Evolution and resurrection ecology of a foundational coastal marsh plant

Stratified storage of dormant seeds in soil can result in natural archives useful for studying evolutionary responses to environmental change. However, few studies leverage soil-stored seed banks as natural archives, in part because of concerns over attrition, bias, and sediment mixing. Here, I examine the persistent seed bank of Schoenoplectus americanus, a foundational brackish marsh sedge, to a) determine whether it can serve as a resource for reconstructing demographic and population genetic/genomic variation, b) whether and how evolution may be occurring across a century. After extracting seeds from radionuclide-dated soil cores taken across the Chesapeake Bay, I “resurrected” age cohorts spanning the 20th century. In Chapter 1, I use microsatellites to assess genetic diversity/differentiation among age cohorts, drawing comparisons to extant plants at the study site and to extant plants in nearby and more distant marshes. I found genotypic differences among cohorts and between cohorts and extant plants. Genetic diversity did not decline with depth, suggesting differentiation is likely not due to attrition. In Chapter 2, I use SNPs to examine population diversity/differentiation for resurrected plants taken from multiple marshes to understand how regional-scale geography interacts with temporal change. I found that location explained genetic clustering better than temporal differences, suggesting that habitat differences between marshes are consequential for S. americanus evolution. In Chapter 3, I deployed 2 resurrected age cohorts from one marsh in a greenhouse experiment to assess phenotypic differences. I cloned plants across triply-crossed conditions: simulated sea level rise (salinity, inundation) and competition. Biomass by treatment did not significantly differ when averaging by cohort. However, variance was smaller for young versus old cohorts, suggesting reduction in phenotypic plasticity across time. I also compared gene expression differences in response to salinity for stem/root tissue and between cohorts. I found that salinity treatment resulted in significantly different expression levels and some evidence of differentiation by age cohort, but only for root tissue. Overall, this work describes complex, geographically variable, and small evolutionary shifts across time in S americanus. This suggests that local population and/or habitat differences mitigate change through time, which may be in response to climate change corollaries.