Following the Aragonese Route, in Navarre, pilgrims will find the Foz de Arbayun Nature Reserve, right in the Sierra Leire. Covering a total area of 1,164 h, it is the largest of the three reserves incorporated into the Foz de Lumbier since 1987. This spectacular and deep canyon cut by the Salazar has a lenght of 6 km and a maximum depth of 385 km. It is considered an area of Special Protection for birds, and has a great geomorphological value.
In the Foz de Arbayún is located one of the biogeographical limits between the atlantic-Pyrenean and the Mediterranean vegetation. The extensions of beeches and pines as well as the colonies of Pyrenean firs predominate in the area. The most humid areas of the Foz contain communities of lindens, ash trees, maples, service trees, hazel trees and elms. There are some autochthonous species that grow in the rock cracks, such as the Saxifraga longifolia, Sarcocapnos enneaphylla, Saponaria glutinosa and Petrocoptis hispanica. At the foot of the gorge, and in the deepest areas grow oaks combined with other kinds of trees (Quercus faginea and Quercus coccifera)
The fauna of the Foz de Arbayún includes the most important colonies of birds of prey in Europe. The colony of vultures is the first in size of Navarre, and the second of the peninsula and the rest of Europe. There are also lammergeyers, Egyptian vultures, different kinds of eagles (Circaëtus gallicus, Hieraëtus pennatus and Aquila chrysaëtus), kites (Milvus milvus, Milvuns migrans), goshawks, eaglets (Circus pygargus and Circus cyaneus), eagle owls, falcons, etc. The mammals are represented by wild boars and roe deers.