Sand and clay

The sediments brought by the river and the sands pushed inland by the sea currents and winds filled the shallow waters in the bay to build a young territory of sand and clay.

There are 3 main ecosystems in Doñana National Park: marshes on clay soils, dunes and forests- scrublands on sandy soil.

The cycle of the sand starts on the beaches of the Atlantic Ocean. The sea currents (probably affected by the construction of harbours and piers in the province) are today changing Doñana coast line. 

The dominating southwest wind called locally "foreño" push the sand north, away from the wet shores. A small number of species of plants adapted to this difficult habitat stop the sand which accumulates around them in small mounts called Embryonic Dunes. These small dunes grow bigger thanks to the continuous supply of sand from the beach to build the first proper line of dunes parallel to the coast line. This line will move away from the sea pushed by the strong winds and grow taller and taller as it get closer and closer to the marshes where the journey of the dry sands end. In rainy years the water will wash the sand away from the marshes and bring it back to the sea to close the cycle.

There are up to six of these dune lines, the gaps among them are colonized by the vegetation, always ready to take advantage of any available space . But pine trees and junipers are not enough to stop the marching dunes. The first are just buried by them while the last have adapted to survive thanks to a sort of floating roots which allow them to ride over the top of the dune. At the rear side of the dune, vegetation will colonize again the empty land left behind. The dunes move at an estimated average rate of 3 - 6 metres a year and the highest goes up to some 35 metres over the sea level. This is the one located closest to the marshes and is locally called "Cerro de los Ánsares" (Geese Hill).

In the interdune spaces (locally called "corrales") and areas of stabilized old sands the vegetation flourish to create different associations depending on the quality of the conditions.

In the south, next to the mouth of the Guadalquivir River, old plantations of Stone Pine spread over the area. Most of the fixed sands to the east are covered by two main kinds of bush associations. The so called locally "Monte Negro" (Black Scrub) in the wetter places where briarwood, gorse, blackberry and flax abound. And the "Monte Blanco" (White Scrub) in poorer soils, where Halimium halimifolium dominates, and other rockroses and aromatic plants grow in a dense covering.

In the north we can still find some patches of the old Cork Oak Forest like that in Matasgordas. 

In the south-eastern edge of the park there are some well preserved patches of Phoenicean Juniper woods, formerly much more extended over the young stabilized dunes.

This is the largest ecosystem with its more than 30,000 has and the one responsible for Doñana´s singularity. Natural evolution of marshes and the man made transformations in the water system have made it dependant on the seasonal rain to get flooded.

The unevenness of the large plain lead to areas with different soil conditions and humidity. Thus different habitats are found in this apparently flat land. 

In the lower areas the water has washed away the salt allowing the domain of the trowel plants like bulrush and the Castañuela Tuber (Scirpus maritimus). In normal years these areas stay flooded from October to May-June.

In some other areas, lower than the average and called locally "lucios" , the salt concentrates preventing the growth of vegetation.

The "marisma seca" (dry marshland), just a few inches higher, stays as a salty steppe most part of the year. Here Quenopodiacea dominate.

There are also some small areas higher than average that are rarely flooded where wild animals and livestock find refugee during high floods. They are locally called "vetas"

All along the edge of the marshes in contact with the sands, we found an extremely important stripe of land. This is what we called "vera" (edge). Here the water accumulated in the water table under the sand dunes is slowly pushed out continuously even during the summer, when no other sources of water are found by the animals.