Short answers, pulled from the story.
Sacred trees and groves were places of religious significance for the Germanic peoples, honored both as individual trees and as groups of trees. The Roman historian Tacitus reported that Germanic cult practices took place exclusively in groves rather than temples. Scholars trace reverence for individual trees to the mythological role of the world tree, Yggdrasil.
Tacitus wrote that the Germanic peoples 'consecrate woods and groves and they apply the name of gods to that mysterious presence which they see only with the eye of devotion.' He described specific examples, including a grove dedicated to the goddess Nerthus and a grove used by the Semnones for regular human sacrifice.
After the nominal Christianisation of the Anglo-Saxons and Saxons in the 7th and 8th centuries, worship at trees and gift-giving to them became punishable crimes. 11th century accounts still describe votive offerings at trees in England and grove worship in Saxony, and later folklore continued the practice through tree spirits such as Askafroa and traditions like the Scandinavian warden tree.
Yggdrasil is the immense ash tree at the center of the Norse cosmos, its branches and roots reaching into the nine worlds. Scholars believe reverence for real-world sacred trees derives from Yggdrasil's mythological role, and other named trees such as Hoddmímis holt, Læraðr, and Mímameiðr are generally considered alternate names for it.
Two possible sites have been identified in Sweden. A birch stump surrounded by animal bones was found under the church on Frösön in Jämtland in 1984 and carbon dated to the late Viking Age, and possible burnt offerings were found on a hill at Lunda near Strängnäs in Södermanland.
Recorded examples include the Grove of Baduhenna in ancient Frisia, the Grove of Nerthus, the Grove of the Semnones, Donar's Oak near Hesse, the Irminsul near Obermarsberg, the sacred tree at Uppsala, and Caill Tomair near Hiberno-Norse Dublin.