imuho, Yggdrasil is astronomical, probably refers to the apparent celestial axis, which is of course the Earth's rotational axis. The horse of Odin was tethered to Yggdrasil, and that horse is probably Ursa Major.
I’m sure there is also astrological symbolism in it too. I believe it is possible it served/represented more than one purpose. Many ancient works had parallel symbolism and hidden symbolism. :)