The video below is of the swearing in ceremony for the "lehendakari" (term referring to the president of Basque Country which literally means "the first one"). The second half of the video (starting at about 2:30 minutes into the video) shows a swearing in ceremony under the Oak tree of Guernica (also spelled Gernika).
The image below is of the coat of arms for the Gernika-Lumo municipality with the Oak tree front and center. There is a strong link between the governance of the Basque Country and the symbolic tree.
For many centuries local councils would meet under special trees to make regional decisions. Over time these local councils gave way to just one council that meet under the tree in Guernica. By 1512 this one tree and its council came to represent the Basque people as a whole. Obviously these oak trees do not last forever but according to tradition new trees are planted from the acorns of the tree that preceded it.
One can only imagine the horror that the Basque people must have felt when in 1937 during the Spanish Civil War the town of Guernica was bombed by the German Condor Legion in support of General Franco. The tree survived the attack but died later of a fungus. The current tree has only been growing since 1986 and was re-located to the site of the original tree in 2005.
Below is the coat of arms for the Basque province of Biscay also with the tree front and center with the shape of a cross in the background and with an oak wreath around the edge.
Oak trees and oak leaves are found on many other symbols representing the Basque people and their ancient culture.