Due to the ancient customs of burning soil to create pastures for cattle and the intense grazing, these mountain ranges suffered from general erosive processes that brought about huge soil losses and an emergence of a straightforward stony substratum throughout the centuries. It’s no wonder that about half the surface covered by semi natural habitats in the park is made up of different types of bushes and pastures and that the wooded areas are small-sized (today is also on a recuperation phase). Besides, forest plantations, especially pines (Pinus insignis, Pinus pinaster and Pinus sylvestris) have a certain entity. The mountainous bushes are dominated by Cytisus spp., gorses (Ulex europaeus, Ulex minor), brooms (Genista spp.), rockroses (Cistus spp.) and ericaceae (Calluna vulgaris, Daboecia cantabrica, Erica spp.), whereas in the highest tops the most characteristic species are Genista florida, Cytisus purgans, Cytisus striatus and the tree heath (Erica arborea). In the east part of the park, there are Erica aragonensis and Genistella tridentata. In the clearings of Pterosparto lasianthi-Ericetum aragonensis appears the Iris boissieri, a beautiful Galician-Portuguese endemic iridacea, one of the most emblematic species in the natural park, which has here the biggest population of the three known in Spain and is considered to be on the verge of extinction (Libro Rojo de la Flora Vascular Amenazada de España, 2003).

The singular biogeographic position of the area, between the Eurosiberian and Mediterranean regions, allows the existence of deciduous tree species, typical of wet climate conditions and perennial species, adapted to situations of hard lack of water in summer. The distribution of the species varies according to altitude, orientation and exposure. On the mountain tops there are small formations of Pyrenean oak (Quercus pyrenaica), birches (Betula celtiberica), some yews (Taxus baccata), hollies (Ilex aquifolium) and rowans (Sorbus aucuparia). In a lower altitude we find the supra-temperate oakwood, characterised by the English oak (Quercus robur) and common bilberry (Vaccinium myrtillus). In the lower zones near to the valleys, we have the Galician Portuguese oakwood with Quercus robur, sweet chestnut (Castanea sativa), buckthorn (Frangula alnus), common hazel (Corylus avellana), laurel (Laurus nobilis), strawberry tree (Arbutus unedo) and cork oak (Quercus suber). All these small woods preserve a great interest and also have rare species such as the Portugal laurel (Prunus lusitanica), which is threatened and confined in Galicia to the Xurés and the River Arnoia valley, and the Eryngium duriaei subsp. juresianum, an umbeliferous on the verge of extinction in Spain which preserves in these mountains the best of its four populations, all of them located in Galicia. In this natura space we can also find black alders (Alnus glutinosa), willows (Salix spp.), narrow-leafed ashes (Fraxinus angustifolia), the small groups of holly oaks and groves of sweet chestnut trees around the towns. 

Other highly-valued vegetable communities, all of them considered as habitats of European interest and therefore included in the Annex I of the Habitats Directive, are the meadowlands, peat bogs, higrophile scrublands and, mainly several pioneering pastures typical of sandy soils. There are two other botanic jewels exclusive of these lands and Portugal: Armeria humilissubsp. odorata and Armeria humilis subsp. Humilis, both of them were recently classified as species on the verge of extinction in “Libro Rojo de la Flora Vascular Amenazada de España” (2003). A great number of endemic and subendemic taxa, less frequent in the autonomous region, meet in these scarce habitats, such as the Bryophytes (Andreaea migistospora and Schistostega pennata) or the vascular plants (Armeria beirana, Festucasummilusitana, Festuca elegans, Silene acutifolia, Centaurea aristata subsp. geresensis, Arabis juressi, Veronica micrantha, Echinospartium ibericum, etc).