biomass Dynamics in Coastal Marshes

The relative abundance of Schoenoplectus americanus in coastal marsh systems plays a crucial role in regulating key processes such as marsh accretion, carbon sequestration, and methane production. Sea level rise, driving increasing salinization and inundation, and subsequent shifts in community composition affect these ecosystem functions. Further, it has been shown that evolutionary changes over the last century in the total root biomass of Schoenoplectus and in its distribution with sediment depth have been large enough to affect such ecosystem-level functions. Schoenoplectus commonly grows in competition with the C4 grass Spartina patens, and it is not known to what extent changes in belowground traits of one species (Schoenoplectus) affect plant community structure, such as the relative distribution of root biomass across species. Here, I explore the effects of intraspecific genetic variation and interspecific interactions on belowground traits by analyzing an experiment exposing 26 Schoenoplectus americanus genotypes, resurrected from century-long seedbanks, to different levels of flooding and salinity in pots with and without the presence of Spartina patens. The experimental environmental treatments drove large differences in species proportion, generating a spectrum of environmental contexts for observing the impact of evolution on the species composition of belowground biomass. Compared to plants grown in monospecific pots, competition greatly reduced belowground biomass and its variance. Within polyculture pots, the impact of a century of evolutionary change in plant traits became increasingly evident as environmental pressures increased: Under benign conditions, the upper sediments of pots grown under low inundation and low salinity, there was no detectable difference across seed age in the relative abundance of Schoenoplectus. With increasing sediment depth, inundation, and (especially) salinity, however, the belowground tissues of Schoenoplectus grown from seeds of older cohorts, dating back as early as the 1800s, made up a lower proportion of total biomass than plants grown from more recent seedbanks. While species proportion will be predominantly driven by the environment and community interactions, evolutionary changes at the margins have the potential to propagate to community-scale changes in species composition. This suggests that evolutionary change, along with changes to the environment and community composition, has the potential to affect predictions of ecosystem resilience