Editing Dune (section)

==Aeolian dunes==

===Aeolian dune shapes===
Five basic dune types are recognized: crescentic, linear, star, dome, and parabolic. Dune areas may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types).<ref name="USG Types of Dunes"/>

====Barchan or crescentic====
{{main|barchan}}
[[File:Barchan in Noachis.jpg|thumbnail|right|Isolated barchan dunes on the surface of [[Mars]]. Dominant wind direction would be from left to right.]]
Barchan dunes are crescent-shaped mounds which are generally wider than they are long. The lee-side slipfaces are on the concave sides of the dunes. These dunes form under winds that blow consistently from one direction (unimodal
winds).<ref name=dkp>{{cite book|title=The Ultimate Visual Family Dictionary|chapter=Geology, Geography, and Meteorology|page=282–283|publisher=[[DK (publisher)|D.K. Pub.]]|year=2012|location=New Delhi|language=en|isbn=978-0-1434-1954-9}}</ref> They form separate crescents when the sand supply is comparatively small. When the sand supply is greater, they may merge into barchanoid ridges, and then transverse dunes (see below).<ref name=Mangimeli/>

Some types of crescentic dunes move more quickly over [[desert]] surfaces than any other type of dune. A group of dunes moved more than 100 metres per year between 1954 and 1959 in [[China]]'s [[Ningxia Province]], and similar speeds have been recorded in the [[Western Desert (Egypt)|Western Desert]] of [[Egypt]]. The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than three kilometres, are in China's [[Taklamakan Desert]].<ref name="USG Types of Dunes">{{cite web|title=Types of Dunes|url=http://pubs.usgs.gov/gip/deserts/dunes/|access-date=8 March 2012|url-status=live|archive-url=https://web.archive.org/web/20120314161113/http://pubs.usgs.gov/gip/deserts/dunes/|archive-date=14 March 2012 |publisher=[[U.S. Geological Survey]] |date=29 October 1997}}</ref>

====Transverse dunes====
Abundant barchan dunes may merge into barchanoid ridges, which then grade into linear (or slightly sinuous) transverse dunes, so called because they lie transverse, or across, the wind direction, with the wind blowing perpendicular to the ridge crest.<ref name=Mangimeli/><ref name=dkp/>

====Seif or longitudinal dunes====
<!--[[Seif dune]] redirects here-->
Seif dunes are linear (or slightly sinuous) dunes with two slip faces. The dunes lie generally parallel to each other<ref name=Mangimeli/><ref name=dkp/> The two slip faces make them sharp-crested. They are called ''seif'' dunes after the Arabic word for "sword". They may be more than 160 kilometres (100 miles) long, and thus easily visible in satellite images (see illustrations).

Seif dunes are associated with bidirectional winds. The long axes and ridges of these dunes extend along the resultant direction of sand movement (hence the name "longitudinal").<ref name=seif_terminology>{{cite book |doi=10.1007/978-1-4614-9213-9_460-2 |chapter=Longitudinal Dunes (or Linear Dunes) |title=Encyclopedia of Planetary Landforms |pages=1–11 |year=2014 |last1=Radebaugh |first1=Jani |last2=Sharma |first2=Priyanka |last3=Korteniemi |first3=Jarmo |last4=Fitzsimmons |first4=Kathryn E. |isbn=978-1-4614-9213-9 }}</ref> Some linear dunes merge to form Y-shaped compound dunes.<ref name="USG Types of Dunes"/>

Formation is debated. [[Ralph Bagnold]], in ''[[The Physics of Blown Sand and Desert Dunes]]'', suggested that some seif dunes form when a barchan dune moves into a bidirectional wind regime, and one arm or wing of the crescent elongates. Others suggest that seif dunes are formed by [[vortex|vortices]] in a unidirectional wind.<ref name=Mangimeli/> In the sheltered troughs between highly developed seif dunes, barchans may be formed, because the wind is constrained to be unidirectional by the dunes.

