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

<p>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 <em>Schoenoplectus americanus</em>, and models of evolutionary and ecosystem change to predict soil surface accretion (<em>i.e.</em>, 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 <em>S. americanus </em>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.</p><p>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 <em>S. americanus</em> 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 <em>S. americanus</em> trait variation by exposing ancestral and descendant genotypes to a variety of global change factors (<em>e.g.</em>, flooding, salinity, elevated atmospheric CO2). I find that although evolution of plasticity is understudied in the literature, it is a common mechanism by which <em>S. americanus</em> 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 <em>S. americanus</em> 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.</p>