Ulukayın is the world tree of Turkic myth: roots in the underworld, trunk in the middle, branches in the sky. It is the axis joining the three realms, and the shaman travels along it — climbing up and going down both happen on the tree.
The birch is not an arbitrary choice: across Siberia and the Altai the birch is sacred, the hoop of the shaman's drum is made from it, and wishing cloths are tied to its branches.
In Balbal, Ulukayın is both a floor (The World Tree, the second in the Middle World) and a skeleton — a board silhouette. The nine tiles of the Birch suit gather around this tree.