<!-- The first image (Rub' al Khali) would be better rotated 90 degrees clockwise to match it with the contrasting image to the right --> 
<gallery mode="packed" heights="150">
File:Rub' al Khali (Arabian Empty Quarter) sand dunes imaged by Terra (EOS AM-1).jpg|Rub' al Khali (Arabian Empty Quarter) sand dunes imaged by Terra (EOS AM-1). Most of these dunes are seif dunes. Their origin from barchans is suggested by the stubby remnant "hooks" seen on many of the dunes. Wind would be from left to right.
File:ISS-31 Linear dunes in the Great Sand Sea in southwest Egypt.jpg|Large linear seif dunes in the [[Great Sand Sea]] in southwest [[Egypt]], seen from the [[International Space Station]]. The distance between each dune is 1.5–2.5&nbsp;km.
File:Longitudinal_dune.jpg|The average-direction-longitudinal model of seif dune formation
File:Tranverse dune.jpg|alt=Transverse dune with wind blowing across crest|By contrast, transverse dunes form with the wind blowing perpendicular to the ridges, and have only one slipface, on the lee side. The stoss side is less steep.
File:Cross-bedding.gif|alt=Animation of wind pushing transverse dunes along. The sand blows from the stoss side down onto the less side, where it is buried by the next layer. The dune thus moves, and a cross-section through it shown diagonal cross-bedding|Transverse dunes lie perpendicular to the wind, which moves them forwards, producing the [[cross-bedding]] shown here.
</gallery>

Seif dunes are common in the Sahara. They range up to {{convert|300|m|ft|abbr=on}} in height and {{convert|300|km|mi|abbr=on}} in length. In the southern third of the Arabian Peninsula, a vast [[Erg (landform)|erg]], called the [[Rub' al Khali]] or Empty Quarter, contains seif dunes that stretch for almost {{convert|200|km|mi|abbr=on}} and reach heights of over {{convert|300|m|ft|abbr=on}}.

Linear [[loess]] hills known as [[paha (landform)|pahas]] are superficially similar. These hills appear to have been formed during the last [[Quaternary glaciation|ice age]] under [[permafrost]] conditions dominated by sparse [[tundra]] vegetation.

====Star====
[[File:Elk and Star Dune (49097789642).jpg|thumb|Star dunes with a rough pyramidal shape and ridges at [[Great Sand Dunes National Park and Preserve]]]]
Star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes.<ref name=dkp/> Star dunes grow upward rather than laterally. They dominate the [[Grand Erg Oriental]] of the Sahara. In other deserts, they occur around the margins of the [[sand sea]]s, particularly near topographic barriers. In the southeast [[Badain Jaran Desert]] of China, the star dunes are up to 500 metres tall and may be the tallest dunes on Earth.

====Dome====
Oval or circular mounds that generally lack a slipface. Dome dunes are rare and occur at the far upwind margins of sand seas.

=====Lunettes=====
Fixed crescentic dunes that form on the leeward margins of [[Dry lake|playas]] and river valleys in arid and semiarid regions in response to the direction (s) of prevailing winds, are known as lunettes, source-bordering dunes, bourrelets and clay dunes. They may be composed of clay, silt, sand, or gypsum, eroded from the basin floor or shore, transported up the concave side of the dune, and deposited on the convex side. Examples in Australia are up to 6.5&nbsp;km long, 1&nbsp;km wide, and up to 50 metres high. They also occur in [[Southern Africa|southern]] and [[West Africa]], and in parts of the western United States, especially Texas.<ref>[[Charles Rowland Twidale|Twidale, C.R.]] & Campbell, E.M. (2005, revised edition): Australian landforms: understanding a low, flat, arid and old landscape. Rosenberg Publishing. pp. 241–3. {{ISBN|1 877058 32 7}}</ref>

====Parabolic====
[[File:Parabolic dune.jpg|thumb|upright=1.5|Schematic of coastal parabolic dunes]]
U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. These dunes are formed from blowout dunes where the erosion of vegetated sand leads to a U-shaped depression. The elongated arms are held in place by vegetation; the largest arm known on Earth reaches 12&nbsp;km. Sometimes these dunes are called U-shaped, [[blowout (geology)|blowout]], or hairpin dunes, and they are well known in coastal deserts. Unlike crescent shaped dunes, their crests point upwind. The bulk of the sand in the dune migrates forward.

