Symbiotic associations involving nitrogen fixation by microorganisms are frequent in dunes. There are nitrogen-fixing bacteria such as Rhizobium, which form a symbiosis with numerous forbs and shrubs in temperate and tropical dune systems. Some of the plants showing nodules are Ulex europaeous, Trifolium spp., Lupinus arbor-eus, and Hippophae rahmnoides in Europe, Acacia shrubs in South Africa, and Chamaecrista chamaecristoides in Mexico.
In foredunes and mobile dunes, pioneer grasses such as Ammophila, Elytrigia, and Uniola show different degrees of infection by vesicular-arbuscular mycorrhizae (VA). The major benefit to these grasses is probably enhanced phosphorus uptake under conditions of phosphorus limitation. They also help in the aggregation of sand particles. Tropical sand dune plants also frequently show symbiosis with mycorrhizae.
Sand dunes are harsh environments where abiotic factors act as filters that determine species survival. The interactions between abiotic and biotic factors in sand dunes change as dunes mature. They are delicate systems in which plant cover, formed by different vegetation structures and species assemblages, maintains the system in a stabilized condition. Higher diversity is found when there are different habitats. Today, these fragile systems are endangered and the urbanization of the coast is increasing. We must find ways to make our activities and dune conservation compatible.
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