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A nabkha or nebkha is a type of sand dune. Other terms used include coppice dune and dune hummock or hummocky dune, but these more accurately refer to similar, but different, sand dune types.〔Langford, R. P. (2000). (Nabkha (coppice dune) fields of south-central New Mexico, USA. ) ''Journal of Arid Environments'' 46(1) 25-41.〕 Authors have also used the terms phytogenic hillock,〔El-Sheikh, M. A., et al. (2010). (Vegetation ecology of phytogenic hillocks (nabkhas) in coastal habitats of Jal Az-Zor National Park, Kuwait: Role of patches and edaphic factors. ) ''Flora'' 205(12) 832-40.〕 bush-mound, shrub-coppice dune, knob dune, dune tumulus, rebdou, nebbe, and takouit.〔Khalaf, F. I., et al. (1995). (Sedimentological and morphological characteristics of some nabkha deposits in the northern coastal plain of Kuwait, Arabia. ) ''Journal of Arid Environments'' 29(3) 267-92.〕 In simplest terms, a nabkha is a sand dune that forms around vegetation.〔〔 It is an aeolian landform, a structure built and shaped by the action of wind.〔 Nabkhas are common and occur in many regions. Well known nabkha fields occur in the Arabian Desert of Kuwait,〔 the Hotan River Basin in Xinjiang, China,〔Wu, S., et al. (2008). (The morphological characteristics and growth mode of nabkha in the basin of Hotan River, Xinjiang. ) ''Geographical Research'' 27(2) 314-22.〕 and New Mexico in the United States and adjacent Chihuahua in Mexico.〔 ==Structure== Many species of sand-dwelling plants form nabkhas. In the Chihuahuan Desert they include soaptree (''Yucca elata''), creosote (''Larrea tridentata''), and atriplex (''Atriplex'' spp.).〔 In China nabkhas form quite often around ''Caragana microphylla'', as well as ''Cleistogenes squarrosa'', ''Leymus chinensis'', ''Caragana stenophylla'', ''Stipa grandis'', and ''S. glareosa'',〔Wang, X., et al. (2006). (Nebkha development and its significance to wind erosion and land degradation in semi-arid northern China. ) ''Journal of Arid Environments'' 65 129-41.〕 plus tamarisks, reeds, and alhagi.〔〔Li, Z., et al. (2010) (Bio-geomorphologic features and growth process of ''Tamarix'' nabkhas in Hotan River Basin, Xinjiang. ) ''Journal of Geographical Sciences'' 20(2) 205-18.〕 In Jal Az-Zor National Park in Kuwait, they occur around ''Nitraria retusa'', ''Zygophyllum qatarense'', ''Haloxylon salicornicum'', and ''Panicum turgidum''.〔 Halophytes such as ''Tamarix aucheriana'', ''Halocnemum strobilaceum'' and ''Salicornia europaea'' have nabkhas in saline soils, while ''Cyperus conglomeratus'', ''Rhanterium epapposum'', ''Astragalus spinosus'', ''Lycium shawii'', and ''Citrulus colocynthis'' are seen in non-saline zones.〔Al-Dousari, A. M., et al. (2008). (Characteristics of nabkhas in relation to dominant perennial plant species in Kuwait. ) ''Kuwait Journal of Science'' 35(1).〕 On the Sinai Peninsula of Egypt they are recorded on ''Artemisia monosperma'', ''Moltkiopsis ciliata'', ''Calligonum polygonoides'', ''Stipagrostis scoparia'', and ''Retama raetam''.〔El‐Bana, M. I., et al. (2007). (Role of host identity in effects of phytogenic mounds on plant assemblages and species richness on coastal arid dunes. ) ''Journal of Vegetation Science'' 18(5) 635-44.〕 In central Asia and surrounding areas they occur on ''Calotropis'', ''Ziziphus'', ''Salvadora'', and ''Heliotropium'' species.〔Cooke, R. U., et al. ''Desert Geomorphology''. Taylor & Francis. 1993. pg. 357.〕 A nabkha is variable in size and shape. The typical example is a steep mound with a flat top. The larger nabkhas of the Chihuahuan Desert reach about 4.3 meters tall by 40 meters wide.〔 The largest known were 10 meters tall〔Milich, L. (Dunes. ) Arid Lands Resource Sciences. The University of Arizona. Tucson. 1998.〕 and a kilometer long; these large dunes have been called mega-nabkhas.〔 Mega-nabkhas in various places have also been called precipitation ridges, bordering dune ridges, and in Dutch, ''randwallen''.〔 Sometimes only the tips of the branches of the plant within protrude from the sand, and sometimes the plant is completely engulfed.〔Saqqa, W. and M. Atallah. (2004). (Characterization of the aeolian terrain facies in Wadi Araba Desert, southwestern Jordan. ) ''Geomorphology'' 62(1) 63-87.〕 Nabkhas may join together and form broad, hilly dune fields, or more often, chains and scattered dune patches. The sand composition is also variable. Silt and clay pellets can be part of the dune.〔 Nabkhas in the Namib Desert are rich in sediment.〔 On the Kuwaiti coastline nabkhas form around ''Nitraria retusa'' growing on sabkhas, a form of salt pan. These dunes are composed of quartz, gypsum, and feldspar sands mixed with bits of volcanic rock and calcite grains. Fragments of mollusc shell and oolites can be found. Traces of garnet, zircon, topaz, and tourmaline occur. Dolomite is common. Coastal nabkhas can be eroded by saltwater, and even washed away completely, leaving the plant behind.〔 The type of plant influences the shape of the nabkha; for example, treasure flower (''Gazania rigens'') forms tall, conical or elongated dunes, while beach daisy (''Arctotheca populifolia'') forms shorter, semi-circular dunes.〔Hesp, P. and A. McLachlan. (2000). (Morphology, dynamics, ecology and fauna of ''Arctotheca populifolia'' and ''Gazania rigens'' nabkha dunes. ) ''Journal of Arid Environments'' 44(2) 155-72.〕 ''Retama raetam'' is a salinity-tolerant woody shrub that can extend roots up to 20 meters deep in the soil, persisting through harsh conditions and eventually building nabkhas up to 8 meters wide.〔 A plant generally cannot hold enough sand to start a nabkha until it is at least 10 to 15 centimeters tall.〔Mares, M. A. ''Encyclopedia of Deserts''. University of Oklahoma Press. 1999. pg. 189.〕〔Ahmed, M. M., et al. (2009). (Chemical and morphological characteristics of phytogenic mounds (nabkhas) in Kuwait. ) ''Arab Gulf Journal of Scientific Research'' 27(3) 114-26.〕 Some sources describe nabkhas as ephemeral features of the landscape, but other research shows that they may last longer than previously thought.〔 Some last over 100 years.〔 Still, a nabkha on an annual plant generally only lasts one season. A nabkha can also be self-limiting if the sand accumulation lifts the plant far enough out of the water table that it dies.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nabkha」の詳細全文を読む スポンサード リンク
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