In plan view, these are U-shaped or V-shaped mounds of well-sorted, very fine to medium sand with elongated arms that extend upwind behind the central part of the dune. There are slipfaces that often occur on the outer side of the nose and on the outer slopes of the arms.

These dunes often occur in semiarid areas where the precipitation is retained in the lower parts of the dune and underlying [[soil]]s. The stability of the dunes was once attributed to the vegetative cover but recent research has pointed to water as the main source of parabolic dune stability. The vegetation that covers them—grasses, shrubs, and trees—help anchor the trailing arms. In inland deserts, parabolic dunes commonly originate and extend downwind from blowouts in sand sheets only partly anchored by vegetation. They can also originate from beach sands and extend inland into vegetated areas in coastal zones and on shores of large lakes.

Most parabolic dunes do not reach heights higher than a few tens of metres except at their nose, where vegetation stops or slows the advance of accumulating sand.

Simple parabolic dunes have only one set of arms that trail upwind, behind the leading nose. Compound parabolic dunes are coalesced features with several sets of trailing arms. Complex parabolic dunes include subsidiary superposed or coalesced forms, usually of barchanoid or linear shapes.

Parabolic dunes, like crescent dunes, occur in areas where very strong winds are mostly unidirectional. Although these dunes are found in areas now characterized by variable wind speeds, the effective winds associated with the growth and migration of both the parabolic and crescent dunes probably are the most consistent in wind direction.

The grain size for these well-sorted, very fine to medium sands is about 0.06 to 0.5&nbsp;mm. Parabolic dunes have loose sand and steep slopes only on their outer flanks. The inner slopes are mostly well packed and anchored by vegetation, as are the corridors between individual dunes. Because all dune arms are oriented in the same direction, and, the inter-dune corridors are generally swept clear of loose sand, the corridors can usually be traversed in between the trailing arms of the dune. However to cross straight over the dune by going over the trailing arms, can be very difficult. Also, traversing the nose is very difficult as well because the nose is usually made up of loose sand without much if any vegetation.

A type of extensive parabolic dune that lacks discernible slipfaces and has mostly coarse grained sand is known as a ''zibar''.<ref>{{cite book|last=Goudie|first=Ron Cooke; Andrew Warren; Andrew|title=Desert geomorphology|year=1996|publisher=UCL Press|location=London|isbn=978-1-85728-017-3|pages=395–396|edition=2. impr.}}</ref> The term zibar comes from the [[Arabic]] word to describe "rolling transverse ridges ... with a hard surface".<ref>{{cite book|last=Goudie|first=Ron Cooke; Andrew Warren; Andrew|title=Desert geomorphology|year=1996|publisher=UCL Press|location=London|isbn=978-1-85728-017-3|pages=395|edition=2. impr.}}</ref> The dunes are small, have low relief, and can be found in many places across the planet from [[Wyoming]] (United States) to Saudi Arabia to Australia.<ref>{{cite web|title=USGS Landform Glossary|url=ftp://ftp-fc.sc.egov.usda.gov/NSSC/Soil_Survey_Handbook/629_glossary.pdf|work=United States Geological Survey|access-date=3 October 2013}}{{dead link|date=December 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Spacing between zibars ranges from 50 to 400 metres and they do not become more than 10 metres high.<ref>{{cite journal |last1=Warren |first1=A. |title=Dunes in the Tenere Desert |journal=The Geographical Journal |date=December 1971 |volume=137 |issue=4 |pages=458–461 |doi=10.2307/1797141|jstor=1797141 |bibcode=1971GeogJ.137..458W }}</ref> The dunes form at about ninety degrees to the prevailing wind which blows away the small, fine-grained sand leaving behind the coarser grained sand to form the crest.<ref>{{cite journal |last1=Nielson |first1=Jamie |last2=Kocurek |first2=Gary |title=Climbing Zibars of the Algodones |journal=Sedimentary Geology |date=June 1986 |volume=48 |issue=1–2 |pages=1–15 |doi=10.1016/0037-0738(86)90078-3|bibcode=1986SedG...48....1N }}</ref>

