Abstract
Salt marshes are increasingly valued for their role in coastal defense. In particular, marsh plants slow the progression of waves, thereby decreasing wave heights, orbital velocities and associated energy. Practical application of these effects has driven substantial research estimating the effects of plants on waves. However, the effects of waves on plants remain understudied, especially regarding plant responses along a wave climate gradient. To begin to understand these responses, we collected above- and belowground plant data and wave data from 60 sites across Mobile Bay, USA, and tributaries and evaluated plant responses along the range of assessed wave climate conditions. Plant responses among the dominant species, Juncus roemerianus and Spartina alterniflora, varied along the wave climate gradient. However, the basal diameter of shoots in both species declined linearly with increasing wave climate. While wave climate had no observable effect on other S. alterniflora parameters, the declining diameter of J. roemerianus shoots along the same gradient was commensurate with a decline in the percentage of live canopy shoots aboveground and an increase in root and rhizome biomass in the active rooting zone belowground. In contrast to previous studies, other responses including the height, biomass and density of aboveground shoots in both species were not related to wave climate. More broadly, these results demonstrate that plant features important for wave attenuation such as shoot diameter can change in response to varying wave conditions. These feedbacks should be incorporated to improve coastal modeling and successes of coastal conservation, restoration and enhancement projects.
Highlights
Coastal wetland plants face many challenges including natural stressors such as salinity (Howard & Mendelssohn 1999), erosion from waves and currents (Green & Coco 2014) and interspecific competition for suitable habitat (Pennings et al 2005), and human-induced threats such as development (e.g. ‘coastal squeeze’, Constantin et al 2019), sediment deprivation (Tweel & Turner 2012) and sea-level rise (Osland et al 2017)
For the first time, above- and belowground fringing marsh plant responses were assessed along a wave climate gradient across a large estuary
We hypothesized that fringing salt marsh plants would respond to increasing wave climate by increasing shoot density, shoot biomass per shoot, basal stem diameter aboveground and rooting depth belowground
Summary
Coastal wetland plants face many challenges including natural stressors such as salinity (Howard & Mendelssohn 1999), erosion from waves and currents (Green & Coco 2014) and interspecific competition for suitable habitat (Pennings et al 2005), and human-induced threats such as development (e.g. ‘coastal squeeze’, Constantin et al 2019), sediment deprivation (Tweel & Turner 2012) and sea-level rise (Osland et al 2017). Sedimentation is generally lower among plants characterized by more flexible stems (e.g. seagrasses), but flexible stems are more likely to adapt a protective ‘shielding posture’ (i.e. lying flat) during high energy wave events, thereby reducing stem breakage and bed erosion (Rupprecht et al 2017) The development of these features and magnitude of their effects on several marsh processes can vary seasonally (Silinski et al 2018, Schoutens et al 2019, Schulze et al 2019, Zhu et al 2020). Plant features that vary seasonally or change in response to various stressors, including waves, have the potential to impact marsh accretion and subsequent persistence
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