Dune movement has only been measured in a few dune systems and most of the published records are based on estimates from maps, the height of sand on fence posts, houses, and trees, etc. The results show that the rate of movement varies considerably among systems, varying from a few centimeters per year to 70 m per month, the latter in New Zealand (personal observation of Patrick Hesp). Dune formation depends on an adequate supply of sand and the wind to transport it. The interaction of wind and vegetation is of primary importance for dune growth. Colonization by plants accelerates dune growth, because surface roughness created by vegetation decreases wind flow and increases sand deposition. Several plants show an inherent capacity to bind sand and are able to develop extensive horizontal and vertical rhizome systems. The growth form and the ecological dynamics of dune plants are important contributors to foredune growth. Rhizomatous growth (as in the grass Ammophila) or sto-loniferous growth (as in Ipomoea or Spinifex) can extend the foredune depositional area by 5-15 m in a few months. Elymus arenaria (Europe) develops vertical rhizomes 150 cm long and Ipomoea pes-caprae (pantropical) can have 25-m-long branches that are buried two or three times along their length. Figure 1 shows species that are able to survive and reproduce successfully under high rates of sand mobility in different parts of the world. In each region, sand-tolerating species have evolved, and they play a very important role in dune formation. Sand deposition produces vigorous growth in some of these species; both plant height and plant cover increase, making these species excellent dune fixers. Many hypotheses have been suggested to explain this response of sand dune plants, but there are few studies in which the explanations are based on experimental evidence. Changes in soil temperature, increased space for root development, higher nutrient and moisture availability, a response to darkness, meristem stimulation, and interactions with endomycorrhizae and nematodes are probably factors that play an important role in this response.
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