====Reversing dunes====
[[File:Libya 4608 Idehan Ubari Dunes Luca Galuzzi 2007.jpg|thumb|left|Reversing dune showing short minor slipface atop the major stoss (upwind) face]]
Occurring wherever winds periodically reverse direction, reversing dunes are varieties of any of the above shapes. These dunes typically have major and minor slipfaces oriented in opposite directions. The minor slipfaces are usually temporary, as they appear after a reverse wind and are generally destroyed when the wind next blows in the dominant direction.<ref name=Mangimeli/>

{{anchor|Draa}}

====Draas====
[[File:350m Düne Deadvlei Sossusvlei.JPG|thumb|Dune Nine in [[Sossusvlei]], Namibia, is over 300m high.]]
Draas are very large-scale dune bedforms; they may be tens or a few hundreds of metres in height, kilometres wide, and hundreds of kilometres in length.<ref name=Mangimeli>{{cite web| publisher = U.S.A. National Park Service| last = Mangimeli| first = John| title = Geology of Sand Dunes| date = 10 September 2007 |url=https://home.nps.gov/whsa/learn/nature/loader.cfm?csModule=security/getfile&PageID=2033934 |access-date=15 January 2021}}</ref> After a draa has reached a certain size, it generally develops superimposed dune forms.<ref name=Lancaster>{{cite journal|title=The development of large aeolian bedforms|first=N.|last=Lancaster|date=1 March 1988|journal=Sedimentary Geology|volume=55|issue=1–2|pages=69–89|doi=10.1016/0037-0738(88)90090-5|bibcode=1988SedG...55...69L}}</ref> They are thought to be more ancient and slower-moving than smaller dunes,<ref name=Mangimeli/> and to form by vertical growth of existing dunes. Draas are widespread in sand seas and are [[Cross-bedding|well-represented in the geological record]].<ref name=Lancaster/>

===Dune complexity===
All these dune shapes may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types).<ref name="USG Types of Dunes"/> Simple dunes are basic forms with the minimum number of slipfaces that define the geometric type. Compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed. Complex dunes are combinations of two or more dune types. A crescentic dune with a star dune superimposed on its crest is the most common complex dune. Simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune, while compound and complex dunes suggest that the intensity and direction of the wind has changed.

===Dune movement===
The sand mass of dunes can move either windward or leeward, depending on if the wind is making contact with the dune from below or above its apogee. If wind hits from above, the sand particles move leeward; the leeward flux of sand is greater than the windward flux. Conversely, if sand hits from below, sand particles move windward. Further, if the wind is carrying sand particles when it hits the dune, the dune's sand particles will saltate more than if the wind had hit the dune without carrying sand particles.<ref>{{Cite journal|last1=Jiang|first1=Hong|last2=Dun|first2=Hongchao|last3=Tong|first3=Ding|last4=Huang|first4=Ning|date=15 April 2017|title=Sand transportation and reverse patterns over leeward face of sand dune|journal=Geomorphology|volume=283|pages=41–47|doi=10.1016/j.geomorph.2016.12.030|bibcode=2017Geomo.283...41J}}</ref>

===Coastal dunes===
[[File:LIVER AAS UDLOEB I JULI 2012 (ubt)-002.JPG|thumb|Coastal dunes covered in grasses around the mouth of the Liver Å river in Denmark]]
[[File:Newborough Dune Rejuvenation.webm|thumb|Newborough Dune Rejuvenation, Wales; video of work done by [[Natural Resources Wales]]; 2015]]

