These woodlands are formed by willows, poplars, alder, ash, elm and sometimes by Pyrenean oak ("Quercus pyrenaica"), lime trees, birches and hazel trees. When the humidity starts to diminish in barren areas of the Ebro valley, the Levante and the southern half of the peninsula, the dryness is often accompanied by an increase of salts in the ground; under such conditions one can find formations of tamarisk shrubs, oleanders and giant reeds ("Saccharum ravenae"), sometimes accompanied by heather. In soils rich in silica but not in salts, such as those of the Sierra Morena and the Montes de Toledo, spurge appears, accompanied, in the warmest places, by oleanders and tamarisks.