Grey dunes are fixed, stable sand dunes that are covered by a continuous layer of herbaceous vegetation. These dunes are typically located 50–100 meters from the ocean shore and are found on the landward side of foredunes (also known as

yellow dune Yellow dunes are sand dunes that develop after the first set of embryo dunes appear. With an average depth of about 5m, with a soil to sand ratio that is small on the soil side (hence yellow dunes). The percentage of exposed sand on these dunes ...

s). Grey dunes are named for their characteristic grey color which is a result of the ground cover of lichen combined with a top soil layer of humus.

Formation

Grey dunes are formed when

pioneer species Pioneer species are hardy species that are the first to colonize barren environments or previously biodiverse steady-state ecosystems that have been disrupted, such as by wildfire. Pioneer flora Some lichens grow on rocks without soil, so ...

inhabit

embryo dunes An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

and begin a successional process toward dune stabilization and

soil formation Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations l ...

. Pioneer plants capture saltating sand grains carried by the wind and deposit them to continue build up the dune. Grey dunes are then formed after the root structures of pioneering plants stabilize foredunes. Sand stabilization allows for dead plant material to be decomposed within the soil, adding nutrients and water-retention abilities to the soil. As soil starts to slowly form, new plant species

colonize Colonization, or colonisation, constitutes large-scale population movements wherein migrants maintain strong links with their, or their ancestors', former country – by such links, gain advantage over other inhabitants of the territory. When ...

the stable dunes ultimately developing more extensive root structures and plant diversity. When the dune has become continuously vegetated, lichen and moss grows in the

understory In forestry and ecology, understory (American English), or understorey (Commonwealth English), also known as underbrush or undergrowth, includes plant life growing beneath the forest canopy without penetrating it to any great extent, but abo ...

, providing a water-repellent surface that reduces

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...

(through rainfall and wind), this results in the official formation of the grey dune.

Pluvial In geology and climatology, a pluvial is either a modern climate characterized by relatively high precipitation or an interval of time of variable length, decades to thousands of years, during which a climate is characterized by relatively high ...

processes dominate the geomorphology of grey dunes, with rain splash and slope wash being the most common processes taking place. Erosion can take place in the form of rain splash and slope wash. Rain splash occurs when a raindrop impacts the bare soil surface and disperses soil particles to the surrounding area. However, as grey dunes begin to develop, lichens and mosses protect the dune surfaces and dissipate raindrop impacts. Slope wash (overland flow) is particularly evident in sparsely vegetated dune slopes. Slope wash occurs when rainfall infiltration is impeded, which can result in rills and

alluvial fans An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...

Dune pH

Depending on the sand composition, a grey dune will become either

acidic In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an ad ...

. An acidic soil substrate will form if the sand is composed completely of

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

grains. Older grey dunes can also exhibit acidic conditions if there has been enough rainfall to leach out the soil nutrients. Calcareous soil substrates form when seashells decompose to create a

lime Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may also refer to: Botany ...

-rich environment. The vegetation on grey dunes is dependent on the pH of the soil substrate.

Vegetation

Many rare and

endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and in ...

plant species can be found within grey dune ecosystems, providing niche habitats for animals and insects. A wide variety of

orchids Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Along with the Asteraceae, they are one of the two largest families of flowering ...

grasses Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns ...

, and

lichens A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.plant communities A plant community is a collection or association of plant species within a designated geographical unit, which forms a relatively uniform patch, distinguishable from neighboring patches of different vegetation types. The components of each plant ...

but species will depend on locations throughout the European continent.

Location

Grey dunes are found along much of the

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located entirel ...

an coast and along large, inland bodies of water. Broadly speaking, they can be found along the Celtic, Baltic, and

North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north ...

Seas, the Eastern and Mid

Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...

coast, the Western and Eastern

Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western Europe, Western and Southern Europe and Anatolia, on the south by North Africa ...

coasts, and the

Black sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...

Loss of Grey Dunes

Grey dunes are highly susceptible to change from both natural and human-induced events. Since the 1960s, grey dune ecosystems have declined from the encroachment of

grass Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns a ...

and shrub species, and more recently, by the invasion by the alien invasive Heath Star Moss ( Campylopus introflexus). Causes of the encroachment have been related to large-scale sand dune stabilization, high

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

deposition from the vegetation, and a decline in rabbit populations within the dunes.{{Cite journal, title = Rapid Improvement of Grey Dunes After Shallow Sod Cutting, last = van Til, first = Mark, date = 2007, journal = Coastline Reports 7, first2 = Annemieke, last2 = Kooijman Grass and shrub encroachment has resulted in the fragmentation of grey dune landscapes (and overall reduction in habitat), a

loss of biodiversity Biodiversity loss includes the worldwide extinction of different species, as well as the local reduction or loss of species in a certain habitat, resulting in a loss of biological diversity. The latter phenomenon can be temporary or permanent, de ...

, and a decline of

and rare species that are endemic to grey dunes.

References