What is meant by ‘shaft of beauty’? What is compared to the shaft?

The shaft of beauty actually mean ‘the beam of beauty’. As the tree grows high with its long narrow stem, it looks like a beautiful tower. Here the stem is compared to a shaft.

In Henry Bunner’s poem ‘The Heart of the Tree’, the ‘shaft of beauty’ is the trunk of the tree.
 
It is compared to a flagpole that holds the flag-like branches high in the sky.