Climate as a control on foredune mode in Southern Australia

Highlights

• This research analyzes the foredune mode with respect to a climate gradient.

• 2668 km of coastline are analyzed in the Great Australian Bight (GAB), Australia.

• The clear and main driver is the rainfall which is strongly related to the latitude.

• Nebkhas and transgressive dunefield predominate with 200–300 mm annual rainfall.

• Discontinuous and continuous foredunes predominate above 400 mm annual rainfall.

Abstract

Foredunes are formed by aeolian sand deposition in vegetation on the backshore of beaches. In this paper, the foredune mode (nebkha, discontinuous foredune, and continuous foredune), and transgressive dunefield development is studied along the Great Australian Bight (GAB), 2668 km of coastline. Orthophotos are used to classify the foredune mode, coastal landforms and the vegetation, through geographic information systems (GIS), with fieldwork support. The results show that the foredune mode is strongly controlled by rainfall and temperature with respect to latitude, and to drift potential with respect to longitude across the GAB. Between 200 and 300 mm annual rainfall, nebkha predominate. When the annual rainfall is between 300 and 400, at latitude 32°, a clear pattern is not observed in foredune mode and this is identified as a transition zone. Discontinuous foredunes and continuous foredunes are strongly represented in regions experiencing above 400 mm annual rainfall. The main contribution of this study is the identification of foredune modes which are not only related to a climatic gradient and latitude, but also related to variations in longitude, vegetation cover and diversity, and dune mobility indices. Finally, there are other environmental relationships between the wind and longitude, where the geomorphology of the bay could be playing an important role.

Introduction

Studies dating back to at least the 1800s show that the world's vegetation distribution is determined largely by climate (Von Humboldt and Bonpland, 1807; De Candolle, 1855; Woodward, 1987). So climate, in combination with other environmental variables, have been extensively used to explain the main vegetation patterns around the world (Salisbury, 1926; Good, 1953; McArthur, 1972; Box, 1981; Walter, 1985; Woodward, 1987). It is recognized from the field of biogeomorphology (Corenblit et al., 2011), that since vegetation started to colonize emerged surfaces, complex changes in Earth surface processes and, consequently, in the landforms, have been produced (Murray et al., 2008; Davies and Gibling, 2009, Davies and Gibling, 2010).

Primarily in desert dune systems, but also some coastal systems, various models have been developed to explain the functioning of sand mobility and development of dunes types with respect to climatic variables, such as: wind (Fryberger and Dean, 1979; Tsoar, 2005; Miot da Silva and Hesp, 2010), a combination of wind, precipitation and temperature (Wasson, 1984; Lancaster, 1988), or wind energy and sand supply (Wasson and Hyde, 1983; Louassa et al., 2018; Lü et al., 2018). In coastal dune systems, foredunes (including nebkhas) and dunefield development and evolution has been shown to depend on a variety of regional or local factors such as wind velocity and direction (Rotnicka, 2011), wave energy and surfzone-beach type, storm intensity, scarping and overwash occurrence, and short to long term coastal dynamics (stability, progradation or retrogradation), vegetation cover and density, and sediment supply (Short and Hesp, 1982; Hesp, 1988; 2002; Davidson-Arnott, 2010; Keijsers et al., 2015; Davidson-Arnott et al., 2018).

Rainfall and temperature patterns are responsible for the different floristic regions on beach-dune systems worldwide (Doing, 1985; Takhtajan, 1986; Brunbjerg et al., 2014). The coastal vegetation, the plant species morphology and their density in the beach-dune systems reflect the local disturbance factors influencing, forming or altering aeolian landforms (Hesp and Martínez, 2007). According to the historical regime of physical disturbance, different plant morphologies facilitate the mobility pattern of the surface sediments and the abundance of species (Dangerfield et al., 1998; Stallins, 2002, Stallins, 2005; Stallins and Parker, 2003; Gumbricht et al., 2004; Nield and Baas, 2008; Corenblit et al., 2011).

There is evidence of the relationship between plant species and the formation of coastal foredunes in different coastal climatic regions, in order (greatest to least), according to the distance with respect to the Equator. In temperate zones, plant species, especially grasses, and generally rhizomatous perennial herbaceous plants and glycophytes, can form nebkhas and continuous foredunes (e.g. Ammophila spp., Spinifex spp., Elymus farctus, Austrofestuca littoralis). If the dunes begin as nebkhas, they can grow in size, merge with other nebkhas and eventually form continuous ridges (Hesp, 2002; Hesp and Walker, 2013; Keijsers et al., 2015; Hesp et al., submitted). Most of these species show variable tolerance to the burial of sand, such as Spinifex and Desmoschoenus spiralis in Australia and New Zealand (temperate zone). However, in arid zones, shrub species usually form nebkhas, but not continuous ridges. These species are mainly halophilic adapted to soils with high salinity as well as the lack of fresh water (e.g. Traganum moquinii, Zygophyllum spp., Suaeda spp., Salsola spp.) (Hernández-Cordero et al., 2015, Hernández-Cordero et al., 2017). In this case, the dimensions of the shrub strongly affect the nebkha morphology (El-Bana et al., 2002). For example, on the arid coasts of the Canary Islands, Traganum moquinii is the predominant plant species, which forms nebkha and nebkha fields. It is a nanophanerophyte and shrubby species that survives and thrives where burial by sand occurs (Hernández-Cordero et al., 2012; Viera-Pérez, 2015). Finally, humid tropical coast are normally characterized by continuous foredunes (Doing, 1985), where dune-building species are commonly creeping plants (e.g. Ipomoea pes-caprae, Canavalia rosea) (Arun et al., 1999), or non-rhizomatous woody species (i.e. Scaevola plumieri) (Pammenter, 1983). Hesp et al. (submitted) determined three typical foredune modes exist on many coasts, namely, nebkha, discontinuous foredunes, and continuous foredunes. Given the range of foredune modes present on a particular coast, and the role of climate in determining to a degree foredune development, this paper aims to determine if the climate, especially rainfall, is a controlling factor in determining the mode of foredune formation and the distribution of either continuous or discontinuous ridges or nebkhas along the Great Australian Bight (GAB), which displays a significant east to west and north to south gradient in rainfall. Vegetation data, sand mobility, climatic data and foredune modes are examined along 2668 km of coastline.