Eco-evolutionary dynamics of coastal marsh responses to rising CO2

Background/Question/Methods

Current projections of the response of coastal marsh ecosystems to rising CO₂ and sea level assume static relationships between environmental stressors and the performance of coastal macrophytes. However, the performance of ecologically dominant plants can rapidly evolve in response to environmental changes. Because plant productivity regulates aggregate ecosystem functions of coastal salt marshes, such as the rate of sediment accretion and the stability of the marsh surface, evolutionary changes in plant performance could conceivably affect the stability of the entire ecosystem. We assessed the rate of genetically-based phenotypic changes in Schoenoplectus americanus, a common Atlantic Coast marsh macrophyte, over the last century by reviving seeds from seed banks buried in dated sediments and estimating plant productivity and tissue allocation in common garden experiments under varying levels of CO₂ and flooding. To determine if observed changes in plant growth were large enough to affect ecosystem function, we parameterized the widely used Marsh Equilibrium Model (MEM) with plant growth and tissue allocation values from our experiments. If the current assumption that evolutionary change in plant traits on the centennial scale is trivial holds true, we would expect no impact of phenotypic changes over the last century on modeled marsh function.

Results/Conclusions

We were able to germinate seeds from as long ago as 1900, but a hierarchical zero-inflated Poisson model estimated an exponential decline in germination rates back in time, resulting in a germination rate for 100 year old seeds of ~2%. We found differences in the impact of atmospheric CO₂ on growth between ancestral and modern cohorts using a growth chamber experiment with CO2 levels set at 400 ppm and 290 ppm. Ancestral genotypes experienced a higher stimulus of aboveground growth under elevated CO₂, and the magnitude of this effect was large enough to induce higher levels of sediment accretion in the MEM model. A crossed CO₂ by flooding experiment in the field also revealed differing impacts of environmental treatments on growth, with the differences between the growth of genotypes similar to the impact of 25 g m^-2 of nitrogen fertilization from a similar experiment. In aggregate, our experiments show that genetically-based shifts in plant phenotype over the last century exist and that they are large enough to potentially impact the ability of coastal ecosystems to respond to the stresses associated with rising CO₂.