Eco-evolutionary Dynamics of Coastal Marshes in Response to Environmental Change

Rapid global environmental change threatens the state and fate of the Earth's ecosystems. Understanding how organisms respond to global environmental change is important to predicting ecosystem-level processes into the future, particularly for organisms that drive elemental cycling and ecosystem structure such as dominant plants. Rapid organismal evolution is a major mechanism for trait change and increasingly has been shown to occur on "ecologically relevant" timescales across diverse ecosystems; however, evolutionary processes are generally not integrated into ecosystem-level predictions. In this dissertation, I use the coastal marsh system as a model for investigating the role of rapid organismal evolution in impacting ecosystem processes. Specifically, I integrate observational data, data collected from common garden experiments with the common marsh sedge Schoenoplectus americanus, and models of evolutionary and ecosystem change to predict soil surface accretion (i.e., building of marsh surface elevation) and carbon accumulation over the course of decades. I employ a unique "resurrection ecology" technique, breaking century-old, soil-stored S. americanus seeds from dormancy to directly assess phenotypic change across the 20th century. My dissertation provides novel insights to the magnitude and consequences of evolutionary trait change on ecosystem processes.

Specifically, in Chapter 2 I assess the role of genetic variation, between-genotype interactions, and evolution in explaining trait variation and, in turn, the impact of trait variation on soil surface accretion and carbon accumulation by coupling a common garden experiment and ecosystem model. I find that belowground traits of S. americanus exhibit high heritable variation and evolutionary change over the course of ~50 years of evolution which substantially alters predicted soil surface accretion and carbon accumulation rates. In Chapter 3 I investigate the role of mean trait evolution and the evolution of plasticity in explaining S. americanus trait variation by exposing ancestral and descendant genotypes to a variety of global change factors (e.g., flooding, salinity, elevated atmospheric CO2). I find that although evolution of plasticity is understudied in the literature, it is a common mechanism by which S. americanus populations respond to rapid environmental change. In Chapter 4 I assess the roles of observational uncertainty and process variance in explaining variation in plant trait data that informs ecosystem-level predictions of accretion and carbon accumulation. I find that the utility of proxy measurements of plants traits varies by species and that process variance can dominate forecasts of ecosystem change. Finally, in Chapter 5 I build a novel eco-evolutionary model of marsh accretion and carbon accumulation that takes into account plasticity and evolution by natural selection of S. americanus root-to-shoot ratio in response to sea-level rise. I find that accounting for trait plasticity and mean trait evolution can alter predictions of marsh accretion and carbon accumulation. Specifically, I find that the evolution of root-to-shoot ratios in response to sea-level rise mitigated the negative plastic response of root-to-shoot ratio to flooding. As a whole, my dissertation highlights the utility of the coastal marsh system in studying eco-evolutionary dynamics, provides novel approaches to studying the role of organismal evolution in driving ecosystem-level processes, and presents exciting evidence that suggests that evolution may be an underappreciated driver of ecosystem change in the Anthropocene.