Short answers, pulled from the story.
The Proto-Germanic word hagaz evolved into Old Norse hagr and Old English hearg to mean altar. Another term vé connects to Gaulish, Latin, and Old Irish words for holiness. A third noun produced Anglo-Saxon hlæw and Middle Low German busk which mean meadow bush or grove respectively.
Tacitus wrote his work Germania during the Roman period describing how Germanic peoples consecrated woods and groves instead of building temples. He reported sacrifices performed in forest clearings by survivors of the Cherusci after their victory at the Battle of the Teutoburg Forest. These accounts were based on reports from Germanicus regarding events like those involving the goddess Nerthus among the Semnones people.
Archaeologists discovered a mouldering birch stump surrounded by animal bones under the church on Frösön in Jämtland during 1984. Possible burnt offerings appeared on a hill at Lunda near Strängnäs in Södermanland where archeologist Gunnar Andersson argued these finds point to sacrificial remnants. Carbon dating placed the Frösön finds in the late Viking Age.
Donar's Oak stood near Hesse Germany as a sacred object held by pagan Saxons before Saint Boniface felled it in the eighth century. Wood from this oak reportedly built a church dedicated to Saint Peter at the same site. The tree was associated with thunder gods and served as a focal point for worship prior to its destruction.
An immense ash tree stands central to the cosmos connecting all nine worlds through its branches and roots. Three wells lie beneath its three roots including Urðarbrunnr where gods assemble daily in a thing assembly. Creatures such as the dragon Níðhöggr and squirrel Ratatoskr live within its structure while stags graze on its leaves.