Genotype by Environment Interactions of a High Marsh Foundation Species Affected by Sea-level Rise
Open AccessTo survive rapid climate change shifts, species must either adapt using evolutionary mechanisms or shift their range with shifting environmental gradients. Mid-Atlantic tidal wetlands are an ideal system in which to study these processes, as accelerated sea level rise (increasing 3-4x faster than the global average) is driving both trait changes in foundation marsh grasses and migration into coastal forests during the process of tree dieback (caused by storms and saltwater intrusion). Using plants along the marsh-to-forest ecotone as a case study, my dissertation aims to understand how plasticity and genotypic variation contribute to plant traits and fitness under rapid climate change. In Chapter 1, I examined the germinable soil seed bank across the marsh-to-forest ecotone under control conditions and with a salinity addition. I found that salt-sensitive tree species are struggling to produce germinable seeds, while marsh plant seeds are dispersing into adjacent pine forest prior to forest dieback. One such plant is Spartina patens, a dominant grass in dune and high salt marsh habitats. To quantify the phenotypic plasticity of this grass as it migrates inland, I conducted a reciprocal transplant of S. patens between marsh and forest habitats at three mid-Atlantic sites (Chapter 2). In the forest understory, S. patens exhibited a strong shift in resource allocation to aboveground traits associated with light acquisition and overall lower fitness. However, the species' high plasticity allowed individuals from the forest that were transplanted into the marsh to recoup significant biomass over a single growing season. Next in a series of observational surveys across the marsh to forest ecotone and experimental manipulations of light availability (Chapter 3), I show that there is a sharp decrease in S. patens flowering density as it progresses along its upland migration front, and that light availability is a strong driver of this flowering limitation. Building on my two previous chapters, I conducted a mesocosm experiment to assess the effects of salt stress and light limitation on S. patens, and the role genetic identity plays in plant responses and trait plasticity (Chapter 4). I found that at the marsh-to-forest ecotone, S. patens trait plasticity is more strongly driven by light availability than by salinity, and that genetic identity impacts both trait means and the degree of trait plasticity between light treatments. Lastly, in a systematic literature review, I evaluated the empirical evidence for genetic swamping: the hypothesis that central to peripheral gene flow is maladaptive to range edge populations (Chapter 5). I found little evidence that central-to-edge gene flow restricts adaptation in edge populations, and some evidence that it has neutral to positive effects on fitness in edge populations, especially those under novel climate change stress. Overall, my dissertation research demonstrates through the case study of migrating marsh plants and sea-level rise that adaptive mechanisms of both genetic and plastic varieties are operating in range edge populations responding to climatic stressors.
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