Coastal dunes<ref name="sloss-etal-2012">{{Cite journal |url=https://www.nature.com/scitable/knowledge/library/coastal-dunes-geomorphology-25822000/ |last1=Sloss |first1=C. R. |last2=Shepherd |first2=M. |last3=Hesp |first3=P |year=2012 |title=Coastal Dunes: Geomorphology |journal=Nature Education Knowledge |volume=3 |number=10 |page=2  |access-date=15 January 2021}}</ref> form when wet sand is deposited along the coast and dries out and is blown along the beach.<ref>{{cite book |last1=Bird |first1=ECF |title=Coasts: an introduction to systematic geomorphology |date=1976 |publisher=Australian National University Press |location=Canberra, Australia |asin=B004750SVK}}</ref> Dunes form where the beach is wide enough to allow for the accumulation of wind-blown sand, and where prevailing '''onshore winds''' tend to blow sand inland. The three key ingredients for coastal dune formation are a large sand supply, winds to move said sand supply, and a place for the sand supply to accumulate.<ref>{{cite book |last1=Goldsmith |first1=Victor |title=Coastal Sedimentary Environments |chapter=Coastal Dunes |date=1978 |doi=10.1007/978-1-4684-0056-4_5 |pages=171–235|isbn=978-1-4684-0058-8 }}</ref> Obstacles—for example, vegetation, pebbles and so on—tend to slow down the wind and lead to the deposition of sand grains.<ref name=Hesp1989>{{Cite journal| last = Hesp | first = P.| year = 1989| title = A review of biological and geomorphological processes involved in the initiation and development of incipient foredunes| journal = Proceedings of the Royal Society of Edinburgh, Section B: Biological Sciences| volume = 96| pages = 181–201| doi =  10.1017/S0269727000010927 }}</ref> These small "incipient dunes or "shadow dunes" tend to grow in the vertical direction if the obstacle slowing the wind can also grow vertically (i.e., vegetation). Coastal dunes expand laterally as a result of lateral growth of coastal plants via seed or [[rhizome]].<ref>{{Cite journal|last=Godfrey|first=P. J.|date=1 September 1977|title=Climate, plant response and development of dunes on barrier beaches along the U.S. east coast|journal=International Journal of Biometeorology|volume=21|issue=3|pages=203–216|doi=10.1007/BF01552874|issn=0020-7128|bibcode=1977IJBm...21..203G|s2cid=85391018}}</ref><ref>{{Cite journal|last1=Goldstein|first1=Evan B.|last2=Moore|first2=Laura J.|last3=Vinent|first3=Orencio Durán|date=8 August 2017|title=Lateral vegetation growth rates exert control on coastal foredune "hummockiness" and coalescing time|journal=Earth Surface Dynamics|volume=5|issue=3|pages=417–427|doi=10.5194/esurf-5-417-2017|issn=2196-6311|doi-access=free}}{{open access}}</ref> Models of coastal dunes suggest that their final equilibrium height is related to the distance between the water line and where vegetation can grow.<ref name=DuranMoore2013>{{Cite journal| last1 = Durán | first1 = O.| last2=Moore|first2= L. J. |year = 2013| title = Vegetation controls on the maximum size of coastal dunes| journal = Proceedings of the National Academy of Sciences| volume = 110 | issue = 43 |  pages = 17217–17222| doi =  10.1073/pnas.1307580110 | pmid=24101481 | pmc=3808624| bibcode = 2013PNAS..11017217D| doi-access = free}}</ref> Coastal dunes can be classified by where they develop, or begin to take shape. Dunes are commonly grouped into either the Primary Dune Group or the Secondary Dune Group.<ref name="sloss-etal-2012"/> Primary dunes gain most of their sand from the beach itself, while secondary dunes gain their sand from the primary dune. Along the Florida Panhandle, most dunes are considered to be foredunes or hummocks.<ref>{{cite journal |last1=Houser |first1=C |last2=Hapke |first2=C |last3=Hamilton |first3=S |title=Controls on coastal dune morphology, shoreline erosion and barrier island response to extreme storms |journal=Geomorphology |date=15 August 2008 |volume=100 |issue=3–4 |pages=223–40|doi=10.1016/j.geomorph.2007.12.007 |bibcode=2008Geomo.100..223H }}</ref><ref>{{cite journal |last1=Claudino-Sales |first1=V |last2=Wang |first2=P |last3=Horwitz |first3=MH |title=Factors controlling the survival of coastal dunes during multiple hurricane impacts in 2004 and 2005: Santa Rosa Barrier Island, Florida |journal=Geomorphology |date=15 March 2008 |volume=95 |issue=3–4 |pages=295–315|doi=10.1016/j.geomorph.2007.06.004 |bibcode=2008Geomo..95..295C }}</ref> Different locations around the globe have dune formations unique to their given coastal profile.

