Vegetation effects on coastal foredune initiation: Wind tunnel experiments and field validation for three dune-building plants
Graphical abstract
Introduction
Ecosystem engineers directly or indirectly modify habitats by changing biotic and abiotic resources or physical habitat structure (Jones et al., 1994). Plants are capable engineers that can physically alter environments both when living and dead (Tanner, 2001; Badano and Cavieres, 2006: Bos et al., 2007; Hall Cushman et al., 2010). Vegetation is particularly important in the development of coastal foredunes, defined as the shore-parallel vegetated dune ridge in the backshore formed by aeolian sand deposition within vegetation (Hesp, 2002). The physical geomorphological processes surrounding foredune evolution have been studied extensively (Hesp and Walker, 2013; Feagin et al., 2015; Elko et al., 2016; Elko et al., 2019). Similarly, the spatial variability of foredune vegetation related to geomorphological processes was appreciated decades ago (Cowles, 1899; Ranwell, 1972; Carter, 1995). However, despite high investment in beach-dune management efforts (Wootton et al., 2016; Elko et al., 2019), the variability and mechanisms surrounding geomorphological processes and foredune vegetation have only recently gained heightened research attention (e.g. Stallins, 2005; Nield and Baas, 2008; Zarnetske et al., 2012; Durán Vinent and Moore, 2014; Zarnetske et al., 2015; Silva et al., 2016; Goldstein et al., 2017; Feagin et al., 2019; Hacker et al., 2019; Mullins et al., 2019). Our understanding of foredune ecogeomorphic feedbacks is limited (Stallins, 2006; Schlacher et al., 2008; Murray et al., 2008; Corenblit et al., 2011; Stallins and Corenblit, 2018). Efforts to model foredune initiation alongside storm response and recovery are thus constrained by an incomplete understanding of vegetation effects (Walker et al., 2017; Jackson and Nordstrom, 2019).
Foredunes are non-linear self-organizing complex adaptive habitats categorized by physical feedbacks between plants and topography (de Castro, 1995; Hesp, 2002; Nield and Baas, 2008; Hesp and Walker, 2013; Balke et al., 2014; Corenblit et al., 2015). Plants create, modify, and stabilize foredunes, while elevation and coastal processes (e.g. waves, overwash) influence vegetation structure and succession (Stallins, 2005; Durán Vinent and Moore, 2014; Zarnetske et al., 2015; Cheplick, 2016). These ecogeomorphic interactions modulate post-storm foredune recovery back to a pre-storm or new system state (Bendix and Hupp, 2000; Murray et al., 2008; Hesp et al., 2011; Wolner et al., 2013; Stallins and Corenblit, 2018). Aeolian sand transport is steered both by topography and vegetation over a range of physical, ecological, and geological timescales (Arens, 1996; Hesp et al., 2015). Across a landscape, beach physical characteristics vary (Durán Vinent and Moore, 2014; Houser and Mathew, 2011) and vegetation is heterogeneously distributed (Hesp, 1989), varying in morphology and density (Arens et al., 2001; Hesp et al., 2019). These characteristics create a spatiotemporally complex heterogenous system (Hilton et al., 2006; Charbonneau et al., 2017; Stallins and Corenblit, 2018).
Topographic heterogeneity is, in part, likely due to plant species-specific morphological traits impacting deposition (Hilton et al., 2006; Houser et al., 2008; Hacker et al., 2011, Hacker et al., 2019). Above- and below-ground differences in plant species morphology and growth habit can yield noticeable species-level differences in the building and stabilizing of already established foredunes (Murray et al., 2008; Hacker et al., 2011; Zarnetske et al., 2012; Duran and Moore, 2013; Charbonneau et al., 2016; Charbonneau et al., 2017; Hacker et al., 2019). For example, some species are associated with more hummocky, erect, taller, or shorter established foredunes (Davies, 1980; Hesp, 1989; Wootton et al., 2005; Hilton et al., 2006; Hacker et al., 2011; Zarnetske et al., 2015; Hacker et al., 2019). Shoots create drag and surface cover, reducing wind and wave erosion (Tanaka et al., 2009; Silva et al., 2016; Feagin et al., 2019) and catch sediment, with species differing in capture efficiency, survival, morphology, establishment, density, and root versus shoot investment (Hesp, 1989; Arens et al., 2001; Zarnetske et al., 2012; Hesp et al., 2019). Changes in plant community structure can thus have cascading consequences on foredune morphology and stability (Wolner et al., 2013; Charbonneau et al., 2017; Bryant et al., 2019). Despite heightened coastal research since the 1960s (Jackson and Nordstrom, 2019), uncertainties remain as to the underlying causes of observed topographic heterogeneity associated with different plant species and densities (van Dijk et al., 1999; Arens et al., 2001; de M Luna et al., 2011; Duran and Moore, 2013; Durán Vinent and Moore, 2014; Zhang et al., 2015; Keijsers et al., 2016; Moore et al., 2016; Hesp et al., 2019).
