Nebkha or not? -Climate control on foredune mode

Highlights

• The presence/absence of foredune mode is determined for a 3,500 km stretch of the African and Canaries Islands coast.

• Foredunes comprise three modes: continuous ridges, discontinuous ridges, and nebkha.

• Foredune mode is strongly correlated with rainfall and floristic region.

• Continuous foredunes occur where rainfall is above ~480 mm and below ~340 mm rainfall only nebkha occur.

Abstract

This study examines the mode of foredune development along ~3,500 km of the west African and the Canary Islands coasts. Foredune modes are classified into either continuous and discontinuous ridges or nebkha (discrete dune mounds). The drivers determining foredune mode including temperature, winds, and sediment supply are investigated and rainfall is the principal driver. Continuous foredunes are found in the region where rainfall is above ~480–500 mm mean annual rainfall. Between around 340 mm and 480 mm the rainfall is not sufficient to sustain laterally continuous pioneer plant communities and discontinuous foredunes predominate. Below ~300 mm rainfall, xerophilous and halophilous shrub plants predominate and only nebkha occur. Our findings support the contention that climate, and particularly rainfall, plays a very significant role in driving foredune mode, and hence coastal morphological evolution.

Introduction

Foredunes are formed by aeolian sand deposition in vegetation on the backshore of beaches (Cowles, 1891; Olson, 1958; Hesp, 2002). Three foredune modes are typically formed alongshore (García-Romero et al., 2019): continuous ridges, discontinuous ridges, and nebkha (aka coppice dunes, hummocks; phytogenic hillocks - formed by sand deposition in isolated or discrete plants). The formation of foredunes depends on the presence of vegetation and a sand supply, and then a variety of other factors including wind regime, surfzone-beach type, plant species present, density and distribution, and impacts and frequencies of storms (Hesp, 2002; Davidson-Arnott, 2010; Keijsers et al., 2015; Davidson-Arnott et al., 2018). The formation of foredunes also depends to a degree on the traits of the plant species present. Annual or bi-annual species (ie, Salsola kali, Cakile sps.) typically produce small ephemeral foredunes. However, if the plant species are perennial, permanent foredunes (including nebkhas) can be formed whose size and shape will depend on the growth habit, plant density, life form and rooting habits of the plants, as well as factors such as wind regime, sediment supply, and age (Hesp, 1989, 1991; Arens, 1996; Hesp and McLachlan, 2000; El-Sheikha et al., 2010; El-Bana et al., 2002; Hesp and Smyth, 2017). Virtually all the species that form foredunes have either rhizomatous roots or are able to produce adventitious roots as a response to burial (Hesp, 1991; Maun, 2009).

The presence of different types of species depends on the floristic region, and is directly related to the climate (Atbib, 1983; Doing, 1985; Takhtajan, 1986; Hesp, 1991; Hesp, 2011; Hesp et al., 2013; Al-Awadhi and Al-Dousari, 2013, Bammi et al., 2014, Gehu, 1998, Gehu and Biondi, 1996; Brunbjerg et al., 2014). In temperate zones, the species which form foredunes are rhizomatous, perennial herbaceous, and glycophyte species, many of them grasses (e.g. Elymus farctus, Ammophila spp., Spinifex sericeus, Austrofestuca littoralis). In the temperate zone, foredunes may begin as nebkhas and when the vegetation spreads laterally, semi-continuous (discontinuous) to continuous ridges are formed (Hesp, 2002; Hesp and Walker, 2013; Keijsers et al., 2015; Ruz et al., 2017a, b). In arid zones, nebkhas predominate, formed by xerophilous and halophilous shrub plants (e.g. Traganum moquinii, Tetraena spp., Suaeda spp, Salsola spp.) adapted to high salinity as a result of the accumulation of saline sea spray not washed away due to lack of rainfall (Hernández-Cordero et al., 2019). The nebkha morphology depends to a large extent on the dimensions of the host shrub (El-Bana et al., 2002), and as plants grow larger over time, so too does the nebkha. On humid tropical coasts, foredune-building species are commonly creeping plants (e.g. Ipomaea pes-caprae, Canavalia rosea) or non-rhizomatous woody species (eg Scaveola plumieri).

Plant density on the backshore will determine whether discrete nebkha, discontinuous or continuous foredunes form. So what drives plant density? First, the climate, as the vegetation cover in rainy and warm weather areas is higher than in arid climate areas (Whittaker 1975; Doing, 1985; Hesp, 2004). Second, the vegetation density will be driven by the availability and distribution of seeds and rhizomes alongshore, serendipitous events such as the degree of juvenile plants survival, number of wave erosion events and wind storms, sand deposition and volume, the subsequent rate and degree of plant colonisation, and presence of rhizomatous versus non-rhizomatous plants (Davies, 1977; McLachlan, 1990; Hesp, 1991, 2002; Maun, 2009; Hesp and Walker, 2013).

At a larger than regional scale, climate zonation may be critical. In the arctic regions, with significant periods of ice and snow (i.e. they are arid zones), nebkha and nebkha fields predominate along the backshore (Mountney and Russell, 2006, 2009; Ruz and Hesp, 2014). Continuous foredunes are only found once climatic conditions are relatively ameliorated, because only a few plants are able to colonise or survive in the extreme climate conditions (Ruz and Hesp, 2014). In contrast, in milder subarctic settings, continuous foredunes are common. In similarity, on arid coastal dunefields, nebkha are common, and continuous foredunes are poorly represented (Khalaf et al., 1995; Edgell, 2006; Hernández-Calvento, 2006; Alonso et al., 2011; Khalaf and Al-Awadhi, 2012; Al-Awadhi and Al-Dousari, 2013; Hernández-Cordero et al., 2015, 2017; Garcia-Romero et al., 2021).

Multiple studies have been conducted on how foredunes evolve. However, only one study (García-Romero et al., 2019) exists on what drives the development of the three foredune modes. Understanding why a foredune is initiated as a continuous dune terrace or ridge (i.e. has laterally continuous plant cover alongshore) and remains as one, or begins as a nebkha and remains as one throughout its development is critical to understanding evolutionary patterns of foredunes and coastal dunes. In order to gain a better appreciation of these evolutionary processes, the main objective of this study was to determine the relationship between foredune mode and climate for an extensive multi-country region along the West African coast and including the Canaries Islands and which embraces multiple climatic regions and gradients.