Coastal sand dunes can provide privacy and/or habitats to support local flora and fauna. Animals such as sand snakes, lizards, and rodents can live in coastal sand dunes, along with insects of all types.<ref>{{cite web |last1=Ronica |first1=D |title=How sand dunes work |url=https://science.howstuffworks.com/environmental/earth/geology/sand-dune3.htm |website=HowStuffWorks |access-date=4 December 2018|date=27 October 2008 }}</ref> Often the vegetation of sand dunes is discussed without acknowledging the importance that coastal dunes have for animals. Further, some animals, such as foxes and feral pigs can use coastal dunes as hunting grounds to find food.<ref>{{cite web |last1=Hill |first1=K |title=Dune Habitats |url=https://naturalhistory2.si.edu/smsfp/IRLSpec/Dunes.htm |website=Smithsonian Marine Station |access-date=4 December 2018}}</ref> Birds are also known to utilize coastal dunes as nesting grounds. All these species find the coastal environment of the sand dune vital to their species' survival.

Over the course of time coastal dunes may be impacted by [[tropical cyclone]]s or other intense storm activity, dependent on their location. Recent work has suggested that coastal dunes tend to evolve toward a high or low morphology depending on the growth rate of dunes relative to storm frequency.<ref>{{Cite journal|last1=Durán Vinent|first1=Orencio|last2=Moore|first2=Laura|date=February 2015|title=Barrier island bistability induced by biophysical interactions|journal=Nature Climate Change|volume=5|issue=2|pages=158–162|doi=10.1038/nclimate2474|issn=1758-6798|bibcode=2015NatCC...5..158D|url=https://cdr.lib.unc.edu/downloads/1257b286c}}</ref><ref>{{Cite journal|last1=Goldstein|first1=Evan B.|last2=Moore|first2=Laura J.|date=2016|title=Stability and bistability in a one-dimensional model of coastal foredune height|journal=Journal of Geophysical Research: Earth Surface|volume=121|issue=5|pages=964–977|doi=10.1002/2015JF003783|issn=2169-9011|bibcode=2016JGRF..121..964G|doi-access=free}}</ref> During a storm event, dunes play a significant role in minimizing wave energy as it moves onshore. As a result, coastal dunes, especially those in the foredune area affected by a [[storm surge]], will retreat or erode.<ref>{{cite journal |last1=Morton |first1=RA |title=Effects of Hurricane Eloise on beach and coastal structures, Florida Panhandle |journal=Geology |date=1 May 1976 |volume=4 |issue=5 |pages=277–80|doi=10.1130/0091-7613(1976)4<277:EOHEOB>2.0.CO;2 |bibcode=1976Geo.....4..277M }}</ref> To counteract the damage from tropical activity on coastal dunes, short term post-storm efforts can be made by individual agencies through fencing to help with sand accumulation.<ref>{{cite journal |last1=Charbonneau |first1=B |last2=Wnek |first2=JP |title=Reactionary fence installation for post-Superstorm Sandy dune recovery |url=https://eartharxiv.org/ntjsc |journal=Eartharxiv ePrints |date=24 September 2018 |doi=10.31223/osf.io/ntjsc |bibcode=2018EaArX....NTJSCC |access-date=4 December 2018 |archive-date=30 July 2020 |archive-url=https://web.archive.org/web/20200730053248/https://eartharxiv.org/ntjsc |url-status=dead }}</ref>

