Biomorphogenic Feedbacks and the Spatial Organization of a Dominant Grass Steer Dune Development

Abstract

Nature-based solutions to mitigate the impact of future climate change depend on restoring biological diversity and natural processes. Coastal foredunes represent the most important natural flood barriers along coastlines worldwide, but their area has been squeezed dramatically because of a continuing urbanization of coastlines, especially in Europe. Dune development is steered by the development of vegetation in interaction with sand fluxes from the beach. Marram grass (Calamagrostis arenaria, formerly Ammophila arenaria) is the main dune building species along most European coasts, but also in other continents where the species was introduced. Engineering of coastal dunes, for instance by building dunes in front of dikes, needs to be based on a solid understanding of the species’ interactions with the environment. Only quantitative approaches enable the further development of mechanistic models and coastal management strategies that encapsulate these biomorphogenic interactions. We here provide a quantitative review of the main biotic and physical interactions that affect marram grass performance, their interactions with sand fluxes and how they eventually shape dune development. Our review highlights that the species’ spatial organization is central to dune development. We further demonstrate this importance by means of remote sensing and a mechanistic model and provide an outlook for further research on the use of coastal dunes as a nature-based solution for coastal protection.

The distribution and impact of an invasive plant species (Senecio inaequidens) on a dune building engineer (Calamagrostis arenaria)

“…If the marram grass is randomly distributed, the parameter is close to 0 and it is positive when the marram grass occurs clustered together. As pointed out by Bonte et al (2021), marram grass distribution is almost always clustered (i.e., high JC values) and rarely random. These two parameters (P and JC) were calculated within four circles with different radii (5 m, 10 m, 20 m, 50 m) around the central marram grass tussock to represent different spatial scales.…”

“…From available vegetation maps of the foredunes (Bonte et al 2021), the proportional cover by marram grass in the vicinity of the central marram grass tussock (P), together with a measure of spatial autocorrelation of marram grass occurrence (normalised join count statistics, JC; Cliff and Ord 1981), were calculated. These two parameters were used to express the spatial configuration of the surrounding marram grass.…”

“…These two parameters (P and JC) were calculated within four circles with different radii (5 m, 10 m, 20 m, 50 m) around the central marram grass tussock to represent different spatial scales. See supplementary material of Bonte et al (2021) for a more in-depth explanation of construction of the vegetation maps and calculations of the spatial parameters.…”

“…This could lead to the invasive species becoming competitively stronger and outcompeting marram grass (i.e., a form of positive density dependence). Such changes in competition could have extensive consequences for coastal dunes and their ecosystem services (Klironomos 2002), especially coastal defence, as they could trigger feedbacks that change system dynamics (Bonte et al 2021).…”

Ammophila Invasion Ecology and Dune Restoration on the West Coast of North America

“…Active sand burial stimulates the production of new shoots from vertical rhizomes [25,26]. Feedbacks between the environment and the spatial arrangement of vegetation determine foredune morphology [27]. In the absence of sand accumulation A. arenaria declines in vigor, a phenomenon much discussed in the European literature and deemed "The Ammophila Problem" [28,29].…”