Eco-Evolutionary Dynamics in a Coastal Marsh Sedge Resurrected from a Century-Long Seed Bank

Tidal wetlands around the world are changing rapidly due to increasing atmospheric CO₂ and sea level rise, among other factors. Current projections of salt marsh ecosystem responses to environmental change assume that the relationship between plant growth and environmental stressors remains constant over time. However, if evolution occurs on a centennial time scale in marsh plants, this could change the magnitude or direction of growth responses to environmental drivers such as CO₂ and flooding, which could in turn impact ecosystem processes. Here, we test whether evolution can change plant and ecosystem responses to CO₂ and sea level using Schoenoplectus americanus, a foundational salt marsh sedge with a 100+ year seed bank. In this dissertation, we present the results of a series of studies on the biology of Schoenoplectus americanus and its growth responses to sea level and rising CO₂. Through germination experiments and hierarchical zero-inflated Poisson model we found that Schoenoplectus americanus seeds can remain viable for up to a century and can be germinated for use in growth experiments. In a growth chamber experiment we investigated the growth of genetically distinct cohorts of Schoenoplectus americanus under pre-industrial and ambient CO₂ levels and found that the ancestral cohort responded more strongly to atmospheric CO₂ than a modern cohort from the same marsh and a modern cohort from a marsh in a different watershed. These results suggest that the magnitude of response to elevated CO₂ can change over time because of evolution. In a field mesocosm experiment we explored the role of evolution in growth responses to elevated CO₂ and sea level treatments. We found that evolution can drive small changes in Schoenoplectus americanus flooding tolerance and productivity under sea level. We used the results of these experiments to simulate marsh accretion under rising sea level and found notable differences in the sustainability of a marsh surface under sea level rise. These studies point to the potential for a century of evolution to alter ecosystem processes and they suggest that leaving out evolution in projections of plant and ecosystem response to environmental change can increase the uncertainty of those predictions.