How much a dune erodes during any storm surge is related to its location on the coastal shoreline and the profile of the beach during a particular season. In those areas with harsher winter weather, during the summer a beach tends to take on more of a convex appearance due to gentler waves, while the same beach in the winter may take on more of a concave appearance. As a result, coastal dunes can get eroded much more quickly in the winter than in the summer. The converse is true in areas with harsher summer weather.<ref>{{cite web |last1=Maine Sea Grant |title=Seasonal changes |url=https://www.seagrant.umaine.edu/coastal-hazards-guide/beaches-and-dunes/learn-more/seasonal-changes |website=Maine Sea Grant College Program |access-date=4 December 2018 |archive-url=https://web.archive.org/web/20181205003653/https://www.seagrant.umaine.edu/coastal-hazards-guide/beaches-and-dunes/learn-more/seasonal-changes |archive-date=5 December 2018 |url-status=dead }}</ref>

There are many threats to these coastal communities. Some coastal dunes, for example ones in San Francisco, have been completely altered by urbanization; reshaping the dune for human use. This puts native species at risk. Another danger, in California and places in the UK specifically, is the introduction of invasive species. Plant species, such as ''[[Carpobrotus edulis]]'', were introduced from South Africa in an attempt to stabilize the dunes and provide horticultural benefits, but instead spread taking land away from native species. ''[[Ammophila arenaria]]'', known as European beachgrass, has a similar story, though it has no horticulture benefits. It has great ground coverage and, as intended, stabilized the dunes but as an unintended side effect prevented native species from thriving in those dunes. One such example is the dune field at [[Point Reyes|Point Reyes, California]]. There are now efforts to get rid of both of these invasive species.<ref>{{Cite web|date=2018-02-06|title=Large-scale removal of beachgrass leads to new life for endangered coastal lupine {{!}} The Source {{!}} Washington University in St. Louis|url=https://source.wustl.edu/2018/02/large-scale-removal-beachgrass-leads-new-life-endangered-coastal-lupine/|access-date=2020-06-09|website=The Source|language=en-US}}</ref><ref>{{Cite web|title=Hottentot Fig Removal & Control {{!}} IWS Ltd|url=https://www.invasiveweedsolutions.co.uk/invasive-weeds/non-native/hottentot-fig/|access-date=2020-06-09|website=Invasive Weed Solutions UK|language=en-GB}}</ref>

====Ecological succession on coastal dunes====
As a dune forms, plant [[Ecological succession|succession]] occurs. The conditions on an ''embryo dune'' are harsh, with [[salt spray]] from the sea carried on strong winds. The dune is well drained and often dry, and composed of calcium carbonate from seashells. Rotting [[seaweed]], brought in by storm waves adds nutrients to allow [[pioneer species]] to colonize the dune. For example, in the [[United Kingdom]] these pioneer species are often [[Ammophila (Poaceae)|marram grass]], [[sea wort grass]] and other sea grasses. These plants are well adapted to the harsh conditions of the foredune, typically having deep roots which reach the [[water table]], [[root nodules]] that produce [[nitrogen]] compounds, and protected [[stoma]], reducing [[transpiration]]. Also, the deep roots bind the sand together, and the dune grows into a [[foredune]] as more sand is blown over the grasses. The grasses add [[nitrogen]] to the soil, meaning other, less hardy plants can then colonize the dunes. Typically these are [[Calluna|heather]], [[Heath (habitat)|heath]]s and [[gorse]]s. These too are adapted to the low [[Soil#Soil moisture|soil water]] content and have small, prickly leaves which reduce transpiration. Heather adds [[humus]] to the soil and is usually replaced by [[conifer]]ous trees, which can tolerate low [[soil pH]], caused by the accumulation and decomposition of organic matter with nitrate leaching.<ref name=Miles1985>{{Cite journal| last = Miles | first = J.| year = 1985| title = The pedogenic effects of different species and vegetation types and the implications of succession| journal = European Journal of Soil Science| volume = 36| issue = 4| pages = 571–584| doi = 10.1111/j.1365-2389.1985.tb00359.x}}</ref> Coniferous forests and [[heathland]] are common [[climax community|climax communities]] for sand dune systems.

