The sand dune environment

The micro-environmental conditions of different soil habitats are influenced by prevailing vegetation, aspect, soil texture, soil colour and other variables that influence the incoming and outgoing solar energy. The variability is especially pronounced in sand dunes because of shifting substrate, burial by sand, bare areas among plants, porous nature of sand and little or no organic matter, especially during the early stages of dune development. Even within a dune system there is disparity in radiative heating of different habitats that is manifested as variation in micro-environmental factors such as relative humidity, temperature, light, moisture content and wind turbulence. The major factor affecting these changes is the establishment of vegetation that stabilizes the surface, adds humus, develops shade, aids in the development of soil structure and reduces the severity of drought on the soil surface. The system changes from an open desert-like sandy substrate on the beach to a mature, well-developed soil system with luxuriant plant communities. The principal topics discussed in this chapter include accounts of micro-environmental factors of coastal sand dunes that influence the growth and reproduction of colonizing species. The water content of the substratum in sandy soils is one of the most important limiting factors in plant growth. Sandy soils have high porosity and after a rain most of the water is drained away from the habitat because of the large interstitial spaces between soil particles and the low capacity of sand to retain water. Evaporation in open dune systems also removes substantial quantities of water. Lichter (1998) showed that evaporation was greater on non-forested dune ridges than on forested areas and the rate of soil drying was influenced by soil depth and dune location. After 3 days of a heavy rainfall there was a drastic decrease in the percentage of moisture (67–80%) at 0–5 cm levels in open habitats compared to only 30–36% in the forested dune ridges. The same measurements at 10–15 cm depths showed much lower reduction in the percentage of moisture. In the swale (slack) even though the evaporative demand was the same, there was actually an increase in moisture because of seepage from the dune ridges.