Examining nebkha formation, one type of precursor to incipient (or embryo) foredune development (Hesp, 2002; Hesp and Walker, 2013), may yield insight into what factors of plant morphology and density are of greatest importance to backshore foredune initiation. Nebkha form from aeolian sand deposition around discrete individuals or groups of plants due to high localized drag and reduced wind velocity (Cooke et al., 1993; Hesp, 2002; Fig. 1). Nebkha vary in size from millimeters to meters, and can grow and merge over time as plants tiller and new nebkha emerge (Hesp, 1989; Cooke et al., 1993; Fig. 1). This deposition can ultimately form a continuous shore-parallel incipient foredune (Hesp, 1984, Hesp, 1989, Hesp, 2002, Hesp, 2013; Fig. 1). Behind plants and the nebkha body, shielding and turbulent eddies create shadow dunes or tails (Hesp, 1981; Hesp and Smyth, 2017). These shadow dunes can vary in size by plant and nebkha shape (Raupach, 1992) and width, independent of plant height and sediment grain size (Hesp, 1981; Hesp and Smyth, 2017). Shadow dune and nebkha morphology are linked, although they are often examined separately (Hesp and Smyth, 2017). When nebkha are referred to in this publication, the nebkha and attached shadow dune complex are grouped as one entity (Charbonneau and Casper, 2018). Similar to studies of foredune ecogeomorphology, nebkha research has frequently focused on established field nebkha (Gillies et al., 2014; Hesp and Smyth, 2017). Nebkha can be thought of as the most basic unit or stage of foredune development whereby underlying physical-biological feedbacks that govern foredune evolution at a greater scale may be illuminated from examining their initiation.
To examine coupled ecogeomorphic relationships of nebkha formation as foredune precursors, we constructed a moveable-bed, unilateral-flow wind tunnel to test how three U.S. East Coast foredune pioneer plant species and their morphological traits, planting density, and planting configuration affect the initial size, shape, and volume of nebkha. We worked with dominant native and invasive foredune plants at natural and managed densities and configurations. After subjecting experimental stands to wind and sediment supply conditions typical of backshore environments, we related the size and shape of each resulting nebkha to the morphological traits of their individual plants (Fig. 1A), as well as to planting density and configuration. We tested the following hypotheses: (1) larger plants create larger nebkha; (2) a taller plant will build a taller, steeper nebkha; and (3) nebkha shape varies by plant species as a function of morphology and not as a function of plant size. Furthermore, we tested the same hypotheses against quantitative field observations to evaluate the ecological relevance of our findings for sandy beach-dune systems. By first understanding the underlying ecogeomorphic feedbacks of nebkha formation, we can more effectively scale up to forecast foredune evolution over time and develop more effective management strategies.
Section snippets
Study species
We worked with three US East Coast dominant foredune building pioneers (Fig. 2A). Erect C3 Ammophila breviligulata (0.66–1 m tall), is native to the US Mid-Atlantic and Great Lakes, and is a Pacific invasive grass (Hacker et al., 2011). C4 bunchgrass Panicum amarum (1–2 m tall) is a US eastern seaboard and Gulf coast native known for high biomass production. Carex kobomugi (15–30 cm tall) is an Asia native sedge, and US invasive, with a low-lying semi-rosette growth form, small petiole angles,
Plant morphological differences across treatments
The LMMs revealed that no plant traits differed between planting densities, but highlighted species differences in morphology and size 2017 and 2018. Generally, for most metrics, C. kobomugi was smallest and P. amarum was largest (Fig. 3). Both years, A. breviligulata had an equal number of leaves as C. kobomugi, and P. amarum had more stems than both (F2,44 = 48.9, P < 0.0001). Both years, C. kobomugi had the least number of stems and P. amarum had the most (F2,32 = 80.02, P < 0.0001) with all
Discussion
Our findings elucidate a strong feedback between plant ecosystem engineers and surface topography at the initial stages of foredune genesis in nebkha formation, both in a wind tunnel and field setting. We show that feedbacks are explained by plant traits. Specifically: (1) larger plants created larger nebkha regardless of species; (2) the anecdotal adage that a taller, steeper plant may build a taller steeper dune, is unsupported at initialization, as stem width in erect grasses better
Conclusions
This research contributes to our fundamental understanding of the role of intraspecific variation in vegetation morphology, density, and configuration in impacting geomorphological processes in aeolian beach-dune systems. We demonstrated that larger plants build larger nebkha, lending experimental support to the commonly held belief that larger plants build larger foredunes. However, taller plants do not necessarily build taller and steeper nebkha. Rather, stem width, a proxy for frontal area,
CRediT authorship contribution statement
Charbonneau secured funding to build the wind tunnel and conduct the research working, with Zarnetske designing its specifications based on previous blueprints. Wnek secured the wind tunnel location, and worked with students and Charbonneau collecting data. Charbonneau organized and oversaw the wind tunnel construction and carried out the research. Zarnetske, Wnek, Casper, and Barber each contributed to experimental design. Dohner and Charbonneau conducted field validation analyses. All authors
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was conducted with Government support under contract FA9550-11-C-0028 and awarded by the Department of Defense, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. The wind tunnel was funded and supported by the US Coastal Research Program (USCRP) with USACE ERDC and USGS (Contract #W912HZ18P0090) organized by the American Shore and Beach Preservation Society. It was additionally funded by USACE ERDC BAA
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