The lowlands are characterized by broadleaf deciduous forests, with English oak (Quercus robur), sweet chestnut (Castanea sativa), European ash (Fraxinus excelsior), lindens (Tilia platyphyllos and Tilia cordata), wych elm (Ulmus glabra), maples (Acer pseudoplatanus and Acer platanoides), Quercus petraea, Picea abies, Abies alba, Alnus glutinosa, Fagus sylvatica, Taxus baccata, Malus sylvestris, Viburnum lantana, Rhamnus cathartica, Ulmus minor, Populus alba, Pinus sylvestris, Betula pendula, Populus tremula, Populus nigra, Salix alba, Juglans regia, and Corylus avellana.

Characteristically Mediterranean evergreen trees and shrubs, including holm oak (Quercus ilex), cork oak (Quercus suber), sweet bay (Laurus nobilis), strawberry tree (Arbutus unedo), Ziziphus spina-christi, Olea europaea, Balanites aegyptiaca, Cupressus sempervirens, Vachellia tortilis, Phoenix canariensis, Phoenix dactylifera, Salix alba, Juglans regia, Populus alba, Populus nigra, Quercus rotundifolia, Quercus ilex, Quercus pubescens, Quercus coccifera, Pistacia atlantica, Pistacia terebinthus, Juniperus communis, Pinus nigra, Pinus pinaster, Pinus pinea, Pinus halepensis, Ceratonia siliqua, Erica arborea, Vachellia flava, Senegalia senegal, Prosopis cineraria, Aerva javanica, Prunus amygdalus, Corylus avellana, and Rhamnus alaternus, can be found in warm and well-drained areas like limestone outcrops.

Maritime pine (Pinus pinaster) is common on Atlantic coastal dunes in northern Portugal and western Galicia, and further inland on rocky dry slopes.

[2] Large mammals include Cantabrian brown bear (Ursus arctos), Iberian wolf (Canis lupus signatus), Cantabrian chamois (Rupicapra pyrenaica parva), wild boar (Sus scrofa), red deer (Cervus elaphus elaphus), and roe deer (Capreolus capreolus).

The reclusive Pyrenean desman (Galemys pyrenaicus) is a vulnerable small mammal that lives here and in the Pyrenees.