Young dunes are called [[yellow dune]]s and dunes which have high humus content are called [[grey dune]]s. Leaching occurs on the dunes, washing humus into the slacks, and the slacks may be much more developed than the exposed tops of the dunes. It is usually in the slacks that more rare species are developed and there is a tendency for the dune slacks' soil to be waterlogged where only marsh plants can survive. In Europe these plants include: creeping willow, cotton grass, [[yellow iris]], reeds, and rushes. As for vertebrates in European dunes, [[natterjack toad]]s sometimes breed here.

==== Coastal dune floral adaptations ====
[[File:Hyypänmäki Hailuoto 2012-06-06 (1).jpg|thumb|Sand dunes of Hyypänmäki in [[Hailuoto]], Finland]]
[[File:Sea dune Erosion at Talace, Wales.webm|thumb|Sea dune erosion at [[Talacre]], [[Wales]]]]
Dune ecosystems are extremely difficult places for plants to survive. This is due to a number of pressures related to their proximity to the ocean and confinement to growth on sandy substrates. These include:
* Little available soil moisture
* Little available [[soil organic matter]]/nutrients/water
* Harsh winds
* Salt spray
* Erosion/shifting and sometimes burial or exposure (from shifting)
* Tidal influences
Plants have evolved many adaptations to cope with these pressures:
* Deep taproot to reach water table ([[Abronia umbellata|Pink Sand Verbena]])
* Shallow but extensive root systems
*[[Rhizome]]s
* Prostrate growth form to avoid wind/salt spray (''Abronia spp''., Beach Primrose)
* [[Krummholz]] growth form ([[Monterrey Cypress]]-not a dune plant but deals with similar pressures)
* Thickened cuticle/Succulence to reduce moisture loss and reduce salt uptake (''Ambrosia/Abronia spp., Calystegia soldanella'')
* Pale leaves to reduce insolation (''Artemisia/Ambrosia spp.'')
* Thorny/Spiky seeds to ensure establishment in vicinity of parent, reduces chances of being blown away or swept out to sea (''Ambrosia chamissonis'')

===Gypsum dunes===
[[File:Aerial view of dunefield, White Sands National Park, New Mexico, United States.png|thumb|Gypsum dune fields, [[White Sands National Park]], New Mexico, United States]]
In deserts where large amounts of limestone mountains surround a [[closed basin]], such as at [[White Sands National Park]] in south-central [[New Mexico]], occasional storm runoff transports dissolved limestone and [[gypsum]] into a low-lying pan within the basin where the water evaporates, depositing the gypsum and forming crystals known as [[Selenite (mineral)|selenite]]. The crystals left behind by this process are eroded by the wind and deposited as vast white dune fields that resemble snow-covered landscapes. These types of dune are rare, and only form in closed arid basins that retain the highly soluble gypsum that would otherwise be washed into the sea.<ref>{{cite web |url=https://www.nps.gov/whsa/learn/geology-of-white-sands.htm |title=Geology of White Sands|publisher=United States National Park Service|access-date=20 January 2021}}</ref>

===Nabkha dunes===
A [[nabkha]], or coppice dune, is a small dune anchored by vegetation. They usually indicate desertification or soil erosion, and serve as nesting and burrow sites for animals.