The primary source of water for dune plants is rainfall. Radiation causes considerable diurnal and nocturnal temperature variations. These fluctuations in soil temperature are sufficient to cause the periodical condensation ofwater vapor in the soil. This produces an increase in water availability from dew that is sufficient to maintain plants in rainless periods. Fog can be another source of water, but in some areas it contains salt. Studies in open dune communities have shown that soil moisture increases to depths of about 60 cm below the dune surface and then tends to fall off. In closed dune communities, where the soil has some organic matter, rainfall is absorbed and held near the surface where it is available to roots. Experiments with Chamaecrista chamaecristoides seedlings, a species that thrives in mobile dunes, showed that they had the ability to withstand total lack of watering for more than 80 days. This probably allows them to survive during the dry months of the year in the dunes of the Gulf of Mexico.
In a wet year, there may be widespread flooding in dune depressions. Blowouts are wind hollows or basins of exposed sand within dunes, called slacks or depressions. They frequently arise through the erosion of deflated areas in poorly vegetated dunes. The deflation limit is reached owing to the presence of water, algae, or the accumulation of coarse immovable material. Deposition occurs around the borders of the blowout and vegetation may recolonize the area. The water table falls during the dry season and recovers during the rainy months and the composition of the plant community reflects this ground-water regime. When the soil is completely flooded, the prevailing anaerobic conditions can influence its chemical composition and the concentration of nutrients, affecting plant survival and growth. Flooding can cause the local extinction of some non-wetland species and facilitate the establishment of others.
The frequency and duration of slack inundation are factors that can alter the distribution of vegetation and plant community composition. When flooding takes place occasionally, on very wet years, many of the plants die, and when the water recedes, colonization takes place again. In wet slacks that flood every year, water-loving plants establish and a completely different set ofspecies is found in these areas. Thus community composition will depend on the differential tolerance of plants to the environmental conditions associated with inundation, particularly anoxia. Species are good indicators of the water table depth. In temperate areas, Erica tetralix, Glyceria maxima, Carex nigra, and Juncus effusus are some of the more common species. In Europe, slacks are very important for endemic and rare species. In tropical regions, Cyperus articulatus, Lippia nodiflora, Hydrocotyle bonariensis (Mexico) and Paspalum maritimum, Fimbrisitylis bahiensis, Marcetia taxifolia (Brazil) are frequently found in these depressions. Thickets are also common and in Mexico they are formed by Pluchea odorata, Chrysobalanus icaco, and Randia laetevirens. In Brazil, there is Ericaceae scrub dominated by Humiria balsmifera, Protium icicariba, and Leucothoe revoluta.
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