The mountain layer is characterised by the presence of beeches and at times, in the Pyrenees, by silver firs ("Abies alba"); these beeches and silver firs occupy the cool slopes with shallow soil. The Mediterranean influence is felt in the presence of Holm oaks ("Quercus ilex") with bay laurel, which are situated on the warmest crests and slopes, especially above chalky soil, where the dryness becomes more pronounced. Improvement by humans has transformed much of this woodland into meadows, which conserve at their edges remnant hedgerows, "setos", of the species of the primitive forest. Clumps of thorny shrubs grow also in glades and clearings, such as the wild rose, blackberry bushes, blackthorn, hawthorn and other more or less thorny shrubs; this role can also be filled by smaller thorny plants, "los piornales", and clumps of broom. The major forests in this area are beech, oak, birch, and fir. Beech forests ("Fagus sylvatica") are found in the mountain layer of the Iberian Eurosiberian region from 800 to 1500 metres up. The soil is cool, as often chalky as siliceous (rich in silica), and nearly always acidified by rain. The layer is characterised by the beech tree. The beech tree projects a deep shadow, and so its dense foliage usually excludes other woodland species. It therefore has little undergrowth. In spite of their Atlantic character, these forests reach Moncayo, in the centre of the peninsula. The southernmost are at the Hayedo de Montejo (in the autonomous community of Madrid) and in the northernmost area of the province of Guadalajara, in the Parque Natural del Hayedo de Tejera Negra (Cantalojas), and Somosierra-Ayllón. The forests seek watercourses and shade, and so their reforestation is very difficult and they are being displaced by the Pyrenean Oak ("Quercus pyrenaica"). The Irati "rainforest", of some 170 square kilometres in the Navarran Pyrenees, is one of the most important beech and fir forests in Europe. Oak forests, above all of pedunculate oak ("Quercus robur"), are the most common in the Atlantic zone. They represent the typical forest floor formation of basal trees, extending to an altitude of some 600 metres.